Multiple authoritative international scientific assessments, based on evidence from climate models and natural analogues, have generally shown that some forms of SRM could reduce global warming and many adverse effects of climate change.[2][3][4] Specifically, controlled stratospheric aerosol injection appears able to greatly moderate most environmental impacts—especially warming—and consequently most ecological, economic, and other impacts of climate change across most regions. However, because warming from greenhouse gases and cooling from SRM would operate differently across latitudes and seasons, a world where global warming would be offset by SRM would have a different climate from one where this warming did not occur in the first place. Furthermore, confidence in the current projections of how SRM would affect regional climate and ecosystems is low.[1]
SRM would pose environmental risks. In addition to its imperfect reduction of climate-change impacts, stratospheric aerosol injection could, for example, slow the recovery of stratospheric ozone.[5] If a significant SRM intervention were to suddenly stop and not be resumed, the cooling would end relatively rapidly, posing serious environmental risks. Some environmental risks remain unknown.
Governing SRM is challenging for multiple reasons, including that several countries would likely be capable of doing it alone.[6] For now, there is no formal international framework designed to regulate SRM, although aspects of existing international law would be applicable. Issues of governance and effectiveness are intertwined, as poorly governed use of SRM might lead to its highly suboptimal implementation.[7] Thus, many questions regarding the acceptable deployment of SRM, or even its research and development, are currently unanswered.
SRM can be deployed on different scales. This graph shows the baseline radiative forcing under three different Representative Concentration Pathway scenarios, and how it would be affected by the deployment of SAI, starting from 2034, to halve the speed of warming by 2100, to halt the warming, or to reverse it entirely.Potential complementary responses to climate change: greenhouse gas emissions abatement, carbon dioxide removal, SRM, and adaptation.
The context for the interest in solar radiation modification (SRM) options is continued high global emissions of greenhouse gases. Human's greenhouse gas emissions have disrupted the Earth's energy budget. Due to elevated atmospheric greenhouse gas concentrations, the net difference between the amount of sunlight absorbed by the Earth and the amount of energy radiated back to space has risen from 1.7W/m2 in 1980, to 3.1W/m2 in 2019.[10] This imbalance, or "radiative forcing," means that the Earth absorbs more energy than it emits, causing global temperatures to rise[11] which will, in turn, have negative impacts on humans and nature.
In principle, net emissions could be reduced and even eliminated achieved through a combination a combination of emission cuts and carbon dioxide removal (together called "mitigation"). However, emissions have persisted, consistently exceeding targets, and experts have raised serious questions regarding the feasibility of large-scale removals.[12][13][14] The 2023 Emissions Gap Report from the UN Environment Programme estimated that even the most optimistic assumptions regarding countries' current conditional emissions policies and pledges has only a 14% chance of limiting global warming to 1.5°C.[15]
SRM would increase Earth's reflection of sunlight by increasing the albedo of the atmosphere or the surface. An increase in planetary albedo of 1% would reduce radiative forcing by 2.35W/m2, eliminating most of global warming from current anthropogenically elevated greenhouse gas concentrations, while a 2% albedo increase would negate the warming effect of doubling the atmospheric carbon dioxide concentration.[16]
SRM could theoretically buy time by slowing the rate of climate change or to eliminate the worst climate impacts until net negative emissions reduce atmospheric greenhouse gas concentrations sufficiently.[citation needed] This is because SRM could, unlike the other responses, cool the planet within months after deployment.[17]
SRM is generally intended to complement, not replace, emissions reduction and carbon dioxide removal. For example, the IPCCSixth Assessment Report says: "There is high agreement in the literature that for addressing climate change risks SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving sustained net zero or net negative CO2 emission levels globally".[1]
In 1974, Russian climatologistMikhail Budyko suggested that if global warming ever became a serious threat, it could be countered with airplane flights in the stratosphere, burning sulfur to make aerosols that would reflect sunlight away.[22][23] Along with carbon dioxide removal, SRM was discussed jointly as geoengineering in a 1992 climate change report from the US National Academies.[24]
David Keith, an American physicist, has worked on solar geoengineering since 1992, when he and Hadi Dowlatabadi published one of the first assessments of the technology and its policy implications, introducing a structured comparison of cost and risk. Keith has consistently argued that geoengineering needs a "systematic research program" to determine whether or not its approaches are feasible.[25][26][27] He has also appealed for international standards of governance and oversight for how such research might proceed.[28]
The first modeled results of SRM were published in 2000.[29] In 2006 Nobel LaureatePaul Crutzen published an influential scholarly paper where he said, "Given the grossly disappointing international political response to the required greenhouse gas emissions, and further considering some drastic results of recent studies, research on the feasibility and environmental consequences of climate engineering [...] should not be tabooed."[30]
Proposed methods
Modeling evidence of the effect of greenhouse gases and SRM on average annual temperature (left column) and precipitation (right column). The first row (a) is moderately high continued greenhouse gas emissions (RCP4.5) at the end of the century. The second row (b) is the same emissions scenario and time, with SRM to reduce global warming to 1.5°C. The third row (c) is the same emissions scenario but in the near future, when global warming would be 1.5°C, with no SRM. The similarity between the second and third rows suggests that SRM could reduce climate change reasonably well.
