Climate engineering

Climate engineering (also called geoengineering) is a term used for both carbon dioxide removal and solar radiation management, also called solar geoengineering, when applied at a planetary scale.: 6–11  However, they have very different geophysical characteristics which is why the Intergovernmental Panel on Climate Change no longer uses this overarching term.: 6–11  Carbon dioxide removal approaches are part of climate change mitigation. Solar geoengineering involves reflecting some sunlight (solar radiation) back to space. All forms of geoengineering are not a standalone solution to climate change, but need to be coupled with other forms of climate change mitigation. Another approach to geoengineering is to increase the Earth's thermal emittance through passive radiative cooling.

Carbon dioxide removal is defined as "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 CO2 sinks and direct air carbon dioxide capture and storage, but excludes natural CO2 uptake not directly caused by human activities."

Some types of climate engineering are highly controversial due to the large uncertainties around effectiveness, side effects and unforeseen consequences. However, the risks of such interventions must be seen in the context of the trajectory of climate change without them.


Climate engineering (or geoengineering) has been used as an umbrella term for both carbon dioxide removal and solar radiation management (or solar geoengineering), when applied at a planetary scale.: 6–11  However, these two methods have very different geophysical characteristics, which is why the Intergovernmental Panel on Climate Change no longer uses this term.: 6–11  This decision was communicated in around 2018, see for example the "Special Report on Global Warming of 1.5 °C".: 550 

Some authors, for example in the mainstream media, also include passive daytime radiative cooling, "ocean geoengineering" and others in the term of climate engineering.

Specific technologies that fall into the "climate engineering" umbrella term include:: 30 

The following methods are not termed "climate engineering" in the latest IPCC assessment report in 2022: 6–11  but are nevertheless included in other publications on this topic:


Carbon dioxide removal

Planting trees is a nature-based way to temporarily remove carbon dioxide from the atmosphere.

Carbon dioxide removal (CDR), also known as carbon removal, greenhouse gas removal (GGR) or negative emissions, is a process in which carbon dioxide gas (CO2) is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products.: 2221  In the context of net zero greenhouse gas emissions targets, CDR is increasingly integrated into climate policy, as an element of climate change mitigation strategies. Achieving net zero emissions will require both deep cuts in emissions and the use of CDR, but CDR is not a current climate solution. In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.: 114 

CDR methods include afforestation, reforestation, agricultural practices that sequester carbon in soils (carbon farming), wetland restoration and blue carbon approaches, bioenergy with carbon capture and storage (BECCS), ocean fertilization, ocean alkalinity enhancement, and direct air capture when combined with storage,: 115  To assess whether negative emissions are achieved by a particular process, comprehensive life cycle analysis and monitoring, reporting, and verification (MRV) of the process must be performed.

Solar geoengineering

refer to caption and image description
Proposed solar geoengineering using a tethered balloon to inject sulfate aerosols into the stratosphere

Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight (solar radiation) would be reflected back to outer space to limit or offset human-caused climate change. There are multiple potential approaches, with stratospheric aerosol injection (SAI) being the most-studied method, followed by marine cloud brightening (MCB). Other methods have been proposed, including a variety of space-based approaches, but they are generally considered less viable, and are not taken seriously by the Intergovernmental Panel on Climate Change. SRM methods could have a rapid cooling effect on atmospheric temperature, but if the intervention were to suddenly stop for any reason, the cooling would soon stop as well. It is estimated that the cooling impact from SAI would cease 1–3 years after the last aerosol injection, while the impact from marine cloud brightening would disappear in just 10 days. Contrastingly, once any carbon dioxide is added to the atmosphere and not removed, its warming impact does not decrease for a century, and some of it will persist for hundreds to thousands of years. As such, solar geoengineering is not a substitute for reducing greenhouse gas emissions but would act as a temporary measure to limit warming while emissions of greenhouse gases are reduced and carbon dioxide is removed.

