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Environmental Chemistry Letters
https://doi.org/10.1007/s10311-020-01059-w
REVIEW
Strategies formitigation ofclimate change: areview
SamerFawzy1· AhmedI.Osman1 · JohnDoran2· DavidW.Rooney1
Received: 4 July 2020 / Accepted: 17 July 2020
© The Author(s) 2020
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 CO2 emissions.
Negative emissions technologies are aiming to capture and sequester atmospheric carbon toreduce 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
Abbreviations
Bio-DME Bio-dimethyl ether
BECCS Bioenergy carbon capture and storage
Bil3 Bismuth triiodide
Ca Calcium
CO2 Carbon dioxide
CO2e Carbon dioxide equivalent
CRED Centre for Research on the Epidemiology of
Disaster
DACCS Direct air carbon capture and storage
Gt Gigatons
GW Gigawatt
HFCs Hydrofluorocarbons
H2 Hydrogen
INDCs Intended nationally determined contributions
IPCC Intergovernmental Panel on Climate Change
IAEA International Atomic Energy Agency
ITMOs Internationally transferred mitigation
outcomes
Fe Iron
CaO Lime
CaCO3 Limestone
CH4 Methane
Mha Megahectare
Mt Million tons
N2O Nitrous oxide
NHRE Non-hydro renewable energy
* Ahmed I. Osman
aosmanahmed01@qub.ac.uk
1 School ofChemistry andChemical Engineering, Queen’s
University Belfast, David Keir Building, Stranmillis Road,
BelfastBT95AG, NorthernIreland,UK
2 The Bryden Centre, Letterkenny Institute ofTechnology,
Letterkenny, Ireland
Environmental Chemistry Letters
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OECD Organization for Economic Co-operation and
Development
PFCs Perfluorocarbons
ha−1 Per hectare
year−1 Per year
REDD+ Reducing emissions from deforestation and
forest degradation
SO2 Sulphur dioxide
SF6 Sulphur hexafluoride
PFCs Perfluorocarbons
t Ton
UNEP United Nations Environment Programme
UNFCCC United Nations Framework Convention on
Climate Change
W/m2 Watt per square meter
Mg Magnesium
Introduction
Status ofclimate change
Climate change is defined as the shift in climate patterns
mainly caused by greenhouse gas emissions. Greenhouse
gas emissions cause heat to be trapped by the earth’s atmos-
phere, and this has been the main driving force behind global
warming. The main sources of such emissions are natural
systems and human activities. Natural systems include for-
est fires, earthquakes, oceans, permafrost, wetlands, mud
volcanoes and volcanoes (Yue and Gao 2018), while human
activities are predominantly related to energy production,
industrial activities and those related to forestry, land use
and land-use change (Edenhofer etal. 2014).Yue and Gao
statistically analysed global greenhouse gas emissions from
natural systems and anthropogenic activities and concluded
that the earth’s natural system can be considered as self-
balancing and that anthropogenic emissions add extra pres-
sure to the earth system (Yue and Gao 2018).
GHG emissions overview
The greenhouse gases widely discussed in the literature and
defined by the Kyoto protocol are carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), and the fluorinated
gases such as hydrofluorocarbons (HFCs), perfluorocar-
bons (PFCs) and sulphur hexafluoride (SF6) (UNFCCC
2008). According to the emissions gap report prepared by
the United Nations Environment Programme (UNEP) in
2019, total greenhouse gas emissions in 2018 amounted to
55.3 GtCO2e, of which 37.5 GtCO2 are attributed to fos-
sil CO2 emissions from energy production and industrial
activities. An increase of 2% in 2018 is noted, as compared
to an annual increase of 1.5% over the past decade for both
total global greenhouse gas and fossil CO2 emissions. The
rise of fossil CO2 emissions in 2018 is mainly driven by
higher energy demand. Furthermore, emissions related to
land-use change amounted to 3.5 GtCO2 in 2018 (UNEP
2019). Together in 2018, fossil-based and land-use-related
CO2 emissions accounted for approximately 74% of the total
global greenhouse gas emissions. Methane (CH4), another
significant greenhouse gas, had an emission rate increase
of 1.7% in 2018 as compared to an annual increase of 1.3%
over the past decade. Nitrous oxide (N2O) emissions, which
are mainly influenced by agricultural and industrial activi-
ties, saw an increase of 0.8% in 2018 as compared to a 1%
annual increase over the past decade. A significant increase
was, however, noted in the fluorinated gases during 2018 at
6.1% as compared to a 4.6% annual increase over the past
decade (UNEP 2019). To put these numbers into perspec-
tive, a recent Intergovernmental Panel on Climate Change
(IPCC) report demonstrated that anthropogenic activities so
far have caused an estimated 1.0°C of global warming above
the pre-industrial level, specifying a likely range between 0.8
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
persist (IPCC 2018).
Climate change impacts, risks andvulnerabilities
An understanding of the severe impact of climate change on
natural and human systems as well as the risks and associ-
ated vulnerabilities is an important starting point in com-
prehending the current state of climate emergency. Changes
in climate indicators, namely temperature, precipitation,
seal-level rise, ocean acidification and extreme weather
conditions have been highlighted in a recent report by the
United Nations Climate Change Secretariat (UNCCS).
Climate hazards reported included droughts, floods, hur-
ricanes, severe storms, heatwaves, wildfires, cold spells
and landslides (UNCCS 2019). According to the Centre for
Research on the Epidemiology of Disasters (CRED), the
world encountered 315 cases of natural disasters in 2018,
mainly climate-related. This included 16 cases of drought,
26 cases of extreme temperature, 127 cases of flooding, 13
cases of landslides, 95 cases of storms and 10 cases of wild-
fire. The number of people affected by natural disasters in
2018 was 68.5 million, with floods, storms and droughts
accounting for 94% of total affected people. In terms of eco-
nomic losses, a total of $131.7 billion was lost in 2018 due
to natural disasters, with storms ($70.8B), floods ($19.7B),
wildfires ($22.8B) and droughts ($9.7B) accounting for
approximately 93% of the total costs. CRED also provides
data on disasters over the past decade, which shows even
higher annual averages in almost all areas, except for wild-
fire cases. The economic losses attributed to wildfires in
2018 alone are approximately equal to the collective losses
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from wildfires incurred over the past decade, which is quite
alarming (CRED 2019). Moreover, wildfires are a direct
source of CO2 emissions. Although wildfires are part of the
natural system, it is clear that human-induced emissions are
directly interfering and amplifying the impact of natural
system emissions. It is evident that human-induced climate
change is a major driving force behind many natural disas-
ters occurring globally.
Furthermore, climate risks such as temperature shifts,
precipitation variability, changing seasonal patterns, changes
in disease distribution, desertification, ocean-related impacts
and soil and coastal degradation contribute to vulnerability
across multiple sectors in many countries (UNCCS 2019).
Sarkodie etal. empirically examined climate change vul-
nerability and adaptation readiness of 192 United Nations
countries and concluded that food, water, health, ecosystem,
human habitat and infrastructure are the most vulnerable
sectors under climate attack while pointing out that Africa
is the most vulnerable region to climate variability (Sarkodie
and Strezov 2019). It is also important to note the intercon-
nected nature of such sectors and the associated impacts.
The 15th edition of the global risks report 2020 prepared
by the world economic forum thoroughly presented a num-
ber of climate realities, laying out areas that are greatly
affected. The risks included loss of life due to health hazards
and natural disasters, as well as excessive stress on ecosys-
tems, especially aquatic/marine systems. Moreover, food
and water security are other areas that are highly impacted.
Increased migration is anticipated due to extreme weather
conditions and disasters as well as rising sea levels. Geopo-
litical tensions and conflicts are likely to arise as countries
aim to extract resources along water and land boundaries.
The report also discusses the negative financial impact on
capital markets as systematic risks soar. Finally, the impact
on trade and supply chains is presented (WEF 2020).
An assessment, recently presented in an Intergovernmen-
tal Panel on Climate Change (IPCC) special report, covered
the impacts and projected risks associated with 2 levels of
global warming, 1.5°C and 2°C. The report investigated the
negative impact of global warming on freshwater sources,
food security and food production systems, ecosystems,
human health, urbanization as well as poverty and chang-
ing structures of communities. The report also investigated
climate change impact on key economic sectors such as tour-
ism, energy and transportation. It is evident that most of
the impacts assessed have lower associated risks at 1.5°C
compared to 2°C warming level. We would likely reach
1.5°C within the next 3 decades and increases in warm-
ing levels beyond this point would amplify risk effects; for
example, water stress would carry double the risk under a
2°C level compared to 1.5°C. An increase of 70% in popu-
lation affected by fluvial floods is projected under the 2°C
scenario compared to 1.5°C, especially in USA, Europe and
Asia. Double or triple rates of species extinction in terres-
trial ecosystems are projected under the 2°C level compared
to 1.5°C (IPCC 2018). It can be simply concluded that the
world is in a current state of climate emergency.
Global climate action
Acknowledgement of climate change realities started in 1979
when the first world climate conference was held in Geneva.
The world climate conference was introduced by the World
Meteorological Organization in response to the observation
of climatic events over the previous decade. The main pur-
pose was to invite technical and scientific experts to review
the latest knowledge on climate change and variability
caused by natural and human systems as well as assess future
impacts and risks to formulate recommendations moving
forward (WMO 1979). This was possibly the first of its kind
conference discussing the adverse effects of climate change.
In 1988, the Intergovernmental Panel on Climate Change
(IPCC) was set up by the World Meteorological Organiza-
tion in collaboration with the United Nations Environment
Programme (UNEP) to provide governments and official
bodies with scientific knowledge and information that can
be used to formulate climate-related policies (IPCC 2013).
Perhaps, the most critical step taken, in terms of action,
was the adoption of the United Nations Framework Conven-
tion on Climate Change (UNFCCC) in 1992, which then
went into force in 1994. Since then, the UNFCCC has been
the main driving force and facilitator of climate action glob-
ally. The main objective of the convention is the stabiliza-
tion of greenhouse gas concentrations in the atmosphere to
prevent severe impacts on the climate system. The conven-
tion set out the commitments to all parties involved, put-
ting major responsibilities on developed countries to imple-
ment national policies to limit anthropogenic emissions and
enhance greenhouse gas sinks. The target was to reduce
emissions by the year 2000 to the levels achieved in the
previous decade. Moreover, committing developed country
parties to assist vulnerable developing country parties finan-
cially and technologically in taking climate action. The con-
vention established the structure, reporting requirements and
mechanism for financial resources, fundamentally setting the
scene for global climate policy (UN 1992). The convention
is currently ratified by 197 countries (UNCCS 2019).
During the third UNFCCC conference of the parties
(COP-3) in 1997, the Kyoto protocol was adopted and
went into force in 2005. The Kyoto protocol introduced
the emission reduction commitments for developed coun-
tries for a five-year commitment period between 2008 and
2012. The protocol laid out all related policies, monitor-
ing and reporting systems, as well as introduced three
market-based mechanisms to achieve those targets. The
protocol introduced two project-based mechanisms, clean
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development mechanism and joint implementation mecha-
nism. The clean development mechanism allows developed
country parties to invest and develop emission reduction
projects in developing countries, to drive sustainable
development in the host country as well as offset carbon
emissions of the investing party. Joint implementation pro-
jects allow developed country parties to develop similar
projects, however, in other developed countries that are
protocol parties, offsetting excess emissions of the invest-
ing party. Furthermore, the protocol introduced an emis-
sions trading mechanism as a platform to facilitate the
trading of annually assigned emissions that are saved by
protocol members to those that exceed their limits (UNF-
CCC 1997). Emission reduction has mainly been achieved
through the introduction of renewable energy, energy effi-
ciency and afforestation/reforestation-related projects.
