Emerging Technologies Terminology — A Living Guide

Emerging climate technologies encompass multiple and often controversial approaches. This guide aims to alert newcomers to some of the pitfalls and complexities in using various terms and expressions, which can lead to miscommunication, and is intended as a supplement to our Glossary.

We start with the primary terms used in the public conversation, and then move onto associated terms with similar challenges. This is a living guide to an active debate; C2G will continue to reassess these definitions over time. Outside input is welcome.

Core Geoengineering Terms


The IPCC in its fifth assessment report (AR5) defined Geoengineering as “a broad set of methods and technologies that aim to deliberately alter the climate system in order to alleviate the impacts of climate change”. Other definitions include that given by the Royal Society’s influential 2009 Report: “the deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change”, in which scale and intentionality are essential elements.

Geoengineering is an umbrella term, generally understood to include both Carbon Dioxide Removal and Solar Radiation Modification technologies. Other terms used to describe these technologies include climate geoengineering, climate intervention, and climate engineering.

In practice, however, the use of the word Geoengineering can be problematic, as it encompasses a wide variety of technologies which pose different challenges. In particular, there are differing views on whether (or which) Carbon Dioxide Removal technologies should be described as Geoengineering.

Geoengineering also contains an implicit assumption that it is possible to engineer the climate, which may underplay the complexity of the earth system. For some, the term Geoengineering evokes broad concerns about humanity overstepping its limits. Many consider its promotion or discussion to contain a stigma. Geoengineering is widely used by Chemtrail Conspiracy theorists, who have adopted social media tags such as #geoengineering as an organizing tool.

In public discussions and advocacy, C2G will generally describe a specific technology (or family of technology) rather than use the term Geoengineering. C2G may use the word Geoengineering if it is appropriate to collate technologies, but will generally specify which technologies it is including.

Carbon Dioxide Removal

Carbon Dioxide Removal (CDR) is an umbrella term for existing deployed and proposed technologies aimed at reducing atmospheric concentrations of carbon dioxide. These include nature-based approaches, such as Afforestation, technical approaches, such as Direct Air Capture, and hybrid approaches, such as Bioenergy with Carbon Capture and Storage. It also includes marine based approaches, such as Iron Fertilization.

The Paris Agreement indirectly recognized a potentially substantially scaled up use of current and new carbon removal technologies. Article Four of the Paris Agreement indirectly references such potential use of Carbon Dioxide Removal when it calls for a balance between anthropogenic emissions by sources and removals by sinks in the second half of the 21st century. The order of magnitude of the scale up of Carbon Removal is directly related to the success of action to reduce emissions.

Whilst experts in the field generally see the value in one overarching term for Carbon Dioxide Removal, they also highlight significant differences between proposed technologies. It is generally worth specifying which technologies are being included in any discussion.

Other umbrella terms include Negative Emissions Technologies (NETs), Carbon Removal, Carbon Drawdown and the more expansive term Greenhouse Gas Removal (used in the UK). C2G uses the term Carbon Dioxide Removal in line with the IPCC’s Special Report on Global Warming of 1.5°C.

There is continued dispute in international climate policy as to whether some, all or none of these technologies should fall under the umbrella of Geoengineering. According to the glossary of the IPCC Fifth Assessment Report: “some Carbon Dioxide Removal methods fall under the category of geoengineering, though this may not be the case for others, with the distinction being based on the magnitude, scale and impact of the particular CDR activities. The boundary between CDR and mitigation is not clear and there could be some overlap between the two given current definitions.”

The IPCC’s Special Report on Global Warming of 1.5°C glossary on geoengineering states: “separate consideration is given to the two main approaches considered as ‘geoengineering’ in some of the literature: solar radiation modification (SRM) and carbon dioxide removal (CDR). Because of this separation, the term ‘geoengineering’ is not used in this report.”

Alternative framings are becoming more popular as these technologies rise up the public agenda, and as some advocates look to separate them from the geoengineering frame. These including Enhanced Mitigation or Mitigation Plus, and Carbon Geoengineering. Some commentators have linked Carbon Removal to the frame of Decarbonization, broadening the concept to include the decarbonization of the atmosphere in addition to the economy.

The IPCC’s Special Report on Global Warming of 1.5°C glossary on geoengineering states that Carbon Dioxide Removal ” is classified as a special type of mitigation.”


Carbon Removal icons

Above: C2G’s icons representing carbon removal technologies / available to use freely.

Solar Radiation Modification

Solar Radiation Modification is an umbrella term for proposed technologies that would reflect more sunlight back into space, or allow more infrared radiation to escape into space, thereby creating a net cooling effect on the earth’s climate.