If solar geoengineering were to cease while greenhouse gas levels remained high, it would lead to "large and extremely rapid" warming and similarly abrupt changes to the water cycle. Rapid termination would significantly increase the threats to biodiversity from climate change. In spite of this risk, solar geoengineering is frequently discussed as a policy option because it is much faster and (in the short run) cheaper than any form of climate change mitigation. While cooling the atmosphere by 1 °C (1.8 °F) through stratospheric aerosol injection would cost at least $18 billion annually (at 2020 USD value), and other approaches also cost tens of billions of dollars or more annually, this would still be "orders of magnitude" cheaper than greenhouse gas mitigation, and the unmitigated effects of climate change would cost far more than that.

Passive daytime radiative cooling

Enhancing the thermal emissivity of Earth through passive daytime radiative cooling has been proposed as an alternative or "third approach" to geoengineering that is "less intrusive" and more predictable or reversible than stratospheric aerosol injection.

Passive daytime radiative cooling (PDRC) can lower temperatures with zero energy consumption or pollution by radiating heat into outer space. Widespread application has been proposed as a solution to global warming.

Passive daytime radiative cooling (PDRC) is a zero-energy building cooling method proposed as a solution to reduce air conditioning, lower urban heat island effect, cool human body temperatures in extreme heat, move toward carbon neutrality and control global warming by enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero energy consumption or pollution. In contrast to compression-based cooling systems that are prevalently used (e.g., air conditioners), consume substantial amounts of energy, have a net heating effect, require ready access to electricity and often require coolants that are ozone-depleting or have a strong greenhouse effect, application of PDRCs may also increase the efficiency of systems benefiting from a better cooling, such like photovoltaic systems, dew collection techniques, and thermoelectric generators.

PDRC surfaces are designed to be high in solar reflectance (to minimize heat gain) and strong in longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window (8–13 µm) to cool temperatures even during the daytime. It is also referred to as passive radiative cooling, daytime passive radiative cooling, radiative sky cooling, photonic radiative cooling, and terrestrial radiative cooling. PDRC differs from solar radiation management because it increases radiative heat emission rather than merely reflecting the absorption of solar radiation.
Video to explain some of the marine geoengineering approaches with a focus on their risks, negative impacts and potential side-effects, as well as on the question of governance of these technologies.

Ocean geoengineering

Ocean geoengineering involves adding material such as lime or iron to the ocean to affect its ability to support marine life and/or sequester CO
. In 2021 the US National Academies of Sciences, Engineering, and Medicine (NASEM) requested $2.5 billion funds for research in the following decade, specifically including field tests.

Ocean liming

Enriching seawater with calcium hydroxide (lime) has been reported to lower ocean acidity, which reduces pressure on marine life such as oysters and absorb CO
. The added lime raised the water's pH, capturing CO
in the form of calcium bicarbonate or as carbonate deposited in mollusk shells. Lime is produced in volume for the cement industry. This was assessed in 2022 in an experiment in Apalachicola, Florida in an attempt to halt declining oyster populations. pH levels increased modestly, as CO
was reduced by 70 ppm.

A 2014 experiment added sodium hydroxide (lye) to part of Australia's Great Barrier Reef. It raised pH levels to nearly preindustrial levels.

However, producing alkaline materials typically releases large amounts of CO
, partially offsetting the sequestration. Alkaline additives become diluted and dispersed in one month, without durable effects, such that if necessary, the program could be ended without leaving long-term effects.

Iron fertilization

Iron fertilization is the intentional introduction of iron-containing compounds (like iron sulfate) to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO2) sequestration from the atmosphere. Iron is a trace element necessary for photosynthesis in plants. It is highly insoluble in sea water and in a variety of locations is the limiting nutrient for phytoplankton growth. Large algal blooms can be created by supplying iron to iron-deficient ocean waters. These blooms can nourish other organisms.

Submarine forest

Another 2022 experiment attempted to sequester carbon using giant kelp planted off the Namibian coast. Whilst this approach has been called "ocean geoengineering" by the researchers it is just another form of carbon dioxide removal via sequestration. Another term that is used to describe this process is blue carbon management and also marine geoengineering.