The Kyoto protocol defines four emission saving units,
each representing one metric ton of CO2 equivalent and are
all tradeable (UNFCCC 2005).
1 Certified emissions reduction unit, obtained through
clean development mechanism projects.
2 Emission reduction unit, obtained through joint imple-
mentation projects.
3 Assigned amount unit, obtained through the trading of
unused assigned emissions between protocol parties.
4 Removal unit, obtained through reforestation-related
projects.
The Kyoto units and general framework introduced laid
the structural foundation of a carbon emissions market and
the concept of carbon pricing. Many national and regional
governments introduced emissions trading schemes; some
are mandatory while others are voluntary. In some cases,
such schemes are linked to Kyoto commitments and regu-
lations. The largest emissions trading scheme introduced
thus far is the European emissions trading scheme (Per-
dan and Azapagic 2011). Villoria-Saez etal. empirically
investigated the effectiveness of greenhouse gas emissions
trading scheme implementation on actual emission reduc-
tions covering six major emitting regions. The investigation
presented a number of findings; first, it is possible to reduce
greenhouse gas emissions by approximately 1.58% annually
upon scheme implementation. Furthermore, after 10years
of implementation, approximately 23.43% of emissions
reduction can be achieved in comparison with a scenario
of non-implementation (Villoria-Sáez etal. 2016). Another
emission abatement instrument widely discussed in the liter-
ature is carbon taxation. There is growing scientific evidence
that carbon taxation is an effective instrument in reducing
greenhouse gas emissions; however, political opposition by
the public and industry is the main reason delaying many
countries in adopting such mechanism (Wang etal. 2016).
In 2012, the Doha amendment to the Kyoto protocol was
adopted, mainly proposing a second commitment period
from 2013 to 2020 as well as updating emissions reduction
targets. The amendment proposed a greenhouse gas emis-
sions reduction target of at least 18% below 1990 levels. The
amendment has not yet entered into force since it has not
been ratified by the minimum number of parties required to
this date (UNFCCC 2012).
During the twenty-first UNFCCC conference of the par-
ties (COP-21) held in Paris in 2015, the Paris agreement was
adopted and entered into force in 2016. The Paris agreement
added further objectives, commitments, enhanced compli-
ance and reporting regulations, as well as support mecha-
nisms to the existing climate change combat framework in
place. The main objective of the agreement is to limit the
global temperature increase to 2°C by 2100 and pursue
efforts to limit the increase to 1.5°C. The agreement aims
to reach global peaking of greenhouse gases as soon as pos-
sible as to strike a balance between human-induced emission
sources and greenhouse gas sinks and reservoirs between
2050 and 2100. The agreement also introduced new binding
commitments, asking all parties to deliver nationally deter-
mined contributions and to enforce national measures to
achieve, and attempt to exceed such commitments. Enhanced
transparency, compliance and clear reporting and commu-
nication are advocated under the agreement. Furthermore,
the agreement encourages voluntary cooperation between
parties beyond mandated initiatives. Moreover, financial
support and technological support, as well as capacity build-
ing initiatives for developing countries, are mandated by the
agreement. Such obligations are to be undertaken by devel-
oped country parties to promote sustainable development
and establish adequate mitigation and adaptation support
measures within vulnerable countries. Perhaps, one of the
most important goals established under the agreement is that
of adaptation and adaptive capacity building concerning the
temperature goal set (UN 2015).
Under article 6 of the agreement, two international mar-
ket mechanisms were introduced, cooperative approaches
and the sustainable development mechanism. These mecha-
nisms are to be utilized by all parties to meet their nation-
ally determined contributions. Cooperative approaches are
a framework that allows parties to utilize internationally
transferred mitigation outcomes (ITMOs) to meet nation-
ally determined contribution goals as well as stimulate sus-
tainable development. On the other hand, the sustainable
development mechanism is a new approach that promotes
mitigation and sustainable development and is perceived as
the successor of the clean development mechanism. There
is still much debate and negotiations on such mechanisms
moving forward (Gao etal. 2019).
Nieto etal. conducted an in-depth systematic analysis of
the effectiveness of the Paris agreement policies through the
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evaluation of 161 intended nationally determined contribu-
tions (INDCs) representing 188 countries. The study investi-
gated sectoral policies in each of these countries and quanti-
fied emissions under such INDCs. The analysis concluded
that a best-case scenario would be an annual global emission
increase of approximately 19.3% in 2030 compared to the
base period (2005–2015). In comparison, if no measures
were taken a 31.5% increase in global emissions is projected.
It is concluded that if the predicted best-case level of emis-
sions is maintained between 2030 and 2050 a temperature
increase of at least 3°C would be realized. Furthermore, a
4°C increase would be assured if annual emissions continue
to increase (Nieto etal. 2018).
To meet the 1.5°C target by the end of the century, the
IPCC stated that by 2030 greenhouse gas emissions should
be maintained at 25–30 GtCO2e year−1. In comparison, the
current unconditional nationally determined contributions
for 2030 are estimated at 52–58 GtCO2e year−1. Based on
pathway modelling for a 1.5°C warming scenario, a 45%
decline in anthropogenic greenhouse gas emissions must be
reached by 2030 as compared to 2010 levels, and net-zero
emissions must be achieved by 2050. To maintain a 2°C
global warming level by the end of the century, emissions
should decline by approximately 25% in 2030 as compared
to 2010 levels and net-zero emissions should be achieved by
2070 (IPCC 2018). There is growing evidence that confirms
that current mitigation efforts, as well as future emissions
commitments, are not sufficient to achieve the temperature
goals set by the Paris agreement (Nieto etal. 2018; Law-
rence etal. 2018). Further measures and new abatement
routes must be explored if an attempt is to be made to
achieve such goals.
Climate change mitigation strategies
Introduction
There are three main climate change mitigation approaches
discussed throughout the literature. First, conventional miti-
gation efforts employ decarbonization technologies and tech-
niques that reduce CO2 emissions, such as renewable energy,
fuel switching, efficiency gains, nuclear power, and carbon
capture storage and utilization. Most of these technologies
are well established and carry an acceptable level of man-
aged risk (Ricke etal. 2017; Victor etal. 2018; Bataille etal.
2018; Mathy etal. 2018; Shinnar and Citro 2008; Bustreo
etal. 2019).
A second route constitutes a new set of technologies and
methods that have been recently proposed. These techniques
are potentially deployed to capture and sequester CO2 from
the atmosphere and are termed negative emissions technolo-
gies, also referred to as carbon dioxide removal methods
(Ricke etal. 2017). The main negative emissions techniques
widely discussed in the literature include bioenergy carbon
capture and storage, biochar, enhanced weathering, direct air
carbon capture and storage, ocean fertilization, ocean alka-
linity enhancement, soil carbon sequestration, afforestation
and reforestation, wetland construction and restoration, as
well as alternative negative emissions utilization and storage
methods such as mineral carbonation and using biomass in
construction (Lawrence etal. 2018; Palmer 2019; McLaren
2012; Yan etal. 2019; McGlashan etal. 2012; Goglio etal.
2020; Lin 2019; Pires 2019; RoyalSociety 2018; Lenzi
2018).
Finally, a third route revolves around the principle of
altering the earth’s radiation balance through the manage-
ment of solar and terrestrial radiation. Such techniques are
termed radiative forcing geoengineering technologies, and
the main objective is temperature stabilization or reduction.
Unlike negative emissions technologies, this is achieved
without altering greenhouse gas concentrations in the atmos-
phere. The main radiative forcing geoengineering techniques
that are discussed in the literature include stratospheric aero-
sol injection, marine sky brightening, cirrus cloud thinning,
space-based mirrors, surface-based brightening and various
radiation management techniques. All these techniques are
still theoretical or at very early trial stages and carry a lot of
uncertainty and risk in terms of practical large-scale deploy-
ment. At the moment, radiative forcing geoengineering tech-
niques are not included within policy frameworks (Lawrence
etal. 2018; Lockley etal. 2019).
Conventional mitigation technologies
As previously discussed, energy-related emissions are the
main driver behind the increased greenhouse gas concen-
tration levels in the atmosphere; hence, conventional miti-
gation technologies and efforts should be focused on both
the supply and demand sides of energy. Mitigation efforts
primarily discussed in the literature cover technologies and
techniques that are deployed in four main sectors, power on
the supply side and industry, transportation and buildings
on the demand side. Within the power sector, decarboniza-
tion can be achieved through the introduction of renewable
energy, nuclear power, carbon capture and storage as well
as supply-side fuel switch to low-carbon fuels such as natu-
ral gas and renewable fuels. Furthermore, mitigation efforts
on the demand side include the efficiency gains achieved
through the deployment of energy-efficient processes and
sector-specific technologies that reduce energy consump-
tion, as well as end-use fuel switch from fossil-based fuels to
renewable fuels, and, moreover, the integration of renewable
power technologies within the energy matrix of such sectors
(Mathy etal. 2018; Hache 2015). This section will review
the literature on decarbonization and efficiency technologies
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and techniques that cover those four main sectors introduced.
Figure1 depicts the conventional mitigation technologies
and techniques discussed in the literature and critically
reviewed in this paper.
Renewable energy
According to a recent global status report on renewables,
the share of renewable energy from the total final energy
consumption globally has been estimated at 18.1% in 2017
(REN21 2019). An array of modern renewable energy tech-
nologies is discussed throughout the literature. The most
prominent technologies include photovoltaic solar power,
concentrated solar power, solar thermal power for heat-
ing and cooling applications, onshore and offshore wind
power, hydropower, marine power, geothermal power, bio-
mass power and biofuels (Mathy etal. 2018; Shinnar and
Citro 2008; Hache 2015; REN21 2019; Hussain etal. 2017;
Østergaard etal. 2020; Shivakumar etal. 2019; Collura etal.
2006; Gude and Martinez-Guerra 2018; Akalın etal. 2017;
Srivastava etal. 2017).
In terms of power production, as of 2018, renewable
energy accounted for approximately 26.2% of global elec-
tricity production. Hydropower accounted for 15.8%, while
wind power’s share was 5.5%, photovoltaic solar power
2.4%, biopower 2.2% and geothermal, concentrated solar
power and marine power accounted for 0.46% of the gen-
erated electricity (REN21 2019). While large-scale hydro-
power leads in terms of generation capacity as well as
production, there has been a significant capacity increase
in photovoltaic solar power and onshore wind power over
the past decade. By the end of 2018, a total of 505 GW of
global installed capacity for photovoltaic solar power has
been noted as compared to 15 GW in 2008. Regarding wind
power, 591 GW of global installed capacity is recorded in
2018 as compared to 121 GW in 2008. Global biopower
capacity has been estimated at 130 GW in 2018 with a total
581 TWh of production in that year. China has maintained
Fig. 1 Major decarbonization technologies which focus on the
reduction of CO2 emissions related to the supply and demand sides
of energy. Conventional mitigation technologies include renewable
energy, nuclear power, carbon capture and storage (CCS) as well as
utilization (CCU), fuel switching and efficiency gains. These tech-
nologies and techniques are mainly deployed in the power, industrial,
transportation and building sectors
Environmental Chemistry Letters
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its position as the largest renewable energy producing coun-
try, from solar, wind and biomass sources. The total share
of renewable energy in global power capacity has reached
approximately 33% in 2018 (REN21 2019).