SRM methods include atmospheric methods, space-based methods and others. The atmospheric methods include:
Injecting reflective aerosols into the stratosphere is the proposed SRM method that has received the most sustained attention. The Intergovernmental Panel on Climate Change concluded that Stratospheric aerosol injection "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5°C."[32] This technique would mimic a cooling phenomenon that occurs naturally by the eruption of volcanoes.[33] Sulfates are the most commonly proposed aerosol, since there is a natural analogue with (and evidence from) volcanic eruptions. Alternative materials such as using photophoretic particles, titanium dioxide, and diamond have been proposed.[34][35][36][37][38] Delivery by custom aircraft appears most feasible, with artillery and balloons sometimes discussed.[39][40][41] The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5–10billion US dollars.[42] This technique could give much more than 3.7W/m2 of globally averaged negative forcing,[43] which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.
Stratospheric aerosol injection is expected to have low direct financial costs of implementation,[44] relative to the expected costs of both unabated climate change and aggressive mitigation.
The most recent Scientific Assessment of Ozone Depletion report in 2022 from the World Meteorological Organization concluded "Stratospheric Aerosol Injection (SAI) has the potential to limit the rise in global surface temperatures by increasing the concentrations of particles in the stratosphere... . However, SAI comes with significant risks and can cause unintended consequences."[4]
Various cloud reflectivity methods have been suggested, such as that proposed by John Latham and Stephen Salter, which works by spraying seawater in the atmosphere to increase the reflectivity of clouds.[45] The extra condensation nuclei created by the spray would change the size distribution of the drops in existing clouds to make them whiter.[46] The sprayers would use fleets of unmanned rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth.[47] The whitening effect is created by using very small cloud condensation nuclei, which whiten the clouds due to the Twomey effect.
This technique can give more than 3.7W/m2 of globally averaged negative forcing,[43] which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.
Natural cirrus clouds are believed to have a net warming effect. These could be dispersed by the injection of various materials. This method is strictly not SRM, as it increases outgoing longwave radiation instead of decreasing incoming shortwave radiation. However, because it shares some of the physical and especially governance characteristics as the other SRM methods, it is often included.[18]
Space-based
The basic function of a space lens to mitigate global warming. The image is simplified, as a 1000 kilometre diameter lens is considered sufficient by most proposals, and would be much smaller than shown. Additionally, a zone plate would only be a few nanometers thick.
