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Strategies for mitigation of climate change: a review

Strategies for mitigation of climate change: a review Climate change is defined as the shift in climate patterns mainly caused by greenhouse gas emissions from natural systems and human activities. So far, anthropogenic activities have caused about 1.0 °C of global warming above the pre-industrial level and this is likely to reach 1.5 °C between 2030 and 2052 if the current emission rates persist. In 2018, the world encountered 315 cases of natural disasters which are mainly related to the climate. Approximately 68.5 million people were affected, and economic losses amounted to $131.7 billion, of which storms, floods, wildfires and droughts accounted for approximately 93%. Economic losses attributed to wildfires in 2018 alone are almost equal to the collective losses from wildfires incurred over the past decade, which is quite alarming. Furthermore, food, water, health, ecosystem, human habitat and infrastructure have been identified as the most vulnerable sectors under climate attack. In 2015, the Paris agreement was introduced with the main objective of limiting global temperature increase to 2 °C by 2100 and pursuing efforts to limit the increase to 1.5 °C. This article reviews the main strategies for climate change abatement, namely conventional mitigation, negative emissions and radiative forcing geoengineering. Conventional mitigation technologies focus on reducing fossil-based CO emissions. Negative emissions technologies are aiming to capture and sequester atmospheric carbon to reduce carbon dioxide levels. Finally, geoengineering techniques of radiative forcing alter the earth’s radiative energy budget to stabilize or reduce global temperatures. It is evident that conventional mitigation efforts alone are not sufficient to meet the targets stipulated by the Paris agreement; therefore, the utilization of alternative routes appears inevitable. While various technologies presented may still be at an early stage of development, biogenic-based sequestration techniques are to a certain extent mature and can be deployed immediately. Keywords Climate change mitigation · Negative emissions technologies · Carbon dioxide removal · Decarbonization technologies · Radiative forcing geoengineering technologies AbbreviationsGt Gigatons Bio-DME Bio-dimethyl etherGW Gigawatt BECCS Bioenergy carbon capture and storageHFCs Hydrofluorocarbons Bil Bismuth triiodide H Hydrogen 3 2 Ca Calcium INDCs Intended nationally determined contributions CO Carbon dioxide IPCC Intergovernmental Panel on Climate Change CO e Carbon dioxide equivalent IAEA International Atomic Energy Agency CRED Centre for Research on the Epidemiology of ITMOs Internationally transferred mitigation Disaster outcomes DACCS Direct air carbon capture and storageFe Iron CaO Lime CaCO Limestone * Ahmed I. Osman CH Methane aosmanahmed01@qub.ac.uk Mha Megahectare Mt Million tons School of Chemistry and Chemical Engineering, Queen’s NO Nitrous oxide University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK NHRE Non-hydro renewable energy The Bryden Centre, Letterkenny Institute of Technology, Letterkenny, Ireland Vol.:(0123456789) 1 3 2070 Environmental Chemistry Letters (2020) 18:2069–2094 OECD Or ganization for Economic Co-operation and total global greenhouse gas and fossil CO emissions. The Development rise of fossil C O emissions in 2018 is mainly driven by PFCs Perfluorocarbons higher energy demand. Furthermore, emissions related to −1 ha Per hectare land-use change amounted to 3.5 GtC O in 2018 (UNEP −1 year Per year 2019). Together in 2018, fossil-based and land-use-related REDD+ Reducing emissions from deforestation and CO emissions accounted for approximately 74% of the total forest degradation global greenhouse gas emissions. Methane (CH ), another SO Sulphur dioxide significant greenhouse gas, had an emission rate increase SF Sulphur hexafluoride of 1.7% in 2018 as compared to an annual increase of 1.3% PFCs Perfluorocarbons over the past decade. Nitrous oxide (N O) emissions, which t Ton are mainly influenced by agricultural and industrial activi- UNEP United Nations Environment Programme ties, saw an increase of 0.8% in 2018 as compared to a 1% UNFCCC United Nations Framework Convention on annual increase over the past decade. A significant increase Climate Change was, however, noted in the fluorinated gases during 2018 at W/m W att per square meter 6.1% as compared to a 4.6% annual increase over the past Mg Magnesium decade (UNEP 2019). To put these numbers into perspec- tive, a recent Intergovernmental Panel on Climate Change (IPCC) report demonstrated that anthropogenic activities so Introduction far have caused an estimated 1.0 °C of global warming above the pre-industrial level, specifying a likely range between 0.8 Status of climate change and 1.2 °C. It is stated that global warming is likely to reach 1.5 °C between 2030 and 2052 if the current emission rates Climate change is defined as the shift in climate patterns persist (IPCC 2018). mainly caused by greenhouse gas emissions. Greenhouse gas emissions cause heat to be trapped by the earth’s atmos- Climate change impacts, risks and vulnerabilities phere, and this has been the main driving force behind global warming. The main sources of such emissions are natural An understanding of the severe impact of climate change on systems and human activities. Natural systems include for- natural and human systems as well as the risks and associ- est fires, earthquakes, oceans, permafrost, wetlands, mud ated vulnerabilities is an important starting point in com- volcanoes and volcanoes (Yue and Gao 2018), while human prehending the current state of climate emergency. Changes activities are predominantly related to energy production, in climate indicators, namely temperature, precipitation, industrial activities and those related to forestry, land use seal-level rise, ocean acidification and extreme weather and land-use change (Edenhofer et al. 2014). Yue and Gao conditions have been highlighted in a recent report by the statistically analysed global greenhouse gas emissions from United Nations Climate Change Secretariat (UNCCS). natural systems and anthropogenic activities and concluded Climate hazards reported included droughts, floods, hur - that the earth’s natural system can be considered as self- ricanes, severe storms, heatwaves, wildfires, cold spells balancing and that anthropogenic emissions add extra pres- and landslides (UNCCS 2019). According to the Centre for sure to the earth system (Yue and Gao 2018). Research on the Epidemiology of Disasters (CRED), the world encountered 315 cases of natural disasters in 2018, GHG emissions overview mainly climate-related. This included 16 cases of drought, 26 cases of extreme temperature, 127 cases of flooding, 13 The greenhouse gases widely discussed in the literature and cases of landslides, 95 cases of storms and 10 cases of wild- defined by the Kyoto protocol are carbon dioxide (CO ), fire. The number of people affected by natural disasters in methane (CH ), nitrous oxide (N O), and the fluorinated 2018 was 68.5 million, with floods, storms and droughts 4 2 gases such as hydrofluorocarbons (HFCs), perfluorocar - accounting for 94% of total affected people. In terms of eco- bons (PFCs) and sulphur hexafluoride (SF ) (UNFCCC nomic losses, a total of $131.7 billion was lost in 2018 due 2008). According to the emissions gap report prepared by to natural disasters, with storms ($70.8B), floods ($19.7B), the United Nations Environment Programme (UNEP) in wildfires ($22.8B) and droughts ($9.7B) accounting for 2019, total greenhouse gas emissions in 2018 amounted to approximately 93% of the total costs. CRED also provides 55.3 GtCO e, of which 37.5 GtCO are attributed to fos- data on disasters over the past decade, which shows even 2 2 sil CO emissions from energy production and industrial higher annual averages in almost all areas, except for wild- activities. An increase of 2% in 2018 is noted, as compared fire cases. The economic losses attributed to wildfires in to an annual increase of 1.5% over the past decade for both 2018 alone are approximately equal to the collective losses 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2071 from wildfires incurred over the past decade, which is quite Asia. Double or triple rates of species extinction in terres- alarming (CRED 2019). Moreover, wildfires are a direct trial ecosystems are projected under the 2 °C level compared source of C O emissions. Although wildfires are part of the to 1.5 °C (IPCC 2018). It can be simply concluded that the natural system, it is clear that human-induced emissions are world is in a current state of climate emergency. directly interfering and amplifying the impact of natural system emissions. It is evident that human-induced climate Global climate action change is a major driving force behind many natural disas- ters occurring globally. Acknowledgement of climate change realities started in 1979 Furthermore, climate risks such as temperature shifts, when the first world climate conference was held in Geneva. precipitation variability, changing seasonal patterns, changes The world climate conference was introduced by the World in disease distribution, desertification, ocean-related impacts Meteorological Organization in response to the observation and soil and coastal degradation contribute to vulnerability of climatic events over the previous decade. The main pur- across multiple sectors in many countries (UNCCS 2019). pose was to invite technical and scientific experts to review Sarkodie et al. empirically examined climate change vul- the latest knowledge on climate change and variability nerability and adaptation readiness of 192 United Nations caused by natural and human systems as well as assess future countries and concluded that food, water, health, ecosystem, impacts and risks to formulate recommendations moving human habitat and infrastructure are the most vulnerable forward (WMO 1979). This was possibly the first of its kind sectors under climate attack while pointing out that Africa conference discussing the adverse effects of climate change. is the most vulnerable region to climate variability (Sarkodie In 1988, the Intergovernmental Panel on Climate Change and Strezov 2019). It is also important to note the intercon- (IPCC) was set up by the World Meteorological Organiza- nected nature of such sectors and the associated impacts. tion in collaboration with the United Nations Environment th The 15 edition of the global risks report 2020 prepared Programme (UNEP) to provide governments and official by the world economic forum thoroughly presented a num- bodies with scientific knowledge and information that can ber of climate realities, laying out areas that are greatly be used to formulate climate-related policies (IPCC 2013). affected. The risks included loss of life due to health hazards Perhaps, the most critical step taken, in terms of action, and natural disasters, as well as excessive stress on ecosys- was the adoption of the United Nations Framework Conven- tems, especially aquatic/marine systems. Moreover, food tion on Climate Change (UNFCCC) in 1992, which then and water security are other areas that are highly impacted. went into force in 1994. Since then, the UNFCCC has been Increased migration is anticipated due to extreme weather the main driving force and facilitator of climate action glob- conditions and disasters as well as rising sea levels. Geopo- ally. The main objective of the convention is the stabiliza- litical tensions and conflicts are likely to arise as countries tion of greenhouse gas concentrations in the atmosphere to aim to extract resources along water and land boundaries. prevent severe impacts on the climate system. The conven- The report also discusses the negative financial impact on tion set out the commitments to all parties involved, put- capital markets as systematic risks soar. Finally, the impact ting major responsibilities on developed countries to imple- on trade and supply chains is presented (WEF 2020). ment national policies to limit anthropogenic emissions and An assessment, recently presented in an Intergovernmen- enhance greenhouse gas sinks. The target was to reduce tal Panel on Climate Change (IPCC) special report, covered emissions by the year 2000 to the levels achieved in the the impacts and projected risks associated with 2 levels of previous decade. Moreover, committing developed country global warming, 1.5 °C and 2 °C. The report investigated the parties to assist vulnerable developing country parties finan- negative impact of global warming on freshwater sources, cially and technologically in taking climate action. The con- food security and food production systems, ecosystems, vention established the structure, reporting requirements and human health, urbanization as well as poverty and chang- mechanism for financial resources, fundamentally setting the ing structures of communities. The report also investigated scene for global climate policy (UN 1992). The convention climate change impact on key economic sectors such as tour- is currently ratified by 197 countries (UNCCS 2019). ism, energy and transportation. It is evident that most of During the third UNFCCC conference of the parties the impacts assessed have lower associated risks at 1.5 °C (COP-3) in 1997, the Kyoto protocol was adopted and compared to 2 °C warming level. We would likely reach went into force in 2005. The Kyoto protocol introduced 1.5 °C within the next 3 decades and increases in warm- the emission reduction commitments for developed coun- ing levels beyond this point would amplify risk effects; for tries for a five-year commitment period between 2008 and example, water stress would carry double the risk under a 2012. The protocol laid out all related policies, monitor- 2 °C level compared to 1.5 °C. An increase of 70% in popu- ing and reporting systems, as well as introduced three lation affected by fluvial floods is projected under the 2 °C market-based mechanisms to achieve those targets. The scenario compared to 1.5 °C, especially in USA, Europe and protocol introduced two project-based mechanisms, clean 1 3 2072 Environmental Chemistry Letters (2020) 18:2069–2094 development mechanism and joint implementation mecha- In 2012, the Doha amendment to the Kyoto protocol was nism. The clean development mechanism allows developed adopted, mainly proposing a second commitment period country parties to invest and develop emission reduction from 2013 to 2020 as well as updating emissions reduction projects in developing countries, to drive sustainable targets. The amendment proposed a greenhouse gas emis- development in the host country as well as offset carbon sions reduction target of at least 18% below 1990 levels. The emissions of the investing party. Joint implementation pro- amendment has not yet entered into force since it has not jects allow developed country parties to develop similar been ratified by the minimum number of parties required to projects, however, in other developed countries that are this date (UNFCCC 2012). protocol parties, offsetting excess emissions of the invest- During the twenty-first UNFCCC conference of the par - ing party. Furthermore, the protocol introduced an emis- ties (COP-21) held in Paris in 2015, the Paris agreement was sions trading mechanism as a platform to facilitate the adopted and entered into force in 2016. The Paris agreement trading of annually assigned emissions that are saved by added further objectives, commitments, enhanced compli- protocol members to those that exceed their limits (UNF- ance and reporting regulations, as well as support mecha- CCC 1997). Emission reduction has mainly been achieved nisms to the existing climate change combat framework in through the introduction of renewable energy, energy effi- place. The main objective of the agreement is to limit the ciency and afforestation/reforestation-related projects. global temperature increase to 2 °C by 2100 and pursue The Kyoto protocol defines four emission saving units, efforts to limit the increase to 1.5 °C. The agreement aims each representing one metric ton of CO equivalent and are to reach global peaking of greenhouse gases as soon as pos- all tradeable (UNFCCC 2005). sible as to strike a balance between human-induced emission sources and greenhouse gas sinks and reservoirs between 1 Certified emissions reduction unit, obtained through 2050 and 2100. The agreement also introduced new binding clean development mechanism projects. commitments, asking all parties to deliver nationally deter- 2 Emission reduction unit, obtained through joint imple- mined contributions and to enforce national measures to mentation projects. achieve, and attempt to exceed such commitments. Enhanced 3 Assigned amount unit, obtained through the trading of transparency, compliance and clear reporting and commu- unused assigned emissions between protocol parties. nication are advocated under the agreement. Furthermore, 4 Removal unit, obtained through reforestation-related the agreement encourages voluntary cooperation between projects. parties beyond mandated initiatives. Moreover, financial support and technological support, as well as capacity build- The Kyoto units and general framework introduced laid ing initiatives for developing countries, are mandated by the the structural foundation of a carbon emissions market and agreement. Such obligations are to be undertaken by devel- the concept of carbon pricing. Many national and regional oped country parties to promote sustainable development governments introduced emissions trading schemes; some and establish adequate mitigation and adaptation support are mandatory while others are voluntary. In some cases, measures within vulnerable countries. Perhaps, one of the such schemes are linked to Kyoto commitments and regu- most important goals established under the agreement is that lations. The largest emissions trading scheme introduced of adaptation and adaptive capacity building concerning the thus far is the European emissions trading scheme (Per- temperature goal set (UN 2015). dan and Azapagic 2011). Villoria-Saez et al. empirically Under article 6 of the agreement, two international mar- investigated the effectiveness of greenhouse gas emissions ket mechanisms were introduced, cooperative approaches trading scheme implementation on actual emission reduc- and the sustainable development mechanism. These mecha- tions covering six major emitting regions. The investigation nisms are to be utilized by all parties to meet their nation- presented a number of findings; first, it is possible to reduce ally determined contributions. Cooperative approaches are greenhouse gas emissions by approximately 1.58% annually a framework that allows parties to utilize internationally upon scheme implementation. Furthermore, after 10 years transferred mitigation outcomes (ITMOs) to meet nation- of implementation, approximately 23.43% of emissions ally determined contribution goals as well as stimulate sus- reduction can be achieved in comparison with a scenario tainable development. On the other hand, the sustainable of non-implementation (Villoria-Sáez et al. 2016). Another development mechanism is a new approach that promotes emission abatement instrument widely discussed in the liter- mitigation and sustainable development and is perceived as ature is carbon taxation. There is growing scientific evidence the successor of the clean development mechanism. There that carbon taxation is an effective instrument in reducing is still much debate and negotiations on such mechanisms greenhouse gas emissions; however, political opposition by moving forward (Gao et al. 2019). the public and industry is the main reason delaying many Nieto et al. conducted an in-depth systematic analysis of countries in adopting such mechanism (Wang et al. 2016). the effectiveness of the Paris agreement policies through the 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2073 evaluation of 161 intended nationally determined contribu- (Ricke et al. 2017). The main negative emissions techniques tions (INDCs) representing 188 countries. The study investi- widely discussed in the literature include bioenergy carbon gated sectoral policies in each of these countries and quanti- capture and storage, biochar, enhanced weathering, direct air fied emissions under such INDCs. The analysis concluded carbon capture and storage, ocean fertilization, ocean alka- that a best-case scenario would be an annual global emission linity enhancement, soil carbon sequestration, afforestation increase of approximately 19.3% in 2030 compared to the and reforestation, wetland construction and restoration, as base period (2005–2015). In comparison, if no measures well as alternative negative emissions utilization and storage were taken a 31.5% increase in global emissions is projected. methods such as mineral carbonation and using biomass in It is concluded that if the predicted best-case level of emis- construction (Lawrence et al. 2018; Palmer 2019; McLaren sions is maintained between 2030 and 2050 a temperature 2012; Yan et al. 2019; McGlashan et al. 2012; Goglio et al. increase of at least 3 °C would be realized. Furthermore, a 2020; Lin 2019; Pires 2019; RoyalSociety 2018; Lenzi 4 °C increase would be assured if annual emissions continue 2018). to increase (Nieto et al. 2018). Finally, a third route revolves around the principle of To meet the 1.5 °C target by the end of the century, the altering the earth’s radiation balance through the manage- IPCC stated that by 2030 greenhouse gas emissions should ment of solar and terrestrial radiation. Such techniques are −1 be maintained at 25–30 GtC Oe year . In comparison, the termed radiative forcing geoengineering technologies, and current unconditional nationally determined contributions the main objective is temperature stabilization or reduction. −1 for 2030 are estimated at 52–58 GtCOe year . Based on Unlike negative emissions technologies, this is achieved pathway modelling for a 1.5 °C warming scenario, a 45% without altering greenhouse gas concentrations in the atmos- decline in anthropogenic greenhouse gas emissions must be phere. The main radiative forcing geoengineering techniques reached by 2030 as compared to 2010 levels, and net-zero that are discussed in the literature include stratospheric aero- emissions must be achieved by 2050. To maintain a 2 °C sol injection, marine sky brightening, cirrus cloud thinning, global warming level by the end of the century, emissions space-based mirrors, surface-based brightening and various should decline by approximately 25% in 2030 as compared radiation management techniques. All these techniques are to 2010 levels and net-zero emissions should be achieved by still theoretical or at very early trial stages and carry a lot of 2070 (IPCC 2018). There is growing evidence that confirms uncertainty and risk in terms of practical large-scale deploy- that current mitigation efforts, as well as future emissions ment. At the moment, radiative forcing geoengineering tech- commitments, are not sufficient to achieve the temperature niques are not included within policy frameworks (Lawrence goals set by the Paris agreement (Nieto et al. 2018; Law- et al. 2018; Lockley et al. 2019). rence et al. 2018). Further measures and new abatement routes must be explored if an attempt is to be made to Conventional mitigation technologies achieve such goals. As previously discussed, energy-related emissions are the main driver behind the increased greenhouse gas concen- Climate change mitigation strategies tration levels in the atmosphere; hence, conventional miti- gation technologies and efforts should be focused on both Introduction the supply and demand sides of energy. Mitigation efforts primarily discussed in the literature cover technologies and There are three main climate change mitigation approaches techniques that are deployed in four main sectors, power on discussed throughout the literature. First, conventional miti- the supply side and industry, transportation and buildings gation efforts employ decarbonization technologies and tech- on the demand side. Within the power sector, decarboniza- niques that reduce C O emissions, such as renewable energy, tion can be achieved through the introduction of renewable fuel switching, efficiency gains, nuclear power, and carbon energy, nuclear power, carbon capture and storage as well capture storage and utilization. Most of these technologies as supply-side fuel switch to low-carbon fuels such as natu- are well established and carry an acceptable level of man- ral gas and renewable fuels. Furthermore, mitigation efforts aged risk (Ricke et al. 2017; Victor et al. 2018; Bataille et al. on the demand side include the efficiency gains achieved 2018; Mathy et al. 2018; Shinnar and Citro 2008; Bustreo through the deployment of energy-efficient processes and et al. 2019). sector-specific technologies that reduce energy consump - A second route constitutes a new set of technologies and tion, as well as end-use fuel switch from fossil-based fuels to methods that have been recently proposed. These techniques renewable fuels, and, moreover, the integration of renewable are potentially deployed to capture and sequester C O from power technologies within the energy matrix of such sectors the atmosphere and are termed negative emissions technolo- (Mathy et al. 2018; Hache 2015). This section will review gies, also referred to as carbon dioxide removal methods the literature on decarbonization and efficiency technologies 1 3 2074 Environmental Chemistry Letters (2020) 18:2069–2094 and techniques that cover those four main sectors introduced. 