Besides the power sector, renewable energy can be
deployed within the industry, transportation and building
sectors. Photovoltaic and thermal solar energy as well as
industrial end-use fuel switch to renewable fuels such as
solid, liquid and gaseous biofuels for combined thermal and
power production are examples of decarbonization efforts
through renewables. Buildings can also benefit from solar as
well as biomass-based technologies for power, heating and
cooling requirements. In relation to the transportation sector,
end-use fuel switch is a determinant to sector decarboni-
zation. Some examples of biofuels are biodiesel, first- and
second-generation bioethanol, bio-hydrogen, bio-methane
and bio-dimethyl ether (bio-DME) (Srivastava etal. 2020;
Chauhan etal. 2009; Hajilary etal. 2019; Osman 2020).
Furthermore, hydrogen produced through electrolysis using
renewable energy is a potential renewable fuel for sector
decarbonization. Another example of sector decarbonization
through renewable energy deployment is electric vehicles
using renewable power (Michalski etal. 2019). Other mitiga-
tion measures within these sectors will be further discussed
in the following section.
Variable renewables, such as solar and wind, are key tech-
nologies with significant decarbonization potential. One of
the main technological challenges associated is the intermit-
tent nature/variability in power production. This has been
overcome by integrating such technologies with storage as
well as other renewable baseload and grid technologies. Sin-
sel etal. discuss four specific challenge areas related to vari-
able renewables, namely quality, flow, stability and balance.
Furthermore, they present a number of solutions that mainly
revolve around flexibility as well as grid technologies for
distributed as well as centralized systems (Sinsel etal. 2020).
Economic, social and policy dimensions play an influ-
encing role in renewable energy technology innovation and
deployment. Pitelis etal. investigated the choice of policy
instruments and its effectiveness in driving renewable energy
technology innovation for 21 Organization for Economic
Co-operation and Development (OECD) countries between
1994 and 2014. The study classified renewable energy poli-
cies into three categories: technology-push, demand-pull
and systemic policy instruments. Furthermore, the study
investigated the impact of each policy classification on inno-
vation activity of various renewable energy technologies:
solar, wind, biomass, geothermal and hydro. The study con-
cluded that not all policy instruments have the same effect
on renewable energy technologies and that each technol-
ogy would require appropriate policies. However, the study
suggested that demand-pull policy instruments are more
effective in driving renewable energy innovation compared
to alternative policy types (Pitelis etal. 2019). On barriers
and drivers of renewable energy deployment, Shivakumar
etal. highlighted various dimensions that may hinder or
enable renewable energy project development. The main
points highlighted revolve around policy, financial access,
government stability and long-term intentions, administra-
tive procedures and support framework or lack thereof, as
well as the profitability of renewable energy investments
(Shivakumar etal. 2019). Seetharaman etal. analysed the
impact of various barriers on renewable energy deployment.
The research confirms that regulatory, social and techno-
logical barriers play a significant role in renewable energy
deployment. The research does not find a significant direct
relationship between economic barriers and project deploy-
ment; however, the interrelated nature between the economic
dimension with regulatory, social and technological barriers
affects deployment, however, indirectly (Seetharaman etal.
2019).
In terms of the relationship between financial accessibil-
ity and renewable energy deployment, Kim etal. empiri-
cally investigated such relationship by analysing a panel data
set of 30 countries during a 13-year period from 2000 to
2013. Statistical evidence shows the positive impact of well-
developed financial markets on renewable energy deploy-
ment and sector growth. Furthermore, the study confirms a
positive and significant relationship between market-based
mechanisms, such as clean development mechanism, with
renewable energy deployment. There is a strong impact on
photovoltaic solar and wind technologies, while the impact
is marginal under biomass and geothermal technologies
(Kim and Park 2016).
Pfeiffer etal. studied the diffusion of non-hydro renew-
able energy (NHRE) technologies in 108 developing coun-
tries throughout a 30-year period from 1980 to 2010. Based
on the results, economic and regulatory policies played a
pivotal role in NHRE deployment, as well as governmen-
tal stability, higher education levels and per capita income.
On the other hand, growth in energy demand, aid and high
local fossil fuel production hindered NHRE diffusion. In
contrast with Kim etal., the study finds weak support to
show that international financing mechanisms and financial
market development positively influenced diffusion (Pfeiffer
and Mulder 2013). The reason may be related to how the
analysis was constructed, different data sets, periods and
statistical methods.
Decarbonization through renewable energy deploy-
ment is extremely significant. Development of renew-
able energy projects should be seen as a top priority. The
areas that would drive decarbonization through renew-
able energy and should be focused upon by policymakers,
financiers and market participants include policy instru-
ments, financial support and accessibility, and market-
based mechanisms to incentivize project developers.
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Moreover, governmental support frameworks, public edu-
cation for social acceptance as well as research and devel-
opment efforts for technological advances and enhanced
efficiencies are important focus areas.
Nuclear power
According to the latest report prepared by the interna-
tional atomic energy agency (IAEA), as of 2018, 450
nuclear energy plants are operational with a total global
installed capacity of 396.4 GW. It is projected that an
increase of 30% in installed capacity will be realized by
2030 (from a base case of 392 GW in 2017). As a low-
case projection scenario, it is estimated that by 2030 a
10% dip might be realized based on the 2017 numbers. On
the long term, it is projected that global capacity might
reach 748 GW by 2050, as a high-case scenario (IAEA
2018). Pravalie etal. provide an interesting review of the
status of nuclear power. The investigation demonstrates
the significant role nuclear power has played in terms
of contribution to global energy production as well as
its decarbonization potential in the global energy sys-
tem. The study presents an estimation of approximately
1.2–2.4 Gt CO2 emissions that are prevented annually
from nuclear power deployment, as alternatively the
power would have been produced through coal or natural
gas combustion. The paper suggests that to be in line with
the 2°C target stipulated by the Paris agreement, nuclear
plant capacity must be expanded to approximately 930
GW by 2050, with a total investment of approximately $
4 trillion (Prăvălie and Bandoc 2018).
Although nuclear energy is considered as a low-carbon
solution for climate change mitigation, it comes with a
number of major disadvantages. First, the capital outlay
and operating costs associated with nuclear power devel-
opment are quite significant. Furthermore, risk of envi-
ronmental radioactive pollution is a major issue related
to nuclear power, which is mainly caused through the
threat of reactor accidents as well as the danger associated
with nuclear waste disposal (Prăvălie and Bandoc 2018;
Abdulla etal. 2019). While conventional fission-based
nuclear plants are suggested to be phased out in future,
the introduction of enhanced fusion-based nuclear tech-
nology may positively contribute to mitigation efforts in
the second half of the century. Fusion power is a new
generation of nuclear power, which is more efficient than
the conventional fission-based technology and does not
carry the hazardous waste disposal risk associated with
conventional fission-based nuclear technology. Further-
more, fusion power is characterized as a zero-emission
technology (Prăvălie and Bandoc 2018; Gi etal. 2020).
Carbon capture, storage andutilization
Carbon capture and storage is a promising technology
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-
ing CO2 gases from processes that rely on fossil fuels such
as coal, oil or gas. The captured CO2 is then transported
and stored in geological reservoirs for very long periods.
The main objective is the reduction in emission levels while
utilizing fossil sources. Three capturing technologies are dis-
cussed in the literature: pre-combustion, post-combustion
and oxyfuel combustion. Each technology carries a specific
process to extracting and capturing CO2. Post-combustion
capture technologies, however, are the most suitable for ret-
rofit projects and havevast application potential. Once CO2
has been successfully captured, it is liquified and transported
through pipelines or ships to suitable storage sites. Based on
the literature, storage options include depleted oil and gas
fields, coal beds and underground saline aquifers not used for
potable water (Vinca etal. 2018). Some of the main draw-
backs of carbon capture and storage include safety in relation
to secured storage and the possibility of leakage. Negative
environmental impacts that may result from onshore stor-
age locations that undergo accidental leakage have been
investigated by Ma etal. The investigation focused on the
impact of leakage on agricultural land (Ma etal. 2020). Risk
of leakage and associated negative impacts have also been
pointed out by Vinca etal. (2018). Other issues related to
this technology include public acceptance (Tcvetkov etal.
2019; Arning etal. 2019) as well as the high deployment
costs associated (Vinca etal. 2018). Another pathway post-
carbon capture is the utilization of the CO2 captured in the
production of chemicals, fuels, microalgae and concrete
building materials, as well as utilization in enhanced oil
recovery (Hepburn etal. 2019; Aresta etal. 2005; Su etal.
2016; Qin etal. 2020).
Large-scale deployment of carbon capture storage and
utilization technologies is yet to be proven. According to
the international energy agency, there are only 2 carbon cap-
ture and storage projects under operation as of 2018, with
a combined annual capture capacity of 2.4 MtCO2. There
are 9 more carbon capture projects under development and
are projected to increase capacity to 11 MtCO2 by 2025;
however, a significant deviation exists from the sustainable
development scenario targeted by the international energy
agency for 2040 which is a capacity of 1488 MtCO2 (IEA
2019a).
Fuel switch andeciency gains
Fuel switching in the power sector from coal to gas, in the
short-term, has been discussed extensively in the literature
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as a potential approach to economically transition to a low-
carbon and hopefully a zero-carbon economy in future (Vic-
tor etal. 2018; Wendling 2019; Pleßmann and Blechinger
2017). The move to natural gas is also applicable to industry,
transportation and building sectors; however, as discussed
previously the switch to renewable fuels is a more sustain-
able approach creating further decarbonization potential in
these sectors.
In addition to fuel switching, efficiency gains are of
extreme significance within mitigation efforts. Efficiency
gains in the power sector are achieved through improve-
ments in thermal power plants by enhancing the efficiency
of fuel combustion as well as improving turbine generator
efficiencies. Furthermore, waste heat recovery for additional
thermal as well as electric production enhances efficiency. In
gas-fired power plants, the utilization of a combined cycle
technology enhances the efficiency significantly. Combined
heat and power units have also played an interesting role in
efficiency gains. Technological advances within transmis-
sion and distribution networks also enhance efficiencies by
reducing losses (REN21 2019).
In industry, there are many potential areas where effi-
ciency gains may be realized. For example, in steel and
cement applications, waste heat can be recovered for onsite
power and heat production through the installation of waste
heat-driven power plants that utilize waste heat from exhaust
gases. For industries that utilize process steam, there is an
excellent opportunity to utilize waste steam pressure to
generate electric power for onsite usage or drive rotating
equipment. The application of back pressure steam turbines
in areas where steam pressure reduction is required can
enhance energy efficiency significantly. The same approach
can be deployed in applications where gas pressure reduc-
tion is required, however, using turboexpanders. Waste gases
from industrial processes can also be utilized to generate
onsite heat and power using micro- and small gas turbines.