There has been a range of proposals to reflect or deflect solar radiation from space, before it even reaches the atmosphere, commonly described as a space sunshade.[35] The most straightforward is to have mirrors orbiting around the Earth—an idea first suggested even before the wider awareness of climate change, with rocketry pioneer Hermann Oberth considering it a way to facilitate terraforming projects in 1923.[48] and this was followed by other books in 1929, 1957 and 1978.[49][50][51] By 1992, the U.S. National Academy of Sciences described a plan to suspend 55,000 mirrors with an individual area of 100 square meters in a Low Earth orbit.[16] Another contemporary plan was to use space dust to replicate Rings of Saturn around the equator, although a large number of satellites would have been necessary to prevent it from dissipating. A 2006 variation on this idea suggested relying entirely on a ring of satellites electromagnetically tethered in the same location. In all cases, sunlight exerts pressure which can displace these reflectors from orbit over time, unless stabilized by enough mass. Yet, higher mass immediately drives up launch costs.[16]
In an attempt to deal with this problem, other researchers have proposed Inner lagrangian point between the Earth and the Sun as an alternative to near-Earth orbits, even though this tends to increase manufacturing or delivery costs instead. In 1989, a paper suggested founding a lunar colony, which would produce and deploy diffraction grating made out of a hundred million tonnes of glass.[52] In 1997, a single, very large mesh of aluminium wires "about one millionth of a millimetre thick" was also proposed.[53][self-published source?] Two other proposals from the early 2000s advocated the use of thin metallic disks 50–60cm in diameter, which would either be launched from the Earth at a rate of once per minute over several decades, or be manufactured from asteroids directly in orbit.[16] When summarizing these options in 2009, the Royal Society concluded that their deployment times are measured in decades and costs in the trillions of USD, meaning that they are "not realistic potential contributors to short-term, temporary measures for avoiding dangerous climate change", and may only be competitive with the other geoengineering approaches when viewed from a genuinely long (a century or more) perspective, as the long lifetime of L1-based approaches could make them cheaper than the need to continually renew atmospheric-based measures over that timeframe.[16]
Relatively few researchers have revisited the subject since that Royal Society review, as it became accepted that space-based approaches would cost about 1000 times more than their terrestrial alternatives.[54] In 2022, the IPCC Sixth Assessment Report had discussed SAI, MCB, CCT and even attempts to alter albedo on the ground or in the ocean, yet completely ignored space-based approaches.[1] There are still some proponents, who argue that unlike stratospheric aerosol injection, space-based approaches are advantageous because they do not interfere directly with the biosphere and ecosystems.[55] After the IPCC report was published, three astronomers have revisited the space dust concept, instead advocating for a lunar colony which would continuously mine the Moon in order to eject lunar dust into space on a trajectory where it would interfere with sunlight streaming towards the Earth. Ejections would have to be near-continuous, as since the dust would scatter in a matter of days, and about 10 million tons would have to be dug out and launched annually.[56] The authors admit that they lack a background in either climate or rocket science, and the proposal may not be logistically feasible.[57]
In 2021, researchers in Sweden considered building solar sails in the near-Earth orbit, which would then arrive to L1 point over 600 days one by one. Once they all form an array in situ, the combined 1.5billion sails would have total area of 3.75 million square kilometers, while their combined mass is estimated in a range between 83 million tons (present-day technology) and 34 million tons (optimal advancements). This proposal would cost between five and ten trillion dollars, but only once launch cost has been reduced to US$50/kg, which represents a massive reduction from the present-day costs of $4400–2700/kg[58] for the most widely used launch vehicles.[59] In July 2022, a pair of researchers from MIT Senseable City Lab, Olivia Borgue and Andreas M. Hein, have instead proposed integrating nanotubes made out of silicon dioxide into ultra-thin polymeric films (described as "space bubbles" in the media [55]), whose semi-transparent nature would allow them to resist the pressure of solar wind at L1 point better than any alternative with the same weight. The use of these "bubbles" would limit the mass of a distributed sunshade roughly the size of Brazil to about 100,000 tons, much lower than the earlier proposals. However, it would still require between 399 and 899 yearly launches of a vehicle such as SpaceX Starship for a period of around 10 years, even though the production of the bubbles themselves would have to be done in space. The flights would not begin until research into production and maintenance of these bubbles is completed, which the authors estimate would require a minimum of 10–15 years. After that, the space shield may be large enough by 2050 to prevent crossing of the 2°C (3.6°F) threshold.[54][55][60]
Others
Cool roof
The albedo of several types of roofs (lower = hotter)
Painting roof materials in white or pale colors to reflect solar radiation, known as 'cool roof' technology, is encouraged by legislation in some areas (notably California).[61] This technique is limited in its ultimate effectiveness by the constrained surface area available for treatment. This technique can give between 0.01 and 0.19W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated.[43] This is small relative to the 3.7W/m2 of positive forcing from a doubling of atmospheric carbon dioxide. Moreover, while in small cases it can be achieved at little or no cost by simply selecting different materials, it can be costly if implemented on a larger scale. A 2009 Royal Society report states that, "the overall cost of a 'white roof method' covering an area of 1% of the land surface (about 1012m2) would be about $300billion/yr, making this one of the least effective and most expensive methods considered."[16] However, it can reduce the need for air conditioning, which emits carbon dioxide and contributes to global warming.