2006; Gude and Martinez-Guerra 2018; Akalın et al. 2017; Figure 1 depicts the conventional mitigation technologies Srivastava et al. 2017). and techniques discussed in the literature and critically In terms of power production, as of 2018, renewable reviewed in this paper. energy accounted for approximately 26.2% of global elec- tricity production. Hydropower accounted for 15.8%, while Renewable energy wind power’s share was 5.5%, photovoltaic solar power 2.4%, biopower 2.2% and geothermal, concentrated solar According to a recent global status report on renewables, power and marine power accounted for 0.46% of the gen- the share of renewable energy from the total final energy erated electricity (REN21 2019). While large-scale hydro- consumption globally has been estimated at 18.1% in 2017 power leads in terms of generation capacity as well as (REN21 2019). An array of modern renewable energy tech- production, there has been a significant capacity increase nologies is discussed throughout the literature. The most in photovoltaic solar power and onshore wind power over prominent technologies include photovoltaic solar power, the past decade. By the end of 2018, a total of 505 GW of concentrated solar power, solar thermal power for heat- global installed capacity for photovoltaic solar power has ing and cooling applications, onshore and offshore wind been noted as compared to 15 GW in 2008. Regarding wind power, hydropower, marine power, geothermal power, bio- power, 591 GW of global installed capacity is recorded in mass power and biofuels (Mathy et al. 2018; Shinnar and 2018 as compared to 121 GW in 2008. Global biopower Citro 2008; Hache 2015; REN21 2019; Hussain et al. 2017; capacity has been estimated at 130 GW in 2018 with a total Østergaard et al. 2020; Shivakumar et al. 2019; Collura et al. 581 TWh of production in that year. China has maintained Fig. 1 Major decarbonization technologies which focus on the utilization (CCU), fuel switching and efficiency gains. These tech- reduction of C O emissions related to the supply and demand sides nologies and techniques are mainly deployed in the power, industrial, of energy. Conventional mitigation technologies include renewable transportation and building sectors energy, nuclear power, carbon capture and storage (CCS) as well as 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2075 its position as the largest renewable energy producing coun- to alternative policy types (Pitelis et al. 2019). On barriers try, from solar, wind and biomass sources. The total share and drivers of renewable energy deployment, Shivakumar of renewable energy in global power capacity has reached et al. highlighted various dimensions that may hinder or approximately 33% in 2018 (REN21 2019). enable renewable energy project development. The main Besides the power sector, renewable energy can be points highlighted revolve around policy, financial access, deployed within the industry, transportation and building government stability and long-term intentions, administra- sectors. Photovoltaic and thermal solar energy as well as tive procedures and support framework or lack thereof, as industrial end-use fuel switch to renewable fuels such as well as the profitability of renewable energy investments solid, liquid and gaseous biofuels for combined thermal and (Shivakumar et al. 2019). Seetharaman et al. analysed the power production are examples of decarbonization efforts impact of various barriers on renewable energy deployment. through renewables. Buildings can also benefit from solar as The research confirms that regulatory, social and techno- well as biomass-based technologies for power, heating and logical barriers play a significant role in renewable energy cooling requirements. In relation to the transportation sector, deployment. The research does not find a significant direct end-use fuel switch is a determinant to sector decarboni- relationship between economic barriers and project deploy- zation. Some examples of biofuels are biodiesel, first- and ment; however, the interrelated nature between the economic second-generation bioethanol, bio-hydrogen, bio-methane dimension with regulatory, social and technological barriers and bio-dimethyl ether (bio-DME) (Srivastava et al. 2020; affects deployment, however, indirectly (Seetharaman et al. Chauhan et al. 2009; Hajilary et al. 2019; Osman 2020). 2019). Furthermore, hydrogen produced through electrolysis using In terms of the relationship between financial accessibil- renewable energy is a potential renewable fuel for sector ity and renewable energy deployment, Kim et al. empiri- decarbonization. Another example of sector decarbonization cally investigated such relationship by analysing a panel data through renewable energy deployment is electric vehicles set of 30 countries during a 13-year period from 2000 to using renewable power (Michalski et al. 2019). Other mitiga- 2013. Statistical evidence shows the positive impact of well- tion measures within these sectors will be further discussed developed financial markets on renewable energy deploy - in the following section. ment and sector growth. Furthermore, the study confirms a Variable renewables, such as solar and wind, are key tech- positive and significant relationship between market-based nologies with significant decarbonization potential. One of mechanisms, such as clean development mechanism, with the main technological challenges associated is the intermit- renewable energy deployment. There is a strong impact on tent nature/variability in power production. This has been photovoltaic solar and wind technologies, while the impact overcome by integrating such technologies with storage as is marginal under biomass and geothermal technologies well as other renewable baseload and grid technologies. Sin- (Kim and Park 2016). sel et al. discuss four specific challenge areas related to vari- Pfeiffer et al. studied the diffusion of non-hydro renew - able renewables, namely quality, flow, stability and balance. able energy (NHRE) technologies in 108 developing coun- Furthermore, they present a number of solutions that mainly tries throughout a 30-year period from 1980 to 2010. Based revolve around flexibility as well as grid technologies for on the results, economic and regulatory policies played a distributed as well as centralized systems (Sinsel et al. 2020). pivotal role in NHRE deployment, as well as governmen- Economic, social and policy dimensions play an influ - tal stability, higher education levels and per capita income. encing role in renewable energy technology innovation and On the other hand, growth in energy demand, aid and high deployment. Pitelis et al. investigated the choice of policy local fossil fuel production hindered NHRE diffusion. In instruments and its effectiveness in driving renewable energy contrast with Kim et al., the study finds weak support to technology innovation for 21 Organization for Economic show that international financing mechanisms and financial Co-operation and Development (OECD) countries between market development positively influenced diffusion (Pfeiffer 1994 and 2014. The study classified renewable energy poli- and Mulder 2013). The reason may be related to how the cies into three categories: technology-push, demand-pull analysis was constructed, different data sets, periods and and systemic policy instruments. Furthermore, the study statistical methods. investigated the impact of each policy classification on inno- Decarbonization through renewable energy deploy- vation activity of various renewable energy technologies: ment is extremely significant. Development of renew- solar, wind, biomass, geothermal and hydro. The study con- able energy projects should be seen as a top priority. The cluded that not all policy instruments have the same effect areas that would drive decarbonization through renew- on renewable energy technologies and that each technol- able energy and should be focused upon by policymakers, ogy would require appropriate policies. However, the study financiers and market participants include policy instru- suggested that demand-pull policy instruments are more ments, financial support and accessibility, and market- effective in driving renewable energy innovation compared based mechanisms to incentivize project developers. 1 3 2076 Environmental Chemistry Letters (2020) 18:2069–2094 Moreover, governmental support frameworks, public edu- Carbon capture, storage and utilization cation for social acceptance as well as research and devel- opment efforts for technological advances and enhanced Carbon capture and storage is a promising technology efficiencies are important focus areas. discussed in the literature as a potential decarbonization approach to be applied to the power as well as the industrial sectors. The technology consists of separating and captur- Nuclear powering CO gases from processes that rely on fossil fuels such as coal, oil or gas. The captured CO is then transported According to the latest report prepared by the interna- and stored in geological reservoirs for very long periods. tional atomic energy agency (IAEA), as of 2018, 450 The main objective is the reduction in emission levels while nuclear energy plants are operational with a total global utilizing fossil sources. Three capturing technologies are dis- installed capacity of 396.4 GW. It is projected that an cussed in the literature: pre-combustion, post-combustion increase of 30% in installed capacity will be realized by and oxyfuel combustion. Each technology carries a specific 2030 (from a base case of 392 GW in 2017). As a low- process to extracting and capturing CO . Post-combustion case projection scenario, it is estimated that by 2030 a capture technologies, however, are the most suitable for ret- 10% dip might be realized based on the 2017 numbers. On rofit projects and have vast application potential. Once CO the long term, it is projected that global capacity might has been successfully captured, it is liquie fi d and transported reach 748 GW by 2050, as a high-case scenario (IAEA through pipelines or ships to suitable storage sites. Based on 2018). Pravalie et al. provide an interesting review of the the literature, storage options include depleted oil and gas status of nuclear power. The investigation demonstrates fields, coal beds and underground saline aquifers not used for the significant role nuclear power has played in terms potable water (Vinca et al. 2018). Some of the main draw- of contribution to global energy production as well as backs of carbon capture and storage include safety in relation its decarbonization potential in the global energy sys- to secured storage and the possibility of leakage. Negative tem. The study presents an estimation of approximately environmental impacts that may result from onshore stor- 1.2–2.4 Gt C O emissions that are prevented annually age locations that undergo accidental leakage have been from nuclear power deployment, as alternatively the investigated by Ma et al. The investigation focused on the power would have been produced through coal or natural impact of leakage on agricultural land (Ma et al. 2020). Risk gas combustion. The paper suggests that to be in line with of leakage and associated negative impacts have also been the 2 °C target stipulated by the Paris agreement, nuclear pointed out by Vinca et al. (2018). Other issues related to plant capacity must be expanded to approximately 930 this technology include public acceptance (Tcvetkov et al. GW by 2050, with a total investment of approximately $ 2019; Arning et al. 2019) as well as the high deployment 4 trillion (Prăvălie and Bandoc 2018). costs associated (Vinca et al. 2018). Another pathway post- Although nuclear energy is considered as a low-carbon carbon capture is the utilization of the CO captured in the solution for climate change mitigation, it comes with a production of chemicals, fuels, microalgae and concrete number of major disadvantages. First, the capital outlay building materials, as well as utilization in enhanced oil and operating costs associated with nuclear power devel- recovery (Hepburn et al. 2019; Aresta et al. 2005; Su et al. opment are quite significant. Furthermore, risk of envi- 2016; Qin et al. 2020). ronmental radioactive pollution is a major issue related Large-scale deployment of carbon capture storage and to nuclear power, which is mainly caused through the utilization technologies is yet to be proven. According to threat of reactor accidents as well as the danger associated the international energy agency, there are only 2 carbon cap- with nuclear waste disposal (Prăvălie and Bandoc 2018; ture and storage projects under operation as of 2018, with Abdulla et al. 2019). While conventional fission-based a combined annual capture capacity of 2.4 MtCO . There nuclear plants are suggested to be phased out in future, are 9 more carbon capture projects under development and the introduction of enhanced fusion-based nuclear tech- are projected to increase capacity to 11 MtC O by 2025; nology may positively contribute to mitigation efforts in however, a significant deviation exists from the sustainable the second half of the century. Fusion power is a new development scenario targeted by the international energy generation of nuclear power, which is more efficient than agency for 2040 which is a capacity of 1488 MtC O (IEA the conventional fission-based technology and does not 2019a). carry the hazardous waste disposal risk associated with conventional fission-based nuclear technology. Further- Fuel switch and efficiency gains more, fusion power is characterized as a zero-emission technology (Prăvălie and Bandoc 2018; Gi et al. 2020). Fuel switching in the power sector from coal to gas, in the short-term, has been discussed extensively in the literature 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2077 as a potential approach to economically transition to a low- within aviation, shipping and rail, although rail is currently carbon and hopefully a zero-carbon economy in future (Vic- one of the most energy-ec ffi ient modes. Ec ffi iency measures tor et al. 2018; Wendling 2019; Pleßmann and Blechinger in the transportation sector can also take other forms. For 2017). The move to natural gas is also applicable to industry, example, travel demand management, to reduce frequency transportation and building sectors; however, as discussed and distance of travel, can be an interesting approach. More- previously the switch to renewable fuels is a more sustain- over, shifting travel to the most efficient modes where pos- able approach creating further decarbonization potential in sible, such as electrified rail, and reducing dependence on these sectors. high-intensity travel methods can play an interesting role in In addition to fuel switching, efficiency gains are of enhancing efficiency (IEA 2019b). extreme significance within mitigation efforts. Efficiency gains in the power sector are achieved through improve- Negative emissions technologies ments in thermal power plants by enhancing the efficiency of fuel combustion as well as improving turbine generator Most of the climate pathways that were investigated by efficiencies. Furthermore, waste heat recovery for additional the Intergovernmental Panel on Climate Change (IPCC) thermal as well as electric production enhances efficiency. In included the deployment of negative emissions technolo- gas-fired power plants, the utilization of a combined cycle gies along with conventional decarbonization technologies technology enhances the efficiency significantly. Combined to assess the feasibility of achieving the targets mandated heat and power units have also played an interesting role in by the Paris agreement. Only two negative emissions tech- efficiency gains. Technological advances within transmis- nologies have been included in the IPCC assessments so far, sion and distribution networks also enhance efficiencies by bioenergy carbon capture and storage as well as afforestation reducing losses (REN21 2019). and reforestation (IPCC 2018). In industry, there are many potential areas where effi- Gasser et al. empirically investigated the potential nega- ciency gains may be realized. For example, in steel and tive emissions needed to limit global warming to less than cement applications, waste heat can be recovered for onsite 2 °C. The analysis utilized an IPCC pathway that is most power and heat production through the installation of waste likely to maintain warming at such level and constructed heat-driven power plants that utilize waste heat from exhaust a number of scenarios based on conventional mitigation gases. For industries that utilize process steam, there is an assumptions in an attempt to quantify the potential nega- excellent opportunity to utilize waste steam pressure to tive emissions efforts required. The results indicated that generate electric power for onsite usage or drive rotating in the best-case scenario, that is under the best assumptions equipment. The application of back pressure steam turbines on conventional mitigation efforts, negative emissions of −1 in areas where steam pressure reduction is required can 0.5–3Gt C year and 50–250 Gt C of storage capacity are enhance energy efficiency significantly. The same approach required. Based on a worst-case scenario, negative emis- −1 can be deployed in applications where gas pressure reduc- sions of 7–11 Gt C year and 1000–1600 Gt C of storage tion is required, however, using turboexpanders. Waste gases capacity are required. (1 Gigaton Carbon = 3.6667 Gigaton from industrial processes can also be utilized to generate CO e) The results indicate the inevitable need for negative onsite heat and power using micro- and small gas turbines. emissions, even at very high rates of conventional mitiga- In addition, further efficiency gains can be realized through tion efforts. Furthermore, the study suggests that negative the deployment of advanced machinery controls in a multi- emissions alone should not be relied upon to meet the 2 °C tude of processes and industrial sectors. target. The investigation concluded that since negative emis- A number of factors influence energy efficiency within sions technologies are still at an infant stage of development, buildings, first the building design as well as materials uti- conventional mitigation technologies should remain focused lized in construction, e.g. insulation and glazing. Further- upon within climate policy, while further financial resources more, appliances, devices and systems used throughout are to be mobilized to accelerate the development of nega- buildings, e.g. heating, cooling and ventilation systems, and tive emissions technologies (Gasser et al. 2015). lighting, play a pivotal role in energy consumption. Effi- It is argued that negative emissions technologies should ciency gains can be realized by utilizing energy-efficient be deployed to remove residual emissions after all conven- systems and appliances as well as improved construction tional decarbonization efforts have been maximized and that materials (REN21 2019; Leibowicz et al. 2018). such approach should be utilized to remove emissions that In the transportation sector, efficiency gains can be real- are difficult to eliminate through conventional methods (Lin ized through the introduction of enhanced and more effi- 2019). It is important to note that negative emissions should cient thermal engines, hybrid and electric vehicles as well be viewed as a complementary suite of technologies and as hydrogen (H ) vehicles (Hache 2015). Furthermore, effi- techniques to conventional decarbonization methods, and ciency gains can be achieved through technological advances not a substitute (Pires 2019). 1 3 2078 Environmental Chemistry Letters (2020) 18:2069–2094 The significant role of negative emissions in meeting cli- straightforward. Biomass biologically captures atmospheric mate targets is understood and appreciated amongst academ- CO through photosynthesis during growth, which is then ics, scientists and policymakers; however, there still remains utilized for energy production through combustion. The C O a debate on the social, economic and technical feasibility emissions realized upon combustion are then captured and as well as the risk associated with large-scale deployment stored in suitable geological reservoirs (Pires 2019; Roy- (Lenzi 2018). This section will carry out an extensive lit- alSociety 2018). This technology can significantly reduce erature review on the main negative emissions technologies greenhouse gas concentration levels by removing C O from and techniques, their current state of development, perceived the atmosphere. The carbon dioxide removal potential of limitations and risks as well as social and policy implica- this technology varies within the literature; however, a con- tions. Figure 2 depicts the major negative emissions technol- servative assessment by Fuss et al. presents an estimated −1 ogies and carbon removal methods discussed in the literature range of 0.5–5 GtC O year by 2050 (Fuss et al. 2018). In and critically reviewed in this article. terms of global estimates for storage capacity, the literature presents a wide range from 200 to 50,000 GtC O (Fuss et al. Bioenergy carbon capture and storage 2018). Cost estimates for carbon dioxide removal through bioenergy carbon capture and storage are in the range of Bioenergy carbon capture and storage, also referred to as $100-$200/tCO (Fuss et al. 2018). BECCS, is one of the prominent negative emissions tech- The biomass feedstocks utilized for this approach can nologies discussed widely in the literature. The Intergovern- either be dedicated energy crops or wastes from agricultural mental Panel on Climate Change (IPCC) heavily relied on or forestry sources. Furthermore, such feedstocks can either bioenergy carbon capture and storage within their assess- be used as dedicated bio-based feedstocks or can be com- ments as a potential route to meet temperature goals (IPCC bined with fossil-based fuels in co-fired power plants (Roy - 2018). The technology is simply an integration of biopower, alSociety 2018). Besides the standard combustion route, the and carbon capture and storage technologies discussed ear- literature suggests that C O can be captured in non-power lier. The basic principle behind the technology is quite bio-based applications, such as during the fermentation Fig. 2 Major negative emissions technologies and techniques which terrestrial weathering, wetland restoration and construction, direct air are deployed to capture and sequester carbon from the atmosphere. carbon capture and storage, ocean alkalinity enhancement and ocean This approach includes bioenergy carbon capture and storage, affores- fertilization tation and reforestation, biochar, soil carbon sequestration, enhanced 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2079 process in ethanol production or the gasification of wood introduce negative emissions as a new class of tradeable pulp effluent, e.g. black liquor, in pulp production (McLaren credits (Fajardy et al. 2019). 2012; Pires 2019). The main challenge associated with this technology is Aor ff estation and reforestation the significant amount of biomass feedstocks required to be an effective emission abatement approach. Under large- During tree growth, C O is captured from the atmosphere scale deployment, resource demand when utilizing dedicated and stored in living biomass, dead organic matter and soils. crops would be quite significant, with high pressure exerted Forestation is thus a biogenic negative emissions technology on land, water as well as nutrient resources. A major issue that plays an important role within climate change abatement would be the direct competition with food and feed crops for efforts. Forestation can be deployed by either establishing land, freshwater and nutrients (RoyalSociety 2018; GNASL new forests, referred to as afforestation, or re-establishing 2018). Heck et al. empirically investigated the large-scale previous forest areas that have undergone deforestation or deployment of bioenergy carbon capture and storage for degradation, which is referred to as reforestation. Depend- climate change abatement and demonstrated its impact on ing on tree species, once forests are established CO uptake freshwater use, land system change, biosphere integrity and may span 20–100 years until trees reach maturity and then biogeochemical flows. Furthermore, the investigation identi- sequestration rates slow down significantly. At that stage, fied the interrelated nature between each of these dimensions forest products can be harvested and utilized. It is argued as well as the associated impacts when any one dimension is that forest management activities and practices have an envi- prioritized (Heck et al. 2018). A sustainable approach to land ronmental impact and should be carefully planned (Royal- use is quite critical in approaching bioenergy carbon capture Society 2018). Harper et al. discuss several advantages and and storage. Competing with food for arable land and chang- co-benefits that are associated with forest-based mitigation ing forest land to dedicated plantations have serious negative which include biodiversity, flood control as well as quality social and environmental effects. Harper et al. argue that the improvement for soil, water and air (Harper et al. 2018). effectiveness of this technology in achieving negative emis- Carbon can be stored in forests for a very long time; how- sions is based on several factors which include previous land ever, permanence is vulnerable due to natural and human cover, the initial carbon gain or loss due to land-use change, disturbances. Natural disasters such as fire, droughts and bioenergy crop yields, and the amount of harvested carbon disease or human-induced deforestation activities are all that is ultimately sequestered. Their empirical investigation risks that negatively impact storage integrity. In general, highlights the negative impact of bioenergy carbon capture biogenic storage has a much shorter lifespan than storage and storage when dedicated plantations replace carbon-dense in geological formations, such as in the case of bioenergy ecosystems (Harper et al. 2018). Another issue discussed in carbon capture and storage (Fuss et al. 2018). Another issue the literature is the albedo effects of biomass cultivation. related to forestation is land requirement as well as compe- This is mainly applicable in high-latitude locations, where tition with other land use. Significant amounts of land are biomass replaces snow cover and reduces radiation reflection required to achieve effective abatement results (RoyalSociety potential which offsets mitigation efforts (Fuss et al. 2018). 