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In human systems, the process of adjustment to actual or expected climate and its effects, to moderate harm or exploit beneficial opportunities. In natural systems, the process of adjustment to actual climate and its effects; human intervention may facilitate adjustment to expected climate and its effects (p542, IPCC, 2018).

A suspension of airborne solid or liquid particles, with a typical size between a few nanometres and 10 micrometres that reside in the atmosphere for at least several hours. Includes both the particles and the suspending gas. Aerosols may be of either natural or anthropogenic origin. Aerosols may influence climate through interactions that scatter and/or absorb radiation and through interactions with cloud microphysics and other cloud properties, or upon deposition on snow or ice covered surfaces thereby altering their albedo and contributing to climate feedback (p542, IPCC, 2018).

Planting of new forests on lands that historically have not contained forests (p542, IPCC, 2018). For a discussion of the term forest, afforestation, reforestation and deforestation, see the IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC, 2000).

The fraction of solar radiation reflected by a surface or object, often expressed as a percentage. The Earth’s planetary albedo varies mainly through varying cloudiness, snow, ice, leaf area and land cover changes (Shepherd, 2009). The higher the albedo, the more the proportion of sunlight that is reflected.

Albedo Modification
An intentional altering of albedo through alterations of surfaces – see Solar Radiation Modification.

Defines Earth’s current geologic time period as being human-influenced, or anthropogenic, based on global evidence that atmospheric, geologic, hydrologic, biosphere and other earth system processes are now altered by humans (EoE, 2020).

Resulting from or produced by human activities (p 543, IPCC, 2018).

Anthropogenic Climate Change
A change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods (UN, 1992).

Anthropogenic Emissions
Emissions of greenhouse gases (GHGs), precursors of GHGs and aerosols caused by human activities. These activities include the burning of fossil fuels, deforestation, land use and land use changes, livestock production, fertilisation, waste management, and industrial processes (p543, IPCC, 2018).

Anthropogenic Removals
The withdrawal of greenhouse gases from the atmosphere because of deliberate human activities. These include enhancing biological sinks of carbon dioxide and using chemical engineering to achieve long term removal and storage (p543, IPCC, 2018).

Anticipatory Governance
A broad-based capacity extended through society that can act on a variety of inputs to govern or manage emerging knowledge-based technologies while such management is still possible. Anticipatory governance motivates the building of capacities to respond in conditions of uncertainty or ambiguity (Guston, 2013).

Atmospheric Climate Intervention (ACI)
See Solar Radiation Modification (SRM).


Stable, carbon-rich material produced by heating biomass in an oxygen-limited environment. Biochar may be added to soils to improve soil functions and to reduce greenhouse gas emissions from biomass and soils, and for carbon sequestration (p543, IPCC, 2018).

The variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems (CBD, 2012).

Energy derived from any form of biomass or its metabolic by-products (p543, IPCC, 2018).

Bioenergy with Carbon Capture and Storage (BECCS)
A carbon dioxide capture and storage (CCS) technique applied to a bioenergy facility. Note that depending on the total emissions of the BECCS supply chain, carbon dioxide can be removed from the atmosphere (p543, IPCC, 2018).

A fuel, generally in liquid form, produced from biomass. Biofuels currently include bioethanol from sugarcane or maize, biodiesel from canola or soybeans, and black liquor from the paper manufacturing process (p543, IPCC, 2018).

Biological Pump
Carbon dioxide (CO₂) fixed by photosynthesis in surface waters sinks below the surface in organic form. As this material settles into the deep ocean by gravity, it is used as food by bacteria and other organisms. They progressively consume it, and as they respire, they reverse the reaction that fixed the carbon, converting it back into CO₂, that is re-released into the water. The combined effect of photosynthesis at the surface followed by respiration deeper in the water column is to remove CO₂ from the surface and re-release it at depth (Shepherd, 2009).

Blue Carbon
Blue carbon is the carbon captured by living organisms in coastal (e.g., mangroves, salt marshes, seagrasses) and marine ecosystems, and stored in biomass and sediments (p543, IPCC, 2018).

Building with Biomass
Using forestry materials in building to extend the duration of carbon storage of natural biomass and opening capacity for additional forestry growth from cleared forests (RS/RAE, 2018).