Radiative cooling
Some papers have proposed the deployment of specific thermal emitters (whether via advanced paint, or printed rolls of material) which would simultaneously reflect sunlight and also emit energy at longwave infrared (LWIR) lengths of 8–20μm, which is too short to be trapped by the greenhouse effect and would radiate into outer space. It has been suggested that to stabilize Earth's energy budget and thus cease warming, 1–2% of the Earth's surface (area equivalent to over half of Sahara) would need to be covered with these emitters, at the deployment cost of $1.25–2.5trillion. While low next to the estimated $20 trillion saved by limiting the warming to 1.5°C (2.7°F) rather than 2°C (3.6°F), it does not include any maintenance costs.[62][63]
Technical problem areas
Aspects of regional scales and seasonal timescales
A moderate magnitude of SRM would bring important aspects of the climate—for example, average and extreme temperature, water availability, and cyclone intensity—closer to their preindustrial values for most of the planet at a subregional resolution.[64] Furthermore, SRM's effect would occur rapidly, unlike those of other responses to climate change. However, even under optimal implementation, some climatic anomalies—especially regarding precipitation—would persist, although mostly at lesser magnitudes than without SRM.[citation needed]
As well as imperfect and geographically uneven cancellation of the climatic effect of greenhouse gases, SRM has other significant technical problems. The IPCCSixth Assessment Report explains some of the risks and uncertainties as follows: "[...] SRM could offset some of the effects of increasing GHGs on global and regional climate, including the carbon and water cycles. However, there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales, and large uncertainties associated with aerosol–cloud–radiation interactions persist. The cooling caused by SRM would increase the global land and ocean CO2 sinks, but this would not stop CO2 from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions."[65]:69[2]
Likewise, a 2023 report from the UN Environment Programme stated, "Climate model results indicate that an operational SRM deployment could fully or partially offset the global mean warming caused by anthropogenic GHG emissions and reduce some climate change hazards in most regions. There could be substantial residual or possible overcompensating climate change at regional scales and seasonal timescales."[3]:14 The report also said: "An operational SRM deployment would introduce new risks to people and ecosystems".[3]:1
SRM would imperfectly compensate for anthropogenic climate changes. Greenhouse gases warm throughout the globe and year, whereas SRM reflects light more effectively at low latitudes and in the hemispheric summer (due to the sunlight's angle of incidence) and only during daytime. Deployment regimes could compensate for this heterogeneity by changing and optimizing injection rates by latitude and season.[66][67]
Impacts on precipitation
Models indicate that SRM would compensate more effectively for temperature than for precipitation.[citation needed] Therefore, using SRM to fully return global mean temperature to a preindustrial level would overcorrect for precipitation changes. This has led to claims that it would dry the planet or even cause drought,[citation needed] but this would depend on the intensity (i.e. radiative forcing) of SRM. Furthermore, soil moisture is more important for plants than average annual precipitation. Because SRM would reduce evaporation, it more precisely compensates for changes to soil moisture than for average annual precipitation.[68] Likewise, the intensity of tropical monsoons is increased by climate change and decreased by SRM.[69]
A net reduction in tropical monsoon intensity might manifest at moderate use of SRM, although to some degree the effect of this on humans and ecosystems would be mitigated by greater net precipitation outside of the monsoon system.[citation needed] This has led to claims that SRM "would disrupt the Asian and African summer monsoons", but the impact would depend on the particular implementation regime.[citation needed]
Maintenance and termination shock
The direct climatic effects of SRM are reversible within short timescales.[17] Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained.[70] If SRM masked significant warming, stopped abruptly, and was not resumed within a year or so, the climate would rapidly warm towards levels which would have existed without the use of SRM, sometimes known as termination shock.[71] The rapid rise in temperature might lead to more severe consequences than a gradual rise of the same magnitude. However, some scholars have argued that this appears preventable because it would be in states' interest to resume any terminated deployment regime, and because infrastructure and knowledge could be made redundant and resilient.[72][73]
Slowing stratospheric ozone recovery
Stratospheric aerosol injection, the most studied SRM technique, using sulphates appears likely to catalyze the destruction of the protective stratospheric ozone layer.[74]
SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification.[75] While not a risk of SRM per se, this points to the limitations of relying on it to the exclusion of emissions reduction.