Glacier stabilization

A proposed "underwater sill" blocking 50% of warm water flows heading for the glacier could have the potential to delay its collapse and the resultant sea level rise by many centuries.

Some engineering interventions have been proposed for Thwaites Glacier and the nearby Pine Island Glacier to stabilize its ice physically, or to preserve it by blocking the flow of warm ocean water, which currently renders the collapse of these two glaciers practically inevitable even without further warming. A proposal from 2018 included building sills at the Thwaites' grounding line to either physically reinforce it, or to block some fraction of warm water flow. The former would be the simplest intervention, yet still equivalent to "the largest civil engineering projects that humanity has ever attempted": it is also only 30% likely to work. Constructions blocking even 50% of the warm water flow are expected to be far more effective, yet far more difficult as well. Further, some researchers dissented, arguing that this proposal could be ineffective, or even accelerate sea level rise. The original authors have suggested attempting this intervention on smaller sites, like the Jakobshavn Glacier in Greenland, as a test run, as well as acknowledging that this intervention cannot prevent sea level rise from the increased ocean heat content, and would be ineffective in the long run without greenhouse gas emission reductions.

In 2023, a modified proposal was tabled: it was proposed that an installation of underwater "curtains", made out of a flexible material and anchored to Amundsen Sea floor would be able to interrupt warm water flow while reducing costs and increasing their longevity (conservatively estimated at 25 years for curtain elements and up to 100 years for the foundations) relative to more rigid structures. With them in place, Thwaites Ice Shelf and Pine Island Ice Shelf would presumably be able to regrow to a state they last had a century ago, thus stabilizing these glaciers. To achieve this, the curtains would have to be placed at a depth of around 600 metres (0.37 miles) (to avoid damage from icebergs which would be regularly drifting above) and be 80 km (50 mi) long. The authors acknowledged that while work on this scale would be unprecedented and face many challenges in the Antarctic (including polar night and the currently insufficient numbers of specialized polar ships and underwater vessels), it would also not require any new technology and there is already experience of laying down pipelines at such depths.


According to climate economist Gernot Wagner the term "geoengineering" is "largely an artefact and a result of the terms frequent use in popular discourse" and "so vague and all-encompassing as to have lost much meaning".: 14 

Interventions at large scale run a greater risk of unintended disruptions of natural systems, resulting in a dilemma that such disruptions might be more damaging than the climate damage that they offset.

Ethical aspects

Climate engineering may reduce the urgency of reducing carbon emissions, a form of moral hazard. Also, most efforts have only temporary effects, which implies rapid rebound if they are not sustained. The Union of Concerned Scientists points to the danger that the technology will become an excuse not to address the root causes of climate change, slow our emissions reductions and start moving toward a low-carbon economy. However, several public opinion surveys and focus groups reported either a desire to increase emission cuts in the presence of climate engineering, or no effect. Other modelling work suggests that the prospect of climate engineering may in fact increase the likelihood of emissions reduction.

If climate engineering can alter the climate, then this raises questions whether humans have the right to deliberately change the climate, and under what conditions. For example, using climate engineering to stabilize temperatures is not the same as doing so to optimize the climate for some other purpose. Some religious traditions express views on the relationship between humans and their surroundings that encourage (to conduct responsible stewardship) or discourage (to avoid hubris) explicit actions to affect climate.

"Just geoegineering theory"

The ethical applications of geoengineering can be grasped by considering the principles laid out in just war theory. In the interests of national security, implementing geoengineering technology creates discrepancies between state actors and their differing priorities. The Just War Theory is used by scholars to measure the morality in warfare, acting as ethical guidance for decision making about the destructive forces of war. The Just War Theory has been modified to the “just geoengineering theory” to offer standards for decision makers to consider geoengineering in practice. The “just geoengineering theory” outlines three moral constraints to climate and the implementation of geoengineering.