Carbon Dioxide (CO₂)
A naturally occurring gas, CO₂ is also a by-product of burning fossil fuels, of burning biomass, of land use changes and of industrial processes. It is the principal anthropogenic greenhouse gas that affects the Earth’s radiative balance (p544, IPCC, 2018).

Carbon Dioxide Capture and Storage (CCS)
A relatively pure stream of carbon dioxide (CO₂) from industrial and energy related sources is separated (captured), conditioned, compressed and transported to a storage location for long-term isolation from the atmosphere (p544, IPCC, 2018). Also known as Carbon Capture and Storage. CCS is not an approach to altering the climate, it only stops additional new CO₂ from entering the atmosphere. Should not be confused with carbon dioxide removal.

Carbon Dioxide Capture and Utilisation (CCU)
A process in which carbon dioxide is captured and then used to produce a new product (p544, IPCC, 2018). Occasionally known as Carbon Dioxide Capture and Use. The carbon captured in new products is not necessarily permanently stored in those products.

Carbon Dioxide Capture, Utilisation and Storage (CCUS)
Recently captured or removed atmospheric carbon dioxide is stored in a product for a climate-relevant time horizon. CCUS is sometimes referred to as carbon dioxide capture and use (adapted from p544, IPCC, 2018). Occasionally known as Carbon Dioxide Capture, Use and Storage.

Carbon Dioxide Removal (CDR)
Anthropogenic activities removing carbon dioxide (CO₂) from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO₂ uptake not directly caused by human activities (p544, IPCC, 2018). CDR is a climate-altering approach that would be used to compensate for residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C following a peak. Also referred to as carbon removal, carbon engineering, carbon drawdown, greenhouse gas removal and negative emissions.

Cirrus Cloud Thinning
A proposed approach for solar radiation modification (SRM). Adding ice nuclei (a class of aerosols) to high- altitude cirrus clouds, with the goal of reducing the density (thinning) of such clouds allowing long wave radiation to escape and creating cooling (Mitchell and Finnegan, 2009).

Term used by some people to attribute aircraft vapor trails to solar radiation modification (SRM). Some believe the activity is widely underway, secretly deployed by governments and with negative effects on the climate and human health (Tingley and Wagner, 2017).

Climate-Altering Approaches
A collective term to describe intentional interventions into the climate system with the purpose of reducing rates of climate change. Such approaches may be existing or theoretical and include carbon dioxide removal and solar radiation modification.

Climate-Altering Techniques
Specific techniques within carbon dioxide removal, solar radiation modification or other approaches, for example, afforestation, Direct Air Carbon Dioxide Capture and Storage or ice restoration with the potential (theoretical or otherwise) to cool the climate.

Climate-Altering Technologies
The application of scientific or engineering knowledge in technologies that help deliver or make climate-altering techniques work.

Climate Change
Climate change refers to a change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or as a result of external forcing from e.g. modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use (p544, IPCC, 2018).

Climate Engineering
Intentional large-scale human interference in the Earth system to combat climate change (Shepherd, 2009). Also known as geoengineering. Climate engineering would use climate-altering techniques.

Climate Neutrality
Concept of a state in which human activities result in no net effect on the climate system. Achieving such a state would require balancing of residual emissions with emission (carbon dioxide) removal as well as accounting for regional or local bio-geophysical effects of human activities that, for example, affect surface albedo or local climate (p545, IPCC, 2018).

Climate Repair
The intentional large-scale human interference in the Earth system using climate-altering techniques with a predetermined target temperature or atmospheric carbon density target in mind (CCRC, 2020).

Code of Conduct for Responsible Geoengineering Research
A code of conduct developed by University of Calgary’s Geoengineering Research Governance Project. It aims to provide practical guidance on the responsible conduct of geoengineering research. A voluntary instrument, based upon existing legal sources, including general principles, rules of customary international law, treaty-based rules, regulations, international decisions, and policy documents (Hubert, 2017).

Conference of the Parties (COP)
The supreme body of United Nations conventions, such as the United Nations Framework Convention on Climate Change, comprising parties with a right to vote that have ratified or acceded to the convention (p546, IPCC, 2018).

Convention on Biological Diversity (CBD)
A convention on the conservation of biological diversity, the sustainable use of its components and the fair and equitable sharing of the benefits arising out of the utilisation of genetic resources (CBD, 1992). A series of CBD Conference of the Parties decisions relating to ‘climate-related geoengineering’ have been taken (CBD, 2017).