Effect on sky and clouds
Managing solar radiation using aerosols or cloud cover would involve changing the ratio between direct and indirect solar radiation. This would affect plant life[76] and solar energy.[77] Visible light, useful for photosynthesis, is reduced proportionally more than is the infrared portion of the solar spectrum due to the mechanism of Mie scattering.[78] As a result, deployment of atmospheric SRM would reduce by at least 2–5% the growth rates of phytoplankton, trees, and crops [79] between now and the end of the century.[80] Uniformly reduced net shortwave radiation would hurt solar photovoltaics by the same >2–5% because of the bandgap of silicon photovoltaics.[81]
Uncertainty regarding effects
Much uncertainty remains about SRM's likely effects.[75] Most of the evidence regarding SRM's expected effects comes from climate models and volcanic eruptions. Some uncertainties in climate models (such as aerosol microphysics, stratospheric dynamics, and sub-grid scale mixing) are particularly relevant to SRM and are a target for future research.[82] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.[83]
Climate change has various effects on agriculture. One of them is the CO2 fertilization effect which affects different crops in different ways. A net increase in agricultural productivity from SRM has been predicted by some studies due to the combination of more diffuse light and carbon dioxide's fertilization effect.[84] Other studies suggest that SRM would have little net effect on agriculture.[85]
There have also been proposals to focus SRM at the poles, in order to combat sea level rise[86] or regional marine cloud brightening (MCB) in order to protect coral reefs from bleaching. However, there is low confidence about the ability to control geographical boundaries of the effect.[1]
SRM might be used in ways that are not optimal. In particular, SRM's climatic effects would be rapid and reversible, which would bring the disadvantage of sudden warming if it were to be stopped suddenly.[87] Similarly, if SRM was very heterogenous, then the climatic responses could be severe and uncertain.
Governance and policy risks
Global governance issues
The potential use of SRM poses several governance challenges because of its high leverage, low apparent direct costs, and technical feasibility as well as issues of power and jurisdiction.[88] Because international law is generally consensual, this creates a challenge of widespread participation being required. Key issues include who will have control over the deployment of SRM and under what governance regime the deployment can be monitored and supervised. A governance framework for SRM must be sustainable enough to contain a multilateral commitment over a long period of time and yet be flexible as information is acquired, the techniques evolve, and interests change through time.
Frank Biermann and other political scientists argue that the current international political system is inadequate for the fair and inclusive governance of SRM deployment on a global scale.[7] Other researchers have suggested that building a global agreement on SRM deployment will be very difficult, and instead power blocs are likely to emerge.[89] There are, however, significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment a rather unlikely event.[90]
Other relevant aspects of the governance of SRM include supporting research, ensuring that it is conducted responsibly, regulating the roles of the private sector and (if any) the military, public engagement, setting and coordinating research priorities, undertaking trusted scientific assessment, building trust, and compensating for possible harms.
Although climate models of SRM rely on some optimal or consistent implementation, leaders of countries and other actors may disagree as to whether, how, and to what degree SRM be used. This could result in suboptimal deployments and exacerbate international tensions.[91] Likewise, blame for perceived local negative impacts from SRM could be a source of international tensions.[92]
There is a risk that countries may start using SRM without proper precaution or research. SRM, at least by stratospheric aerosol injection, appears to have low direct implementation costs relative to its potential impact, and many countries have the financial and technical resources to undertake SRM.[6] Some have suggested that SRM could be within reach of a lone "Greenfinger", a wealthy individual who takes it upon him or herself to be the "self-appointed protector of the planet".[93] Others argue that states will insist on maintaining control of SRM.[94]
The existence of SRM may reduce the political and social impetus for climate change mitigation.[95] This has often been called a potential "moral hazard", although such language is not precise. Some modelling work suggests that the threat of SRM may in fact increase the likelihood of emissions reduction.[96][97][98][99]
Advocacy
In 2024, Professor David Keith stated that in the last year or so, there has been far more engagement with SRM from senior political leaders than was previously the case.[100] Other countries have expressed a range of views at intergovernmental forums such as the UN Environment Assembly.
The leading argument supportive of SRM research is that the risks of likely anthropogenic climate change are great and imminent enough to warrant research and evaluation of a wide range of responses, even one with limitations and risks of its own. Leading this effort have been some climate scientists (such as James Hansen), some of whom have endorsed one or both public letters that support further SRM research.[101][102]
Scientific organizations that have called for further research in SRM include:
A few nongovernmental organizations actively support SRM research and governance dialogues. The Degrees Initiative works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."[111] Among other activities, it provides grants to scientists in the Global South. SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts."[112] The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM.[113] The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal,[114] although it ended operations in 2023.