Other terms include Solar Geoengineering, Albedo Modification, Solar Radiation Management, Radiation Modification Measures, Radiative Forcing Geoengineering or simply Geoengineering. Alternative framings include Sunlight Reduction or Reflection, and Extreme Adaptation. C2G uses the umbrella term Solar Radiation Modification, following its adoption by the IPCC’s Special Report on Global Warming of 1.5°C.

Solar Radiation Modification encompasses a range of approaches, including Stratospheric Aerosol Injection, Marine Cloud Brightening, Cirrus Cloud Thinning and Surface Albedo Modification (e.g. ice restoration, or painting roofs white).

Whilst experts in the field see value in one overarching term for these technologies, they also highlight their significant differences. Solar Geoengineering, or just Geoengineering, is sometimes been used to refer solely to Stratospheric Aerosol Injection, rather than as an umbrella term. The inclusion of some technologies in this category may be open to debate: for example, efforts to restore lost Arctic sea ice.

It is usually worth specifying which technologies are being included in any discussion about Solar Radiation Modification, in order to avoid miscommunication.


Solar geoengineering icons

Above: C2G’s icons representing solar geoengineering technologies / available to use freely.

Terms of Significant Relevance to the Geoengineering Discourse

Climate Restoration

A term used by some to describe actions they believe could restore the earth system to a safe, sustainable and productive state. (Note: there is no scientific consensus on what this state would be.)

Some have used the term with regards to technologies which might otherwise be described as Solar Radiation Modification, e.g. with regards to the “restoration” of Arctic sea ice. This reframing is not universally accepted.

Counter Geo-engineering

The idea that a country might seek or threaten to counteract the effect of Solar Radiation Modification through technical means. This could include countervailing a cooling impact with a warming agent, or neutralising it with a physical disruption.

Geoengineering Governance

What constitutes the effective governance of proposed Geoengineering technologies remains open to debate. In C2G’s view, it includes regulation, broad participation in decision making, transparency and access to information at the international, national, and subnational levels, and needs to apply to research, testing and deployment.

Good governance needs to take a precautionary, deliberative and evidence-based risk management approach, which weighs potential outcomes against the reality of a warming world. This risk-risk approach needs to examine the totality of the risks of mitigation, adaptation, Carbon Dioxide Removal and Solar Radiation Modification, and be measured against implementation of the Sustainable Development agenda. It must take account of different perceptions of risk by different stakeholders, and be informed by sound ethical principles, which consider the international justice of actions with different impacts in different geographical locations, as well as justice for future generations.


The IPCC’s Special Report on Global Warming of 1.5°C defines Mitigation (of climate change) as “a human intervention to reduce emissions or enhance the sinks of greenhouse gases. Note that this encompasses carbon dioxide removal (CDR) options.” Mitigation is the first priority for action to address climate change, according to scientific consensus.

As largescale Carbon Dioxide Removal technologies rise up the agenda, some supporters are seeking to delink them from the broader concept of Geoengineering, and rather site them within the spectrum of Mitigation.

Some believe that if these new Carbon Dioxide Removal technologies are described as Geoengineering, important actors in international climate policy may find it difficult to join that conversation, or avoid it altogether. If these new technologies are described as Mitigation, or some variation thereof, those same actors might feel more able to engage, without fear of abandoning the core message that Mitigation remains paramount.

Moral Hazard

A term used in the Geoengineering context to describe the risk that debating or researching Carbon Dioxide Removal or Solar Radiation Modification technologies might lead society to believe it can reduce the urgency of Mitigation efforts, and potentially perpetuate reliance on fossil fuels.

Excessive optimism about the extent or success of future Mitigation efforts may also be considered a moral hazard, by reducing interest in potential additional technologies to achieve temperature targets or reduce climate impacts. See C2G’s blog optimism and prudence in geoengineering governance for a discussion of these issues.

Outdoor Experimentation / Testing / Deployment of Solar Radiation Modification

Differentiating outdoor experimentation from testing and deployment is particularly important for Solar Radiation Modification. With outdoor experiments expected to begin soon for Stratospheric Aerosol Injection, Marine Cloud Brightening and Arctic Restoration, defining the lines becomes an issue for urgent consideration.

Scientists say it is not possible to test certain Solar Radiation Modification technologies at a scale below what effectively amounts to deployment, but that small scale, low-impact outdoor experimentation is possible. There are nonetheless concerns that outdoor experimentation could lead down a Slippery Slope to eventual deployment.