Counter Geoengineering
The idea that a country might seek or threaten to counteract the cooling effect of solar radiation modification through technical means. Two theoretical types of counter geoengineering have been suggested: countervailing with a warming agent and neutralising with a physical disruption (Parker et al., 2018).


The process by which countries, individuals or other entities aim to achieve zero fossil carbon existence. Typically refers to a reduction of the carbon emissions associated with electricity, industry and transport (p546, IPCC, 2018).

The changing of forest to non-forest. For a discussion of the term forest, afforestation, reforestation and deforestation, see the IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC, 2000).

The deliberate use of a climate-altering technique with the intention of affecting a change in the global climate.

Direct Air Capture (DAC)
Using engineered systems to capture atmospheric carbon dioxide for subsequent usage or storage (RS/RAE 2018).

Direct Air Capture and Carbon-Dioxide Storage (DACCS)
Chemical process by which carbon dioxide is captured directly from the ambient air, with subsequent storage. Also known as direct air capture and storage (DACS) (p547, IPCC, 2018).

Distributive Justice
Justice in the allocation of economic and non-economic costs and benefits across society (of engineered climates for example), seeks to avoid inequalities (p547 & p552, IPCC, 2018).


Earth System Modelling
Earth system models seek to simulate all relevant aspects of the Earth system. They include physical, chemical and biological processes (GESAMP, 2019).

Enhanced Oil Recovery (EOR) with Carbon Dioxide (CO₂)
The injection of carbon dioxide (CO₂) into oil wells to increase extraction efficiency. The injecting of CO₂ could be considered sequestration. The practice is unlikely to have climate effects. The processing and burning of the extracted oil are likely to produce more emissions than the volume of CO₂ that is sequestered by the EOR in the same space. Further, how the CO₂ is captured or manufactured and transported to the well will have implications for the emission reductions efficiency of the process (Nwidee et al., 2016).

Enhanced Weathering
Enhancing the removal of carbon dioxide from the atmosphere through dissolution of silicate and carbonate rocks by grinding these minerals to small particles and actively applying them to soils, coasts or oceans (p548, IPCC, 2018).

Environmental Modification Convention (ENMOD)
The Environmental Modification Convention, formally the Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques, 1976 (ENMOD, 1976). Prohibits military or any other hostile use of environmental modification techniques.


Growing new trees and improving the management of existing forests. As forests grow they absorb carbon dioxide from the atmosphere and store it in living biomass, dead organic matter and soils (RS/RAE, 2018). Includes afforestation and reforestation.


The deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change (Shepherd, 2009). Also known as climate engineering. Theoretical approaches would include the use of carbon dioxide removal and solar radiation modification or other climate-altering techniques.

Geoengineering Governance
The application of the full range of means for deciding, managing, implementing and monitoring policies and measures about geoengineering. See Governance.

Global Thermostat
A metaphor regarding decision making about who, when and how much cooling might be delivered using climate-altering techniques (for a discussion see Rickles et al. (2018)).

A comprehensive and inclusive concept of the full range of means for deciding, managing, implementing and monitoring policies and measures. Whereas government is defined strictly in terms of the nation-state, the more inclusive concept of governance recognises the contributions of various levels of government (global, international, regional, sub-national and local) and the contributing roles of the private sector, of nongovernmental actors, and of civil society to addressing the many types of issues facing the global community (p550, IPCC, 2018).

Greenhouse Gases (GHGs)
Greenhouse gases (GHGs) are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect (p550, IPCC, 2018). GHGs include carbon dioxide, methane and nitrous oxide.

Greenhouse Gas Removal (GGR)
Withdrawal of a greenhouse gas (GHG) and/or its precursor from the atmosphere by a sink (p551, IPCC, 2018). GGR includes natural and engineering techniques that are purposefully deployed to remove GHGs by way of seeking to address a cause of climate change. Carbon dioxide removal is one kind of GGR.


Human Rights
Rights that are inherent to all human beings, universal, inalienable, and indivisible, typically expressed and guaranteed by law. They include the right to life; economic, social, and cultural rights; and the right to development and self-determination. Based upon the definition by the United Nations Office of the High Commissioner for Human Rights (UNOHCR, 2020).


Ice Restoration
A theoretical climate-altering approach that would drive the creation of new ice in cold regions, in particular on ocean surfaces. Techniques suggested in the literature include the use of billions of manufactured reflective floating silica spheres, the creation of micro bubbles, vast reflective rafts of foam, using wind power to cool and spray water in winter and submarines acting as ice factories (FCR, 2019).