Some critics claim that political conservatives, opponents of action to reduce greenhouse gas emissions, and fossil fuel firms are major advocates of SRM research.[115][116] However, only a handful of conservatives and opponents of climate action have expressed support, and there is no evidence that fossil fuel firms are involved in SRM research.[117] Instead, claims of fossil-fuel industry support typically conflate SRM and carbon dioxide removal—where fossil fuel firms are involved—under the broader term of climate engineering.[citation needed]
Opposition to deployment and research
Opposition to SRM has come from various academics and groups.[118] The most common concern is that SRM could lessen climate change mitigation efforts. Opponents of SRM research often emphasize that reductions of greenhouse gas emissions would also bring co-benefits (for example reduced air pollution) and that consideration of SRM could prevent these outcomes.[119]
In 2021, researchers at Harvard put plans for a SRM test on hold after Indigenous Sámi people objected to the test taking place in their homeland.[123][124] Although the test would not have involved any atmospheric experiments, members of the Saami Council spoke out against the lack of consultation and SRM more broadly. Speaking at a panel organized by the Center for International Environmental Law and other groups, Saami Council Vice President Åsa Larsson Blind said, "This goes against our worldview that we as humans should live and adapt to nature."
By 2024, U.S. government agencies were operating an airborne early warning system for detecting small concentrations of aerosols to determine where other countries might be carrying out geoengineering attempts, thought to have unpredictable effects on climate.[125]
The Climate Overshoot Commission is a group of global, eminent, and independent figures.[126] It investigated and developed a comprehensive strategy to reduce climate risks. The Commission is not supporting deployment of SRM. In fact, it recommends a "a moratorium on the deployment of solar radiation modification (SRM) and large-scale outdoor experiment". But it also says that "governance of SRM research should be expanded".[127]:15
Proposed international non-use agreement on solar geoengineering
In 2022, a dozen academics launched a political campaign for national policies of "no public funding, no outdoor experiments, no patents, no deployment, and no support in international institutions... including in assessments by the Intergovernmental Panel on Climate Change."[118] The proponents call this an International Non-Use Agreement on Solar Geoengineering.
The advocates’ core argument is that, because SRM would be global in effect and some countries are much more powerful than others, it is “not governable in a globally inclusive and just manner within the current international political system.”[118] They therefore oppose the “normalization” of SRM and call on countries, intergovernmental organizations, and others to adopt the proposal’s five elements.
On the day that the academic article was published, the authors also launched a campaign calling for others to endorse the proposal.[128] Their open letter emphasized, in addition to the governance challenges, that SRM’s risks are “poorly understood and can never be fully known” and that its potential would threaten commitments to reducing greenhouse gas emissions.[129] As of March 2024, nearly 500 academics[130] and 60 advocacy organizations[131] have endorsed the proposal. Among the latter is Climate Action Network, itself a coalition of more than 1900 political organizations. The position from Climate Action Network included a footnote that excluded the Environmental Defense Fund and the Natural Resources Defense Council.[132]
Research funding
As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually.[133] Almost all research into SRM has to date consisted of computer modeling or laboratory tests,[134] and there are calls for more research funding as the science is poorly understood.[135][18]:17
Country activities
Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom[136] and Germany,[137]:58 support some SRM research even if they do not see it as a current climate policy option. For example, the German Federal Government does have an explicit position on SRM and stated in 2023 in a strategy document climate foreign policy: "Due to the uncertainties, implications and risks, the German Government is not currently considering solar radiation management (SRM) as a climate policy option". The document also stated: "Nonetheless, in accordance with the precautionary principle we will continue to analyse and assess the extensive scientific, technological, political, social and ethical risks and implications of SRM, in the context of technology-neutral basic research as distinguished from technology development for use at scale".[137]:58
Major academic institutions, including Harvard University, have begun research into SRM,[138] with NOAA alone investing $22 million from 2019 to 2022, though few outdoor tests have been run to date.[139]The Degrees Initiative is a UK registered charity,[140] established to build capacity in developing countries to evaluate SRM.[141]
Some countries, such as the U.S., Germany, China, Finland, Norway, and Japan, as well as the European Union, have funded SRM research.[142]
In 2021, the National Academies of Sciences, Engineering, and Medicine released their consensus study report Recommendations for Solar Geoengineering Research and Research Governance. The report recommended an initial investment into SRM research of $100–200 million over five years.[18]:17
International collaborations
Under the World Climate Research Programme there is a Lighthouse Activity called Research on Climate Intervention as of 2024. This will include research on all possible climate interventions (another term for climate engineering): "large-scale Carbon Dioxide Removal (CDR; also known as Greenhouse Gas Removal, or Negative Emissions Technologies) and Solar Radiation Modification (SRM; also known as Solar Reflection Modification, Albedo Modification, or Radiative Forcing Management)".[143]
Philanthropic and venture capitalist activities
There are also research activities on SRM that are funded by philanthropy. According to Bloomberg News, as of 2024 several American billionaires are funding research into SRM: "A growing number of Silicon Valley founders and investors are backing research into blocking the sun by spraying reflective particles high in the atmosphere or making clouds brighter."[144] The article listed the following billionaires as being notable geoengineering research supporters: Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, Dustin Moskovitz.