2018). Fuss et al. discuss another issue and that is the albedo In terms of technology readiness, bioenergy technologies effect. Forests in high latitudes would actually be counter - are to a certain extent well developed; however, carbon cap- productive, accelerating local warming as well as ice and ture and storage are still at an early stage. Technology risk snow cover loss. They argue that tropical areas would be the is mainly associated with storage integrity and the potential most suitable zones to host forestation projects. However, of leakage as discussed previously on carbon capture and competition with agriculture and other sectors for land will storage. Furthermore, Mander et al. discuss the technical be another problem. Based on global tropical boundary limi- difficulties in scaling deployment within a short period. tations, an estimated total area of 500 Mha is argued to be Besides, they question whether this technology can deliver suitable for forestation deployment. This would allow for a its abatement potential within the projected time frame. In global carbon dioxide removal potential of 0.5–3.6 GtCO −1 terms of policy, it is argued that a strong framework, as well year by 2050. Removal costs are estimated at $5–$50/tCO as adequate incentives, need to be in place to properly push (Fuss et al. 2018). the technology forward (Mander et al. 2017). Commercial In terms of technology readiness, afforestation and refor - logic may not be enough to drive forward global deploy- estation have already been widely adopted on a global level ment. Financial viability of such projects will depend on a and have already been integrated within climate policies utilitarian carbon market that caters for negative emissions through the Kyoto protocol’s clean development mecha- as well as an appropriate carbon price that incentivizes nism programme since the 1990s. To drive forward forest- deployment (Hansson et al. 2019). Therefore, policy should based mitigation efforts, the protocol introduced removal look at ways to strengthen carbon pricing mechanisms and units which allowed forestation projects to yield tradeable 1 3 2080 Environmental Chemistry Letters (2020) 18:2069–2094 credits. Despite the early policy measures, forest-based In terms of resource requirements, biochar produc- mitigation efforts accounted for a small fraction of emis- tion would require vast amounts of land to have an effec- sions at that time. Forest-based abatement projects have tive impact on greenhouse gas concentration levels. Land also been introduced through national regulations as well is required for feedstock cultivation, as well as for biochar as voluntary systems such as the reducing emissions from dispersal acting as a carbon sink. While land for dedicated deforestation and forest degradation (REDD+) programme biomass cultivation may create competition issues with that was introduced by the United Nations in 2008. However, agriculture and other land-use sectors, same as the case of carbon sequestration through forestation remained insignifi- bioenergy carbon capture and storage, there would be no cant, as it only accounted for 0.5% of the total carbon traded issues with areas required for biochar dispersal. This would in 2013 (Gren and Aklilu 2016). The effectiveness of the be the case as long as the biochar is technically matched REDD+ programme is argued in the literature after more with the type of crop, soil and growing conditions related than 10 years of its introduction. Hein et al. present a num- to the specific cropping system. Besides soil, Schmidt ber of arguments around the programme’s poor track record et al. introduced other carbon sink applications for biochar in achieving its intended purpose of emissions reduction. which include construction materials, wastewater treatment However, despite the uncertainty and weaknesses discussed, and electronics, as long as the product does not thermally REDD+ implementation intentions have been indicated by degrade or oxidize throughout its life cycle (Schmidt et al. 56 countries in their INDC submissions under the Paris 2019). Furthermore, it has been argued in the literature that agreement (Hein et al. 2018). Permanence, sequestration marginal and degraded lands can potentially be utilized for uncertainty, the availability of efficient financing mecha- dedicated plantations, relieving pressure on land that can nisms as well as monitoring, reporting and verification be used for other purposes. Moreover, using waste biomass systems are all difficulties associated around forest-based eliminates the need for land and provides a waste disposal abatement projects (Gren and Aklilu 2016). solution; however, competition over waste for other purposes increases feedstock availability risk as well as price vola- Biochar tility. Biomass availability is one of the limiting factors to successful large-scale deployment of biochar projects (Roy- Biochar has recently gained considerable recognition as a alSociety 2018). viable approach for carbon capture and permanent storage In addition to the beneficial effect of capturing and stor - and is considered as one of the promising negative emis-ing CO from the atmosphere, there is growing evidence sions technologies. Biochar is produced from biomass, e.g. in the literature that biochar also has an impact on other dedicated crops, agricultural residues and forestry residues, greenhouse gas emissions such as CH and N O. Although 4 2 through a thermochemical conversion process. It is produced the literature shows a positive impact in many occasions, through pyrolysis, a process of heating in the absence of in terms of reduced emissions, Semida et al. present mixed oxygen, as well as through gasification and hydrothermal results, where the application of biochar has positive as well carbonization (Matovic 2011; Oni et al. 2020; Osman et al. as negative effects on CH and N O emissions. This is spe- 4 2 2020a, b). The carbon captured by biomass through CO cific to the cropping system as well as the type of biochar uptake during plant growth is then processed into a char that utilized and its processing conditions (Semida et al. 2019). can be applied to soils for extended periods. The conversion Xiao et al. also present conflicting results regarding biochar process stores biomass carbon in a form that is very stable application, which is very specific to the condition of the and resistant to decomposition. Stability in soils is perhaps soils amended with biochar (Xiao et al. 2019). Impact on the most important property of biochar that makes it a solid greenhouse gas emissions should, therefore, be studied on carbon removal technology. Although considered more sta- a case-by-case basis. ble than soil organic carbon, there are certain uncertain- Another benefit that is widely discussed in the literature ties around decomposition rates of various types of biochar, is the positive effects associated with biochar application to which depends on the feedstock used and process conditions soils. It is argued that soil quality and fertility are signifi- utilized (Osman et al. 2019; Chen et al. 2019). Depending cantly enhanced. Improvement in nutrient cycling, reduction on the feedstock used, it is estimated that this technology in nutrient leaching from the soil and an increase in water can potentially remove between 2.1 and 4.8 tC O /tonne of and nutrient retention as well as stimulation of soil microbial biochar (RoyalSociety 2018). Carbon removal potential, as activity are all co-benefits associated with biochar applica- well as costs, varies greatly in the literature; however, a con- tion. However, this is mainly dependent on biochar physical servative range is provided by Fuss et al. It is estimated that and chemical properties. Such properties are defined by the by 2050 global carbon reduction removal potential achieved type of feedstock utilized, pyrolysis conditions, as well as −1 through biochar can be in the range of 0.3–2 Gt C O  year , other processing conditions. Furthermore, despite the gen- with costs ranging from $90 to $120/tCO (Fuss et al. 2018). eral perception that biochar positively impacts plant growth 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2081 and production, which is true in a large number of cases, carbon sequestration promotes enhanced soil fertility and there is evidence that biochar application may hinder plant health as well as improves crop yields due to organic carbon growth in certain cropping systems. This is based on the accumulation within soils (Fuss et al. 2018). Various land type of biochar, the quantity applied and the specific crops management practices that promote soil carbon sequestration under cultivation and sometimes management practices. The are discussed in the literature which include cropping system evidence is mixed, and therefore careful analysis should be intensity and rotation practices, zero-tillage and conserva- carried out to successfully match biochar with appropriate tion tillage practices, nutrient management, mulching and carbon sinks (Oni et al. 2020; Semida et al. 2019; El-Naggar use of crop residues and manure, incorporation of biochar, et al. 2019; Maraseni 2010; Purakayastha et al. 2019; Xu use of organic fertilizers and water management (Royal- et al. 2019). Society 2018; Srivastava 2012; Farooqi et al. 2018). Fur- Concerning the risks associated with large-scale deploy- thermore, the impact of perennial cropping systems on soil ment, albedo effect is mentioned in the literature. With high carbon sequestration is well documented in the literature. application rates of biochar to the soil surface, e.g. 30–60 Agostini et al. investigated the impact of herbaceous and tons/ha, it is argued that a decrease in surface reflectiv - woody perennial cropping systems on soil organic carbon ity would increase soil temperature, which in turn would and confirmed an increase in soil organic carbon levels by −1 −1 reduce the beneficial effect of carbon sequestration through 1.14–1.88 tCha  year for herbaceous crops and 0.63–0.72 −1 −1 this route (RoyalSociety 2018; Fuss et al. 2018). Other risks tCha  year for woody crops. It is reported that these val- and challenges associated include the risk of reversibility ues are well above the proposed sequestration requirement −1 −1 and challenges in monitoring, reporting and verification. (0.25 tCha  year ) to make the crop carbon neutral once Moreover, limited policy incentives and support, as well converted to biofuels (Agostini et al. 2015). The positive as lack of carbon pricing mechanisms that incorporate CO impact of perennial cropping systems on soil carbon seques- removal through biochar (Ernsting et al. 2011), hinder this tration is supported and documented in the literature by sev- technology’s potential for large-scale commercialization. eral other investigations (Nakajima et al. 2018; Sarkhot et al. Pourhashem et al. examined the role of government policy 2012). in accelerating biochar adoption and identified three types The main issues related to this approach revolve around of existing policy instruments that can be used to stimulate permanence, sink saturation as well as the impact on other biochar deployment in the USA: commercial financial incen - greenhouse gas emissions. According to Fuss et  al., the tives, non-financial incentives and research and development potential of carbon removal through soil carbon sequestra- funding (Pourhashem et al. 2019). With the current techno- tion is time-limited. Once soils reach a level of saturation, logical advancements, in particular blockchain, a number of further sequestration is no longer achieved. This may take start-ups are developing carbon removal platforms to drive 10–100 years depending on soil type and climatic condi- forward voluntary carbon offsets for consumers and corpora- tions. However, the Intergovernmental Panel on Climate tions. A Finnish start-up, Puro.earth, has introduced biochar Change (IPCC) defined a default saturation period of as a net-negative technology. Once verified through the com- 20 years (Fuss et al. 2018). Once saturation is reached, land pany’s verification system, the carbon removal certificates management practices need to be maintained indefinitely to generated by biochar producers are auctioned to potential mitigate reversal. A disadvantage to this would be the ongo- offset parties. However, until carbon removal is adequately ing costs with no further removal benefits. Risks of revers- monetized and supported through sufficient policy instru- ibility are significant and weaken this approach’s storage ments, biochar project development will probably not reach integrity. Another negative effect discussed in the literature the scale required to have a profound impact within the time is the impact of soil carbon sequestration on other green- frame mandated by international policy. house gas emissions, mainly CH and N O; however, this 4 2 effect is reported to be negligible (Fuss et al. 2018). Soil carbon sequestration By 2050, the global carbon dioxide removal potential discussed in the literature is estimated between 2.3 and 5.3 −1 Soil carbon sequestration is the process of capturing atmos- GtCO  year at costs ranging from $0 to $100 t/CO (Fuss 2 2 pheric CO through changing land management practices to et  al. 2018). While soil carbon sequestration is ready for increase soil carbon content. The level of carbon concentra- large-scale deployment, since many of such practices are tion within the soil is determined by the balance of inputs, already being used, lack of knowledge, resistance to change e.g. residues, litter, roots and manure, and the carbon losses as well as lack of policy and financial incentives are identi- realized through respiration which is mainly influenced by fied as barriers for scalability. Challenges around monitor - soil disturbance. Practices that increase inputs and/or reduce ing, reporting and verification, as well as concerns about losses drive soil carbon sequestration (RoyalSociety 2018; sink saturation and potential reversibility, have been the main Fuss et al. 2018). It is well noted in the literature that soil reasons behind slow policy action. However, non-climate 1 3 2082 Environmental Chemistry Letters (2020) 18:2069–2094 policies have mainly promoted land management practices technology is more energy- and material-intensive due to the to improve soil quality, fertility and productivity as well as fact that capturing CO from ambient air is much more dif- prevent land degradation (RoyalSociety 2018). While pol- ficult compared to capturing CO from highly concentrated icy and market-based mechanisms are required to push this flue gas streams. Direct air carbon capture is three times approach forward, international voluntary carbon removal energy-intensive compared to conventional carbon capture platforms are emerging. A US-based platform (Nori) is per ton of C O removed (Gambhir and Tavoni 2019). How- based on the concept of soil carbon sequestration and oper- ever, direct air carbon capture and storage plants are more ates by linking consumers and businesses that wish to offset e fl xible and can be located anywhere, provided that low-car - their carbon footprint with farmers that oe ff r carbon removal bon energy and adequate transportation and storage facili- certificates that have been audited through an independent ties are available. In terms of technology readiness, a lot of verification party. Using blockchain technology, this com- processes are currently being developed and are either under pany is one step further in fighting the challenges associated laboratory-scale or pilot-scale phases. Technology develop- with monitoring, reporting and verification systems. ers are mainly working on reducing energy requirements as this is one of the main challenges to deployment and scal- Direct air carbon capture and storage ability (RoyalSociety 2018). The global potential for carbon dioxide removal has Direct air carbon capture and storage, also referred to as been estimated by Fuss et al. to be in the range of 0.5–5 −1 DACCS in the literature, is emerging as a potential synthetic GtCO  year by 2050, and this may potentially go up to 40 −1 CO removal technology. The underlying principle behind GtCO  year by the end of the century if the unexpected 2 2 this technology is the use of chemical bonding to remove challenges associated with large-scale deployment are atmospheric CO directly from the air and then store it in overcome. Furthermore, CO removal costs are estimated 2 2 geological reservoirs or utilize it for other purposes such at $600–$1000/tCO initially, moving down to the range as the production of chemicals or mineral carbonates. C O of $100–$300/tCO as the technology matures (Fuss et al. 2 2 is captured from the air by allowing ambient air to get in 2018). Currently, there are no policy instruments to support contact with chemicals known as sorbents. Furthermore, the this technology, similar to many of the negative emissions sorbents are then regenerated by applying heat or water to technologies discussed (RoyalSociety 2018). release the CO for storage or utilization. There are mainly two processes by which sorbents work: first through absorp- Ocean fertilization tion, where the C O dissolves in the sorbent material, typi- cally using liquid sorbents such as potassium hydroxide or Ocean fertilization is the process of adding nutrients, sodium hydroxide; second through adsorption, whereby the macro such as phosphorus and nitrates as well as micro CO adheres to the sorbent, typically using solid materials such as iron, to the upper surface of the ocean to enhance such as amines (Pires 2019; GNASL 2018; Gambhir and CO uptake by promoting biological activity. Microscopic Tavoni 2019; Liu et al. 2018). Both processes require ther- organisms, called phytoplankton, found at the surface layer mal energy to regenerate the sorbent and release the CO ; of oceans are an important contributor to the concept of oce- however, it is important to note that less energy is required anic carbon sequestration. The sequestered CO , in the form under the adsorption route (Gambhir and Tavoni 2019). A of organic marine biomass, is naturally transported to the key issue widely discussed in the literature is the significant deep ocean; this process is termed “the biological pump”. energy required by direct air carbon capture and storage It is important to note that this downward flow is to a cer - plants. Besides the energy required for sorbent regenera- tain extent balanced by oceanic carbon respiration. Similar tion, energy is required for fans, pumps as well as compres- to land-based plants, phytoplankton utilizes light, C O as sors for pressurizing the C O . It is of course very important well as nutrients to grow. In the natural system, nutrients to utilize low-carbon energy sources, preferably renewable are available in the ocean as a consequence of death and energy as well as sources of waste heat, to drive the opera- decomposition of marine life. Hence, marine production is tion (Fuss et al. 2018). Another major drawback highlighted limited by the availability of recycled nutrients in the ocean. in the literature is the significant cost associated with devel- The idea behind ocean fertilization is to introduce additional oping direct air carbon capture and storage projects (Fuss nutrients to increase the magnitude of biological produc- et al. 2018). The major risk associated with this technology tion, which in turn increases C O uptake rate as compared is CO storage integrity, similar to that of carbon capture to the natural rate of respiration creating a carbon-negative and storage and bioenergy carbon capture and storage (Roy- atmospheric balance (RoyalSociety 2018; Williamson et al. alSociety 2018). 2012). Although there is not much information in the lit- Gambhir et al. compare direct air carbon capture and stor- erature regarding carbon removal potential, it is estimated age to carbon capture and storage and explain that the former that ocean fertilization can potentially sequester up to 3.7 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2083 −1 GtCO  year by 2100 with a total global storage capacity Besides the carbon removal potential associated with of 70–300 GtC O (RoyalSociety 2018). In terms of poten- enhanced weathering, the literature presents a number of tial abatement costs, a range between $2 and $457/tCO has positive side effects. This includes favourable impact on soil been estimated in the literature (Fuss et al. 2018). hydrological properties, a source for plant nutrients allowing Side effects of ocean fertilization that are discussed in the lower dependence on conventional fertilizers, increase in literature include ocean acidification, deep and mid-water water pH, enhanced soil health, increase in biomass produc- oxygen decrease or depletion, increase in production of fur- tion and an opportunity to reduce dependence on conven- ther greenhouse gases, unpredictable impact on food cycles, tional pesticides. Such benefits depend on the type of rock creation of toxic algal blooms as well as mixed effects on and its application rate, climate, soil and cropping system the seafloor and upper ocean ecosystems (Fuss et al. 2018; (RoyalSociety 2018; Fuss et al. 2018; de Oliveira Garcia Williamson et al. 2012). Furthermore, the environmental, et al. 2019; Strefler et al. 2018). economic and social effects as well as the energy and mate- In terms of technology readiness, enhanced weathering rial resources associated with fertilizer production, trans- can be practically deployed at the moment. Current land portation and distribution are significant. Moreover, accord- management practices incorporate the application of granu- ing to Fuss et al., uncertainty around permeance is a major lar materials, e.g. lime. Existing equipment can be utilized drawback. Permanence depends on whether the sequestered with no additional investment in equipment or infrastructure. carbon, in organic form, remains dissolved in the different The technologies related to quarrying, crushing and grinding layers of the ocean or whether sedimentation allows it to are well developed, and there would not be issues with scal- settle within long-term oceanic compartments for extended ability. However, under large-scale deployment, the energy periods (Fuss et al. 2018). The issue with permeance, impact required for extraction, production and transportation would on ecosystems, low sequestration efficiency, as well as lack be quite significant (RoyalSociety 2018). Careful attention of adequate monitoring, reporting and verification systems, should be paid to the carbon footprint of enhanced weather- do not support the concept that ocean fertilization is an ing operations to assess actual sequestration potential. Lefe- effective climate change abatement approach (Fuss et al. bevre et al. investigated carbon sequestration through EW 2018; Williamson et al. 2012). in Brazil by conducting a life cycle assessment to identify the carbon removal potential using basalt on agricultural Enhanced terrestrial weathering land in Sao Paolo. The investigation presented several key findings, first, that the operation emits 75 kg of CO per ton In the natural system, silicate rocks decompose; this is a of CO removed through enhanced weathering and 135 kg process termed weathering. This chemical reaction con-of CO per ton of CO removed through carbonation. This 2 2 sumes atmospheric CO and releases metal ions as well as is based on a distance of 65 km between the production site carbonate and/or bicarbonate ions. The dissolved ions are and the field on which the ground rock is applied. The results transported through groundwater streams through to rivers indicate a maximum road travel distance of 540 km for car- and eventually end up in the ocean where they are stored as bonation and 990 km for enhanced weathering, above which alkalinity, or they precipitate in the land system as carbon- the emissions offset the potential benefits realized from such ate minerals. Enhanced weathering is an approach that can activity. It is concluded that transportation is a major draw- accelerate this weathering process to enhance CO uptake on back which places limitations on the potential viability of a much shorter timescale. This is achieved through milling this technology. Furthermore, the results suggest a capture silicate rocks to increase its reactive surface and enhance rate of approximately 0.11–0.2 tCO e/ton of basaltic rock its mineral dissolution rate. The ground material is then applied (Lefebvre et al. 2019). applied to croplands providing a multitude of co-benefits Another approach to reducing pressure on the resources (RoyalSociety 2018; Bach et al. 2019). Kantola et al. discuss required for extraction is to utilize silicate wastes from the potential of applying this approach to bioenergy crop- various industries. Potential materials include wastes from ping systems (Kantola et al. 2017). According to Fuss et al., mining operations, cement, steel, aluminium, and coal or enhanced weathering promotes the sequestration of atmos- biomass combustion activities (Renforth 2019). However, pheric carbon in two forms, inorganic and organic. Inorganic this needs to be carefully assessed as potentially there is carbon is sequestered through the production of alkalinity a risk of releasing heavy metals into soils if inappropriate and carbonates, as discussed above. Organic carbon, on the materials are used (Fuss et al. 2018). Another risk associated other hand, is sequestered when additional carbon sequestra- with enhanced weathering is the potential health risk from tion is realized from enhanced biomass production, through the respiration of fine dust in the production and applica- photosynthesis, as a result of the nutrients that are naturally tion of finely ground rock materials (Strefler et al. 2018). released from the rocks (Fuss et al. 2018). Furthermore, uncertainties about the impacts of enhanced 1 3 2084 Environmental Chemistry Letters (2020) 18:2069–2094 weathering on microbial and marine biodiversity require the ocean’s acidity (Renforth and Henderson 2017). It is further investigation (RoyalSociety 2018). discussed in the literature that oceanic pH has a significant In terms of permanence, the sequestered C O can be impact on CO partial pressure for a given inorganic carbon 2 2 stored in several earth pools. Initially, C O can be stored content, which is the sum of carbon concentrations in car- as dissolved inorganic carbon, alkalinity, in soils as well as bonic acid, carbonate and bicarbonate ions (Kheshgi 1995). in groundwater. Depending on conditions, precipitation of Increasing ocean alkalinity is argued to decrease the sur- carbonate minerals in the soil can take place and such min- face ocean partial pressure, promoting further oceanic CO erals can be stored for an extended period (in the order of uptake, with a major positive side effect of reducing ocean 10  years) (Fuss et al. 2018). If precipitation does not take acidification. As alkalinity increases, more carbonic acid place, the dissolved inorganic carbon will be transported to is converted to bicarbonate and carbonate ions and greater the ocean through water streams, where it would be stored amounts of carbon are stored in inorganic form (Renforth as alkalinity, providing a number of additional benefits and and Henderson 2017). challenges to the oceanic pool. Based on an extensive litera- There are several approaches discussed in the literature ture assessment, Fuss et al. estimate global carbon removal on how an increase in oceanic alkalinity can be achieved. −1 potential of 2–4 GtCO   year by 2050 at a cost ranging The concept of enhanced weathering is the first approach to from $50 to $200/tCO (Fuss et  al. 2018). Strefler et  al. increase alkalinity within oceans. As previously discussed, conducted a techno-economic investigation on the carbon dissolved inorganic carbon in the form of bicarbonate and removal potential and costs of enhanced weathering using carbonate ions is a product of enhanced terrestrial weather- two rock types (dunite and basaltic rock). The results are ing. If precipitation does not occur, the bicarbonate and car- inline and support the estimates presented by Fuss et al. in bonate ions are transported through water streams and end terms of removal potential as well as costs. Furthermore, up in the ocean, increasing its alkalinity. Another approach is the investigation highlighted the dimensions that influence the addition of alkaline silicate rocks directly into the ocean, removal potential and cost, mainly being rock grain size and whereby finely ground rocks are added to the seawater for weathering rates. Finally, the study indicated that climates CO uptake and carbon storage in the form of bicarbonate that are warm and humid with lands that lack sufficient nutri- and carbonate ions, further enhancing alkalinity as well as ents are the most appropriate areas for enhanced weathering inducing additional atmospheric CO absorption (Bach et al. activities (Strefler et al. 2018). 