In addition, further efficiency gains can be realized through
the deployment of advanced machinery controls in a multi-
tude of processes and industrial sectors.
A number of factors influence energy efficiency within
buildings, first the building design as well as materials uti-
lized in construction, e.g. insulation and glazing. Further-
more, appliances, devices and systems used throughout
buildings, e.g. heating, cooling and ventilation systems, and
lighting, play a pivotal role in energy consumption. Effi-
ciency gains can be realized by utilizing energy-efficient
systems and appliances as well as improved construction
materials (REN21 2019; Leibowicz etal. 2018).
In the transportation sector, efficiency gains can be real-
ized through the introduction of enhanced and more effi-
cient thermal engines, hybrid and electric vehicles as well
as hydrogen (H2) vehicles (Hache 2015). Furthermore, effi-
ciency gains can be achieved through technological advances
within aviation, shipping and rail, although rail is currently
one of the most energy-efficient modes. Efficiency measures
in the transportation sector can also take other forms. For
example, travel demand management, to reduce frequency
and distance of travel, can be an interesting approach. More-
over, shifting travel to the most efficient modes where pos-
sible, such as electrified rail, and reducing dependence on
high-intensity travel methods can play an interesting role in
enhancing efficiency (IEA 2019b).
Negative emissions technologies
Most of the climate pathways that were investigated by
the Intergovernmental Panel on Climate Change (IPCC)
included the deployment of negative emissions technolo-
gies along with conventional decarbonization technologies
to assess the feasibility of achieving the targets mandated
by the Paris agreement. Only two negative emissions tech-
nologies have been included in the IPCC assessments so far,
bioenergy carbon capture and storage as well as afforestation
and reforestation (IPCC 2018).
Gasser etal. empirically investigated the potential nega-
tive emissions needed to limit global warming to less than
2°C. The analysis utilized an IPCC pathway that is most
likely to maintain warming at such level and constructed
a number of scenarios based on conventional mitigation
assumptions in an attempt to quantify the potential nega-
tive emissions efforts required. The results indicated that
in the best-case scenario, that is under the best assumptions
on conventional mitigation efforts, negative emissions of
0.5–3Gt C year−1 and 50–250 Gt C of storage capacity are
required. Based on a worst-case scenario, negative emis-
sions of 7–11 Gt C year−1 and 1000–1600 Gt C of storage
capacity are required. (1 Gigaton Carbon = 3.6667 Gigaton
CO2e) The results indicate the inevitable need for negative
emissions, even at very high rates of conventional mitiga-
tion efforts. Furthermore, the study suggests that negative
emissions alone should not be relied upon to meet the 2°C
target. The investigation concluded that since negative emis-
sions technologies are still at an infant stage of development,
conventional mitigation technologies should remain focused
upon within climate policy, while further financial resources
are to be mobilized to accelerate the development of nega-
tive emissions technologies (Gasser etal. 2015).
It is argued that negative emissions technologies should
be deployed to remove residual emissions after all conven-
tional decarbonization efforts have been maximized and that
such approach should be utilized to remove emissions that
are difficult to eliminate through conventional methods (Lin
2019). It is important to note that negative emissions should
be viewed as a complementary suite of technologies and
techniques to conventional decarbonization methods, and
not a substitute (Pires 2019).
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The significant role of negative emissions in meeting cli-
mate targets is understood and appreciated amongst academ-
ics, scientists and policymakers; however, there still remains
a debate on the social, economic and technical feasibility
as well as the risk associated with large-scale deployment
(Lenzi 2018). This section will carry out an extensive lit-
erature review on the main negative emissions technologies
and techniques, their current state of development, perceived
limitations and risks as well as social and policy implica-
tions. Figure2 depicts the major negative emissions technol-
ogies and carbon removal methods discussed in the literature
and critically reviewed in this article.
Bioenergy carbon capture andstorage
Bioenergy carbon capture and storage, also referred to as
BECCS, is one of the prominent negative emissions tech-
nologies discussed widely in the literature. The Intergovern-
mental Panel on Climate Change (IPCC) heavily relied on
bioenergy carbon capture and storage within their assess-
ments as a potential route to meet temperature goals (IPCC
2018). The technology is simply an integration of biopower,
and carbon capture and storage technologies discussed ear-
lier. The basic principle behind the technology is quite
straightforward. Biomass biologically captures atmospheric
CO2 through photosynthesis during growth, which is then
utilized for energy production through combustion. The CO2
emissions realized upon combustion are then captured and
stored in suitable geological reservoirs (Pires 2019; Roy-
alSociety 2018). This technology can significantly reduce
greenhouse gas concentration levels by removing CO2 from
the atmosphere. The carbon dioxide removal potential of
this technology varies within the literature; however, a con-
servative assessment by Fuss etal. presents an estimated
range of 0.5–5 GtCO2 year−1 by 2050 (Fuss etal. 2018). In
terms of global estimates for storage capacity, the literature
presents a wide range from 200 to 50,000 GtCO2 (Fuss etal.
2018). Cost estimates for carbon dioxide removal through
bioenergy carbon capture and storage are in the range of
$100-$200/tCO2 (Fuss etal. 2018).
The biomass feedstocks utilized for this approach can
either be dedicated energy crops or wastes from agricultural
or forestry sources. Furthermore, such feedstocks can either
be used as dedicated bio-based feedstocks or can be com-
bined with fossil-based fuels in co-fired power plants (Roy-
alSociety 2018). Besides the standard combustion route, the
literature suggests that CO2 can be captured in non-power
bio-based applications, such as during the fermentation
Fig. 2 Major negative emissions technologies and techniques which
are deployed to capture and sequester carbon from the atmosphere.
This approach includes bioenergy carbon capture and storage, affores-
tation and reforestation, biochar, soil carbon sequestration, enhanced
terrestrial weathering, wetland restoration and construction, direct air
carbon capture and storage, ocean alkalinity enhancement and ocean
fertilization
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process in ethanol production or the gasification of wood
pulp effluent, e.g. black liquor, in pulp production (McLaren
2012; Pires 2019).
The main challenge associated with this technology is
the significant amount of biomass feedstocks required to
be an effective emission abatement approach. Under large-
scale deployment, resource demand when utilizing dedicated
crops would be quite significant, with high pressure exerted
on land, water as well as nutrient resources. A major issue
would be the direct competition with food and feed crops for
land, freshwater and nutrients (RoyalSociety 2018; GNASL
2018). Heck etal. empirically investigated the large-scale
deployment of bioenergy carbon capture and storage for
climate change abatement and demonstrated its impact on
freshwater use, land system change, biosphere integrity and
biogeochemical flows. Furthermore, the investigation identi-
fied the interrelated nature between each of these dimensions
as well as the associated impacts when any one dimension is
prioritized (Heck etal. 2018). A sustainable approach to land
use is quite critical in approaching bioenergy carbon capture
and storage. Competing with food for arable land and chang-
ing forest land to dedicated plantations have serious negative
social and environmental effects. Harper etal. argue that the
effectiveness of this technology in achieving negative emis-
sions is based on several factors which include previous land
cover, the initial carbon gain or loss due to land-use change,
bioenergy crop yields, and the amount of harvested carbon
that is ultimately sequestered. Their empirical investigation
highlights the negative impact of bioenergy carbon capture
and storage when dedicated plantations replace carbon-dense
ecosystems (Harper etal. 2018). Another issue discussed in
the literature is the albedo effects of biomass cultivation.
This is mainly applicable in high-latitude locations, where
biomass replaces snow cover and reduces radiation reflection
potential which offsets mitigation efforts (Fuss etal. 2018).
In terms of technology readiness, bioenergy technologies
are to a certain extent well developed; however, carbon cap-
ture and storage are still at an early stage. Technology risk
is mainly associated with storage integrity and the potential
of leakage as discussed previously on carbon capture and
storage. Furthermore, Mander etal. discuss the technical
difficulties in scaling deployment within a short period.
Besides, they question whether this technology can deliver
its abatement potential within the projected time frame. In
terms of policy, it is argued that a strong framework, as well
as adequate incentives, need to be in place to properly push
the technology forward (Mander etal. 2017). Commercial
logic may not be enough to drive forward global deploy-
ment. Financial viability of such projects will depend on a
utilitarian carbon market that caters for negative emissions
as well as an appropriate carbon price that incentivizes
deployment (Hansson etal. 2019). Therefore, policy should
look at ways to strengthen carbon pricing mechanisms and
introduce negative emissions as a new class of tradeable
credits (Fajardy etal. 2019).
Aorestation andreforestation
During tree growth, CO2 is captured from the atmosphere
and stored in living biomass, dead organic matter and soils.
Forestation is thus a biogenic negative emissions technology
that plays an important role within climate change abatement
efforts. Forestation can be deployed by either establishing
new forests, referred to as afforestation, or re-establishing
previous forest areas that have undergone deforestation or
degradation, which is referred to as reforestation. Depend-
ing on tree species, once forests are established CO2 uptake
may span 20–100years until trees reach maturity and then
sequestration rates slow down significantly. At that stage,
forest products can be harvested and utilized. It is argued
that forest management activities and practices have an envi-
ronmental impact and should be carefully planned (Royal-
Society 2018). Harper etal. discuss several advantages and
co-benefits that are associated with forest-based mitigation
which include biodiversity, flood control as well as quality
improvement for soil, water and air (Harper etal. 2018).
Carbon can be stored in forests for a very long time; how-
ever, permanence is vulnerable due to natural and human
disturbances. Natural disasters such as fire, droughts and
disease or human-induced deforestation activities are all
risks that negatively impact storage integrity. In general,
biogenic storage has a much shorter lifespan than storage
in geological formations, such as in the case of bioenergy
carbon capture and storage (Fuss etal. 2018). Another issue
related to forestation is land requirement as well as compe-
tition with other land use. Significant amounts of land are
required to achieve effective abatement results (RoyalSociety
2018). Fuss etal. discuss another issue and that is the albedo
effect. Forests in high latitudes would actually be counter-
productive, accelerating local warming as well as ice and
snow cover loss. They argue that tropical areas would be the
most suitable zones to host forestation projects. However,
competition with agriculture and other sectors for land will
be another problem. Based on global tropical boundary limi-
tations, an estimated total area of 500 Mha is argued to be
suitable for forestation deployment. This would allow for a
global carbon dioxide removal potential of 0.5–3.6 GtCO2
year−1 by 2050. Removal costs are estimated at $5–$50/tCO2
(Fuss etal. 2018).
In terms of technology readiness, afforestation and refor-
estation have already been widely adopted on a global level
and have already been integrated within climate policies
through the Kyoto protocol’s clean development mecha-
nism programme since the 1990s. To drive forward forest-
based mitigation efforts, the protocol introduced removal
units which allowed forestation projects to yield tradeable
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credits. Despite the early policy measures, forest-based
mitigation efforts accounted for a small fraction of emis-
sions at that time. Forest-based abatement projects have
also been introduced through national regulations as well
as voluntary systems such as the reducing emissions from
deforestation and forest degradation (REDD+) programme
that was introduced by the United Nations in 2008. However,
carbon sequestration through forestation remained insignifi-
cant, as it only accounted for 0.5% of the total carbon traded
in 2013 (Gren and Aklilu 2016). The effectiveness of the
REDD+ programme is argued in the literature after more
than 10years of its introduction. Hein etal. present a num-
ber of arguments around the programme’s poor track record
in achieving its intended purpose of emissions reduction.