Jus ad climate describes a state must be facing a major climate change relate emergency to justify the use of geoengineering, like self-defense in a just war. Issues arise as there are neither financial no moral estimates constituting “major emergencies”. Further, states are tasked with common but differentiated responsibilities, where small scale disasters in one state may be more detrimental in another. Second, jus in climate states the method chosen is least environmentally harmful designed to achieve minimum ecological disruption to offset climate change effects. These assumptions rely on subjective scientific and environmental judgement to understand the level of ecological disruption. Finally, jus post climate calls for ending geoengineering deployment as soon as possible and restoring the ecosystem to its previous state. With little available data on the effects of geoengineering, it is logical to assume use would need to continue indefinitely or global cooling will not be achieved.

Society and culture

Public perception

A large 2018 study used an online survey to investigate public perceptions of six climate engineering methods in the United States, United Kingdom, Australia, and New Zealand. Public awareness of climate engineering was low; less than a fifth of respondents reported prior knowledge. Perceptions of the six climate engineering methods proposed (three from the carbon dioxide removal group and three from the solar geoengineering group) were largely negative and frequently associated with attributes like 'risky', 'artificial' and 'unknown effects'. Carbon dioxide removal methods were preferred over solar geoengineering. Public perceptions were remarkably stable with only minor differences between the different countries in the surveys.

Some environmental organizations (such as Friends of the Earth and Greenpeace) have been reluctant to endorse or oppose solar geoengineering, but are often more supportive of nature-based carbon dioxide removal projects, such as afforestation and peatland restoration.

Existing Litigation

Mild domestic legal guidance leaves international law to assist in governing national security. However, the assistance is limited, and various environmental conventions and war treaties are required to aid in policy makers decisions.

Environmental Laws

The 1972 London Dumping Convention and the 1982 UN Convention of the Law and the Sea both address marine pollution and ocean iron fertilization (OIF) geoengineering. Articles state security actions and intentions must be named to ensure the interests of the state are not over another state in deploying geoengineering strategies. The 1979 Convention on Long Range Transboundary Air Pollution discloses to limitation of solar radiation geoengineering. In 1992, the Convention of Biological Diversity clarified the processes that affect ecological biodiversity.

Laws of War

In certain ways, the implementation and environmental effects of geoengineering classify as threatening to state if certain limitations are met by another state. Therefore, norms of wartime behavior apply to the impacts of geoengineering technology. The 1977 Environmental Modification Convention (ENMOD) permits any military to use environmental modification on any other state. Although not disclosed as a method, if deployment if geoengineering becomes matter of national economic or scientific policy, then military involvement is governed.

From 1974 to 1977 Protocol I, an amendment of the Geneva Conventions implies the protection of victims of international armed conflict and the protection of the national environment. Additionally, prohibiting militaries from attacking resources necessary for the survival of a population. The intentions clearly stating that the health of the environment is critical to the growth of society and are not prejudiced to war.

Financial Aspects and Funding Trends

Government and Private Funding

Investment into climate engineering is extensively linked to financial dynamics, with contributions from both the public and private sectors. Government funding for climate engineering increased noticeably over the last decade, peaking in 2014 and then declining. Conversely, private funding has remained relatively stable, with notable contributions from institutions such as Harvard's Solar Geoengineering Research Program and the Carnegie Climate Geoengineering Governance Initiative, with a significant increase in funding observed in 2016. Harvard University has emerged as a key player in privately funded climate engineering research. Harvard's contributions to solar engineering total at least $100,000 and include research, governance, policy, public engagement, and advocacy. The university's dedication to climate engineering can be seen in its extensive tracking of projects from 2008 to 2018, which resulted in several million dollars in funding. Harvard currently holds the most funds, with approximately $16 million allocated for research from 2017 to 2024, demonstrating its leadership in advancing climate engineering initiatives.

Mixed Funding

While collaboration between the public and private sectors exists, it accounts for a small percentage of total funds. Because of its small contribution to the overall financial landscape, patterns in mixed sources funding remain difficult to discern.