2019). Another approach to increasing alkalinity was pro- At the moment, enhanced weathering is not included in posed by Kheshgi in the mid-1990s and that is the addition any carbon markets and does not have any policy support. of lime (CaO) to the ocean surface. The main drawback of Further research on social and environmental implications this approach is the energy required for the calcination of as well as adequate monitoring, reporting and verification limestone as well as the CO emissions realized (Kheshgi systems needs to be developed for this approach to gain 1995). Another approach discussed in the literature is the traction (RoyalSociety 2018). Moreover, integration within accelerated weathering of limestone. This concept includes carbon markets and adequate carbon pricing are required to utilizing a reactor and reacting limestone (CaCO ) with sea- incentivize deployment. water and a gas stream that is high in CO concentration to facilitate mineral dissolution. The main drawback of this Ocean alkalinity enhancement approach is the excessive water requirement (Renforth and Henderson 2017). Finally, the last approach to enhancing Ocean alkalinity enhancement has been discussed in the alkalinity was introduced by House et al. whereby an alka- literature as a potential route to inorganic carbon capture line solution is produced through an electrochemical method and storage within the ocean. The ocean already absorbs (House et al. 2009). Besides the challenges associated with a significant amount of atmospheric CO annually, mainly each of the approaches presented, challenges around the through two routes. First, through the diffusion of CO from impact of alkalinity enhancement on the oceanic ecosystem the atmosphere into the water, based on the differences of is still an area that needs further investigation. Furthermore, CO partial pressure between the atmosphere and the ocean. issues are raised around monitoring and regulations related The second route is through photosynthesis of phytoplank- to oceanic modifications (Renforth and Henderson 2017). ton discussed earlier. This section will mainly focus on CO In terms of permanence, carbon can be stored for oceanic uptake through diffusion that is governed by the extended periods, in the order of 10   years, in the form oceanic partial pressure of C O . When CO moves from the of dissolved inorganic carbon. The ocean currently stores 2 2 atmosphere into the ocean, the gas reacts with water to form approximately 140,000 GtCO , and with some changes in its carbonic acid, which further dissociates into bicarbonate and chemistry, it may be able to store in the order of trillions of carbonate ions, where dissolved inorganic carbon is stored. tons of CO (Renforth and Henderson 2017). There is, how- This reaction also releases hydrogen ions, which increases ever, a risk of reversal pointed out if mineral precipitation 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2085 takes place, reducing the carbon carrying capacity of the sink with significant sequestration rates of 9.9 MtCO and water (RoyalSociety 2018). According to Renforth et al., the 16.5 MtCO , respectively, over the entire period of study. cost of removing C O through ocean alkalinity enhancement This illustrates the high impact of management activities is estimated between $10 and $190/tCO , depending on the on the carbon sequestration potential of wetland habitats approach utilized in producing, transporting and distributing (Pindilli et al. 2018). the alkaline material (Renforth and Henderson 2017). Cur- Carbon sequestration and storage potential vary amongst rently, no policies or carbon pricing mechanisms incentivize different types of wetlands; for example, the estimated car - −1 −1 the pursuit of climate change abatement through this tech- bon sequestration rate is 6.3 ± 4.8 tCOe ha   year for −1 −1 nique, and there is still a need for field trials before deploy - mangroves, 8.0 ± 8.5 tCO2e ha year for salt marshes and −1 −1 ing such approach on a large scale. 4.4 ± 0.95 tCOe ha year for seagrass meadows. Within these habitats, the soil organic carbon accumulated in the top −1 −1 Wetland restoration and construction one metre amounted to 1060tC O e ha , 917 tC O ha and 2 2 −1 500 tCO ha for mangroves, salt marshes and seagrasses, Wetlands are high carbon density ecosystems that facilitate respectively (Sapkota and White 2020). The estimated cost atmospheric carbon sequestration through photosynthesis of carbon abatement through wetland restoration and con- and subsequent storage in above-ground and below-ground struction ranges between $10 and $100/tCO (RoyalSociety biomass as well as soil organic matter (Villa and Bernal 2018). According to Sapkota et al., several attempts have 2018). Examples of wetlands include peatlands as well as been made to include wetland-related offsets within exist- coastal habitats such as mangrove forests, tidal marshes ing voluntary and compliance carbon markets, including the and seagrass meadows, also referred to as blue carbon development of protocols and methodologies. A number of ecosystems. Furthermore, constructed wetlands have been methodologies have already been certified in the USA by discussed in the literature as a valid solution to wastewater various voluntary markets. However, despite the efforts, a treatment. While peatlands and coastal wetlands are esti- few wetland restoration carbon offsets have been transacted mated to store between 44 and 71% of the world’s terres- so far (Sapkota and White 2020). trial biological carbon, such carbon stocks are vulnerable to deterioration due to habitat degradation. Risks leading to Alternative negative emissions utilization and storage carbon loss, similar to forests, are caused by anthropogenic techniques activities as well as natural disasters. Restoration efforts usually revolve around rewetting the ecosystems as well as Mineral carbonation is a process by which C O is chemically further applicable measures (RoyalSociety 2018). A major reacted with minerals to form stable carbonates that can be drawback discussed in the literature is the substantial emis- safely stored below-ground or utilized in many applications sions of non-CO greenhouse gases such as CH and N O (Olajire 2013; Wang et al. 2020). It very much resembles 2 4 2 associated with wetland habitats. A number of investiga- the natural weathering process of converting silicate rocks tions emphasize the importance of incorporating the nega- to carbonates, but at a much faster rate. The literature dis- tive impact of non-CO greenhouse gases in evaluating the cusses two main routes for mineral carbonation, an ex situ sequestration benefits associated with a specific wetland res- industrial process above-ground that includes grinding and toration or construction project, as a specific site can either pre-treatment of minerals pre-reaction, or an in situ process be a net carbon sink or a greenhouse gas source. This is with direct injection of CO in silicate rocks below-ground based on various environmental and habitat management (RoyalSociety 2018; Olajire 2013; Galina et al. 2019). Sili- conditions (de Klein and van der Werf 2014; Gallant et al. cate rocks that contain high concentrations of calcium (Ca), 2020). Pindilli et al. conducted an empirical investigation magnesium (Mg) and iron (Fe) are the most suitable ele- on the impact of peatland restoration and management on ments to react with CO to form stable carbonates. Further- the carbon sequestration potential of a 54,000 ha protected more, industrial wastes that contain concentrations of such habitat over a 50-year period. The research modelled four elements such as slag from steel plants and fly ash from coal scenarios: the first scenario included no management, the combustion plants are also adequate materials to utilize for second added the impact of a catastrophic fire under no the carbonation process (Galina et al. 2019). Cost estimates management, the third incorporated current management under ex situ carbonation range from $50 to $300/tCO , practices, while the final scenario promoted increased man- while in situ carbonation is estimated at approximately $17/ agement activities. The results derived from this investi- tCO (RoyalSociety 2018). An interesting utilization route of gation showed that under the first two scenarios the peat- mineral carbonates is the replacement of conventional aggre- land is declared a net source of C O emissions, emitting gates in concrete production. Substituting aggregates with 2.4  MtCO and 6.5 MtCO , respectively. Under the third mineral carbonates in conjunction with C O curing to speed 2 2 2 and fourth scenarios, the peatland is declared a net carbon up the curing process and achieve higher strength concrete 1 3 2086 Environmental Chemistry Letters (2020) 18:2069–2094 material is a promising approach to sequester C O in the very large amount of sulphur dioxide gas (SO ) was ejected, 2 2 built environment (RoyalSociety 2018). Mineral carbonation between 15 and 30 million tons, which induced sunlight using CO that has been captured through direct air carbon reflectively and reduced global temperatures by 0.4–0.5 °C capture or bioenergy carbon capture systems can be con- (Zhang et al. 2015). Stratospheric aerosol injection is a solar sidered as a carbon-negative process since CO is removed radiation management technology that aims to mimic the from the atmosphere and safely stored in carbonate form cooling effect caused by the volcanic eruption by artificially in geological formations, or in the built environment if the injecting reflecting aerosol particles in the stratosphere carbonates are utilized in construction. It is also important (Lawrence et al. 2018; Zhang et al. 2015). Through model- to note that mineral carbonation can also be coupled with ling and past volcanic eruption data, the maximum potential carbon capture and storage technologies but would not be cooling from this approach is estimated between 2 and 5 W/ considered as a negative emissions technique if the CO uti- m (Lawrence et al. 2018). Smith et al. investigated the tech- lized is fossil-based. nology’s tactics and costs during the first 15 years of deploy - Another approach discussed in the literature is the utiliza- ment starting in 2033. They surveyed potential deployment tion of biomass materials in construction, while this is not techniques and concluded that an aircraft-based delivery sys- a new concept, technological advancements in thermal and tem is the most efficient method to deploy stratospheric aero- chemical treatments have mainly focused on increasing the sol injection. However, a new purpose-built high-altitude variety and number of materials that can be utilized in dif- aircraft will need to be developed for this purpose as current ferent applications within the building industry. The basic models, even with modifications will not be sufficient. In an principle behind this approach is that carbon is sequestered attempt to reduce anthropogenically driven radiative forcing through photosynthesis, where the resulting biomass can rate by half, Smith et al. calculated initial costs for deploy- then be utilized in construction allowing carbon to be stored ment to be in the range of $3.5 billion with average annual for decades in the built environment, e.g. building struc- operating costs of $2.25 billion (approximately $1500/t SO tures, insulation and furniture. The potential CO removal injected) (Smith and Wagner 2018). The main issue behind −1 is estimated at approximately 0.5–1 GtCO  year , through this technique is the uncertainty of the side effects and the replacing conventional construction materials (RoyalSociety harmful consequences of deployment, with a specific nega- 2018). Besides the removal potential, by replacing conven- tive impact on the hydrological cycle as well as stratospheric tional building materials such as steel and cement further ozone depletion (Zhang et al. 2015). It is important to note emission reductions can be realized since these are carbon- that while this approach will provide temporary temperature intensive materials. Estimates of 14–31% reduction in global reduction it should not be considered a long-term solution. CO emissions and 12–19% reduction in global fossil fuel This approach is still at a very early stage of research and consumption can be realized through this approach (Royal- development (Lawrence et al. 2018). Society 2018). However, significant sustainable forestation projects are required. Marine sky brightening Radiative forcing geoengineering technologies Marine sky brightening, also known as marine cloud bright- ening or cloud albedo enhancement, is another solar radia- Radiative forcing geoengineering techniques are a set of tion management technology that aims to maintain or reduce technologies that aim to alter the earth’s radiative energy global temperatures by enhancing cloud reflectivity. This is budget to stabilize or reduce global temperatures. This is achieved through cloud seeding with seawater particles or achieved by either increasing the earth’s reflectivity by with chemicals (Zhang et al. 2015). The main idea behind increasing shortwave solar radiation that is reflected to this technique is that seawater is sprayed into the air creat- space, termed solar radiation management, or by enhancing ing small droplets that easily evaporate leaving behind salt longwave radiation that is emitted by the earth’s surfaces to crystals that increase low-altitude cloud reflectivity above space, termed terrestrial radiation management (Lawrence oceans (Ming et al. 2014). The potential cooling effect has et al. 2018). This section briefly describes the various radia- been estimated between 0.8 and 5.4 W/m , due to uncer- tive forcing geoengineering techniques discussed in the lit- tainty, limited knowledge and spatial considerations (Law- erature. Figure 3 depicts the main techniques discussed in rence et al. 2018). While this technique seems simple and the literature and reviewed in this article. straightforward, Latham et al. highlighted a number of prob- lems associated with marine sky brightening. This includes Stratospheric aerosol injection the lack of spraying system that is capable of generating seawater particles of the size and quantities required, as well Back in 1991, a very large volcanic eruption took place in as further technical problems that are associated with the the Philippines (Mount Pinatubo). During the eruption, a physical outcome of this approach as a result of the complex 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2087 Fig. 3 Major radiative forcing geoengineering technologies that injection, marine sky brightening, cirrus cloud thinning, space-based aim to alter the earth’s radiative energy budget to stabilize or reduce mirrors and surface-based brightening global temperatures. These technologies include stratospheric aerosol nature of cloud characteristics. Another challenge would be to be reduced from approximately $10,000/kg to less than to undertake extensive trials and properly understand and $100/kg (Lawrence et al. 2018). Moreover, risks such as overcome potential side effects (Latham et al. 2012). Again, those associated with space debris and asteroid collisions this approach is still at an infant stage and will require exten- or those associated with technical and communication sive field research and development moving forward. failures need to be appropriately catered for (Lawrence et al. 2018). Space‑based mirrors Sunshade using space-based mirrors is a solar radiation Surface‑based brightening management technique discussed in the literature that aims to reflect part of the incoming solar radiation to reduce Another solar radiation management approach discussed global temperatures. For this approach to technically be in the literature is the brightening of the earth surface to deployed, space mirrors or reflectors need to be trans- increase the earth’s albedo and thus reduce global tempera- ported into orbit around the earth or placed at the Lagran- tures. This has been suggested through painting urban roofs gian L1 location between the earth and the sun, where the and roads in white, as well as covering deserts and glaciers gravitational fields are in balance allowing the reflectors with plastic sheets that are highly reflective, and, further - to remain stationary (Zhang et al. 2015; Kosugi 2010). more, by placing reflective floating panels over water bodies While this approach can have a considerable cooling effect (Ming et al. 2014). According to Lawrence et al., based on based on model simulations, development of such tech- an extensive literature review, the cooling potential for this nology is still at a very infant stage. The major drawback approach is too limited. Furthermore, substantial negative associated with this approach is the economic feasibility side effects are associated, such as disruption of desert eco- of transporting materials into space. For this technology systems (Lawrence et al. 2018). to be economically feasible, material transport costs need 1 3 2088 Environmental Chemistry Letters (2020) 18:2069–2094 Cirrus cloud thinning Bibliometric analysis of research on climate change mitigation Cirrus cloud thinning is a terrestrial radiation manage- ment technique that aims to increase longwave radia- Bibliometric analysis is a statistical tool that can be used tion that is emitted from the earth’s surface to space to to quantitatively analyse the current state of scientific stabilize or reduce global temperatures. Cirrus clouds research, by highlighting gaps in the literature as well as are high-altitude ice clouds that play a significant role trends. The Web of Science (WoS) core collection data- within the earth’s radiation budget, having an impact on base was used in this analysis. The following search meth- the earth’s hydrological cycle as well as surface tempera- odology was used to retrieve relevant research for further tures. Cirrus clouds absorb terrestrial radiation as well as evaluation. Please note that the search was refined to a reflect incoming solar radiation; however, in general, they 5-year timespan from 2015 to 2020 to specifically evaluate induce an average net warming effect from the imbal- scientific research efforts related to climate change mitiga- ance between incoming and outgoing radiative forcings tion after the Paris agreement in 2015. (Kärcher 2017). The basic principle behind this technique Search Methodology: is the injection of aerosols into cirrus clouds to reduce You searched for: TOPIC: (“Climate change mitiga- its optical thickness as well as its lifetime to increase tion”) OR TOPIC: (“climate change abatement”) OR terrestrial radiation emission to space. This approach TOPIC: (“Decarbonization Technologies”) OR TOPIC: would require regular cloud injection, so an efficient and (“Bioenergy Carbon Capture & Storage”) OR TOPIC: cost-effective delivery method needs to be in places such (“Afforestation & Reforestation”) OR TOPIC: (“Soil Car - as dedicated aircrafts or drones. Bismuth triiodide (Bil ) bon Sequestration”) OR TOPIC: (“Direct Air Carbon Cap- has been proposed as an effective cloud seeding mate- ture & Storage”) OR TOPIC: (“Ocean Fertilization”) OR rial; however, its toxicity needs to be taken into account. TOPIC: (“Enhanced Terrestrial Weathering”) OR TOPIC: Sea salt is another proposed option, yet it is not found to (“Ocean Alkalinity Enhancement”) OR TOPIC: (“Wetland be as effective as Bil (Lawrence et al. 2018). Based on Restoration & Construction”) OR TOPIC: (“Stratospheric model simulations, the maximum cooling effect through Aerosol Injection”) OR TOPIC: (“Marine Sky Brighten- this approach has been estimated to be in the range of ing”) OR TOPIC: (“Space-Based Sunshade/Mirrors”) 2–3.5 W/m (Lawrence et al. 2018). According to Law- OR TOPIC: (“Surface-Based Brightening”) OR TOPIC: rence et al., there are no published costs for cirrus cloud (“Cirrus Cloud Thinning”) OR TOPIC: (“Carbon Dioxide thinning and this approach still requires further research Removal Techniques”) OR TOPIC: (“Radiative Forcing to understand side effects as well as to conduct appro- Geoengineering”) priate research on potential delivery methods (Lawrence Timespan: Last 5  years. Indexes: SCI-EXPANDED, et al. 2018). SSCI, A&HCI, CPCI-S, CPCI-SSH, ESCI. Results: A total of 3993 papers were retrieved (3386 articles, 362 reviews, 201 proceedings papers, 71 early Miscellaneous radiation management techniques access and 61 editorial materials) The results obtained were then analysed using Ming et  al. proposed several theoretical technologies VOSviewer software by plotting network and density vis- that target terrestrial radiation, mainly by creating ther- ualization maps as shown in Fig. 4. The maps are based mal bridges to bypass the greenhouse gas insulating layer on keyword co-occurrences. The visualization maps high- and be able to transfer thermal radiation out to space. The light various trends related to climate change mitigation, research paper presented several concepts which include where areas related to biomass, carbon sequestration, transferring surface hot air to the troposphere, transfer- especially soil carbon sequestration, and biochar have ring latent and sensible heat to the top of the troposphere, received high attention over the past 5 years. Furthermore, transferring surface-sensible heat to the troposphere, as research related to policy, energy and in particular renew- well as transferring cold air to the earth surface. For each able energy has also received much attention. Although concept, conceptual technologies are proposed. Some of research on climate change mitigation is trending, a gap in the technologies discussed are systems that transfer heat the literature can be highlighted regarding research related beyond the earth system while generating energy, termed to specific mitigation technologies. It is also evident from metrological reactors by the authors (Ming et al. 2014). the literature that radiative forcing geoengineering tech- While the idea of thermal bridging is interesting, the tech- nologies have not received much attention. nologies and concepts introduced require further research, development and extensive field trials. 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2089 Fig. 4 Bibliometric analysis of research on climate change mitiga- change mitigation by highlighting trends and gaps in the literature tion: a network visualization map and b density visualization map, during 5 years between 2015 and 2020 showing the recent state of scientific research on the topic of climate 1 3 2090 Environmental Chemistry Letters (2020) 18:2069–2094 efficiently utilize financial resources and policy support Conclusion as most of the related technologies can be deployed imme- diately; however, efficient carbon pricing mechanisms that Based on the current state of climate emergency, imme- focus on carbon removal need to be aggressively devel- diate development of viable mitigation and adaptation oped and introduced. Furthermore, funding for technology mechanisms is of extreme importance. An extensive lit- research and development is also a very important aspect erature review covered three main strategies to tackling moving forward. climate change, conventional mitigation technologies, negative emissions technologies as well as radiative forc- Acknowledgements Authors would like to acknowledge the support ing geoengineering technologies. It is important to clarify given by the EPSRC project “Advancing Creative Circular Economies that there is no ultimate solution to tackle climate change for Plastics via Technological-Social Transitions” (ACCEPT Transi- and that all technologies and techniques discussed in this tions, EP/S025545/1). The authors wish to acknowledge the support of The Bryden Centre project (Project ID VA5048) which was awarded review if technically and economically are viable should by The European Union’s INTERREG VA Programme, managed by be deployed. As previously discussed, decarbonization the Special EU Programmes Body (SEUPB), with match funding pro- efforts alone are not sufficient to meet the targets stipu- vided by the Department for the Economy in Northern Ireland and the lated by the Paris agreement; therefore, the utilization of Department of Business, Enterprise and Innovation in the Republic of Ireland. an alternative abatement approach is inevitable. While the concept of radiative forcing geoengineering in terms Compliance with ethical standards of managing the earth’s radiation budget is interesting, it is not a long-term solution, as it does not solve the root Conflict of interest The author declares no competing financial inter - cause of the problem. It may, however, buy some time until ests. greenhouse gas concentrations are stabilized and reduced. However, the technologies to be deployed are still to be Open Access This article is licensed under a Creative Commons Attri- developed and tested and side effects adequately catered bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long for, which may be a lengthy process. Negative emissions as you give appropriate credit to the original author(s) and the source, technologies, on the other hand, provide a solid solution provide a link to the Creative Commons licence, and indicate if changes in combination with the current decarbonization efforts. were made. The images or other third party material in this article are While some of the negative emissions technologies pre- included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in sented in the literature review may still be at an early the article’s Creative Commons licence and your intended use is not stage of development, biogenic-based sequestration tech- permitted by statutory regulation or exceeds the permitted use, you will niques are to a certain extent mature and can be deployed need to obtain permission directly from the copyright holder. To view a immediately. Capturing C O through photosynthesis is a copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. straightforward and solid process; however, it needs to be effectively integrated within a technological framework as presented in the review. 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Springer Journals