Intergenerational Justice
Balancing the needs of current and future generations. For a discussion of intergenerational justice and solar radiation modification see Burns (2011).

Integrated Assessment Models (IAMs)
Integrated Assessment Models (IAMs) integrate knowledge from two or more domains into a single framework. They are one of the main tools for undertaking integrated assessments of climate. IAMs used in respect of climate change mitigation may include representations of multiple sectors of the economy, such as energy, land use and land-use change; interactions between sectors; the economy as a whole; associated greenhouse gas emissions and sinks; and reduced representations of the climate system. This class of model is used to assess linkages between economic, social and technological development and the evolution of the climate system. Another class of IAM additionally includes representations of the costs associated with climate change impacts but includes less detailed representations of economic systems. These can be used to assess impacts and mitigation in a cost–benefit framework and have been used to estimate the social cost of carbon (p552, IPCC, 2018).

Intergovernmental Panel on Climate Change (IPCC)
An intergovernmental scientific body which provides regular assessments of the scientific basis of climate change, its impacts and future risks, and policy relevant but not prescriptive options for adaptation and mitigation. Created by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988, the objective of the IPCC is to provide governments at all levels with scientific information that they can use to develop climate policies. IPCC reports are also a key input into international climate change negotiations (IPCC, 2020).

Iron Fertilisation
See Ocean Fertilisation.


Kyoto Protocol
The Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) is an international treaty adopted in December 1997 in Kyoto, Japan, at the Third Session of the Conference of the Parties (COP) to the UNFCCC. It contains legally binding commitments, in addition to those non-binding included in the UNFCCC (UN, 1997). Parties to the Protocol agree to reduce or limit their future emissions in quantitative terms. In the accounting process the removal of carbon by sinks from direct human-induced land-use change and forestry activities are included. Parties can choose to include net removals of carbon from certain additional activities, including forest management, cropland management, and revegetation (UN, 1997).


London Convention/London Protocol (LC/LP)
The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972, known as the London Convention, protects the marine environment. Its objective is to promote the effective control of all sources of marine pollution and to prevent pollution of the sea by dumping of wastes and other matter. Currently, 87 States are Parties to this Convention. In 1996, the London Protocol modernised the Convention and it will, eventually, replace it. Under the Protocol all dumping is prohibited, including for the purposes of ’geoengineering’, except for possibly acceptable wastes on a ‘reverse list’. The Protocol entered into force in 2006 and there are currently 53 Parties to the Protocol (LC/LP, 2006). In 2013, an amendment to the Protocol sought to establish a stable, legally binding framework for the regulation of marine geoengineering (as described in a new Annex), while also allowing for regulatory flexibility and adaptability based on a precautionary approach. The amendment was adopted by consensus but is not yet in force, with only six contracting Parties having submitted their instruments of acceptance so far.


Marine Cloud Brightening (MCB)
A proposed solar radiation modification technique. Clouds over the oceans would be engineered, by spraying sea-salt into the clouds to enlarge the naturally present suspended particles, making them brighter, increasing the amount of sunlight that is deflected back out into space, hence achieving cooling. In relatively dust-free parts of the marine atmosphere, increasing the number of cloud-condensation nuclei (particles around which droplets of water coalesce to form clouds) would raise cloud albedo significantly and may also increase the cloud longevity (NRC, 2015).

Methane (CH₄)
A greenhouse gas, significant emissions of methane occur because of animal husbandry and agriculture and their management represents a major mitigation option (p554, IPCC, 2018).

Mineral Carbonation
Accelerating the conversion of silicate rocks to carbonates either above or below the surface to provide permanent storage for carbon dioxide (RS/RAE, 2018).

A human intervention to reduce emissions or enhance the sinks of greenhouse gases. Note that this encompasses carbon dioxide removal options (p554, IPCC, 2018), but not solar radiation modification.

Mitigation Deterrence
Mitigation deterrence is the recognition of a potential negative side effect of carbon dioxide removal (CDR). It is the risk that pursuing CDR could discourage, deter or delay other mitigation efforts. It is similar to the rebound effect – the reduction in expected gains from new technologies that increase the efficiency of resource use, because of behavioural or other systemic responses (McLaren, 2020).