However, despite the uncertainty and weaknesses discussed,
REDD+ implementation intentions have been indicated by
56 countries in their INDC submissions under the Paris
agreement (Hein etal. 2018). Permanence, sequestration
uncertainty, the availability of efficient financing mecha-
nisms as well as monitoring, reporting and verification
systems are all difficulties associated around forest-based
abatement projects (Gren and Aklilu 2016).
Biochar
Biochar has recently gained considerable recognition as a
viable approach for carbon capture and permanent storage
and is considered as one of the promising negative emis-
sions technologies. Biochar is produced from biomass, e.g.
dedicated crops, agricultural residues and forestry residues,
through a thermochemical conversion process. It is produced
through pyrolysis, a process of heating in the absence of
oxygen, as well as through gasification and hydrothermal
carbonization (Matovic 2011; Oni etal. 2020; Osman etal.
2020a, b). The carbon captured by biomass through CO2
uptake during plant growth is then processed into a char that
can be applied to soils for extended periods. The conversion
process stores biomass carbon in a form that is very stable
and resistant to decomposition. Stability in soils is perhaps
the most important property of biochar that makes it a solid
carbon removal technology. Although considered more sta-
ble than soil organic carbon, there are certain uncertain-
ties around decomposition rates of various types of biochar,
which depends on the feedstock used and process conditions
utilized (Osman etal. 2019; Chen etal. 2019). Depending
on the feedstock used, it is estimated that this technology
can potentially remove between 2.1 and 4.8 tCO2/tonne of
biochar (RoyalSociety 2018). Carbon removal potential, as
well as costs, varies greatly in the literature; however, a con-
servative range is provided by Fuss etal. It is estimated that
by 2050 global carbon reduction removal potential achieved
through biochar can be in the range of 0.3–2 Gt CO2year−1,
with costs ranging from $90 to $120/tCO2 (Fuss etal. 2018).
In terms of resource requirements, biochar produc-
tion would require vast amounts of land to have an effec-
tive impact on greenhouse gas concentration levels. Land
is required for feedstock cultivation, as well as for biochar
dispersal acting as a carbon sink. While land for dedicated
biomass cultivation may create competition issues with
agriculture and other land-use sectors, same as the case of
bioenergy carbon capture and storage, there would be no
issues with areas required for biochar dispersal. This would
be the case as long as the biochar is technically matched
with the type of crop, soil and growing conditions related
to the specific cropping system. Besides soil, Schmidt
etal. introduced other carbon sink applications for biochar
which include construction materials, wastewater treatment
and electronics, as long as the product does not thermally
degrade or oxidize throughout its life cycle (Schmidt etal.
2019). Furthermore, it has been argued in the literature that
marginal and degraded lands can potentially be utilized for
dedicated plantations, relieving pressure on land that can
be used for other purposes. Moreover, using waste biomass
eliminates the need for land and provides a waste disposal
solution; however, competition over waste for other purposes
increases feedstock availability risk as well as price vola-
tility. Biomass availability is one of the limiting factors to
successful large-scale deployment of biochar projects (Roy-
alSociety 2018).
In addition to the beneficial effect of capturing and stor-
ing CO2 from the atmosphere, there is growing evidence
in the literature that biochar also has an impact on other
greenhouse gas emissions such as CH4 and N2O. Although
the literature shows a positive impact in many occasions,
in terms of reduced emissions, Semida etal. present mixed
results, where the application of biochar has positive as well
as negative effects on CH4 and N2O emissions. This is spe-
cific to the cropping system as well as the type of biochar
utilized and its processing conditions (Semida etal. 2019).
Xiao etal. also present conflicting results regarding biochar
application, which is very specific to the condition of the
soils amended with biochar (Xiao etal. 2019). Impact on
greenhouse gas emissions should, therefore, be studied on
a case-by-case basis.
Another benefit that is widely discussed in the literature
is the positive effects associated with biochar application to
soils. It is argued that soil quality and fertility are signifi-
cantly enhanced. Improvement in nutrient cycling, reduction
in nutrient leaching from the soil and an increase in water
and nutrient retention as well as stimulation of soil microbial
activity are all co-benefits associated with biochar applica-
tion. However, this is mainly dependent on biochar physical
and chemical properties. Such properties are defined by the
type of feedstock utilized, pyrolysis conditions, as well as
other processing conditions. Furthermore, despite the gen-
eral perception that biochar positively impacts plant growth
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and production, which is true in a large number of cases,
there is evidence that biochar application may hinder plant
growth in certain cropping systems. This is based on the
type of biochar, the quantity applied and the specific crops
under cultivation and sometimes management practices. The
evidence is mixed, and therefore careful analysis should be
carried out to successfully match biochar with appropriate
carbon sinks (Oni etal. 2020; Semida etal. 2019; El-Naggar
etal. 2019; Maraseni 2010; Purakayastha etal. 2019; Xu
etal. 2019).
Concerning the risks associated with large-scale deploy-
ment, albedo effect is mentioned in the literature. With high
application rates of biochar to the soil surface, e.g. 30–60
tons/ha, it is argued that a decrease in surface reflectiv-
ity would increase soil temperature, which in turn would
reduce the beneficial effect of carbon sequestration through
this route (RoyalSociety 2018; Fuss etal. 2018). Other risks
and challenges associated include the risk of reversibility
and challenges in monitoring, reporting and verification.
Moreover, limited policy incentives and support, as well
as lack of carbon pricing mechanisms that incorporate CO2
removal through biochar (Ernsting etal. 2011), hinder this
technology’s potential for large-scale commercialization.
Pourhashem etal. examined the role of government policy
in accelerating biochar adoption and identified three types
of existing policy instruments that can be used to stimulate
biochar deployment in the USA: commercial financial incen-
tives, non-financial incentives and research and development
funding (Pourhashem etal. 2019). With the current techno-
logical advancements, in particular blockchain, a number of
start-ups are developing carbon removal platforms to drive
forward voluntary carbon offsets for consumers and corpora-
tions. A Finnish start-up, Puro.earth, has introduced biochar
as a net-negative technology. Once verified through the com-
pany’s verification system, the carbon removal certificates
generated by biochar producers are auctioned to potential
offset parties. However, until carbon removal is adequately
monetized and supported through sufficient policy instru-
ments, biochar project development will probably not reach
the scale required to have a profound impact within the time
frame mandated by international policy.
Soil carbon sequestration
Soil carbon sequestration is the process of capturing atmos-
pheric CO2 through changing land management practices to
increase soil carbon content. The level of carbon concentra-
tion within the soil is determined by the balance of inputs,
e.g. residues, litter, roots and manure, and the carbon losses
realized through respiration which is mainly influenced by
soil disturbance. Practices that increase inputs and/or reduce
losses drive soil carbon sequestration (RoyalSociety 2018;
Fuss etal. 2018). It is well noted in the literature that soil
carbon sequestration promotes enhanced soil fertility and
health as well as improves crop yields due to organic carbon
accumulation within soils (Fuss etal. 2018). Various land
management practices that promote soil carbon sequestration
are discussed in the literature which include cropping system
intensity and rotation practices, zero-tillage and conserva-
tion tillage practices, nutrient management, mulching and
use of crop residues and manure, incorporation of biochar,
use of organic fertilizers and water management (Royal-
Society 2018; Srivastava 2012; Farooqi etal. 2018). Fur-
thermore, the impact of perennial cropping systems on soil
carbon sequestration is well documented in the literature.
Agostini etal. investigated the impact of herbaceous and
woody perennial cropping systems on soil organic carbon
and confirmed an increase in soil organic carbon levels by
1.14–1.88 tCha−1year−1 for herbaceous crops and 0.63–0.72
tCha−1year−1 for woody crops. It is reported that these val-
ues are well above the proposed sequestration requirement
(0.25 tCha−1year−1) to make the crop carbon neutral once
converted to biofuels (Agostini etal. 2015). The positive
impact of perennial cropping systems on soil carbon seques-
tration is supported and documented in the literature by sev-
eral other investigations (Nakajima etal. 2018; Sarkhot etal.
2012).
The main issues related to this approach revolve around
permanence, sink saturation as well as the impact on other
greenhouse gas emissions. According to Fuss etal., the
potential of carbon removal through soil carbon sequestra-
tion is time-limited. Once soils reach a level of saturation,
further sequestration is no longer achieved. This may take
10–100years depending on soil type and climatic condi-
tions. However, the Intergovernmental Panel on Climate
Change (IPCC) defined a default saturation period of
20years (Fuss etal. 2018). Once saturation is reached, land
management practices need to be maintained indefinitely to
mitigate reversal. A disadvantage to this would be the ongo-
ing costs with no further removal benefits. Risks of revers-
ibility are significant and weaken this approach’s storage
integrity. Another negative effect discussed in the literature
is the impact of soil carbon sequestration on other green-
house gas emissions, mainly CH4 and N2O; however, this
effect is reported to be negligible (Fuss etal. 2018).
By 2050, the global carbon dioxide removal potential
discussed in the literature is estimated between 2.3 and 5.3
GtCO2year−1 at costs ranging from $0 to $100 t/CO2 (Fuss
etal. 2018). While soil carbon sequestration is ready for
large-scale deployment, since many of such practices are
already being used, lack of knowledge, resistance to change
as well as lack of policy and financial incentives are identi-
fied as barriers for scalability. Challenges around monitor-
ing, reporting and verification, as well as concerns about
sink saturation and potential reversibility, have been the main
reasons behind slow policy action. However, non-climate
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policies have mainly promoted land management practices
to improve soil quality, fertility and productivity as well as
prevent land degradation (RoyalSociety 2018). While pol-
icy and market-based mechanisms are required to push this
approach forward, international voluntary carbon removal
platforms are emerging. A US-based platform (Nori) is
based on the concept of soil carbon sequestration and oper-
ates by linking consumers and businesses that wish to offset
their carbon footprint with farmers that offer carbon removal
certificates that have been audited through an independent
verification party. Using blockchain technology, this com-
pany is one step further in fighting the challenges associated
with monitoring, reporting and verification systems.
Direct air carbon capture andstorage
Direct air carbon capture and storage, also referred to as
DACCS in the literature, is emerging as a potential synthetic
CO2 removal technology. The underlying principle behind
this technology is the use of chemical bonding to remove
atmospheric CO2 directly from the air and then store it in
geological reservoirs or utilizeit for other purposes such
as the production of chemicals or mineral carbonates. CO2
is captured from the air by allowing ambient air to get in
contact with chemicals known as sorbents. Furthermore, the
sorbents are then regenerated by applying heat or water to
release the CO2 for storage or utilization. There are mainly
two processes by which sorbents work: first through absorp-
tion, where the CO2 dissolves in the sorbent material, typi-
cally using liquid sorbents such as potassium hydroxide or
sodium hydroxide; second through adsorption, whereby the
CO2 adheres to the sorbent, typically using solid materials
such as amines (Pires 2019; GNASL 2018; Gambhir and
Tavoni 2019; Liu etal. 2018). Both processes require ther-
mal energy to regenerate the sorbent and release the CO2;
however, it is important to note that less energy is required
under the adsorption route (Gambhir and Tavoni 2019). A
key issue widely discussed in the literature is the significant
energy required by direct air carbon capture and storage
plants. Besides the energy required for sorbent regenera-
tion, energy is required for fans, pumps as well as compres-
sors for pressurizing the CO2. It is of course very important
to utilize low-carbon energy sources, preferably renewable
energy as well as sources of waste heat, to drive the opera-
tion (Fuss etal. 2018). Another major drawback highlighted
in the literature is the significant cost associated with devel-
oping direct air carbon capture and storage projects (Fuss
etal. 2018). The major risk associated with this technology
is CO2 storage integrity, similar to that of carbon capture
and storage and bioenergy carbon capture and storage (Roy-
alSociety 2018).