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Copyright © The Author(s) 2020
ISSN
1610-3653
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1610-3661
DOI
10.1007/s10311-020-01059-w
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Abstract

Climate change is defined as the shift in climate patterns mainly caused by greenhouse gas emissions from natural systems and human activities. So far, anthropogenic activities have caused about 1.0 °C of global warming above the pre-industrial level and this is likely to reach 1.5 °C between 2030 and 2052 if the current emission rates persist. In 2018, the world encountered 315 cases of natural disasters which are mainly related to the climate. Approximately 68.5 million people were affected, and economic losses amounted to $131.7 billion, of which storms, floods, wildfires and droughts accounted for approximately 93%. Economic losses attributed to wildfires in 2018 alone are almost equal to the collective losses from wildfires incurred over the past decade, which is quite alarming. Furthermore, food, water, health, ecosystem, human habitat and infrastructure have been identified as the most vulnerable sectors under climate attack. In 2015, the Paris agreement was introduced with the main objective of limiting global temperature increase to 2 °C by 2100 and pursuing efforts to limit the increase to 1.5 °C. This article reviews the main strategies for climate change abatement, namely conventional mitigation, negative emissions and radiative forcing geoengineering. Conventional mitigation technologies focus on reducing fossil-based CO emissions. Negative emissions technologies are aiming to capture and sequester atmospheric carbon to reduce carbon dioxide levels. Finally, geoengineering techniques of radiative forcing alter the earth’s radiative energy budget to stabilize or reduce global temperatures. It is evident that conventional mitigation efforts alone are not sufficient to meet the targets stipulated by the Paris agreement; therefore, the utilization of alternative routes appears inevitable. While various technologies presented may still be at an early stage of development, biogenic-based sequestration techniques are to a certain extent mature and can be deployed immediately. Keywords Climate change mitigation · Negative emissions technologies · Carbon dioxide removal · Decarbonization technologies · Radiative forcing geoengineering technologies AbbreviationsGt Gigatons Bio-DME Bio-dimethyl etherGW Gigawatt BECCS Bioenergy carbon capture and storageHFCs Hydrofluorocarbons Bil Bismuth triiodide H Hydrogen 3 2 Ca Calcium INDCs Intended nationally determined contributions CO Carbon dioxide IPCC Intergovernmental Panel on Climate Change CO e Carbon dioxide equivalent IAEA International Atomic Energy Agency CRED Centre for Research on the Epidemiology of ITMOs Internationally transferred mitigation Disaster outcomes DACCS Direct air carbon capture and storageFe Iron CaO Lime CaCO Limestone * Ahmed I. Osman CH Methane aosmanahmed01@qub.ac.uk Mha Megahectare Mt Million tons School of Chemistry and Chemical Engineering, Queen’s NO Nitrous oxide University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK NHRE Non-hydro renewable energy The Bryden Centre, Letterkenny Institute of Technology, Letterkenny, Ireland Vol.:(0123456789) 1 3 2070 Environmental Chemistry Letters (2020) 18:2069–2094 OECD Or ganization for Economic Co-operation and total global greenhouse gas and fossil CO emissions. The Development rise of fossil C O emissions in 2018 is mainly driven by PFCs Perfluorocarbons higher energy demand. Furthermore, emissions related to −1 ha Per hectare land-use change amounted to 3.5 GtC O in 2018 (UNEP −1 year Per year 2019). Together in 2018, fossil-based and land-use-related REDD+ Reducing emissions from deforestation and CO emissions accounted for approximately 74% of the total forest degradation global greenhouse gas emissions. Methane (CH ), another SO Sulphur dioxide significant greenhouse gas, had an emission rate increase SF Sulphur hexafluoride of 1.7% in 2018 as compared to an annual increase of 1.3% PFCs Perfluorocarbons over the past decade. Nitrous oxide (N O) emissions, which t Ton are mainly influenced by agricultural and industrial activi- UNEP United Nations Environment Programme ties, saw an increase of 0.8% in 2018 as compared to a 1% UNFCCC United Nations Framework Convention on annual increase over the past decade. A significant increase Climate Change was, however, noted in the fluorinated gases during 2018 at W/m W att per square meter 6.1% as compared to a 4.6% annual increase over the past Mg Magnesium decade (UNEP 2019). To put these numbers into perspec- tive, a recent Intergovernmental Panel on Climate Change (IPCC) report demonstrated that anthropogenic activities so Introduction far have caused an estimated 1.0 °C of global warming above the pre-industrial level, specifying a likely range between 0.8 Status of climate change and 1.2 °C. It is stated that global warming is likely to reach 1.5 °C between 2030 and 2052 if the current emission rates Climate change is defined as the shift in climate patterns persist (IPCC 2018). mainly caused by greenhouse gas emissions. Greenhouse gas emissions cause heat to be trapped by the earth’s atmos- Climate change impacts, risks and vulnerabilities phere, and this has been the main driving force behind global warming. The main sources of such emissions are natural An understanding of the severe impact of climate change on systems and human activities. Natural systems include for- natural and human systems as well as the risks and associ- est fires, earthquakes, oceans, permafrost, wetlands, mud ated vulnerabilities is an important starting point in com- volcanoes and volcanoes (Yue and Gao 2018), while human prehending the current state of climate emergency. Changes activities are predominantly related to energy production, in climate indicators, namely temperature, precipitation, industrial activities and those related to forestry, land use seal-level rise, ocean acidification and extreme weather and land-use change (Edenhofer et al. 2014). Yue and Gao conditions have been highlighted in a recent report by the statistically analysed global greenhouse gas emissions from United Nations Climate Change Secretariat (UNCCS). natural systems and anthropogenic activities and concluded Climate hazards reported included droughts, floods, hur - that the earth’s natural system can be considered as self- ricanes, severe storms, heatwaves, wildfires, cold spells balancing and that anthropogenic emissions add extra pres- and landslides (UNCCS 2019). According to the Centre for sure to the earth system (Yue and Gao 2018). Research on the Epidemiology of Disasters (CRED), the world encountered 315 cases of natural disasters in 2018, GHG emissions overview mainly climate-related. This included 16 cases of drought, 26 cases of extreme temperature, 127 cases of flooding, 13 The greenhouse gases widely discussed in the literature and cases of landslides, 95 cases of storms and 10 cases of wild- defined by the Kyoto protocol are carbon dioxide (CO ), fire. The number of people affected by natural disasters in methane (CH ), nitrous oxide (N O), and the fluorinated 2018 was 68.5 million, with floods, storms and droughts 4 2 gases such as hydrofluorocarbons (HFCs), perfluorocar - accounting for 94% of total affected people. In terms of eco- bons (PFCs) and sulphur hexafluoride (SF ) (UNFCCC nomic losses, a total of $131.7 billion was lost in 2018 due 2008). According to the emissions gap report prepared by to natural disasters, with storms ($70.8B), floods ($19.7B), the United Nations Environment Programme (UNEP) in wildfires ($22.8B) and droughts ($9.7B) accounting for 2019, total greenhouse gas emissions in 2018 amounted to approximately 93% of the total costs. CRED also provides 55.3 GtCO e, of which 37.5 GtCO are attributed to fos- data on disasters over the past decade, which shows even 2 2 sil CO emissions from energy production and industrial higher annual averages in almost all areas, except for wild- activities. An increase of 2% in 2018 is noted, as compared fire cases. The economic losses attributed to wildfires in to an annual increase of 1.5% over the past decade for both 2018 alone are approximately equal to the collective losses 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2071 from wildfires incurred over the past decade, which is quite Asia. Double or triple rates of species extinction in terres- alarming (CRED 2019). Moreover, wildfires are a direct trial ecosystems are projected under the 2 °C level compared source of C O emissions. Although wildfires are part of the to 1.5 °C (IPCC 2018). It can be simply concluded that the natural system, it is clear that human-induced emissions are world is in a current state of climate emergency. directly interfering and amplifying the impact of natural system emissions. It is evident that human-induced climate Global climate action change is a major driving force behind many natural disas- ters occurring globally. Acknowledgement of climate change realities started in 1979 Furthermore, climate risks such as temperature shifts, when the first world climate conference was held in Geneva. precipitation variability, changing seasonal patterns, changes The world climate conference was introduced by the World in disease distribution, desertification, ocean-related impacts Meteorological Organization in response to the observation and soil and coastal degradation contribute to vulnerability of climatic events over the previous decade. The main pur- across multiple sectors in many countries (UNCCS 2019). pose was to invite technical and scientific experts to review Sarkodie et al. empirically examined climate change vul- the latest knowledge on climate change and variability nerability and adaptation readiness of 192 United Nations caused by natural and human systems as well as assess future countries and concluded that food, water, health, ecosystem, impacts and risks to formulate recommendations moving human habitat and infrastructure are the most vulnerable forward (WMO 1979). This was possibly the first of its kind sectors under climate attack while pointing out that Africa conference discussing the adverse effects of climate change. is the most vulnerable region to climate variability (Sarkodie In 1988, the Intergovernmental Panel on Climate Change and Strezov 2019). It is also important to note the intercon- (IPCC) was set up by the World Meteorological Organiza- nected nature of such sectors and the associated impacts. tion in collaboration with the United Nations Environment th The 15 edition of the global risks report 2020 prepared Programme (UNEP) to provide governments and official by the world economic forum thoroughly presented a num- bodies with scientific knowledge and information that can ber of climate realities, laying out areas that are greatly be used to formulate climate-related policies (IPCC 2013). affected. The risks included loss of life due to health hazards Perhaps, the most critical step taken, in terms of action, and natural disasters, as well as excessive stress on ecosys- was the adoption of the United Nations Framework Conven- tems, especially aquatic/marine systems. Moreover, food tion on Climate Change (UNFCCC) in 1992, which then and water security are other areas that are highly impacted. went into force in 1994. Since then, the UNFCCC has been Increased migration is anticipated due to extreme weather the main driving force and facilitator of climate action glob- conditions and disasters as well as rising sea levels. Geopo- ally. The main objective of the convention is the stabiliza- litical tensions and conflicts are likely to arise as countries tion of greenhouse gas concentrations in the atmosphere to aim to extract resources along water and land boundaries. prevent severe impacts on the climate system. The conven- The report also discusses the negative financial impact on tion set out the commitments to all parties involved, put- capital markets as systematic risks soar. Finally, the impact ting major responsibilities on developed countries to imple- on trade and supply chains is presented (WEF 2020). ment national policies to limit anthropogenic emissions and An assessment, recently presented in an Intergovernmen- enhance greenhouse gas sinks. The target was to reduce tal Panel on Climate Change (IPCC) special report, covered emissions by the year 2000 to the levels achieved in the the impacts and projected risks associated with 2 levels of previous decade. Moreover, committing developed country global warming, 1.5 °C and 2 °C. The report investigated the parties to assist vulnerable developing country parties finan- negative impact of global warming on freshwater sources, cially and technologically in taking climate action. The con- food security and food production systems, ecosystems, vention established the structure, reporting requirements and human health, urbanization as well as poverty and chang- mechanism for financial resources, fundamentally setting the ing structures of communities. The report also investigated scene for global climate policy (UN 1992). The convention climate change impact on key economic sectors such as tour- is currently ratified by 197 countries (UNCCS 2019). ism, energy and transportation. It is evident that most of During the third UNFCCC conference of the parties the impacts assessed have lower associated risks at 1.5 °C (COP-3) in 1997, the Kyoto protocol was adopted and compared to 2 °C warming level. We would likely reach went into force in 2005. The Kyoto protocol introduced 1.5 °C within the next 3 decades and increases in warm- the emission reduction commitments for developed coun- ing levels beyond this point would amplify risk effects; for tries for a five-year commitment period between 2008 and example, water stress would carry double the risk under a 2012. The protocol laid out all related policies, monitor- 2 °C level compared to 1.5 °C. An increase of 70% in popu- ing and reporting systems, as well as introduced three lation affected by fluvial floods is projected under the 2 °C market-based mechanisms to achieve those targets. The scenario compared to 1.5 °C, especially in USA, Europe and protocol introduced two project-based mechanisms, clean 1 3 2072 Environmental Chemistry Letters (2020) 18:2069–2094 development mechanism and joint implementation mecha- In 2012, the Doha amendment to the Kyoto protocol was nism. The clean development mechanism allows developed adopted, mainly proposing a second commitment period country parties to invest and develop emission reduction from 2013 to 2020 as well as updating emissions reduction projects in developing countries, to drive sustainable targets. The amendment proposed a greenhouse gas emis- development in the host country as well as offset carbon sions reduction target of at least 18% below 1990 levels. The emissions of the investing party. Joint implementation pro- amendment has not yet entered into force since it has not jects allow developed country parties to develop similar been ratified by the minimum number of parties required to projects, however, in other developed countries that are this date (UNFCCC 2012). protocol parties, offsetting excess emissions of the invest- During the twenty-first UNFCCC conference of the par - ing party. Furthermore, the protocol introduced an emis- ties (COP-21) held in Paris in 2015, the Paris agreement was sions trading mechanism as a platform to facilitate the adopted and entered into force in 2016. The Paris agreement trading of annually assigned emissions that are saved by added further objectives, commitments, enhanced compli- protocol members to those that exceed their limits (UNF- ance and reporting regulations, as well as support mecha- CCC 1997). Emission reduction has mainly been achieved nisms to the existing climate change combat framework in through the introduction of renewable energy, energy effi- place. The main objective of the agreement is to limit the ciency and afforestation/reforestation-related projects. global temperature increase to 2 °C by 2100 and pursue The Kyoto protocol defines four emission saving units, efforts to limit the increase to 1.5 °C. The agreement aims each representing one metric ton of CO equivalent and are to reach global peaking of greenhouse gases as soon as pos- all tradeable (UNFCCC 2005). sible as to strike a balance between human-induced emission sources and greenhouse gas sinks and reservoirs between 1 Certified emissions reduction unit, obtained through 2050 and 2100. The agreement also introduced new binding clean development mechanism projects. commitments, asking all parties to deliver nationally deter- 2 Emission reduction unit, obtained through joint imple- mined contributions and to enforce national measures to mentation projects. achieve, and attempt to exceed such commitments. Enhanced 3 Assigned amount unit, obtained through the trading of transparency, compliance and clear reporting and commu- unused assigned emissions between protocol parties. nication are advocated under the agreement. Furthermore, 4 Removal unit, obtained through reforestation-related the agreement encourages voluntary cooperation between projects. parties beyond mandated initiatives. Moreover, financial support and technological support, as well as capacity build- The Kyoto units and general framework introduced laid ing initiatives for developing countries, are mandated by the the structural foundation of a carbon emissions market and agreement. Such obligations are to be undertaken by devel- the concept of carbon pricing. Many national and regional oped country parties to promote sustainable development governments introduced emissions trading schemes; some and establish adequate mitigation and adaptation support are mandatory while others are voluntary. In some cases, measures within vulnerable countries. Perhaps, one of the such schemes are linked to Kyoto commitments and regu- most important goals established under the agreement is that lations. The largest emissions trading scheme introduced of adaptation and adaptive capacity building concerning the thus far is the European emissions trading scheme (Per- temperature goal set (UN 2015). dan and Azapagic 2011). Villoria-Saez et al. empirically Under article 6 of the agreement, two international mar- investigated the effectiveness of greenhouse gas emissions ket mechanisms were introduced, cooperative approaches trading scheme implementation on actual emission reduc- and the sustainable development mechanism. These mecha- tions covering six major emitting regions. The investigation nisms are to be utilized by all parties to meet their nation- presented a number of findings; first, it is possible to reduce ally determined contributions. Cooperative approaches are greenhouse gas emissions by approximately 1.58% annually a framework that allows parties to utilize internationally upon scheme implementation. Furthermore, after 10 years transferred mitigation outcomes (ITMOs) to meet nation- of implementation, approximately 23.43% of emissions ally determined contribution goals as well as stimulate sus- reduction can be achieved in comparison with a scenario tainable development. On the other hand, the sustainable of non-implementation (Villoria-Sáez et al. 2016). Another development mechanism is a new approach that promotes emission abatement instrument widely discussed in the liter- mitigation and sustainable development and is perceived as ature is carbon taxation. There is growing scientific evidence the successor of the clean development mechanism. There that carbon taxation is an effective instrument in reducing is still much debate and negotiations on such mechanisms greenhouse gas emissions; however, political opposition by moving forward (Gao et al. 2019). the public and industry is the main reason delaying many Nieto et al. conducted an in-depth systematic analysis of countries in adopting such mechanism (Wang et al. 2016). the effectiveness of the Paris agreement policies through the 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2073 evaluation of 161 intended nationally determined contribu- (Ricke et al. 2017). The main negative emissions techniques tions (INDCs) representing 188 countries. The study investi- widely discussed in the literature include bioenergy carbon gated sectoral policies in each of these countries and quanti- capture and storage, biochar, enhanced weathering, direct air fied emissions under such INDCs. The analysis concluded carbon capture and storage, ocean fertilization, ocean alka- that a best-case scenario would be an annual global emission linity enhancement, soil carbon sequestration, afforestation increase of approximately 19.3% in 2030 compared to the and reforestation, wetland construction and restoration, as base period (2005–2015). In comparison, if no measures well as alternative negative emissions utilization and storage were taken a 31.5% increase in global emissions is projected. methods such as mineral carbonation and using biomass in It is concluded that if the predicted best-case level of emis- construction (Lawrence et al. 2018; Palmer 2019; McLaren sions is maintained between 2030 and 2050 a temperature 2012; Yan et al. 2019; McGlashan et al. 2012; Goglio et al. increase of at least 3 °C would be realized. Furthermore, a 2020; Lin 2019; Pires 2019; RoyalSociety 2018; Lenzi 4 °C increase would be assured if annual emissions continue 2018). to increase (Nieto et al. 2018). Finally, a third route revolves around the principle of To meet the 1.5 °C target by the end of the century, the altering the earth’s radiation balance through the manage- IPCC stated that by 2030 greenhouse gas emissions should ment of solar and terrestrial radiation. Such techniques are −1 be maintained at 25–30 GtC Oe year . In comparison, the termed radiative forcing geoengineering technologies, and current unconditional nationally determined contributions the main objective is temperature stabilization or reduction. −1 for 2030 are estimated at 52–58 GtCOe year . Based on Unlike negative emissions technologies, this is achieved pathway modelling for a 1.5 °C warming scenario, a 45% without altering greenhouse gas concentrations in the atmos- decline in anthropogenic greenhouse gas emissions must be phere. The main radiative forcing geoengineering techniques reached by 2030 as compared to 2010 levels, and net-zero that are discussed in the literature include stratospheric aero- emissions must be achieved by 2050. To maintain a 2 °C sol injection, marine sky brightening, cirrus cloud thinning, global warming level by the end of the century, emissions space-based mirrors, surface-based brightening and various should decline by approximately 25% in 2030 as compared radiation management techniques. All these techniques are to 2010 levels and net-zero emissions should be achieved by still theoretical or at very early trial stages and carry a lot of 2070 (IPCC 2018). There is growing evidence that confirms uncertainty and risk in terms of practical large-scale deploy- that current mitigation efforts, as well as future emissions ment. At the moment, radiative forcing geoengineering tech- commitments, are not sufficient to achieve the temperature niques are not included within policy frameworks (Lawrence goals set by the Paris agreement (Nieto et al. 2018; Law- et al. 2018; Lockley et al. 2019). rence et al. 2018). Further measures and new abatement routes must be explored if an attempt is to be made to Conventional mitigation technologies achieve such goals. As previously discussed, energy-related emissions are the main driver behind the increased greenhouse gas concen- Climate change mitigation strategies tration levels in the atmosphere; hence, conventional miti- gation technologies and efforts should be focused on both Introduction the supply and demand sides of energy. Mitigation efforts primarily discussed in the literature cover technologies and There are three main climate change mitigation approaches techniques that are deployed in four main sectors, power on discussed throughout the literature. First, conventional miti- the supply side and industry, transportation and buildings gation efforts employ decarbonization technologies and tech- on the demand side. Within the power sector, decarboniza- niques that reduce C O emissions, such as renewable energy, tion can be achieved through the introduction of renewable fuel switching, efficiency gains, nuclear power, and carbon energy, nuclear power, carbon capture and storage as well capture storage and utilization. Most of these technologies as supply-side fuel switch to low-carbon fuels such as natu- are well established and carry an acceptable level of man- ral gas and renewable fuels. Furthermore, mitigation efforts aged risk (Ricke et al. 2017; Victor et al. 2018; Bataille et al. on the demand side include the efficiency gains achieved 2018; Mathy et al. 2018; Shinnar and Citro 2008; Bustreo through the deployment of energy-efficient processes and et al. 2019). sector-specific technologies that reduce energy consump - A second route constitutes a new set of technologies and tion, as well as end-use fuel switch from fossil-based fuels to methods that have been recently proposed. These techniques renewable fuels, and, moreover, the integration of renewable are potentially deployed to capture and sequester C O from power technologies within the energy matrix of such sectors the atmosphere and are termed negative emissions technolo- (Mathy et al. 2018; Hache 2015). This section will review gies, also referred to as carbon dioxide removal methods the literature on decarbonization and efficiency technologies 1 3 2074 Environmental Chemistry Letters (2020) 18:2069–2094 and techniques that cover those four main sectors introduced. 