SRM research initiatives, or non-profit knowledge hubs, include for example SRM360 which is "supporting an informed, evidence-based discussion of sunlight reflection methods (SRM)".[145] Funding comes from the LAD Climate Fund.[146][147]David Keith, a long-term proponent of SRM,[25][26][27] is one of the members of the advisory board.[148]
Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development".[149] Their funding is "entirely by grants or donations from a number of leading philanthropies focused on addressing climate change": Outlier Projects, Navigation Fund, Astera Institute, Open Philanthropy, Crankstart, Matt Cohler, Richard and Sabine Wood.[149]
Deployment activities
Some startups in the private sector have secured funding for potential SRM deployment. One such example is Make Sunsets,[150] which began launching balloons containing helium and sulfur dioxide. The startup sells cooling credits, claiming that each US$10 credit would offset the warming effect of one ton of carbon dioxide warming for a year.[151] Based in California, Make Sunsets conducted some of its activities in Mexico. In response to these activities, which were conducted without prior notification or consent, the Mexican government announced measures to prohibit SRM experiments within its borders.[152] Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking.[100]
Mexico has announced that it will prohibit "experimental practices with solar geoengineering",[152] although it remains unclear what this policy will include and whether the policy has actually been implemented.
Society and culture
There have been a handful of studies into attitudes to and opinions of SRM. These generally find low levels of awareness, uneasiness with the implementation of SRM, cautious support of research, and a preference for greenhouse gas emissions reduction.[153][154] Although most public opinion studies have polled residents of developed countries, those that have examined residents of developing countries—which tend to be more vulnerable to climate change impacts—find slightly greater levels of support there.[155][156][157]
The largest assessment of public opinion and perception of SRM, which had over 30,000 respondents in 30 countries, found that "Global South publics are significantly more favorable about potential benefits and express greater support for climate-intervention technologies." Though the assessment also found Global South publics had greater concern the technologies could undermine climate-mitigation.[158]
The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.
The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of Jupiter, or from its host star as in the case of the Earth. In the case of Earth, the Sun emits shortwave radiation (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits longwave radiation that is mostly absorbed by greenhouse gases. The absorption of longwave radiation prevents it from reaching space, reducing the rate at which the Earth can cool off.
Global warming potential (GWP) is an index to measure how much infrared thermal radiation a greenhouse gas would absorb over a given time frame after it has been added to the atmosphere. The GWP makes different greenhouse gases comparable with regard to their "effectiveness in causing radiative forcing". It is expressed as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide, which is taken as a reference gas. Therefore, the GWP has a value of 1 for CO2. For other gases it depends on how strongly the gas absorbs infrared thermal radiation, how quickly the gas leaves the atmosphere, and the time frame being considered.
Cloud feedback is a type of climate change feedback, where the overall cloud frequency, height, and the relative fraction of the different types of clouds are altered due to climate change, and these changes then affect the Earth's energy balance. On their own, clouds are already an important part of the climate system, as they consist of water vapor, which acts as a greenhouse gas and so contributes to warming; at the same time, they are bright and reflective of the Sun, which causes cooling. Clouds at low altitudes have a stronger cooling effect, and those at high altitudes have a stronger warming effect. Altogether, clouds make the Earth cooler than it would have been without them.