Gambhir etal. compare direct air carbon capture and stor-
age to carbon capture and storage and explain that the former
technology is more energy- and material-intensive due to the
fact that capturing CO2 from ambient air is much more dif-
ficult compared to capturing CO2 from highly concentrated
flue gas streams. Direct air carbon capture is three times
energy-intensive compared to conventional carbon capture
per ton of CO2 removed (Gambhir and Tavoni 2019). How-
ever, direct air carbon capture and storage plants are more
flexible and can be located anywhere, provided that low-car-
bon energy and adequate transportation and storage facili-
ties are available. In terms of technology readiness, a lot of
processes are currently being developed and are either under
laboratory-scale or pilot-scale phases. Technology develop-
ers are mainly working on reducing energy requirements as
this is one of the main challenges to deployment and scal-
ability (RoyalSociety 2018).
The global potential for carbon dioxide removal has
been estimated by Fuss etal. to be in the range of 0.5–5
GtCO2year−1 by 2050, and this may potentially go up to 40
GtCO2year−1 by the end of the century if the unexpected
challenges associated with large-scale deployment are
overcome. Furthermore, CO2 removal costs are estimated
at $600–$1000/tCO2 initially, moving down to the range
of $100–$300/tCO2 as the technology matures (Fuss etal.
2018). Currently, there are no policy instruments to support
this technology, similar to many of the negative emissions
technologies discussed (RoyalSociety 2018).
Ocean fertilization
Ocean fertilization is the process of adding nutrients,
macro such as phosphorus and nitrates as well as micro
such as iron, to the upper surface of the ocean to enhance
CO2 uptake by promoting biological activity. Microscopic
organisms, called phytoplankton, found at the surface layer
of oceans are an important contributor to the concept of oce-
anic carbon sequestration. The sequestered CO2, in the form
of organic marine biomass, is naturally transported to the
deep ocean; this process is termed “the biological pump”.
It is important to note that this downward flow is to a cer-
tain extent balanced by oceanic carbon respiration. Similar
to land-based plants, phytoplankton utilizes light, CO2 as
well as nutrients to grow. In the natural system, nutrients
are available in the ocean as a consequence of death and
decomposition of marine life. Hence, marine production is
limited by the availability of recycled nutrients in the ocean.
The idea behind ocean fertilization is to introduce additional
nutrients to increase the magnitude of biological produc-
tion, which in turn increases CO2 uptake rate as compared
to the natural rate of respiration creating a carbon-negative
atmospheric balance (RoyalSociety 2018; Williamson etal.
2012). Although there is not much information in the lit-
erature regarding carbon removal potential, it is estimated
that ocean fertilization can potentially sequester up to 3.7
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GtCO2year−1 by 2100 with a total global storage capacity
of 70–300 GtCO2 (RoyalSociety 2018). In terms of poten-
tial abatement costs, a range between $2 and $457/tCO2 has
been estimated in the literature (Fuss etal. 2018).
Side effects of ocean fertilization that are discussed in the
literature include ocean acidification, deep and mid-water
oxygen decrease or depletion, increase in production of fur-
ther greenhouse gases, unpredictable impact on food cycles,
creation of toxic algal blooms as well as mixed effects on
the seafloor and upper ocean ecosystems (Fuss etal. 2018;
Williamson etal. 2012). Furthermore, the environmental,
economic and social effects as well as the energy and mate-
rial resources associated with fertilizer production, trans-
portation and distribution are significant. Moreover, accord-
ing to Fuss etal., uncertainty around permeance is a major
drawback. Permanence depends on whether the sequestered
carbon, in organic form, remains dissolved in the different
layers of the ocean or whether sedimentation allows it to
settle within long-term oceanic compartments for extended
periods (Fuss etal. 2018). The issue with permeance, impact
on ecosystems, low sequestration efficiency, as well as lack
of adequate monitoring, reporting and verification systems,
do not support the concept that ocean fertilization is an
effective climate change abatement approach (Fuss etal.
2018; Williamson etal. 2012).
Enhanced terrestrial weathering
In the natural system, silicate rocks decompose; this is a
process termed weathering. This chemical reaction con-
sumes atmospheric CO2 and releases metal ions as well as
carbonate and/or bicarbonate ions. The dissolved ions are
transported through groundwater streams through to rivers
and eventually end up in the ocean where they are stored as
alkalinity, or they precipitate in the land system as carbon-
ate minerals. Enhanced weathering is an approach that can
accelerate this weathering process to enhance CO2 uptake on
a much shorter timescale. This is achieved through milling
silicate rocks to increase its reactive surface and enhance
its mineral dissolution rate. The ground material is then
applied to croplands providing a multitude of co-benefits
(RoyalSociety 2018; Bach etal. 2019). Kantola etal. discuss
the potential of applying this approach to bioenergy crop-
ping systems (Kantola etal. 2017). According to Fuss etal.,
enhanced weathering promotes the sequestration of atmos-
pheric carbon in two forms, inorganic and organic. Inorganic
carbon is sequestered through the production of alkalinity
and carbonates, as discussed above. Organic carbon, on the
other hand, is sequestered when additional carbon sequestra-
tion is realized from enhanced biomass production, through
photosynthesis, as a result of the nutrients that are naturally
released from the rocks (Fuss etal. 2018).
Besides the carbon removal potential associated with
enhanced weathering, the literature presents a number of
positive side effects. This includes favourable impact on soil
hydrological properties, a source for plant nutrients allowing
lower dependence on conventional fertilizers, increase in
water pH, enhanced soil health, increase in biomass produc-
tion and an opportunity to reduce dependence on conven-
tional pesticides. Such benefits depend on the type of rock
and its application rate, climate, soil and cropping system
(RoyalSociety 2018; Fuss etal. 2018; de Oliveira Garcia
etal. 2019; Strefler etal. 2018).
In terms of technology readiness, enhanced weathering
can be practically deployed at the moment. Current land
management practices incorporate the application of granu-
lar materials, e.g. lime. Existing equipment can be utilized
with no additional investment in equipment or infrastructure.
The technologies related to quarrying, crushing and grinding
are well developed, and there would not be issues with scal-
ability. However, under large-scale deployment, the energy
required for extraction, production and transportation would
be quite significant (RoyalSociety 2018). Careful attention
should be paid to the carbon footprint of enhanced weather-
ing operations to assess actual sequestration potential. Lefe-
bevre etal. investigated carbon sequestration through EW
in Brazil by conducting a life cycle assessment to identify
the carbon removal potential using basalt on agricultural
land in Sao Paolo. The investigation presented several key
findings, first, that the operation emits 75kg of CO2 per ton
of CO2 removed through enhanced weathering and 135kg
of CO2 per ton of CO2 removed through carbonation. This
is based on a distance of 65km between the production site
and the field on which the ground rock is applied. The results
indicate a maximum road travel distance of 540km for car-
bonation and 990km for enhanced weathering, above which
the emissions offset the potential benefits realized from such
activity. It is concluded that transportation is a major draw-
back which places limitations on the potential viability of
this technology. Furthermore, the results suggest a capture
rate of approximately 0.11–0.2 tCO2e/ton of basaltic rock
applied (Lefebvre etal. 2019).
Another approach to reducing pressure on the resources
required for extraction is to utilize silicate wastes from
various industries. Potential materials include wastes from
mining operations, cement, steel, aluminium, and coal or
biomass combustion activities (Renforth 2019). However,
this needs to be carefully assessed as potentially there is
a risk of releasing heavy metals into soils if inappropriate
materials are used (Fuss etal. 2018). Another risk associated
with enhanced weathering is the potential health risk from
the respiration of fine dust in the production and applica-
tion of finely ground rock materials (Strefler etal. 2018).
Furthermore, uncertainties about the impacts of enhanced
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weathering on microbial and marine biodiversity require
further investigation (RoyalSociety 2018).
In terms of permanence, the sequestered CO2 can be
stored in several earth pools. Initially, CO2 can be stored
as dissolved inorganic carbon, alkalinity, in soils as well as
in groundwater. Depending on conditions, precipitation of
carbonate minerals in the soil can take place and such min-
erals can be stored for an extended period (in the order of
106years) (Fuss etal. 2018). If precipitation does not take
place, the dissolved inorganic carbon will be transported to
the ocean through water streams, where it would be stored
as alkalinity, providing a number of additional benefits and
challenges to the oceanic pool. Based on an extensive litera-
ture assessment, Fuss etal. estimate global carbon removal
potential of 2–4 GtCO2year−1 by 2050 at a cost ranging
from $50 to $200/tCO2 (Fuss etal. 2018). Strefler etal.
conducted a techno-economic investigation on the carbon
removal potential and costs of enhanced weathering using
two rock types (dunite and basaltic rock). The results are
inline and support the estimates presented by Fuss etal. in
terms of removal potential as well as costs. Furthermore,
the investigation highlighted the dimensions that influence
removal potential and cost, mainly being rock grain size and
weathering rates. Finally, the study indicated that climates
that are warm and humid with lands that lack sufficient nutri-
ents are the most appropriate areas for enhanced weathering
activities (Strefler etal. 2018).
At the moment, enhanced weathering is not included in
any carbon markets and does not have any policy support.
Further research on social and environmental implications
as well as adequate monitoring, reporting and verification
systems needs to be developed for this approach to gain
traction (RoyalSociety 2018). Moreover, integration within
carbon markets and adequate carbon pricing are required to
incentivize deployment.
Ocean alkalinity enhancement
Ocean alkalinity enhancement has been discussed in the
literature as a potential route to inorganic carbon capture
and storage within the ocean. The ocean already absorbs
a significant amount of atmospheric CO2 annually, mainly
through two routes. First, through the diffusion of CO2 from
the atmosphere into the water, based on the differences of
CO2 partial pressure between the atmosphere and the ocean.
The second route is through photosynthesis of phytoplank-
ton discussed earlier. This section will mainly focus on CO2
oceanic uptake through diffusion that is governed by the
oceanic partial pressure of CO2. When CO2 moves from the
atmosphere into the ocean, the gas reacts with water to form
carbonic acid, which further dissociates into bicarbonate and
carbonate ions, where dissolved inorganic carbon is stored.
This reaction also releases hydrogen ions, which increases
the ocean’s acidity (Renforth and Henderson 2017). It is
discussed in the literature that oceanic pH has a significant
impact on CO2 partial pressure for a given inorganic carbon
content, which is the sum of carbon concentrations in car-
bonic acid, carbonate and bicarbonate ions (Kheshgi 1995).