2006; Gude and Martinez-Guerra 2018; Akalın et al. 2017; Figure 1 depicts the conventional mitigation technologies Srivastava et al. 2017). and techniques discussed in the literature and critically In terms of power production, as of 2018, renewable reviewed in this paper. energy accounted for approximately 26.2% of global elec- tricity production. Hydropower accounted for 15.8%, while Renewable energy wind power’s share was 5.5%, photovoltaic solar power 2.4%, biopower 2.2% and geothermal, concentrated solar According to a recent global status report on renewables, power and marine power accounted for 0.46% of the gen- the share of renewable energy from the total final energy erated electricity (REN21 2019). While large-scale hydro- consumption globally has been estimated at 18.1% in 2017 power leads in terms of generation capacity as well as (REN21 2019). An array of modern renewable energy tech- production, there has been a significant capacity increase nologies is discussed throughout the literature. The most in photovoltaic solar power and onshore wind power over prominent technologies include photovoltaic solar power, the past decade. By the end of 2018, a total of 505 GW of concentrated solar power, solar thermal power for heat- global installed capacity for photovoltaic solar power has ing and cooling applications, onshore and offshore wind been noted as compared to 15 GW in 2008. Regarding wind power, hydropower, marine power, geothermal power, bio- power, 591 GW of global installed capacity is recorded in mass power and biofuels (Mathy et al. 2018; Shinnar and 2018 as compared to 121 GW in 2008. Global biopower Citro 2008; Hache 2015; REN21 2019; Hussain et al. 2017; capacity has been estimated at 130 GW in 2018 with a total Østergaard et al. 2020; Shivakumar et al. 2019; Collura et al. 581 TWh of production in that year. China has maintained Fig. 1 Major decarbonization technologies which focus on the utilization (CCU), fuel switching and efficiency gains. These tech- reduction of C O emissions related to the supply and demand sides nologies and techniques are mainly deployed in the power, industrial, of energy. Conventional mitigation technologies include renewable transportation and building sectors energy, nuclear power, carbon capture and storage (CCS) as well as 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2075 its position as the largest renewable energy producing coun- to alternative policy types (Pitelis et al. 2019). On barriers try, from solar, wind and biomass sources. The total share and drivers of renewable energy deployment, Shivakumar of renewable energy in global power capacity has reached et al. highlighted various dimensions that may hinder or approximately 33% in 2018 (REN21 2019). enable renewable energy project development. The main Besides the power sector, renewable energy can be points highlighted revolve around policy, financial access, deployed within the industry, transportation and building government stability and long-term intentions, administra- sectors. Photovoltaic and thermal solar energy as well as tive procedures and support framework or lack thereof, as industrial end-use fuel switch to renewable fuels such as well as the profitability of renewable energy investments solid, liquid and gaseous biofuels for combined thermal and (Shivakumar et al. 2019). Seetharaman et al. analysed the power production are examples of decarbonization efforts impact of various barriers on renewable energy deployment. through renewables. Buildings can also benefit from solar as The research confirms that regulatory, social and techno- well as biomass-based technologies for power, heating and logical barriers play a significant role in renewable energy cooling requirements. In relation to the transportation sector, deployment. The research does not find a significant direct end-use fuel switch is a determinant to sector decarboni- relationship between economic barriers and project deploy- zation. Some examples of biofuels are biodiesel, first- and ment; however, the interrelated nature between the economic second-generation bioethanol, bio-hydrogen, bio-methane dimension with regulatory, social and technological barriers and bio-dimethyl ether (bio-DME) (Srivastava et al. 2020; affects deployment, however, indirectly (Seetharaman et al. Chauhan et al. 2009; Hajilary et al. 2019; Osman 2020). 2019). Furthermore, hydrogen produced through electrolysis using In terms of the relationship between financial accessibil- renewable energy is a potential renewable fuel for sector ity and renewable energy deployment, Kim et al. empiri- decarbonization. Another example of sector decarbonization cally investigated such relationship by analysing a panel data through renewable energy deployment is electric vehicles set of 30 countries during a 13-year period from 2000 to using renewable power (Michalski et al. 2019). Other mitiga- 2013. Statistical evidence shows the positive impact of well- tion measures within these sectors will be further discussed developed financial markets on renewable energy deploy - in the following section. ment and sector growth. Furthermore, the study confirms a Variable renewables, such as solar and wind, are key tech- positive and significant relationship between market-based nologies with significant decarbonization potential. One of mechanisms, such as clean development mechanism, with the main technological challenges associated is the intermit- renewable energy deployment. There is a strong impact on tent nature/variability in power production. This has been photovoltaic solar and wind technologies, while the impact overcome by integrating such technologies with storage as is marginal under biomass and geothermal technologies well as other renewable baseload and grid technologies. Sin- (Kim and Park 2016). sel et al. discuss four specific challenge areas related to vari- Pfeiffer et al. studied the diffusion of non-hydro renew - able renewables, namely quality, flow, stability and balance. able energy (NHRE) technologies in 108 developing coun- Furthermore, they present a number of solutions that mainly tries throughout a 30-year period from 1980 to 2010. Based revolve around flexibility as well as grid technologies for on the results, economic and regulatory policies played a distributed as well as centralized systems (Sinsel et al. 2020). pivotal role in NHRE deployment, as well as governmen- Economic, social and policy dimensions play an influ - tal stability, higher education levels and per capita income. encing role in renewable energy technology innovation and On the other hand, growth in energy demand, aid and high deployment. Pitelis et al. investigated the choice of policy local fossil fuel production hindered NHRE diffusion. In instruments and its effectiveness in driving renewable energy contrast with Kim et al., the study finds weak support to technology innovation for 21 Organization for Economic show that international financing mechanisms and financial Co-operation and Development (OECD) countries between market development positively influenced diffusion (Pfeiffer 1994 and 2014. The study classified renewable energy poli- and Mulder 2013). The reason may be related to how the cies into three categories: technology-push, demand-pull analysis was constructed, different data sets, periods and and systemic policy instruments. Furthermore, the study statistical methods. investigated the impact of each policy classification on inno- Decarbonization through renewable energy deploy- vation activity of various renewable energy technologies: ment is extremely significant. Development of renew- solar, wind, biomass, geothermal and hydro. The study con- able energy projects should be seen as a top priority. The cluded that not all policy instruments have the same effect areas that would drive decarbonization through renew- on renewable energy technologies and that each technol- able energy and should be focused upon by policymakers, ogy would require appropriate policies. However, the study financiers and market participants include policy instru- suggested that demand-pull policy instruments are more ments, financial support and accessibility, and market- effective in driving renewable energy innovation compared based mechanisms to incentivize project developers. 1 3 2076 Environmental Chemistry Letters (2020) 18:2069–2094 Moreover, governmental support frameworks, public edu- Carbon capture, storage and utilization cation for social acceptance as well as research and devel- opment efforts for technological advances and enhanced Carbon capture and storage is a promising technology efficiencies are important focus areas. discussed in the literature as a potential decarbonization approach to be applied to the power as well as the industrial sectors. The technology consists of separating and captur- Nuclear powering CO gases from processes that rely on fossil fuels such as coal, oil or gas. The captured CO is then transported According to the latest report prepared by the interna- and stored in geological reservoirs for very long periods. tional atomic energy agency (IAEA), as of 2018, 450 The main objective is the reduction in emission levels while nuclear energy plants are operational with a total global utilizing fossil sources. Three capturing technologies are dis- installed capacity of 396.4 GW. It is projected that an cussed in the literature: pre-combustion, post-combustion increase of 30% in installed capacity will be realized by and oxyfuel combustion. Each technology carries a specific 2030 (from a base case of 392 GW in 2017). As a low- process to extracting and capturing CO . Post-combustion case projection scenario, it is estimated that by 2030 a capture technologies, however, are the most suitable for ret- 10% dip might be realized based on the 2017 numbers. On rofit projects and have vast application potential. Once CO the long term, it is projected that global capacity might has been successfully captured, it is liquie fi d and transported reach 748 GW by 2050, as a high-case scenario (IAEA through pipelines or ships to suitable storage sites. Based on 2018). Pravalie et al. provide an interesting review of the the literature, storage options include depleted oil and gas status of nuclear power. The investigation demonstrates fields, coal beds and underground saline aquifers not used for the significant role nuclear power has played in terms potable water (Vinca et al. 2018). Some of the main draw- of contribution to global energy production as well as backs of carbon capture and storage include safety in relation its decarbonization potential in the global energy sys- to secured storage and the possibility of leakage. Negative tem. The study presents an estimation of approximately environmental impacts that may result from onshore stor- 1.2–2.4 Gt C O emissions that are prevented annually age locations that undergo accidental leakage have been from nuclear power deployment, as alternatively the investigated by Ma et al. The investigation focused on the power would have been produced through coal or natural impact of leakage on agricultural land (Ma et al. 2020). Risk gas combustion. The paper suggests that to be in line with of leakage and associated negative impacts have also been the 2 °C target stipulated by the Paris agreement, nuclear pointed out by Vinca et al. (2018). Other issues related to plant capacity must be expanded to approximately 930 this technology include public acceptance (Tcvetkov et al. GW by 2050, with a total investment of approximately $ 2019; Arning et al. 2019) as well as the high deployment 4 trillion (Prăvălie and Bandoc 2018). costs associated (Vinca et al. 2018). Another pathway post- Although nuclear energy is considered as a low-carbon carbon capture is the utilization of the CO captured in the solution for climate change mitigation, it comes with a production of chemicals, fuels, microalgae and concrete number of major disadvantages. First, the capital outlay building materials, as well as utilization in enhanced oil and operating costs associated with nuclear power devel- recovery (Hepburn et al. 2019; Aresta et al. 2005; Su et al. opment are quite significant. Furthermore, risk of envi- 2016; Qin et al. 2020). ronmental radioactive pollution is a major issue related Large-scale deployment of carbon capture storage and to nuclear power, which is mainly caused through the utilization technologies is yet to be proven. According to threat of reactor accidents as well as the danger associated the international energy agency, there are only 2 carbon cap- with nuclear waste disposal (Prăvălie and Bandoc 2018; ture and storage projects under operation as of 2018, with Abdulla et al. 2019). While conventional fission-based a combined annual capture capacity of 2.4 MtCO . There nuclear plants are suggested to be phased out in future, are 9 more carbon capture projects under development and the introduction of enhanced fusion-based nuclear tech- are projected to increase capacity to 11 MtC O by 2025; nology may positively contribute to mitigation efforts in however, a significant deviation exists from the sustainable the second half of the century. Fusion power is a new development scenario targeted by the international energy generation of nuclear power, which is more efficient than agency for 2040 which is a capacity of 1488 MtC O (IEA the conventional fission-based technology and does not 2019a). carry the hazardous waste disposal risk associated with conventional fission-based nuclear technology. Further- Fuel switch and efficiency gains more, fusion power is characterized as a zero-emission technology (Prăvălie and Bandoc 2018; Gi et al. 2020). Fuel switching in the power sector from coal to gas, in the short-term, has been discussed extensively in the literature 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2077 as a potential approach to economically transition to a low- within aviation, shipping and rail, although rail is currently carbon and hopefully a zero-carbon economy in future (Vic- one of the most energy-ec ffi ient modes. Ec ffi iency measures tor et al. 2018; Wendling 2019; Pleßmann and Blechinger in the transportation sector can also take other forms. For 2017). The move to natural gas is also applicable to industry, example, travel demand management, to reduce frequency transportation and building sectors; however, as discussed and distance of travel, can be an interesting approach. More- previously the switch to renewable fuels is a more sustain- over, shifting travel to the most efficient modes where pos- able approach creating further decarbonization potential in sible, such as electrified rail, and reducing dependence on these sectors. high-intensity travel methods can play an interesting role in In addition to fuel switching, efficiency gains are of enhancing efficiency (IEA 2019b). extreme significance within mitigation efforts. Efficiency gains in the power sector are achieved through improve- Negative emissions technologies ments in thermal power plants by enhancing the efficiency of fuel combustion as well as improving turbine generator Most of the climate pathways that were investigated by efficiencies. Furthermore, waste heat recovery for additional the Intergovernmental Panel on Climate Change (IPCC) thermal as well as electric production enhances efficiency. In included the deployment of negative emissions technolo- gas-fired power plants, the utilization of a combined cycle gies along with conventional decarbonization technologies technology enhances the efficiency significantly. Combined to assess the feasibility of achieving the targets mandated heat and power units have also played an interesting role in by the Paris agreement. Only two negative emissions tech- efficiency gains. Technological advances within transmis- nologies have been included in the IPCC assessments so far, sion and distribution networks also enhance efficiencies by bioenergy carbon capture and storage as well as afforestation reducing losses (REN21 2019). and reforestation (IPCC 2018). In industry, there are many potential areas where effi- Gasser et al. empirically investigated the potential nega- ciency gains may be realized. For example, in steel and tive emissions needed to limit global warming to less than cement applications, waste heat can be recovered for onsite 2 °C. The analysis utilized an IPCC pathway that is most power and heat production through the installation of waste likely to maintain warming at such level and constructed heat-driven power plants that utilize waste heat from exhaust a number of scenarios based on conventional mitigation gases. For industries that utilize process steam, there is an assumptions in an attempt to quantify the potential nega- excellent opportunity to utilize waste steam pressure to tive emissions efforts required. The results indicated that generate electric power for onsite usage or drive rotating in the best-case scenario, that is under the best assumptions equipment. The application of back pressure steam turbines on conventional mitigation efforts, negative emissions of −1 in areas where steam pressure reduction is required can 0.5–3Gt C year and 50–250 Gt C of storage capacity are enhance energy efficiency significantly. The same approach required. Based on a worst-case scenario, negative emis- −1 can be deployed in applications where gas pressure reduc- sions of 7–11 Gt C year and 1000–1600 Gt C of storage tion is required, however, using turboexpanders. Waste gases capacity are required. (1 Gigaton Carbon = 3.6667 Gigaton from industrial processes can also be utilized to generate CO e) The results indicate the inevitable need for negative onsite heat and power using micro- and small gas turbines. emissions, even at very high rates of conventional mitiga- In addition, further efficiency gains can be realized through tion efforts. Furthermore, the study suggests that negative the deployment of advanced machinery controls in a multi- emissions alone should not be relied upon to meet the 2 °C tude of processes and industrial sectors. target. The investigation concluded that since negative emis- A number of factors influence energy efficiency within sions technologies are still at an infant stage of development, buildings, first the building design as well as materials uti- conventional mitigation technologies should remain focused lized in construction, e.g. insulation and glazing. Further- upon within climate policy, while further financial resources more, appliances, devices and systems used throughout are to be mobilized to accelerate the development of nega- buildings, e.g. heating, cooling and ventilation systems, and tive emissions technologies (Gasser et al. 2015). lighting, play a pivotal role in energy consumption. Effi- It is argued that negative emissions technologies should ciency gains can be realized by utilizing energy-efficient be deployed to remove residual emissions after all conven- systems and appliances as well as improved construction tional decarbonization efforts have been maximized and that materials (REN21 2019; Leibowicz et al. 2018). such approach should be utilized to remove emissions that In the transportation sector, efficiency gains can be real- are difficult to eliminate through conventional methods (Lin ized through the introduction of enhanced and more effi- 2019). It is important to note that negative emissions should cient thermal engines, hybrid and electric vehicles as well be viewed as a complementary suite of technologies and as hydrogen (H ) vehicles (Hache 2015). Furthermore, effi- techniques to conventional decarbonization methods, and ciency gains can be achieved through technological advances not a substitute (Pires 2019). 1 3 2078 Environmental Chemistry Letters (2020) 18:2069–2094 The significant role of negative emissions in meeting cli- straightforward. Biomass biologically captures atmospheric mate targets is understood and appreciated amongst academ- CO through photosynthesis during growth, which is then ics, scientists and policymakers; however, there still remains utilized for energy production through combustion. The C O a debate on the social, economic and technical feasibility emissions realized upon combustion are then captured and as well as the risk associated with large-scale deployment stored in suitable geological reservoirs (Pires 2019; Roy- (Lenzi 2018). This section will carry out an extensive lit- alSociety 2018). This technology can significantly reduce erature review on the main negative emissions technologies greenhouse gas concentration levels by removing C O from and techniques, their current state of development, perceived the atmosphere. The carbon dioxide removal potential of limitations and risks as well as social and policy implica- this technology varies within the literature; however, a con- tions. Figure 2 depicts the major negative emissions technol- servative assessment by Fuss et al. presents an estimated −1 ogies and carbon removal methods discussed in the literature range of 0.5–5 GtC O year by 2050 (Fuss et al. 2018). In and critically reviewed in this article. terms of global estimates for storage capacity, the literature presents a wide range from 200 to 50,000 GtC O (Fuss et al. Bioenergy carbon capture and storage 2018). Cost estimates for carbon dioxide removal through bioenergy carbon capture and storage are in the range of Bioenergy carbon capture and storage, also referred to as $100-$200/tCO (Fuss et al. 2018). BECCS, is one of the prominent negative emissions tech- The biomass feedstocks utilized for this approach can nologies discussed widely in the literature. The Intergovern- either be dedicated energy crops or wastes from agricultural mental Panel on Climate Change (IPCC) heavily relied on or forestry sources. Furthermore, such feedstocks can either bioenergy carbon capture and storage within their assess- be used as dedicated bio-based feedstocks or can be com- ments as a potential route to meet temperature goals (IPCC bined with fossil-based fuels in co-fired power plants (Roy - 2018). The technology is simply an integration of biopower, alSociety 2018). Besides the standard combustion route, the and carbon capture and storage technologies discussed ear- literature suggests that C O can be captured in non-power lier. The basic principle behind the technology is quite bio-based applications, such as during the fermentation Fig. 2 Major negative emissions technologies and techniques which terrestrial weathering, wetland restoration and construction, direct air are deployed to capture and sequester carbon from the atmosphere. carbon capture and storage, ocean alkalinity enhancement and ocean This approach includes bioenergy carbon capture and storage, affores- fertilization tation and reforestation, biochar, soil carbon sequestration, enhanced 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2079 process in ethanol production or the gasification of wood introduce negative emissions as a new class of tradeable pulp effluent, e.g. black liquor, in pulp production (McLaren credits (Fajardy et al. 2019). 2012; Pires 2019). The main challenge associated with this technology is Aor ff estation and reforestation the significant amount of biomass feedstocks required to be an effective emission abatement approach. Under large- During tree growth, C O is captured from the atmosphere scale deployment, resource demand when utilizing dedicated and stored in living biomass, dead organic matter and soils. crops would be quite significant, with high pressure exerted Forestation is thus a biogenic negative emissions technology on land, water as well as nutrient resources. A major issue that plays an important role within climate change abatement would be the direct competition with food and feed crops for efforts. Forestation can be deployed by either establishing land, freshwater and nutrients (RoyalSociety 2018; GNASL new forests, referred to as afforestation, or re-establishing 2018). Heck et al. empirically investigated the large-scale previous forest areas that have undergone deforestation or deployment of bioenergy carbon capture and storage for degradation, which is referred to as reforestation. Depend- climate change abatement and demonstrated its impact on ing on tree species, once forests are established CO uptake freshwater use, land system change, biosphere integrity and may span 20–100 years until trees reach maturity and then biogeochemical flows. Furthermore, the investigation identi- sequestration rates slow down significantly. At that stage, fied the interrelated nature between each of these dimensions forest products can be harvested and utilized. It is argued as well as the associated impacts when any one dimension is that forest management activities and practices have an envi- prioritized (Heck et al. 2018). A sustainable approach to land ronmental impact and should be carefully planned (Royal- use is quite critical in approaching bioenergy carbon capture Society 2018). Harper et al. discuss several advantages and and storage. Competing with food for arable land and chang- co-benefits that are associated with forest-based mitigation ing forest land to dedicated plantations have serious negative which include biodiversity, flood control as well as quality social and environmental effects. Harper et al. argue that the improvement for soil, water and air (Harper et al. 2018). effectiveness of this technology in achieving negative emis- Carbon can be stored in forests for a very long time; how- sions is based on several factors which include previous land ever, permanence is vulnerable due to natural and human cover, the initial carbon gain or loss due to land-use change, disturbances. Natural disasters such as fire, droughts and bioenergy crop yields, and the amount of harvested carbon disease or human-induced deforestation activities are all that is ultimately sequestered. Their empirical investigation risks that negatively impact storage integrity. In general, highlights the negative impact of bioenergy carbon capture biogenic storage has a much shorter lifespan than storage and storage when dedicated plantations replace carbon-dense in geological formations, such as in the case of bioenergy ecosystems (Harper et al. 2018). Another issue discussed in carbon capture and storage (Fuss et al. 2018). Another issue the literature is the albedo effects of biomass cultivation. related to forestation is land requirement as well as compe- This is mainly applicable in high-latitude locations, where tition with other land use. Significant amounts of land are biomass replaces snow cover and reduces radiation reflection required to achieve effective abatement results (RoyalSociety potential which offsets mitigation efforts (Fuss et al. 2018). 