Some scientists have proposed specific physical delineators whereby any impact below a certain level is to be considered experimentation, and anything above a certain higher level is to be considered deployment. There is no consensus yet on these delineators.

However the lines are defined, in public communication it is important to clarify whether an activity constitutes outdoor experimentation (which may be acceptable, if properly governed) or testing and deployment (which at present would not.)

Slippery Slope, Foot in the Door

This describes the concern that that if something relatively harmless is allowed in the present, it may start a trend that results in something potentially harmful occurring in the future.

In the context of Geoengineering, it has been used to suggest that limited outdoor experimentation today will lead to more widespread outdoor testing, and eventually to full deployment, with each step in the chain facilitating the next in a way that may not be stopped, or in which stopping it would become increasingly difficult.

In its fallacious form, this argument is also sometimes referred to as “the fallacy of the camel’s nose”; in which no logical proof is provided that infringement in one area will necessarily lead to infringement in another.

Soft Geoengineering

A term promoted by some researchers to describe Geoengineering efforts that are reversible in a short time-frame, with primarily local effects, using widely naturally occurring materials. The expression has been used, for example, to describe Ice911, a proposed project to restore Arctic sea ice.  There is no consensus on the use of this term.

Technology Lock-in

The concern that once a technological standard is selected, it may be locked into the market, even though a better alternative might emerge. In the context of Carbon Dioxide Removal, this has been used to critique the proposed large-scale deployment of Bioenergy with Carbon Capture and Storage (BECCS) included in numerous integrated assessment models. Similar issues arise around decisions to build largescale infrastructure with long timelines.

In communications, it is important to be aware of the potential to promote lock-in by favouring the discussion of certain technologies over other technologies, which might give an incorrect impression that they are more advanced or appropriate than alternatives.

Temperature Overshoot

Many climate models cited by the IPCC foresee temperatures rising beyond 1.5-2°C, before a combination of Decarbonisation and Carbon Dioxide Removal technologies might bring them back down.

This is known as a Temperature Overshot, and may last for several decades if not longer, entailing increasingly dangerous and potentially irreversible consequences.

There are also concerns that normalizing the idea of a Temperature Overshoot creates a risk that temperature targets are no longer seen as strict upper limits.

This has led to some calls for largescale Carbon Dioxide Removal to be put in place sooner than some models had envisaged, e.g. well before 2050, in order to avoid an overshoot.

Termination Risk / Termination Shock

One objection to Solar Radiation Modification technologies is that if their deployment were suddenly terminated, whether by decision or by accident, global temperatures could rapidly rise back to what the greenhouse gas levels in the atmosphere command. The less the underlying causes of climate change are addressed by the time of termination, the higher the rebound (or temperature whiplash).

A quick, large rebound could cause devastating damage to ecosystems. This concern came to prominence in 2018 with widespread media coverage of a study on the potential impact on biodiversity in Nature Ecology and Evolution.

Some researchers suggest that conclusions about the risk of Termination Shock are premature. They suggest there would be a high level of political motivation to avoid it, that there could be gradual deployment and ramp down, and that it would be relatively easy to design a system that would be highly robust and resilient against most proposed drivers of premature termination. This would nonetheless require global and regional governance mechanisms that would take a long time to develop.

Weather Modification

At least one NGO and some media outlets have included Weather Modification in their consideration of Geoengineering. This is not generally accepted in the academic and policy community, as Weather Modification is local in intention, action and impact (rather than on an earth system level) and is quickly reversible.

At a very large scale some Weather Modification programmes may have regional and transboundary environmental and socio-economic impacts and would need to be governed appropriately.


The Chemtrails Conspiracy

Debate on Geoengineering brings with it a specific challenge: the Chemtrails Conspiracy community, which believes deliberate Stratospheric Aerosol Injection is already widely underway, with an allegedly negative impact on the climate and on human health. This community often describes aircraft condensation trails (or ‘contrails’) as Chemtrails, which it cites as evidence of secret spraying programmes.

These ideas have significant traction, and Chemtrailers are very active on social media. Their positions consequently can influence the debate around Geoengineering Governance. A Harvard study in 2017, solar geoengineering and the chemtrails conspiracy on social media, found that 10% of Americans declare the Chemtrails Conspiracy “completely” true; a further 20–30% as “somewhat” true, with no apparent difference by party affiliation or strength of partisanship. Conspiratorial views accounted for ~ 60% of Geoengineering discourse on social media over the past decade.

As far as C2G is aware, no deployment of Stratospheric Aerosol Injection technologies is underway. For more details, see “Surveyed scientists debunk chemtrails conspiracy theory”.

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