Moral Hazard Argument
The idea that by deploying climate-altering techniques to address the causes and effects of climate change, the impetus to tackle it through reducing greenhouse gas emissions is lessened (see Wagner (2019)) for a discussion of moral hazard in relation to solar radiation modification (SRM)).

Mount Pinatubo
Volcano in the Philippines whose eruption in 1991 lowered global temperatures by up to 0.5°C over the following two-years, providing evidence for prospective stratospheric aerosol injection solar radiation modification (Soden, 2002).


Nature-Based Approaches
A climate-altering approach. The intentional, sustainable intervention in the natural environment with the purpose of enhancing or accelerating the removal of greenhouse gases from the atmosphere. Techniques include afforestation and peatland and wetland restoration.

Nature-Based Solutions (NBS)
Actions to protect, sustainably manage, and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits (IUCN, 2016). Some NBS are specifically directed at removing carbon dioxide from the atmosphere, see for example afforestation, these are also known as nature-based approaches.

Nationally Determined Contributions (NDCs)
A term used under the United Nations Framework Convention on Climate Change whereby a Party to the Paris Agreement outlines its plans for reducing its emissions. Some countries NDCs also address how they will adapt to climate change impacts, and what support they need from, or will provide to, other countries to adopt low-carbon pathways and to build climate resilience (p554, IPCC, 2018).

Negative Emissions
Removal of greenhouse gases from the atmosphere by deliberate human activities, i.e. in addition to the removal that would occur via natural carbon cycle processes (p554, IPCC, 2018).

Negative Emissions Technologies (NETs)
Climate-altering technologies designed to remove greenhouse gases from the atmosphere. For a discussion of NETs see McLaren (2012).

Net Negative Emissions
A situation of net negative emissions is achieved when, as result of human activities, more greenhouse gases are removed from the atmosphere than are emitted into it (p555, IPCC, 2018). Also known as negative emissions.

Net Zero Emissions
Net zero emissions are achieved when emissions of greenhouse gases (GHGs) to the atmosphere are balanced by anthropogenic removals. Where multiple GHGs are involved, the quantification of net-zero emissions depends on the climate metric chosen to compare emissions of different gases (such as global warming potential, global temperature change potential, and others, as well as the chosen time horizon) (p555, IPCC, 2018).

Nitrous Oxide (N₂O)
A greenhouse gas. The main anthropogenic source of nitrous oxide (N₂O) is agriculture, but important contributions also come from sewage treatment, fossil fuel combustion, and chemical industrial processes. N₂O is also produced naturally from a wide variety of biological sources in soil and water, particularly microbial action in wet tropical forests (p555, IPCC, 2018). N₂O may be a by-product of some nature-based approaches to removing carbon such as afforestation.


Ocean Acidification
Ocean acidification refers to a reduction in the pH of the ocean over an extended period, typically decades or longer, which is caused primarily by uptake of carbon dioxide (CO₂) from the atmosphere but can also be caused by other chemical additions or subtractions from the ocean (p555, IPCC, 2011).

Ocean Alkalinity Enhancement
Increasing ocean concentration of ions like calcium to increase uptake of carbon dioxide (CO₂) into the ocean, and reverse acidification (RA/RE, 2018). Adding additional alkalinity to the surface of the ocean will decrease the relative pressure of CO₂ in the water and, as a result, increase the uptake of CO₂ by the ocean from the atmosphere. Enhancing alkalinity would also help reduce the effects of ocean acidification on the marine ecosystem (GESAMP, 2019).

Ocean Fertilisation
Deliberate increase of nutrient supply to the near-surface ocean in order to enhance biological production through which additional carbon dioxide (CO₂) from the atmosphere is sequestered (p555, IPCC, 2018). Photosynthesis by plankton in the ocean currently removes around 40 Gt CO₂ per year from the ocean surface and transports it downward to the deep ocean (RS/RAE 2018). This so-called ‘biological pump’ is limited by the abundance of photosynthesising life which in turn is constrained by the supply of micronutrients such as iron (or macro nutrients including nitrates and phosphates). Iron ocean fertilisation seeks to address this shortfall by introducing additional micronutrients (GESAMP, 2019).

Outdoor Experimentation
The testing of climate-altering technologies or techniques through experiments which are not expected to have discernible climate-scale effects. Such activities are not deployments of the technologies or techniques (for a discussion of framing experimentation see Bellamy (2014)).