Investment Challenges

Securing funding for climate engineering research is an intricate process that faces legal, ethical, and legislative challenges, particularly when it comes to the introduction of chemicals into shared global waters. Striking a balance between advancing research and ensuring ethical practices within a strong legal framework remains a critical challenge in addressing climate change.

  • Legal and Ethical Issues: Issues such as 'dumping' protocols for marine geoengineering, raise concerns about the introduction of chemicals into shared global waters. Resolving these policies requires a global consensus, navigating issues of shared but differentiated responsibility.
  • Legislation and Oversight: Congressional initiatives, such as the bill introduced by Congressman Jerry McNerney, seek to allow agencies such as the National Oceanic and Atmospheric Administration (NOAA) to pioneer climate engineering research. The absence of specific international laws governing climate interventions, on the other hand, poses significant challenges.
  • Stratospheric Research: Concerns have been raised about the international legality of stratospheric research for CO2 reduction. Given that the majority of global "aero activity" occurs in the troposphere, which is outside the purview of existing international laws, questions about oversight and regulation of such endeavors arise. This among other things could create friction for investment incetives.


Several organizations have investigated climate engineering with a view to evaluating its potential, including the US Congress, the US National Academy of Sciences, Engineering, and Medicine, the Royal Society, the UK Parliament, the Institution of Mechanical Engineers, and the Intergovernmental Panel on Climate Change. The IMechE report examined a small subset of proposed methods (air capture, urban albedo and algal-based CO2 capture techniques), and its main conclusions were that climate engineering should be researched and trialed at the small scale alongside a wider decarbonization of the economy.

The Royal Society review examined a wide range of proposed climate engineering methods and evaluated them in terms of effectiveness, affordability, timeliness, and safety (assigning qualitative estimates in each assessment). The key recommendations reports were that "Parties to the UNFCCC should make increased efforts towards mitigating and adapting to climate change, and in particular to agreeing to global emissions reductions", and that "[nothing] now known about geoengineering options gives any reason to diminish these efforts". Nonetheless, the report also recommended that "research and development of climate engineering options should be undertaken to investigate whether low-risk methods can be made available if it becomes necessary to reduce the rate of warming this century".

In 2009, a review examined the scientific plausibility of proposed methods rather than the practical considerations such as engineering feasibility or economic cost. The authors found that "[air] capture and storage shows the greatest potential, combined with afforestation, reforestation and bio-char production", and noted that "other suggestions that have received considerable media attention, in particular, "ocean pipes" appear to be ineffective". They concluded that "[climate] geoengineering is best considered as a potential complement to the mitigation of CO2 emissions, rather than as an alternative to it".

In 2015, the US National Academy of Sciences, Engineering, and Medicine concluded a 21-month project to study the potential impacts, benefits, and costs of climate engineering. The differences between these two classes of climate engineering "led the committee to evaluate the two types of approaches separately in companion reports, a distinction it hopes carries over to future scientific and policy discussions." The resulting study titled Climate Intervention was released in February 2015 and consists of two volumes: Reflecting Sunlight to Cool Earth and Carbon Dioxide Removal and Reliable Sequestration. According to their brief about the study:

Climate intervention is no substitute for reductions in carbon dioxide emissions and adaptation efforts aimed at reducing the negative consequences of climate change. However, as our planet enters a period of changing climate never before experienced in recorded human history, interest is growing in the potential for deliberate intervention in the climate system to counter climate change... Carbon dioxide removal strategies address a key driver of climate change, but research is needed to fully assess if any of these technologies could be appropriate for large-scale deployment. Albedo modification strategies could rapidly cool the planet's surface but pose environmental and other risks that are not well understood and therefore should not be deployed at climate-altering scales; more research is needed to determine if albedo modification approaches could be viable in the future.

In June 2023 the US government released a report that recommended conducting research on stratospheric aerosol injection and marine cloud brightening.

See also

This page was last updated at 2024-02-05 22:51 UTC. Update now. View original page.

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