2018). Fuss et al. discuss another issue and that is the albedo In terms of technology readiness, bioenergy technologies effect. Forests in high latitudes would actually be counter - are to a certain extent well developed; however, carbon cap- productive, accelerating local warming as well as ice and ture and storage are still at an early stage. Technology risk snow cover loss. They argue that tropical areas would be the is mainly associated with storage integrity and the potential most suitable zones to host forestation projects. However, of leakage as discussed previously on carbon capture and competition with agriculture and other sectors for land will storage. Furthermore, Mander et al. discuss the technical be another problem. Based on global tropical boundary limi- difficulties in scaling deployment within a short period. tations, an estimated total area of 500 Mha is argued to be Besides, they question whether this technology can deliver suitable for forestation deployment. This would allow for a its abatement potential within the projected time frame. In global carbon dioxide removal potential of 0.5–3.6 GtCO −1 terms of policy, it is argued that a strong framework, as well year by 2050. Removal costs are estimated at $5–$50/tCO as adequate incentives, need to be in place to properly push (Fuss et al. 2018). the technology forward (Mander et al. 2017). Commercial In terms of technology readiness, afforestation and refor - logic may not be enough to drive forward global deploy- estation have already been widely adopted on a global level ment. Financial viability of such projects will depend on a and have already been integrated within climate policies utilitarian carbon market that caters for negative emissions through the Kyoto protocol’s clean development mecha- as well as an appropriate carbon price that incentivizes nism programme since the 1990s. To drive forward forest- deployment (Hansson et al. 2019). Therefore, policy should based mitigation efforts, the protocol introduced removal look at ways to strengthen carbon pricing mechanisms and units which allowed forestation projects to yield tradeable 1 3 2080 Environmental Chemistry Letters (2020) 18:2069–2094 credits. Despite the early policy measures, forest-based In terms of resource requirements, biochar produc- mitigation efforts accounted for a small fraction of emis- tion would require vast amounts of land to have an effec- sions at that time. Forest-based abatement projects have tive impact on greenhouse gas concentration levels. Land also been introduced through national regulations as well is required for feedstock cultivation, as well as for biochar as voluntary systems such as the reducing emissions from dispersal acting as a carbon sink. While land for dedicated deforestation and forest degradation (REDD+) programme biomass cultivation may create competition issues with that was introduced by the United Nations in 2008. However, agriculture and other land-use sectors, same as the case of carbon sequestration through forestation remained insignifi- bioenergy carbon capture and storage, there would be no cant, as it only accounted for 0.5% of the total carbon traded issues with areas required for biochar dispersal. This would in 2013 (Gren and Aklilu 2016). The effectiveness of the be the case as long as the biochar is technically matched REDD+ programme is argued in the literature after more with the type of crop, soil and growing conditions related than 10 years of its introduction. Hein et al. present a num- to the specific cropping system. Besides soil, Schmidt ber of arguments around the programme’s poor track record et al. introduced other carbon sink applications for biochar in achieving its intended purpose of emissions reduction. which include construction materials, wastewater treatment However, despite the uncertainty and weaknesses discussed, and electronics, as long as the product does not thermally REDD+ implementation intentions have been indicated by degrade or oxidize throughout its life cycle (Schmidt et al. 56 countries in their INDC submissions under the Paris 2019). Furthermore, it has been argued in the literature that agreement (Hein et al. 2018). Permanence, sequestration marginal and degraded lands can potentially be utilized for uncertainty, the availability of efficient financing mecha- dedicated plantations, relieving pressure on land that can nisms as well as monitoring, reporting and verification be used for other purposes. Moreover, using waste biomass systems are all difficulties associated around forest-based eliminates the need for land and provides a waste disposal abatement projects (Gren and Aklilu 2016). solution; however, competition over waste for other purposes increases feedstock availability risk as well as price vola- Biochar tility. Biomass availability is one of the limiting factors to successful large-scale deployment of biochar projects (Roy- Biochar has recently gained considerable recognition as a alSociety 2018). viable approach for carbon capture and permanent storage In addition to the beneficial effect of capturing and stor - and is considered as one of the promising negative emis-ing CO from the atmosphere, there is growing evidence sions technologies. Biochar is produced from biomass, e.g. in the literature that biochar also has an impact on other dedicated crops, agricultural residues and forestry residues, greenhouse gas emissions such as CH and N O. Although 4 2 through a thermochemical conversion process. It is produced the literature shows a positive impact in many occasions, through pyrolysis, a process of heating in the absence of in terms of reduced emissions, Semida et al. present mixed oxygen, as well as through gasification and hydrothermal results, where the application of biochar has positive as well carbonization (Matovic 2011; Oni et al. 2020; Osman et al. as negative effects on CH and N O emissions. This is spe- 4 2 2020a, b). The carbon captured by biomass through CO cific to the cropping system as well as the type of biochar uptake during plant growth is then processed into a char that utilized and its processing conditions (Semida et al. 2019). can be applied to soils for extended periods. The conversion Xiao et al. also present conflicting results regarding biochar process stores biomass carbon in a form that is very stable application, which is very specific to the condition of the and resistant to decomposition. Stability in soils is perhaps soils amended with biochar (Xiao et al. 2019). Impact on the most important property of biochar that makes it a solid greenhouse gas emissions should, therefore, be studied on carbon removal technology. Although considered more sta- a case-by-case basis. ble than soil organic carbon, there are certain uncertain- Another benefit that is widely discussed in the literature ties around decomposition rates of various types of biochar, is the positive effects associated with biochar application to which depends on the feedstock used and process conditions soils. It is argued that soil quality and fertility are signifi- utilized (Osman et al. 2019; Chen et al. 2019). Depending cantly enhanced. Improvement in nutrient cycling, reduction on the feedstock used, it is estimated that this technology in nutrient leaching from the soil and an increase in water can potentially remove between 2.1 and 4.8 tC O /tonne of and nutrient retention as well as stimulation of soil microbial biochar (RoyalSociety 2018). Carbon removal potential, as activity are all co-benefits associated with biochar applica- well as costs, varies greatly in the literature; however, a con- tion. However, this is mainly dependent on biochar physical servative range is provided by Fuss et al. It is estimated that and chemical properties. Such properties are defined by the by 2050 global carbon reduction removal potential achieved type of feedstock utilized, pyrolysis conditions, as well as −1 through biochar can be in the range of 0.3–2 Gt C O  year , other processing conditions. Furthermore, despite the gen- with costs ranging from $90 to $120/tCO (Fuss et al. 2018). eral perception that biochar positively impacts plant growth 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2081 and production, which is true in a large number of cases, carbon sequestration promotes enhanced soil fertility and there is evidence that biochar application may hinder plant health as well as improves crop yields due to organic carbon growth in certain cropping systems. This is based on the accumulation within soils (Fuss et al. 2018). Various land type of biochar, the quantity applied and the specific crops management practices that promote soil carbon sequestration under cultivation and sometimes management practices. The are discussed in the literature which include cropping system evidence is mixed, and therefore careful analysis should be intensity and rotation practices, zero-tillage and conserva- carried out to successfully match biochar with appropriate tion tillage practices, nutrient management, mulching and carbon sinks (Oni et al. 2020; Semida et al. 2019; El-Naggar use of crop residues and manure, incorporation of biochar, et al. 2019; Maraseni 2010; Purakayastha et al. 2019; Xu use of organic fertilizers and water management (Royal- et al. 2019). Society 2018; Srivastava 2012; Farooqi et al. 2018). Fur- Concerning the risks associated with large-scale deploy- thermore, the impact of perennial cropping systems on soil ment, albedo effect is mentioned in the literature. With high carbon sequestration is well documented in the literature. application rates of biochar to the soil surface, e.g. 30–60 Agostini et al. investigated the impact of herbaceous and tons/ha, it is argued that a decrease in surface reflectiv - woody perennial cropping systems on soil organic carbon ity would increase soil temperature, which in turn would and confirmed an increase in soil organic carbon levels by −1 −1 reduce the beneficial effect of carbon sequestration through 1.14–1.88 tCha  year for herbaceous crops and 0.63–0.72 −1 −1 this route (RoyalSociety 2018; Fuss et al. 2018). Other risks tCha  year for woody crops. It is reported that these val- and challenges associated include the risk of reversibility ues are well above the proposed sequestration requirement −1 −1 and challenges in monitoring, reporting and verification. (0.25 tCha  year ) to make the crop carbon neutral once Moreover, limited policy incentives and support, as well converted to biofuels (Agostini et al. 2015). The positive as lack of carbon pricing mechanisms that incorporate CO impact of perennial cropping systems on soil carbon seques- removal through biochar (Ernsting et al. 2011), hinder this tration is supported and documented in the literature by sev- technology’s potential for large-scale commercialization. eral other investigations (Nakajima et al. 2018; Sarkhot et al. Pourhashem et al. examined the role of government policy 2012). in accelerating biochar adoption and identified three types The main issues related to this approach revolve around of existing policy instruments that can be used to stimulate permanence, sink saturation as well as the impact on other biochar deployment in the USA: commercial financial incen - greenhouse gas emissions. According to Fuss et  al., the tives, non-financial incentives and research and development potential of carbon removal through soil carbon sequestra- funding (Pourhashem et al. 2019). With the current techno- tion is time-limited. Once soils reach a level of saturation, logical advancements, in particular blockchain, a number of further sequestration is no longer achieved. This may take start-ups are developing carbon removal platforms to drive 10–100 years depending on soil type and climatic condi- forward voluntary carbon offsets for consumers and corpora- tions. However, the Intergovernmental Panel on Climate tions. A Finnish start-up, Puro.earth, has introduced biochar Change (IPCC) defined a default saturation period of as a net-negative technology. Once verified through the com- 20 years (Fuss et al. 2018). Once saturation is reached, land pany’s verification system, the carbon removal certificates management practices need to be maintained indefinitely to generated by biochar producers are auctioned to potential mitigate reversal. A disadvantage to this would be the ongo- offset parties. However, until carbon removal is adequately ing costs with no further removal benefits. Risks of revers- monetized and supported through sufficient policy instru- ibility are significant and weaken this approach’s storage ments, biochar project development will probably not reach integrity. Another negative effect discussed in the literature the scale required to have a profound impact within the time is the impact of soil carbon sequestration on other green- frame mandated by international policy. house gas emissions, mainly CH and N O; however, this 4 2 effect is reported to be negligible (Fuss et al. 2018). Soil carbon sequestration By 2050, the global carbon dioxide removal potential discussed in the literature is estimated between 2.3 and 5.3 −1 Soil carbon sequestration is the process of capturing atmos- GtCO  year at costs ranging from $0 to $100 t/CO (Fuss 2 2 pheric CO through changing land management practices to et  al. 2018). While soil carbon sequestration is ready for increase soil carbon content. The level of carbon concentra- large-scale deployment, since many of such practices are tion within the soil is determined by the balance of inputs, already being used, lack of knowledge, resistance to change e.g. residues, litter, roots and manure, and the carbon losses as well as lack of policy and financial incentives are identi- realized through respiration which is mainly influenced by fied as barriers for scalability. Challenges around monitor - soil disturbance. Practices that increase inputs and/or reduce ing, reporting and verification, as well as concerns about losses drive soil carbon sequestration (RoyalSociety 2018; sink saturation and potential reversibility, have been the main Fuss et al. 2018). It is well noted in the literature that soil reasons behind slow policy action. However, non-climate 1 3 2082 Environmental Chemistry Letters (2020) 18:2069–2094 policies have mainly promoted land management practices technology is more energy- and material-intensive due to the to improve soil quality, fertility and productivity as well as fact that capturing CO from ambient air is much more dif- prevent land degradation (RoyalSociety 2018). While pol- ficult compared to capturing CO from highly concentrated icy and market-based mechanisms are required to push this flue gas streams. Direct air carbon capture is three times approach forward, international voluntary carbon removal energy-intensive compared to conventional carbon capture platforms are emerging. A US-based platform (Nori) is per ton of C O removed (Gambhir and Tavoni 2019). How- based on the concept of soil carbon sequestration and oper- ever, direct air carbon capture and storage plants are more ates by linking consumers and businesses that wish to offset e fl xible and can be located anywhere, provided that low-car - their carbon footprint with farmers that oe ff r carbon removal bon energy and adequate transportation and storage facili- certificates that have been audited through an independent ties are available. In terms of technology readiness, a lot of verification party. Using blockchain technology, this com- processes are currently being developed and are either under pany is one step further in fighting the challenges associated laboratory-scale or pilot-scale phases. Technology develop- with monitoring, reporting and verification systems. ers are mainly working on reducing energy requirements as this is one of the main challenges to deployment and scal- Direct air carbon capture and storage ability (RoyalSociety 2018). The global potential for carbon dioxide removal has Direct air carbon capture and storage, also referred to as been estimated by Fuss et al. to be in the range of 0.5–5 −1 DACCS in the literature, is emerging as a potential synthetic GtCO  year by 2050, and this may potentially go up to 40 −1 CO removal technology. The underlying principle behind GtCO  year by the end of the century if the unexpected 2 2 this technology is the use of chemical bonding to remove challenges associated with large-scale deployment are atmospheric CO directly from the air and then store it in overcome. Furthermore, CO removal costs are estimated 2 2 geological reservoirs or utilize it for other purposes such at $600–$1000/tCO initially, moving down to the range as the production of chemicals or mineral carbonates. C O of $100–$300/tCO as the technology matures (Fuss et al. 2 2 is captured from the air by allowing ambient air to get in 2018). Currently, there are no policy instruments to support contact with chemicals known as sorbents. Furthermore, the this technology, similar to many of the negative emissions sorbents are then regenerated by applying heat or water to technologies discussed (RoyalSociety 2018). release the CO for storage or utilization. There are mainly two processes by which sorbents work: first through absorp- Ocean fertilization tion, where the C O dissolves in the sorbent material, typi- cally using liquid sorbents such as potassium hydroxide or Ocean fertilization is the process of adding nutrients, sodium hydroxide; second through adsorption, whereby the macro such as phosphorus and nitrates as well as micro CO adheres to the sorbent, typically using solid materials such as iron, to the upper surface of the ocean to enhance such as amines (Pires 2019; GNASL 2018; Gambhir and CO uptake by promoting biological activity. Microscopic Tavoni 2019; Liu et al. 2018). Both processes require ther- organisms, called phytoplankton, found at the surface layer mal energy to regenerate the sorbent and release the CO ; of oceans are an important contributor to the concept of oce- however, it is important to note that less energy is required anic carbon sequestration. The sequestered CO , in the form under the adsorption route (Gambhir and Tavoni 2019). A of organic marine biomass, is naturally transported to the key issue widely discussed in the literature is the significant deep ocean; this process is termed “the biological pump”. energy required by direct air carbon capture and storage It is important to note that this downward flow is to a cer - plants. Besides the energy required for sorbent regenera- tain extent balanced by oceanic carbon respiration. Similar tion, energy is required for fans, pumps as well as compres- to land-based plants, phytoplankton utilizes light, C O as sors for pressurizing the C O . It is of course very important well as nutrients to grow. In the natural system, nutrients to utilize low-carbon energy sources, preferably renewable are available in the ocean as a consequence of death and energy as well as sources of waste heat, to drive the opera- decomposition of marine life. Hence, marine production is tion (Fuss et al. 2018). Another major drawback highlighted limited by the availability of recycled nutrients in the ocean. in the literature is the significant cost associated with devel- The idea behind ocean fertilization is to introduce additional oping direct air carbon capture and storage projects (Fuss nutrients to increase the magnitude of biological produc- et al. 2018). The major risk associated with this technology tion, which in turn increases C O uptake rate as compared is CO storage integrity, similar to that of carbon capture to the natural rate of respiration creating a carbon-negative and storage and bioenergy carbon capture and storage (Roy- atmospheric balance (RoyalSociety 2018; Williamson et al. alSociety 2018). 2012). Although there is not much information in the lit- Gambhir et al. compare direct air carbon capture and stor- erature regarding carbon removal potential, it is estimated age to carbon capture and storage and explain that the former that ocean fertilization can potentially sequester up to 3.7 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2083 −1 GtCO  year by 2100 with a total global storage capacity Besides the carbon removal potential associated with of 70–300 GtC O (RoyalSociety 2018). In terms of poten- enhanced weathering, the literature presents a number of tial abatement costs, a range between $2 and $457/tCO has positive side effects. This includes favourable impact on soil been estimated in the literature (Fuss et al. 2018). hydrological properties, a source for plant nutrients allowing Side effects of ocean fertilization that are discussed in the lower dependence on conventional fertilizers, increase in literature include ocean acidification, deep and mid-water water pH, enhanced soil health, increase in biomass produc- oxygen decrease or depletion, increase in production of fur- tion and an opportunity to reduce dependence on conven- ther greenhouse gases, unpredictable impact on food cycles, tional pesticides. Such benefits depend on the type of rock creation of toxic algal blooms as well as mixed effects on and its application rate, climate, soil and cropping system the seafloor and upper ocean ecosystems (Fuss et al. 2018; (RoyalSociety 2018; Fuss et al. 2018; de Oliveira Garcia Williamson et al. 2012). Furthermore, the environmental, et al. 2019; Strefler et al. 2018). economic and social effects as well as the energy and mate- In terms of technology readiness, enhanced weathering rial resources associated with fertilizer production, trans- can be practically deployed at the moment. Current land portation and distribution are significant. Moreover, accord- management practices incorporate the application of granu- ing to Fuss et al., uncertainty around permeance is a major lar materials, e.g. lime. Existing equipment can be utilized drawback. Permanence depends on whether the sequestered with no additional investment in equipment or infrastructure. carbon, in organic form, remains dissolved in the different The technologies related to quarrying, crushing and grinding layers of the ocean or whether sedimentation allows it to are well developed, and there would not be issues with scal- settle within long-term oceanic compartments for extended ability. However, under large-scale deployment, the energy periods (Fuss et al. 2018). The issue with permeance, impact required for extraction, production and transportation would on ecosystems, low sequestration efficiency, as well as lack be quite significant (RoyalSociety 2018). Careful attention of adequate monitoring, reporting and verification systems, should be paid to the carbon footprint of enhanced weather- do not support the concept that ocean fertilization is an ing operations to assess actual sequestration potential. Lefe- effective climate change abatement approach (Fuss et al. bevre et al. investigated carbon sequestration through EW 2018; Williamson et al. 2012). in Brazil by conducting a life cycle assessment to identify the carbon removal potential using basalt on agricultural Enhanced terrestrial weathering land in Sao Paolo. The investigation presented several key findings, first, that the operation emits 75 kg of CO per ton In the natural system, silicate rocks decompose; this is a of CO removed through enhanced weathering and 135 kg process termed weathering. This chemical reaction con-of CO per ton of CO removed through carbonation. This 2 2 sumes atmospheric CO and releases metal ions as well as is based on a distance of 65 km between the production site carbonate and/or bicarbonate ions. The dissolved ions are and the field on which the ground rock is applied. The results transported through groundwater streams through to rivers indicate a maximum road travel distance of 540 km for car- and eventually end up in the ocean where they are stored as bonation and 990 km for enhanced weathering, above which alkalinity, or they precipitate in the land system as carbon- the emissions offset the potential benefits realized from such ate minerals. Enhanced weathering is an approach that can activity. It is concluded that transportation is a major draw- accelerate this weathering process to enhance CO uptake on back which places limitations on the potential viability of a much shorter timescale. This is achieved through milling this technology. Furthermore, the results suggest a capture silicate rocks to increase its reactive surface and enhance rate of approximately 0.11–0.2 tCO e/ton of basaltic rock its mineral dissolution rate. The ground material is then applied (Lefebvre et al. 2019). applied to croplands providing a multitude of co-benefits Another approach to reducing pressure on the resources (RoyalSociety 2018; Bach et al. 2019). Kantola et al. discuss required for extraction is to utilize silicate wastes from the potential of applying this approach to bioenergy crop- various industries. Potential materials include wastes from ping systems (Kantola et al. 2017). According to Fuss et al., mining operations, cement, steel, aluminium, and coal or enhanced weathering promotes the sequestration of atmos- biomass combustion activities (Renforth 2019). However, pheric carbon in two forms, inorganic and organic. Inorganic this needs to be carefully assessed as potentially there is carbon is sequestered through the production of alkalinity a risk of releasing heavy metals into soils if inappropriate and carbonates, as discussed above. Organic carbon, on the materials are used (Fuss et al. 2018). Another risk associated other hand, is sequestered when additional carbon sequestra- with enhanced weathering is the potential health risk from tion is realized from enhanced biomass production, through the respiration of fine dust in the production and applica- photosynthesis, as a result of the nutrients that are naturally tion of finely ground rock materials (Strefler et al. 2018). released from the rocks (Fuss et al. 2018). Furthermore, uncertainties about the impacts of enhanced 1 3 2084 Environmental Chemistry Letters (2020) 18:2069–2094 weathering on microbial and marine biodiversity require the ocean’s acidity (Renforth and Henderson 2017). It is further investigation (RoyalSociety 2018). discussed in the literature that oceanic pH has a significant In terms of permanence, the sequestered C O can be impact on CO partial pressure for a given inorganic carbon 2 2 stored in several earth pools. Initially, C O can be stored content, which is the sum of carbon concentrations in car- as dissolved inorganic carbon, alkalinity, in soils as well as bonic acid, carbonate and bicarbonate ions (Kheshgi 1995). in groundwater. Depending on conditions, precipitation of Increasing ocean alkalinity is argued to decrease the sur- carbonate minerals in the soil can take place and such min- face ocean partial pressure, promoting further oceanic CO erals can be stored for an extended period (in the order of uptake, with a major positive side effect of reducing ocean 10  years) (Fuss et al. 2018). If precipitation does not take acidification. As alkalinity increases, more carbonic acid place, the dissolved inorganic carbon will be transported to is converted to bicarbonate and carbonate ions and greater the ocean through water streams, where it would be stored amounts of carbon are stored in inorganic form (Renforth as alkalinity, providing a number of additional benefits and and Henderson 2017). challenges to the oceanic pool. Based on an extensive litera- There are several approaches discussed in the literature ture assessment, Fuss et al. estimate global carbon removal on how an increase in oceanic alkalinity can be achieved. −1 potential of 2–4 GtCO   year by 2050 at a cost ranging The concept of enhanced weathering is the first approach to from $50 to $200/tCO (Fuss et  al. 2018). Strefler et  al. increase alkalinity within oceans. As previously discussed, conducted a techno-economic investigation on the carbon dissolved inorganic carbon in the form of bicarbonate and removal potential and costs of enhanced weathering using carbonate ions is a product of enhanced terrestrial weather- two rock types (dunite and basaltic rock). The results are ing. If precipitation does not occur, the bicarbonate and car- inline and support the estimates presented by Fuss et al. in bonate ions are transported through water streams and end terms of removal potential as well as costs. Furthermore, up in the ocean, increasing its alkalinity. Another approach is the investigation highlighted the dimensions that influence the addition of alkaline silicate rocks directly into the ocean, removal potential and cost, mainly being rock grain size and whereby finely ground rocks are added to the seawater for weathering rates. Finally, the study indicated that climates CO uptake and carbon storage in the form of bicarbonate that are warm and humid with lands that lack sufficient nutri- and carbonate ions, further enhancing alkalinity as well as ents are the most appropriate areas for enhanced weathering inducing additional atmospheric CO absorption (Bach et al. activities (Strefler et al. 2018). 