Oxford Principles
A list of five high-level principles for the governance of research, development, and any eventual deployment of climate-altering techniques. Drafted by an ad-hoc group of academics and submitted to a United Kingdom House of Commons Select Committee on Science and Technology inquiry into how geoengineering should be governed. The principles subsequently became known as the ‘Oxford Principles’ and have been influential in framing debates (Rayner et al., 2013).

The temporary exceedance of a specified level of global warming, such as 1.5°C. Climate-altering approaches are suggested to limit or make an overshoot temporary, i.e., by anthropogenically removing excess carbon dioxide (p555 & 559, IPCC, 2018). In addition to using carbon dioxide removal to address overshoot (e.g., in the IPCC pathways (IPCC, 2018)), solar radiation modification is also being considered by some as a temporary overshoot measure (MacMartin, 2018).

Ozone (O₃)
Ozone is a gaseous atmospheric constituent. In the troposphere, it is created both naturally and by photochemical reactions involving gases resulting from human activities (smog). Tropospheric ozone acts as a greenhouse gas. In the stratosphere, it is created by the interaction between solar ultraviolet radiation and molecular oxygen (O₂). Stratospheric ozone plays a dominant role in the stratospheric radiative balance. Its concentration is highest in the ozone layer (p555, IPCC, 2018). Some stratospheric aerosol injection solar radiation modification particle candidates may harm stratospheric ozone, whilst some may enhance it (Keith, 2013).


Paris Agreement
An international agreement under the United Nations Framework Convention on Climate Change. Agreed in 2015 and entered into force in 2016, a key goal of the Agreement is “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”. Achieving this goal would significantly reduce the risks and impacts of climate change. Article 4 of the Agreement also sets the goal of achieving a balance between anthropogenic emissions by sources and removals by sinks, including through the use of climate-altering approaches, by the second half of the 21st century, and achieving net-negative thereafter (UN, 2015). The Agreement also aims to strengthen countries capacity to deal with the impacts of climate change (UN, 2015).

Procedural Justice
Justice in the way outcomes are brought about including who participates and is heard in the processes of decision making (p556 & 552 IPCC, 2018).


Radiative Forcing (RF)
The change in the net, downward minus upward, radiative flux (expressed in Watts per square meter) at the tropopause or top of atmosphere due to a change in a driver of climate change (p556, IPCC, 2018). Drivers include changes in the concentration of carbon dioxide, the output of the Sun or albedo through natural changes or, theoretically, through solar radiation modification. Negative radiative forcing indicates that more energy is sent back than is received, leading to cooling.

Planting of forests on lands that have previously contained forests but that have been converted to some other use (p557, IPCC, 2018). For a discussion of the term forest, afforestation, reforestation and deforestation, see the IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC, 2000).

See Carbon Dioxide Removal (CDR) and Greenhouse Gas Removal (GGR).

Residual emissions
Residual emissions are the emissions remaining after all technically and economically feasible opportunities to reduce emissions in all covered scopes and sectors have been implemented (CAPF, 2020), for example some aviation emissions. Whilst the Intergovernmental Panel on Climate Change do not provide a definition of residual emissions, the Panel uses the term to describe emissions that would remain, after all other emission reduction measures other than carbon dioxide removal had been taken (SPM, para C.3, IPCC, 2018).

In the context of the assessment of climate impacts, the term risk is often used to refer to the potential for adverse consequences of a climate-related hazard, or of adaptation or mitigation responses to such a hazard, on lives, livelihoods, health and wellbeing, ecosystems and species, economic, social and cultural assets, services (including ecosystem services), and infrastructure. Risk results from the interaction of vulnerability (of the affected system), its exposure over time (to the hazard), as well as the (climate-related) hazard and the likelihood of its occurrence (p557, IPCC, 2018).

Risk Management
Plans, actions, strategies or policies to reduce the likelihood and/or consequences of risks or to respond to consequences (p557, IPCC, 2018).


The secure storage of a substance. In the case of sequestration of carbon dioxide, this is generally taken to mean secure storage for a minimum of 100 years (GESAMP, 2019).

Any process, activity or mechanism which removes a greenhouse gas (GHG), an aerosol or a precursor of a GHG from the atmosphere (UN, 1992).

Slippery Slope
The concept that a course of action, once embarked upon is, hard to reverse, and seems to lead inevitably from one action or result to another with unintended consequences (Webster’s, 2020). The concept suggests that, if research into a climate-altering technique is undertaken, it may inevitably lead to its eventual development and deployment (for a discussion see Bellamy and Healey, 2018).