Increasing ocean alkalinity is argued to decrease the sur-
face ocean partial pressure, promoting further oceanic CO2
uptake, with a major positive side effect of reducing ocean
acidification. As alkalinity increases, more carbonic acid
is converted to bicarbonate and carbonate ions and greater
amounts of carbon are stored in inorganic form (Renforth
and Henderson 2017).
There are several approaches discussed in the literature
on how an increase in oceanic alkalinity can be achieved.
The concept of enhanced weathering is the first approach to
increase alkalinity within oceans. As previously discussed,
dissolved inorganic carbon in the form of bicarbonate and
carbonate ions is a product of enhanced terrestrial weather-
ing. If precipitation does not occur, the bicarbonate and car-
bonate ions are transported through water streams and end
up in the ocean, increasing its alkalinity. Another approach is
the addition of alkaline silicate rocks directly into the ocean,
whereby finely ground rocks are added to the seawater for
CO2 uptake and carbon storage in the form of bicarbonate
and carbonate ions, further enhancing alkalinity as well as
inducing additional atmospheric CO2 absorption (Bach etal.
2019). Another approach to increasing alkalinity was pro-
posed by Kheshgi in the mid-1990s and that is the addition
of lime (CaO) to the ocean surface. The main drawback of
this approach is the energy required for the calcination of
limestone as well as the CO2 emissions realized (Kheshgi
1995). Another approach discussed in the literature is the
accelerated weathering of limestone. This concept includes
utilizing a reactor and reacting limestone (CaCO3) with sea-
water and a gas stream that is high in CO2 concentration to
facilitate mineral dissolution. The main drawback of this
approach is the excessive water requirement (Renforth and
Henderson 2017). Finally, the last approach to enhancing
alkalinity was introduced by House etal. whereby an alka-
line solution is produced through an electrochemical method
(House etal. 2009). Besides the challenges associated with
each of the approaches presented, challenges around the
impact of alkalinity enhancement on the oceanic ecosystem
is still an area that needs further investigation. Furthermore,
issues are raised around monitoring and regulations related
to oceanic modifications (Renforth and Henderson 2017).
In terms of permanence, carbon can be stored for
extended periods, in the order of 104years, in the form
of dissolved inorganic carbon. The ocean currently stores
approximately 140,000 GtCO2, and with some changes in its
chemistry, it may be able to store in the order of trillions of
tons of CO2 (Renforth and Henderson 2017). There is, how-
ever, a risk of reversal pointed out if mineral precipitation
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takes place, reducing the carbon carrying capacity of the
water (RoyalSociety 2018). According to Renforth etal., the
cost of removing CO2 through ocean alkalinity enhancement
is estimated between $10 and $190/tCO2, depending on the
approach utilized in producing, transporting and distributing
the alkaline material (Renforth and Henderson 2017). Cur-
rently, no policies or carbon pricing mechanisms incentivize
the pursuit of climate change abatement through this tech-
nique, and there is still a need for field trials before deploy-
ing such approach on a large scale.
Wetland restoration andconstruction
Wetlands are high carbon density ecosystems that facilitate
atmospheric carbon sequestration through photosynthesis
and subsequent storage in above-ground and below-ground
biomass as well as soil organic matter (Villa and Bernal
2018). Examples of wetlands include peatlands as well as
coastal habitats such as mangrove forests, tidal marshes
and seagrass meadows, also referred to as blue carbon
ecosystems. Furthermore, constructed wetlands have been
discussed in the literature as a valid solution to wastewater
treatment. While peatlands and coastal wetlands are esti-
mated to store between 44 and 71% of the world’s terres-
trial biological carbon, such carbon stocks are vulnerable
to deterioration due to habitat degradation. Risks leading to
carbon loss, similar to forests, are caused by anthropogenic
activities as well as natural disasters. Restoration efforts
usually revolve around rewetting the ecosystems as well as
further applicable measures (RoyalSociety 2018). A major
drawback discussed in the literature is the substantial emis-
sions of non-CO2 greenhouse gases such as CH4 and N2O
associated with wetland habitats. A number of investiga-
tions emphasize the importance of incorporating the nega-
tive impact of non-CO2 greenhouse gases in evaluating the
sequestration benefits associated with a specific wetland res-
toration or construction project, as a specific site can either
be a net carbon sink or a greenhouse gas source. This is
based on various environmental and habitat management
conditions (de Klein and van der Werf 2014; Gallant etal.
2020). Pindilli etal. conducted an empirical investigation
on the impact of peatland restoration and management on
the carbon sequestration potential of a 54,000ha protected
habitat over a 50-year period. The research modelled four
scenarios: the first scenario included no management, the
second added the impact of a catastrophic fire under no
management, the third incorporated current management
practices, while the final scenario promoted increased man-
agement activities. The results derived from this investi-
gation showed that under the first two scenarios the peat-
land is declared a net source of CO2 emissions, emitting
2.4MtCO2 and 6.5MtCO2, respectively. Under the third
and fourth scenarios, the peatland is declared a net carbon
sink with significant sequestration rates of 9.9MtCO2 and
16.5MtCO2, respectively, over the entire period of study.
This illustrates the high impact of management activities
on the carbon sequestration potential of wetland habitats
(Pindilli etal. 2018).
Carbon sequestration and storage potential vary amongst
different types of wetlands; for example, the estimated car-
bon sequestration rate is 6.3 ± 4.8 tCO2e ha−1year−1 for
mangroves, 8.0 ± 8.5 tCO2e ha−1 year−1 for salt marshes and
4.4 ± 0.95 tCO2e ha−1 year−1 for seagrass meadows. Within
these habitats, the soil organic carbon accumulated in the top
one metre amounted to 1060tCO2e ha −1, 917 tCO2 ha−1 and
500 tCO2 ha−1 for mangroves, salt marshes and seagrasses,
respectively (Sapkota and White 2020). The estimated cost
of carbon abatement through wetland restoration and con-
struction ranges between $10 and $100/tCO2 (RoyalSociety
2018). According to Sapkota etal., several attempts have
been made to include wetland-related offsets within exist-
ing voluntary and compliance carbon markets, including the
development of protocols and methodologies. A number of
methodologies have already been certified in the USA by
various voluntary markets. However, despite the efforts, a
few wetland restoration carbon offsets have been transacted
so far (Sapkota and White 2020).
Alternative negative emissions utilization andstorage
techniques
Mineral carbonation is a process by which CO2 is chemically
reacted with minerals to form stable carbonates that can be
safely stored below-ground or utilized in many applications
(Olajire 2013; Wang etal. 2020). It very much resembles
the natural weathering process of converting silicate rocks
to carbonates, but at a much faster rate. The literature dis-
cusses two main routes for mineral carbonation, an ex situ
industrial process above-ground that includes grinding and
pre-treatment of minerals pre-reaction, or an insitu process
with direct injection of CO2 in silicate rocks below-ground
(RoyalSociety 2018; Olajire 2013; Galina etal. 2019). Sili-
cate rocks that contain high concentrations of calcium (Ca),
magnesium (Mg) and iron (Fe) are the most suitable ele-
ments to react with CO2 to form stable carbonates. Further-
more, industrial wastes that contain concentrations of such
elements such as slag from steel plants and fly ash from coal
combustion plants are also adequate materials to utilize for
the carbonation process (Galina etal. 2019). Cost estimates
under ex situ carbonation range from $50 to $300/tCO2,
while insitu carbonation is estimated at approximately $17/
tCO2 (RoyalSociety 2018). An interesting utilization route of
mineral carbonates is the replacement of conventional aggre-
gates in concrete production. Substituting aggregates with
mineral carbonates in conjunction with CO2 curing to speed
up the curing process and achieve higher strength concrete
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material is a promising approach to sequester CO2 in the
built environment (RoyalSociety 2018). Mineral carbonation
using CO2 that has been captured through direct air carbon
capture or bioenergy carbon capture systems can be con-
sidered as a carbon-negative process since CO2 is removed
from the atmosphere and safely stored in carbonate form
in geological formations, or in the built environment if the
carbonates are utilized in construction. It is also important
to note that mineral carbonation can also be coupled with
carbon capture and storage technologies but would not be
considered as a negative emissions technique if the CO2 uti-
lized is fossil-based.
Another approach discussed in the literature is the utiliza-
tion of biomass materials in construction, while this is not
a new concept, technological advancements in thermal and
chemical treatments have mainly focused on increasing the
variety and number of materials that can be utilized in dif-
ferent applications within the building industry. The basic
principle behind this approach is that carbon is sequestered
through photosynthesis, where the resulting biomass can
then be utilized in construction allowing carbon to be stored
for decades in the built environment, e.g. building struc-
tures, insulation and furniture. The potential CO2 removal
is estimated at approximately 0.5–1 GtCO2year−1, through
replacing conventional construction materials (RoyalSociety
2018). Besides the removal potential, by replacing conven-
tional building materials such as steel and cement further
emission reductions can be realized since these are carbon-
intensive materials. Estimates of 14–31% reduction in global
CO2 emissions and 12–19% reduction in global fossil fuel
consumption can be realized through this approach (Royal-
Society 2018). However, significant sustainable forestation
projects are required.
Radiative forcing geoengineering technologies
Radiative forcing geoengineering techniques are a set of
technologies that aim to alter the earth’s radiative energy
budget to stabilize or reduce global temperatures. This is
achieved by either increasing the earth’s reflectivity by
increasing shortwave solar radiation that is reflected to
space, termed solar radiation management, or by enhancing
longwave radiation that is emitted by the earth’s surfaces to
space, termed terrestrial radiation management (Lawrence
etal. 2018). This section briefly describes the various radia-
tive forcing geoengineering techniques discussed in the lit-
erature. Figure3 depicts the main techniques discussed in
the literature and reviewed in this article.
Stratospheric aerosol injection
Back in 1991, a very large volcanic eruption took place in
the Philippines (Mount Pinatubo). During the eruption, a
very large amount of sulphur dioxide gas (SO2) was ejected,
between 15 and 30 million tons, which induced sunlight
reflectively and reduced global temperatures by 0.4–0.5°C
(Zhang etal. 2015). Stratospheric aerosol injection is a solar
radiation management technology that aims to mimic the
cooling effect caused by the volcanic eruption by artificially
injecting reflecting aerosol particles in the stratosphere
(Lawrence etal. 2018; Zhang etal. 2015). Through model-
ling and past volcanic eruption data, the maximum potential
cooling from this approach is estimated between 2 and 5W/
m2 (Lawrence etal. 2018). Smith etal. investigated the tech-
nology’s tactics and costs during the first 15years of deploy-
ment starting in 2033. They surveyed potential deployment
techniques and concluded that an aircraft-based delivery sys-
tem is the most efficient method to deploy stratospheric aero-
sol injection. However, a new purpose-built high-altitude
aircraft will need to be developed for this purpose as current
models, even with modifications will not be sufficient. In an
attempt to reduce anthropogenically driven radiative forcing
rate by half, Smith etal. calculated initial costs for deploy-
ment to be in the range of $3.5 billion with average annual
operating costs of $2.25 billion (approximately $1500/t SO2
injected) (Smith and Wagner 2018). The main issue behind
this technique is the uncertainty of the side effects and the
harmful consequences of deployment, with a specific nega-
tive impact on the hydrological cycle as well as stratospheric
ozone depletion (Zhang etal. 2015). It is important to note
that while this approach will provide temporary temperature
reduction it should not be considered a long-term solution.