2019). Another approach to increasing alkalinity was pro- At the moment, enhanced weathering is not included in posed by Kheshgi in the mid-1990s and that is the addition any carbon markets and does not have any policy support. of lime (CaO) to the ocean surface. The main drawback of Further research on social and environmental implications this approach is the energy required for the calcination of as well as adequate monitoring, reporting and verification limestone as well as the CO emissions realized (Kheshgi systems needs to be developed for this approach to gain 1995). Another approach discussed in the literature is the traction (RoyalSociety 2018). Moreover, integration within accelerated weathering of limestone. This concept includes carbon markets and adequate carbon pricing are required to utilizing a reactor and reacting limestone (CaCO ) with sea- incentivize deployment. water and a gas stream that is high in CO concentration to facilitate mineral dissolution. The main drawback of this Ocean alkalinity enhancement approach is the excessive water requirement (Renforth and Henderson 2017). Finally, the last approach to enhancing Ocean alkalinity enhancement has been discussed in the alkalinity was introduced by House et al. whereby an alka- literature as a potential route to inorganic carbon capture line solution is produced through an electrochemical method and storage within the ocean. The ocean already absorbs (House et al. 2009). Besides the challenges associated with a significant amount of atmospheric CO annually, mainly each of the approaches presented, challenges around the through two routes. First, through the diffusion of CO from impact of alkalinity enhancement on the oceanic ecosystem the atmosphere into the water, based on the differences of is still an area that needs further investigation. Furthermore, CO partial pressure between the atmosphere and the ocean. issues are raised around monitoring and regulations related The second route is through photosynthesis of phytoplank- to oceanic modifications (Renforth and Henderson 2017). ton discussed earlier. This section will mainly focus on CO In terms of permanence, carbon can be stored for oceanic uptake through diffusion that is governed by the extended periods, in the order of 10   years, in the form oceanic partial pressure of C O . When CO moves from the of dissolved inorganic carbon. The ocean currently stores 2 2 atmosphere into the ocean, the gas reacts with water to form approximately 140,000 GtCO , and with some changes in its carbonic acid, which further dissociates into bicarbonate and chemistry, it may be able to store in the order of trillions of carbonate ions, where dissolved inorganic carbon is stored. tons of CO (Renforth and Henderson 2017). There is, how- This reaction also releases hydrogen ions, which increases ever, a risk of reversal pointed out if mineral precipitation 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2085 takes place, reducing the carbon carrying capacity of the sink with significant sequestration rates of 9.9 MtCO and water (RoyalSociety 2018). According to Renforth et al., the 16.5 MtCO , respectively, over the entire period of study. cost of removing C O through ocean alkalinity enhancement This illustrates the high impact of management activities is estimated between $10 and $190/tCO , depending on the on the carbon sequestration potential of wetland habitats approach utilized in producing, transporting and distributing (Pindilli et al. 2018). the alkaline material (Renforth and Henderson 2017). Cur- Carbon sequestration and storage potential vary amongst rently, no policies or carbon pricing mechanisms incentivize different types of wetlands; for example, the estimated car - −1 −1 the pursuit of climate change abatement through this tech- bon sequestration rate is 6.3 ± 4.8 tCOe ha   year for −1 −1 nique, and there is still a need for field trials before deploy - mangroves, 8.0 ± 8.5 tCO2e ha year for salt marshes and −1 −1 ing such approach on a large scale. 4.4 ± 0.95 tCOe ha year for seagrass meadows. Within these habitats, the soil organic carbon accumulated in the top −1 −1 Wetland restoration and construction one metre amounted to 1060tC O e ha , 917 tC O ha and 2 2 −1 500 tCO ha for mangroves, salt marshes and seagrasses, Wetlands are high carbon density ecosystems that facilitate respectively (Sapkota and White 2020). The estimated cost atmospheric carbon sequestration through photosynthesis of carbon abatement through wetland restoration and con- and subsequent storage in above-ground and below-ground struction ranges between $10 and $100/tCO (RoyalSociety biomass as well as soil organic matter (Villa and Bernal 2018). According to Sapkota et al., several attempts have 2018). Examples of wetlands include peatlands as well as been made to include wetland-related offsets within exist- coastal habitats such as mangrove forests, tidal marshes ing voluntary and compliance carbon markets, including the and seagrass meadows, also referred to as blue carbon development of protocols and methodologies. A number of ecosystems. Furthermore, constructed wetlands have been methodologies have already been certified in the USA by discussed in the literature as a valid solution to wastewater various voluntary markets. However, despite the efforts, a treatment. While peatlands and coastal wetlands are esti- few wetland restoration carbon offsets have been transacted mated to store between 44 and 71% of the world’s terres- so far (Sapkota and White 2020). trial biological carbon, such carbon stocks are vulnerable to deterioration due to habitat degradation. Risks leading to Alternative negative emissions utilization and storage carbon loss, similar to forests, are caused by anthropogenic techniques activities as well as natural disasters. Restoration efforts usually revolve around rewetting the ecosystems as well as Mineral carbonation is a process by which C O is chemically further applicable measures (RoyalSociety 2018). A major reacted with minerals to form stable carbonates that can be drawback discussed in the literature is the substantial emis- safely stored below-ground or utilized in many applications sions of non-CO greenhouse gases such as CH and N O (Olajire 2013; Wang et al. 2020). It very much resembles 2 4 2 associated with wetland habitats. A number of investiga- the natural weathering process of converting silicate rocks tions emphasize the importance of incorporating the nega- to carbonates, but at a much faster rate. The literature dis- tive impact of non-CO greenhouse gases in evaluating the cusses two main routes for mineral carbonation, an ex situ sequestration benefits associated with a specific wetland res- industrial process above-ground that includes grinding and toration or construction project, as a specific site can either pre-treatment of minerals pre-reaction, or an in situ process be a net carbon sink or a greenhouse gas source. This is with direct injection of CO in silicate rocks below-ground based on various environmental and habitat management (RoyalSociety 2018; Olajire 2013; Galina et al. 2019). Sili- conditions (de Klein and van der Werf 2014; Gallant et al. cate rocks that contain high concentrations of calcium (Ca), 2020). Pindilli et al. conducted an empirical investigation magnesium (Mg) and iron (Fe) are the most suitable ele- on the impact of peatland restoration and management on ments to react with CO to form stable carbonates. Further- the carbon sequestration potential of a 54,000 ha protected more, industrial wastes that contain concentrations of such habitat over a 50-year period. The research modelled four elements such as slag from steel plants and fly ash from coal scenarios: the first scenario included no management, the combustion plants are also adequate materials to utilize for second added the impact of a catastrophic fire under no the carbonation process (Galina et al. 2019). Cost estimates management, the third incorporated current management under ex situ carbonation range from $50 to $300/tCO , practices, while the final scenario promoted increased man- while in situ carbonation is estimated at approximately $17/ agement activities. The results derived from this investi- tCO (RoyalSociety 2018). An interesting utilization route of gation showed that under the first two scenarios the peat- mineral carbonates is the replacement of conventional aggre- land is declared a net source of C O emissions, emitting gates in concrete production. Substituting aggregates with 2.4  MtCO and 6.5 MtCO , respectively. Under the third mineral carbonates in conjunction with C O curing to speed 2 2 2 and fourth scenarios, the peatland is declared a net carbon up the curing process and achieve higher strength concrete 1 3 2086 Environmental Chemistry Letters (2020) 18:2069–2094 material is a promising approach to sequester C O in the very large amount of sulphur dioxide gas (SO ) was ejected, 2 2 built environment (RoyalSociety 2018). Mineral carbonation between 15 and 30 million tons, which induced sunlight using CO that has been captured through direct air carbon reflectively and reduced global temperatures by 0.4–0.5 °C capture or bioenergy carbon capture systems can be con- (Zhang et al. 2015). Stratospheric aerosol injection is a solar sidered as a carbon-negative process since CO is removed radiation management technology that aims to mimic the from the atmosphere and safely stored in carbonate form cooling effect caused by the volcanic eruption by artificially in geological formations, or in the built environment if the injecting reflecting aerosol particles in the stratosphere carbonates are utilized in construction. It is also important (Lawrence et al. 2018; Zhang et al. 2015). Through model- to note that mineral carbonation can also be coupled with ling and past volcanic eruption data, the maximum potential carbon capture and storage technologies but would not be cooling from this approach is estimated between 2 and 5 W/ considered as a negative emissions technique if the CO uti- m (Lawrence et al. 2018). Smith et al. investigated the tech- lized is fossil-based. nology’s tactics and costs during the first 15 years of deploy - Another approach discussed in the literature is the utiliza- ment starting in 2033. They surveyed potential deployment tion of biomass materials in construction, while this is not techniques and concluded that an aircraft-based delivery sys- a new concept, technological advancements in thermal and tem is the most efficient method to deploy stratospheric aero- chemical treatments have mainly focused on increasing the sol injection. However, a new purpose-built high-altitude variety and number of materials that can be utilized in dif- aircraft will need to be developed for this purpose as current ferent applications within the building industry. The basic models, even with modifications will not be sufficient. In an principle behind this approach is that carbon is sequestered attempt to reduce anthropogenically driven radiative forcing through photosynthesis, where the resulting biomass can rate by half, Smith et al. calculated initial costs for deploy- then be utilized in construction allowing carbon to be stored ment to be in the range of $3.5 billion with average annual for decades in the built environment, e.g. building struc- operating costs of $2.25 billion (approximately $1500/t SO tures, insulation and furniture. The potential CO removal injected) (Smith and Wagner 2018). The main issue behind −1 is estimated at approximately 0.5–1 GtCO  year , through this technique is the uncertainty of the side effects and the replacing conventional construction materials (RoyalSociety harmful consequences of deployment, with a specific nega- 2018). Besides the removal potential, by replacing conven- tive impact on the hydrological cycle as well as stratospheric tional building materials such as steel and cement further ozone depletion (Zhang et al. 2015). It is important to note emission reductions can be realized since these are carbon- that while this approach will provide temporary temperature intensive materials. Estimates of 14–31% reduction in global reduction it should not be considered a long-term solution. CO emissions and 12–19% reduction in global fossil fuel This approach is still at a very early stage of research and consumption can be realized through this approach (Royal- development (Lawrence et al. 2018). Society 2018). However, significant sustainable forestation projects are required. Marine sky brightening Radiative forcing geoengineering technologies Marine sky brightening, also known as marine cloud bright- ening or cloud albedo enhancement, is another solar radia- Radiative forcing geoengineering techniques are a set of tion management technology that aims to maintain or reduce technologies that aim to alter the earth’s radiative energy global temperatures by enhancing cloud reflectivity. This is budget to stabilize or reduce global temperatures. This is achieved through cloud seeding with seawater particles or achieved by either increasing the earth’s reflectivity by with chemicals (Zhang et al. 2015). The main idea behind increasing shortwave solar radiation that is reflected to this technique is that seawater is sprayed into the air creat- space, termed solar radiation management, or by enhancing ing small droplets that easily evaporate leaving behind salt longwave radiation that is emitted by the earth’s surfaces to crystals that increase low-altitude cloud reflectivity above space, termed terrestrial radiation management (Lawrence oceans (Ming et al. 2014). The potential cooling effect has et al. 2018). This section briefly describes the various radia- been estimated between 0.8 and 5.4 W/m , due to uncer- tive forcing geoengineering techniques discussed in the lit- tainty, limited knowledge and spatial considerations (Law- erature. Figure 3 depicts the main techniques discussed in rence et al. 2018). While this technique seems simple and the literature and reviewed in this article. straightforward, Latham et al. highlighted a number of prob- lems associated with marine sky brightening. This includes Stratospheric aerosol injection the lack of spraying system that is capable of generating seawater particles of the size and quantities required, as well Back in 1991, a very large volcanic eruption took place in as further technical problems that are associated with the the Philippines (Mount Pinatubo). During the eruption, a physical outcome of this approach as a result of the complex 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2087 Fig. 3 Major radiative forcing geoengineering technologies that injection, marine sky brightening, cirrus cloud thinning, space-based aim to alter the earth’s radiative energy budget to stabilize or reduce mirrors and surface-based brightening global temperatures. These technologies include stratospheric aerosol nature of cloud characteristics. Another challenge would be to be reduced from approximately $10,000/kg to less than to undertake extensive trials and properly understand and $100/kg (Lawrence et al. 2018). Moreover, risks such as overcome potential side effects (Latham et al. 2012). Again, those associated with space debris and asteroid collisions this approach is still at an infant stage and will require exten- or those associated with technical and communication sive field research and development moving forward. failures need to be appropriately catered for (Lawrence et al. 2018). Space‑based mirrors Sunshade using space-based mirrors is a solar radiation Surface‑based brightening management technique discussed in the literature that aims to reflect part of the incoming solar radiation to reduce Another solar radiation management approach discussed global temperatures. For this approach to technically be in the literature is the brightening of the earth surface to deployed, space mirrors or reflectors need to be trans- increase the earth’s albedo and thus reduce global tempera- ported into orbit around the earth or placed at the Lagran- tures. This has been suggested through painting urban roofs gian L1 location between the earth and the sun, where the and roads in white, as well as covering deserts and glaciers gravitational fields are in balance allowing the reflectors with plastic sheets that are highly reflective, and, further - to remain stationary (Zhang et al. 2015; Kosugi 2010). more, by placing reflective floating panels over water bodies While this approach can have a considerable cooling effect (Ming et al. 2014). According to Lawrence et al., based on based on model simulations, development of such tech- an extensive literature review, the cooling potential for this nology is still at a very infant stage. The major drawback approach is too limited. Furthermore, substantial negative associated with this approach is the economic feasibility side effects are associated, such as disruption of desert eco- of transporting materials into space. For this technology systems (Lawrence et al. 2018). to be economically feasible, material transport costs need 1 3 2088 Environmental Chemistry Letters (2020) 18:2069–2094 Cirrus cloud thinning Bibliometric analysis of research on climate change mitigation Cirrus cloud thinning is a terrestrial radiation manage- ment technique that aims to increase longwave radia- Bibliometric analysis is a statistical tool that can be used tion that is emitted from the earth’s surface to space to to quantitatively analyse the current state of scientific stabilize or reduce global temperatures. Cirrus clouds research, by highlighting gaps in the literature as well as are high-altitude ice clouds that play a significant role trends. The Web of Science (WoS) core collection data- within the earth’s radiation budget, having an impact on base was used in this analysis. The following search meth- the earth’s hydrological cycle as well as surface tempera- odology was used to retrieve relevant research for further tures. Cirrus clouds absorb terrestrial radiation as well as evaluation. Please note that the search was refined to a reflect incoming solar radiation; however, in general, they 5-year timespan from 2015 to 2020 to specifically evaluate induce an average net warming effect from the imbal- scientific research efforts related to climate change mitiga- ance between incoming and outgoing radiative forcings tion after the Paris agreement in 2015. (Kärcher 2017). The basic principle behind this technique Search Methodology: is the injection of aerosols into cirrus clouds to reduce You searched for: TOPIC: (“Climate change mitiga- its optical thickness as well as its lifetime to increase tion”) OR TOPIC: (“climate change abatement”) OR terrestrial radiation emission to space. This approach TOPIC: (“Decarbonization Technologies”) OR TOPIC: would require regular cloud injection, so an efficient and (“Bioenergy Carbon Capture & Storage”) OR TOPIC: cost-effective delivery method needs to be in places such (“Afforestation & Reforestation”) OR TOPIC: (“Soil Car - as dedicated aircrafts or drones. Bismuth triiodide (Bil ) bon Sequestration”) OR TOPIC: (“Direct Air Carbon Cap- has been proposed as an effective cloud seeding mate- ture & Storage”) OR TOPIC: (“Ocean Fertilization”) OR rial; however, its toxicity needs to be taken into account. TOPIC: (“Enhanced Terrestrial Weathering”) OR TOPIC: Sea salt is another proposed option, yet it is not found to (“Ocean Alkalinity Enhancement”) OR TOPIC: (“Wetland be as effective as Bil (Lawrence et al. 2018). Based on Restoration & Construction”) OR TOPIC: (“Stratospheric model simulations, the maximum cooling effect through Aerosol Injection”) OR TOPIC: (“Marine Sky Brighten- this approach has been estimated to be in the range of ing”) OR TOPIC: (“Space-Based Sunshade/Mirrors”) 2–3.5 W/m (Lawrence et al. 2018). According to Law- OR TOPIC: (“Surface-Based Brightening”) OR TOPIC: rence et al., there are no published costs for cirrus cloud (“Cirrus Cloud Thinning”) OR TOPIC: (“Carbon Dioxide thinning and this approach still requires further research Removal Techniques”) OR TOPIC: (“Radiative Forcing to understand side effects as well as to conduct appro- Geoengineering”) priate research on potential delivery methods (Lawrence Timespan: Last 5  years. Indexes: SCI-EXPANDED, et al. 2018). SSCI, A&HCI, CPCI-S, CPCI-SSH, ESCI. Results: A total of 3993 papers were retrieved (3386 articles, 362 reviews, 201 proceedings papers, 71 early Miscellaneous radiation management techniques access and 61 editorial materials) The results obtained were then analysed using Ming et  al. proposed several theoretical technologies VOSviewer software by plotting network and density vis- that target terrestrial radiation, mainly by creating ther- ualization maps as shown in Fig. 4. The maps are based mal bridges to bypass the greenhouse gas insulating layer on keyword co-occurrences. The visualization maps high- and be able to transfer thermal radiation out to space. The light various trends related to climate change mitigation, research paper presented several concepts which include where areas related to biomass, carbon sequestration, transferring surface hot air to the troposphere, transfer- especially soil carbon sequestration, and biochar have ring latent and sensible heat to the top of the troposphere, received high attention over the past 5 years. Furthermore, transferring surface-sensible heat to the troposphere, as research related to policy, energy and in particular renew- well as transferring cold air to the earth surface. For each able energy has also received much attention. Although concept, conceptual technologies are proposed. Some of research on climate change mitigation is trending, a gap in the technologies discussed are systems that transfer heat the literature can be highlighted regarding research related beyond the earth system while generating energy, termed to specific mitigation technologies. It is also evident from metrological reactors by the authors (Ming et al. 2014). the literature that radiative forcing geoengineering tech- While the idea of thermal bridging is interesting, the tech- nologies have not received much attention. nologies and concepts introduced require further research, development and extensive field trials. 1 3 Environmental Chemistry Letters (2020) 18:2069–2094 2089 Fig. 4 Bibliometric analysis of research on climate change mitiga- change mitigation by highlighting trends and gaps in the literature tion: a network visualization map and b density visualization map, during 5 years between 2015 and 2020 showing the recent state of scientific research on the topic of climate 1 3 2090 Environmental Chemistry Letters (2020) 18:2069–2094 efficiently utilize financial resources and policy support Conclusion as most of the related technologies can be deployed imme- diately; however, efficient carbon pricing mechanisms that Based on the current state of climate emergency, imme- focus on carbon removal need to be aggressively devel- diate development of viable mitigation and adaptation oped and introduced. Furthermore, funding for technology mechanisms is of extreme importance. An extensive lit- research and development is also a very important aspect erature review covered three main strategies to tackling moving forward. climate change, conventional mitigation technologies, negative emissions technologies as well as radiative forc- Acknowledgements Authors would like to acknowledge the support ing geoengineering technologies. It is important to clarify given by the EPSRC project “Advancing Creative Circular Economies that there is no ultimate solution to tackle climate change for Plastics via Technological-Social Transitions” (ACCEPT Transi- and that all technologies and techniques discussed in this tions, EP/S025545/1). The authors wish to acknowledge the support of The Bryden Centre project (Project ID VA5048) which was awarded review if technically and economically are viable should by The European Union’s INTERREG VA Programme, managed by be deployed. As previously discussed, decarbonization the Special EU Programmes Body (SEUPB), with match funding pro- efforts alone are not sufficient to meet the targets stipu- vided by the Department for the Economy in Northern Ireland and the lated by the Paris agreement; therefore, the utilization of Department of Business, Enterprise and Innovation in the Republic of Ireland. an alternative abatement approach is inevitable. While the concept of radiative forcing geoengineering in terms Compliance with ethical standards of managing the earth’s radiation budget is interesting, it is not a long-term solution, as it does not solve the root Conflict of interest The author declares no competing financial inter - cause of the problem. It may, however, buy some time until ests. greenhouse gas concentrations are stabilized and reduced. However, the technologies to be deployed are still to be Open Access This article is licensed under a Creative Commons Attri- developed and tested and side effects adequately catered bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long for, which may be a lengthy process. Negative emissions as you give appropriate credit to the original author(s) and the source, technologies, on the other hand, provide a solid solution provide a link to the Creative Commons licence, and indicate if changes in combination with the current decarbonization efforts. were made. The images or other third party material in this article are While some of the negative emissions technologies pre- included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in sented in the literature review may still be at an early the article’s Creative Commons licence and your intended use is not stage of development, biogenic-based sequestration tech- permitted by statutory regulation or exceeds the permitted use, you will niques are to a certain extent mature and can be deployed need to obtain permission directly from the copyright holder. To view a immediately. Capturing C O through photosynthesis is a copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. straightforward and solid process; however, it needs to be effectively integrated within a technological framework as presented in the review. 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