Soil Carbon Sequestration
Changing agricultural practices such as tillage or crop rotations to increase the soil carbon content (RS/RAE 2018).

Solar Geoengineering
See Solar Radiation Modification (SRM).

Solar Radiation Management (SRM)
See Solar Radiation Modification (SRM).

Solar Radiation Modification (SRM)
Solar Radiation Modification (SRM) is an approach to altering the climate and refers to the intentional modification of the Earth’s shortwave radiative budget with the aim of reducing warming. Artificial injection of stratospheric aerosols, marine cloud brightening, cirrus cloud thinning and land surface albedo modification are examples of proposed SRM methods (IPCC, 2018). SRM does not fall within the definitions of mitigation and adaptation (p558, IPCC, 2012). SRM is also referred to as solar radiation management, solar geoengineering, atmospheric climate intervention (ACI) or albedo enhancement.

Space-based Methods
Theoretical engineered approaches proposed to reduce the amount of solar energy reaching Earth by positioning sunshields in space to reflect or deflect the solar radiation (Shepherd, 2009).

The highly stratified region of the atmosphere above the troposphere extending from about 10 km (ranging from 9 km at high latitudes to 16 km in the tropics on average) to about 50 km altitude (p559, IPCC, 2018). The layer of the atmosphere where some forms of solar radiation modification are proposed to be deployed (see stratospheric aerosol injection) (Keith, 2013).

Stratospheric Aerosol Injection (SAI)
A proposed solar radiation modification (SRM) technique, involving the introduction of aerosols into the stratosphere with the objective of scattering sunlight back to space and lower temperatures. Occasionally called Stratospheric Aerosol SRM and Stratospheric Aerosol Geoengineering (for an overview see Irvine, et al. (2016)).

Sustainable Development Goals (SDGs)
Adopted by United Nations Member States in 2015. There are 17 SDGs, to be achieved by 2030, which call for action in global partnership. They recognise that ending poverty and other deprivations must go hand-in-hand with strategies that improve health and education, reduce inequality, and spur economic growth – all while tackling climate change and working to preserve our oceans and forests (UN, 2020).


Termination Risk
The idea that, having been implemented for a significant period, if a solar radiation modification intervention subsequently failed or was abruptly stopped, that there would be a very swift and sustained rise in temperature (an upward ‘step’, rather than a ‘spike’) and a rapid transition to the much warmer climate associated with the higher carbon dioxide levels (Shepherd, 2009). Such rapid warming would create shocks for ecosystems and biodiversity and would be economically and socially disruptive (Trisos et al., 2018). Also known as Termination Shock or Bounce Back.

Tipping Point
A level of change in system properties beyond which a system reorganises, often abruptly, and does not return to the initial state even if the drivers of the change are abated. For the climate system, it refers to a critical threshold when global or regional climate changes from one stable state to another stable state (p559, IPCC, 2018). It has been suggested that climate-altering approaches may help avoid climate tipping points (Keith, 2013).


United Nations Environment Assembly (UNEA)
The world’s highest-level decision-making body on the environment. It addresses the critical environmental challenges facing the world. The Environment Assembly meets biennially to set priorities for global environmental policies and develop international environmental law. Through its resolutions and calls to action, the Assembly provides leadership and catalyses intergovernmental action on the environment (UNEA, 2020). At its 4th session in March 2019, a draft resolution sponsored by Switzerland and a dozen countries invited the United Nations Environment Programme to produce an assessment of ‘geoengineering’, however, the resolution was withdrawn after negotiations failed to result in consensus (Perrez, 2020).

United Nations Environment Programme (UNEP)
Global environmental authority that promotes the coherent implementation of the environmental dimension of sustainable development within the United Nations system. Serves as an authoritative advocate for the global environment (UNEP, 2020). Its governing body is the UN Environment Assembly.

United Nations Educational, Scientific and Cultural Organization (UNESCO)
UNESCO seeks to build peace through international cooperation in Education, the Sciences and Culture (UNESCO, 2020). UNESCO’s programmes contribute to the achievement of the Sustainable Development Goals (SGDs) defined in Agenda 2030, adopted by the United Nations General Assembly (UNGA) in 2015.