This approach is still at a very early stage of research and
development (Lawrence etal. 2018).
Marine sky brightening
Marine sky brightening, also known as marine cloud bright-
ening or cloud albedo enhancement, is another solar radia-
tion management technology that aims to maintain or reduce
global temperatures by enhancing cloud reflectivity. This is
achieved through cloud seeding with seawater particles or
with chemicals (Zhang etal. 2015). The main idea behind
this technique is that seawater is sprayed into the air creat-
ing small droplets that easily evaporate leaving behind salt
crystals that increase low-altitude cloud reflectivity above
oceans (Ming etal. 2014). The potential cooling effect has
been estimated between 0.8 and 5.4W/m2, due to uncer-
tainty, limited knowledge and spatial considerations (Law-
rence etal. 2018). While this technique seems simple and
straightforward, Latham etal. highlighted a number of prob-
lems associated with marine sky brightening. This includes
the lack of spraying system that is capable of generating
seawater particles of the size and quantities required, as well
as further technical problems that are associated with the
physical outcome of this approach as a result of the complex
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1 3
nature of cloud characteristics. Another challenge would be
to undertake extensive trials and properly understand and
overcome potential side effects (Latham etal. 2012). Again,
this approach is still at an infant stage and will require exten-
sive field research and development moving forward.
Space‑based mirrors
Sunshade using space-based mirrors is a solar radiation
management technique discussed in the literature that aims
to reflect part of the incoming solar radiation to reduce
global temperatures. For this approach to technically be
deployed, space mirrors or reflectors need to be trans-
ported into orbit around the earth or placed at the Lagran-
gian L1 location between the earth and the sun, where the
gravitational fields are in balance allowing the reflectors
to remain stationary (Zhang etal. 2015; Kosugi 2010).
While this approach can have a considerable cooling effect
based on model simulations, development of such tech-
nology is still at a very infant stage. The major drawback
associated with this approach is the economic feasibility
of transporting materials into space. For this technology
to be economically feasible, material transport costs need
to be reduced from approximately $10,000/kg to less than
$100/kg (Lawrence etal. 2018). Moreover, risks such as
those associated with space debris and asteroid collisions
or those associated with technical and communication
failures need to be appropriately catered for (Lawrence
etal. 2018).
Surface‑based brightening
Another solar radiation management approach discussed
in the literature is the brightening of the earth surface to
increase the earth’s albedo and thus reduce global tempera-
tures. This has been suggested through painting urban roofs
and roads in white, as well as covering deserts and glaciers
with plastic sheets that are highly reflective, and, further-
more, by placing reflective floating panels over water bodies
(Ming etal. 2014). According to Lawrence etal., based on
an extensive literature review, the cooling potential for this
approach is too limited. Furthermore, substantial negative
side effects are associated, such as disruption of desert eco-
systems (Lawrence etal. 2018).
Fig. 3 Major radiative forcing geoengineering technologies that
aim to alter the earth’s radiative energy budget to stabilize or reduce
global temperatures. These technologies include stratospheric aerosol
injection, marine sky brightening, cirrus cloud thinning, space-based
mirrors and surface-based brightening
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Cirrus cloud thinning
Cirrus cloud thinning is a terrestrial radiation manage-
ment technique that aims to increase longwave radia-
tion that is emitted from the earth’s surface to space to
stabilize or reduce global temperatures. Cirrus clouds
are high-altitude ice clouds that play a significant role
within the earth’s radiation budget, having an impact on
the earth’s hydrological cycle as well as surface tempera-
tures. Cirrus clouds absorb terrestrial radiation as well as
reflect incoming solar radiation; however, in general, they
induce an average net warming effect from the imbal-
ance between incoming and outgoing radiative forcings
(Kärcher 2017). The basic principle behind this technique
is the injection of aerosols into cirrus clouds to reduce
its optical thickness as well as its lifetime to increase
terrestrial radiation emission to space. This approach
would require regular cloud injection, so an efficient and
cost-effective delivery method needs to be in places such
as dedicated aircrafts or drones. Bismuth triiodide (Bil3)
has been proposed as an effective cloud seeding mate-
rial; however, its toxicity needs to be taken into account.
Sea salt is another proposed option, yet it is not found to
be as effective as Bil3 (Lawrence etal. 2018). Based on
model simulations, the maximum cooling effect through
this approach has been estimated to be in the range of
2–3.5W/m2 (Lawrence etal. 2018). According to Law-
rence etal., there are no published costs for cirrus cloud
thinning and this approach still requires further research
to understand side effects as well as to conduct appro-
priate research on potential delivery methods (Lawrence
etal. 2018).
Miscellaneous radiation management techniques
Ming etal. proposed several theoretical technologies
that target terrestrial radiation, mainly by creating ther-
mal bridges to bypass the greenhouse gas insulating layer
and be able to transfer thermal radiation out to space. The
research paper presented several concepts which include
transferring surface hot air to the troposphere, transfer-
ring latent and sensible heat to the top of the troposphere,
transferring surface-sensible heat to the troposphere, as
well as transferring cold air to the earth surface. For each
concept, conceptual technologies are proposed. Some of
the technologies discussed are systems that transfer heat
beyond the earth system while generating energy, termed
metrological reactors by the authors (Ming etal. 2014).
While the idea of thermal bridging is interesting, the tech-
nologies and concepts introduced require further research,
development and extensive field trials.
Bibliometric analysis ofresearch onclimate change
mitigation
Bibliometric analysis is a statistical tool that can be used
to quantitatively analyse the current state of scientific
research, by highlighting gaps in the literature as well as
trends. The Web of Science (WoS) core collection data-
base was used in this analysis. The following search meth-
odology was used to retrieve relevant research for further
evaluation. Please note that the search was refined to a
5-year timespan from 2015 to 2020 to specifically evaluate
scientific research efforts related to climate change mitiga-
tion after the Paris agreement in 2015.
Search Methodology:
You searched for: TOPIC: (“Climate change mitiga-
tion”) OR TOPIC: (“climate change abatement”) OR
TOPIC: (“Decarbonization Technologies”) OR TOPIC:
(“Bioenergy Carbon Capture & Storage”) OR TOPIC:
(“Afforestation & Reforestation”) OR TOPIC: (“Soil Car-
bon Sequestration”) OR TOPIC: (“Direct Air Carbon Cap-
ture & Storage”) OR TOPIC: (“Ocean Fertilization”) OR
TOPIC: (“Enhanced Terrestrial Weathering”) OR TOPIC:
(“Ocean Alkalinity Enhancement”) OR TOPIC: (“Wetland
Restoration & Construction”) OR TOPIC: (“Stratospheric
Aerosol Injection”) OR TOPIC: (“Marine Sky Brighten-
ing”) OR TOPIC: (“Space-Based Sunshade/Mirrors”)
OR TOPIC: (“Surface-Based Brightening”) OR TOPIC:
(“Cirrus Cloud Thinning”) OR TOPIC: (“Carbon Dioxide
Removal Techniques”) OR TOPIC: (“Radiative Forcing
Geoengineering”)
Timespan: Last 5years. Indexes: SCI-EXPANDED,
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
access and 61 editorial materials)
The results obtained were then analysed using
VOSviewer software by plotting network and density vis-
ualization maps as shown in Fig.4. The maps are based
on keyword co-occurrences. The visualization maps high-
light various trends related to climate change mitigation,
where areas related to biomass, carbon sequestration,
especially soil carbon sequestration, and biochar have
received high attention over the past 5years. Furthermore,
research related to policy, energy and in particular renew-
able energy has also received much attention. Although
research on climate change mitigation is trending, a gap in
the literature can be highlighted regarding research related
to specific mitigation technologies. It is also evident from
the literature that radiative forcing geoengineering tech-
nologies have not received much attention.
Environmental Chemistry Letters
1 3
Fig. 4 Bibliometric analysis of research on climate change mitiga-
tion: a network visualization map and b density visualization map,
showing the recent state of scientific research on the topic of climate
change mitigation by highlighting trends and gaps in the literature
during 5years between 2015 and 2020
Environmental Chemistry Letters
1 3
Conclusion
Based on the current state of climate emergency, imme-
diate development of viable mitigation and adaptation
mechanisms is of extreme importance. An extensive lit-
erature review covered three main strategies to tackling
climate change, conventional mitigation technologies,
negative emissions technologies as well as radiative forc-
ing geoengineering technologies. It is important to clarify
that there is no ultimate solution to tackle climate change
and that all technologies and techniques discussed in this
review if technically and economically are viable should
be deployed. As previously discussed, decarbonization
efforts alone are not sufficient to meet the targets stipu-
lated by the Paris agreement; therefore, the utilization of
an alternative abatement approach is inevitable. While
the concept of radiative forcing geoengineering in terms
of managing the earth’s radiation budget is interesting, it
is not a long-term solution, as it does not solve the root
cause of the problem. It may, however, buy some time until
greenhouse gas concentrations are stabilized and reduced.
However, the technologies to be deployed are still to be
developed and tested and side effects adequately catered
for, which may be a lengthy process. Negative emissions
technologies, on the other hand, provide a solid solution
in combination with the current decarbonization efforts.
While some of the negative emissions technologies pre-
sented in the literature review may still be at an early
stage of development, biogenic-based sequestration tech-
niques are to a certain extent mature and can be deployed
immediately. Capturing CO2 through photosynthesis is a
straightforward and solid process; however, it needs to be
effectively integrated within a technological framework
as presented in the review. The challenge at the moment
is that carbon pricing for negative emissions is at a very
infant stage, mainly available through voluntary markets
for a very small number of carbon removal methods and
technically non-existent for most of the technologies dis-
cussed. Currently, carbon pricing would be insufficient to
economically sustain carbon removal projects, apart from
the existing framework for afforestation and reforestation
projects. As carbon markets mature and offer incentives
for carbon removal, this may change in near future. In
order to aggressively drive negative emissions projects,
policymakers and governments should devise appropriate
policy instruments and support frameworks with a spe-
cial focus on carbon pricing. Furthermore, the financial
industry should provide enhanced financial support and
accessibility as well as introduce efficient market-based
mechanisms to incentivize project developers to establish
carbon removal projects. At the very moment, biogenic-
based sequestration projects are in a good position to
efficiently utilize financial resources and policy support
as most of the related technologies can be deployed imme-
diately; however, efficient carbon pricing mechanisms that
focus on carbon removal need to be aggressively devel-
oped and introduced. Furthermore, funding for technology
research and development is also a very important aspect
moving forward.
Acknowledgements Authors would like to acknowledge the support
given by the EPSRC project “Advancing Creative Circular Economies
for Plastics via Technological-Social Transitions” (ACCEPT Transi-
tions, EP/S025545/1). The authors wish to acknowledge the support of
The Bryden Centre project (Project ID VA5048) which was awarded
by The European Union’s INTERREG VA Programme,managed by
the Special EU Programmes Body (SEUPB), with match funding pro-
vided by the Department for the Economy in Northern Ireland and the
Department of Business, Enterprise and Innovation in the Republic
of Ireland.
Compliance with ethical standards
Conflict of interest The author declares no competing financial inter-
ests.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.
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