United Nations Framework Convention on Climate Change (UNFCCC)
The UNFCCC was adopted and opened for signature in 1992. It entered into force in March 1994 and has 197 Parties. The Convention’s objective is the “stabilisation of greenhouse gas (GHG) concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (UN, 1992). The provisions of the Convention are pursued through two additional treaties: the Kyoto Protocol and the Paris Agreement.

United Nations General Assembly (UNGA)
All 193 member states of the United Nations (UN) are represented in the General Assembly to discuss and work together on international issues covered by the Charter of the UN such as development, peace and security and international law. Every year in September, all the Members meet at the Headquarters in New York for the General Assembly session (UNGA, 2020).

United Nations Security Council (UNSC)
The Security Council has primary responsibility for the maintenance of international peace and security. It has five permanent Members with a veto power and 10 additional rotating Members, and each Member has one vote. All Member States are obligated to comply with Council decisions. The Council takes the lead in determining the existence of a threat to the peace or act of aggression. It calls upon the parties to a dispute to settle it by peaceful means and recommends methods of adjustment or terms of settlement. In some cases, the Security Council can resort to imposing sanctions or even authorise the use of force to maintain or restore international peace and security (UNSC, 2020).


Weather Modification
Operational programmes to modify the weather – including to disperse fog, enhance rain and snowfall, and suppress hail (WMO, 2015). Weather Modification techniques are not approaches to altering the climate (WMO, 2014).

World Meteorological Organization (WMO)
An intergovernmental organisation (IGO) established in 1950 with a membership of 193 Member States and Territories. The specialised agency of the United Nations for meteorology (weather and climate), operational hydrology and related geophysical sciences. Activities include the monitoring of weather modification projects (WMO, 2020).


AR5 5th Assessment Report of the IPCC
BAU Business as Usual
BECCS Bioenergy with Carbon Capture and Storage
C2G The Carnegie Climate Governance Initiative
CBD Convention on Biological Diversity
CCN Cloud Condensation Nuclei
CCS Carbon Capture and Storage/Sequestration
CDR Carbon Dioxide Removal
COP Conference of Parties
CLRTBAP Convention on Long-Range Trans Boundary Air Pollution
DAC Direct Air Capture
DACCS Direct Air Capture and Carbon-Dioxide Storage
ENMOD Environmental Modification Convention
ETC Action Group on Erosion, Technology and Concentration
Gt Gigatonnes
GEOMIP Geoengineering Model Intercalibration Project
GESAMP Joint Group of Experts on Scientific Aspects of Marine Protection
GRGP Geoengineering Research Governance Project
GGR Greenhouse Gas Removal
GGH Greenhouse Gasses
IPCC Intergovernmental Panel on Climate Change
LC London Convention 1972
LP London Protocol 1996
MCB Marine Cloud Brightening
NBS Nature Based Solutions
NETs Negative Emissions Technologies
OF Ocean Fertilisation
OFAF Ocean Fertilisation Assessment Framework (of the LC/LP)
SAI Stratospheric Aerosol Injection
SDGs Sustainable Development Goals (UN)
SCoPeX Stratospheric Controlled Perturbation Experiment
SPICE Stratospheric Particle Injection for Climate Engineering Research Project
SR1.5 Special Report on Global Warming of 1.5°C (IPCC, 2018)
SRM Solar Radiation Modification
SRMGI Solar Radiation Management Governance Initiative
UN United Nations
UNCLOS United Nations Convention on the Law of the Sea
UNEP United Nations Environment Programme
UNESCO United Nations Educational, Scientific and Cultural Organisation
UNFCCC United Nations Framework Convention on Climate Change
WCRP World Climate Research Programme
WMO World Meteorological Organization

Scientific notation

C Carbon
CO₂ Carbon dioxide
CO₂e Carbon dioxide equivalents. A measure used to compare warming levels between CO₂ and other greenhouse gases
Gigatonne 1,000,000,000 tonnes (1 billion)
Ha Hectare
Megatonne 1,000,000 tonnes (1 million)
pa Per annum
pH chemical measure of the acidity or basicity/alkalinity of a solution
Ppm parts per million
tCO₂/MtCO₂/GtCO₂ tonnes/megatons/ gigatons of carbon dioxide
Tonne Measure of weight, 1,000 kilograms
TWh Terawatt hours
Watts/m² Watts per square meter
TRL Technology Readiness Level
μm Micro-meters (1 μm = 0.001mm)


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