Sulfur dioxide or sulphur dioxide is the chemical compound with the formula SO 2. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by volcanic activity and is produced as a by-product of copper extraction and the burning of sulfur-bearing fossil fuels.
The sulfate or sulphate ion is a polyatomic anion with the empirical formula SO2−4. Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many are prepared from that acid.
Global dimming is a decline in the amount of sunlight reaching the Earth's surface. It is caused by atmospheric particulate matter, predominantly sulfate aerosols, which are components of air pollution. Global dimming was observed soon after the first systematic measurements of solar irradiance began in the 1950s. This weakening of visible sunlight proceeded at the rate of 4–5% per decade until the 1980s. During these years, air pollution increased due to post-war industrialization. Solar activity did not vary more than the usual during this period.
Radiative forcing is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases and aerosols, and changes in surface albedo and solar irradiance. In more technical terms, it is defined as "the change in the net, downward minus upward, radiative flux due to a change in an external driver of climate change." These external drivers are distinguished from feedbacks and variability that are internal to the climate system, and that further influence the direction and magnitude of imbalance. Radiative forcing on Earth is meaningfully evaluated at the tropopause and at the top of the stratosphere. It is quantified in units of watts per square meter, and often summarized as an average over the total surface area of the globe.
Planetary engineering is the development and application of technology for the purpose of influencing the environment of a planet. Planetary engineering encompasses a variety of methods such as terraforming, seeding, and geoengineering.
Climate engineering is the intentional large-scale alteration of the planetary environment to counteract anthropogenic climate change. The term has been used as an umbrella term for both carbon dioxide removal and solar radiation modification when applied at a planetary scale. However, these two processes have very different characteristics, and are now often discussed separately. Carbon dioxide removal techniques remove carbon dioxide from the atmosphere, and are part of climate change mitigation. Solar radiation modification is the reflection of some sunlight back to space to cool the earth. Some publications include passive radiative cooling as a climate engineering technology. The media tends to also use climate engineering for other technologies such as glacier stabilization, ocean liming, and iron fertilization of oceans. The latter would modify carbon sequestration processes that take place in oceans.
Climate change mitigation (or decarbonisation) is action to limit the greenhouse gases in the atmosphere that cause climate change. Climate change mitigation actions include conserving energy and replacing fossil fuels with clean energy sources. Secondary mitigation strategies include changes to land use and removing carbon dioxide (CO2) from the atmosphere. Current climate change mitigation policies are insufficient as they would still result in global warming of about 2.7 °C by 2100, significantly above the 2015 Paris Agreement's goal of limiting global warming to below 2 °C.
This is a list of climate change topics.
Arctic geoengineering is a type of climate engineering in which polar climate systems are intentionally manipulated to reduce the undesired impacts of climate change. As a proposed solution to climate change, arctic geoengineering is relatively new and has not been implemented on a large scale. It is based on the principle that Arctic albedo plays a significant role in regulating the Earth's temperature and that there are large-scale engineering solutions that can help maintain Earth's hemispheric albedo. According to researchers, projections of sea ice loss, when adjusted to account for recent rapid Arctic shrinkage, indicate that the Arctic will likely be free of summer sea ice sometime between 2059 and 2078. Advocates for Arctic geoengineering believe that climate engineering methods can be used to prevent this from happening.
Marine cloud brightening also known as marine cloud seeding and marine cloud engineering is a proposed solar radiation management technique that would make clouds brighter, reflecting a small fraction of incoming sunlight back into space in order to offset anthropogenic global warming. Along with stratospheric aerosol injection, it is one of the two solar radiation management methods that may most feasibly have a substantial climate impact. The intention is that increasing the Earth's albedo, in combination with greenhouse gas emissions reduction, carbon dioxide removal, and adaptation, would reduce climate change and its risks to people and the environment. If implemented, the cooling effect is expected to be felt rapidly and to be reversible on fairly short time scales. However, technical barriers remain to large-scale marine cloud brightening. There are also risks with such modification of complex climate systems.
Carbon dioxide removal (CDR) is a process in which carbon dioxide is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products. This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated into climate policy, as an element of climate change mitigation strategies. Achieving net zero emissions will require first and foremost deep and sustained cuts in emissions, and then—in addition—the use of CDR. In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).
Stratospheric aerosol injection (SAI) is a proposed method of solar geoengineering to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects. The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] method that it could limit warming to below 1.5 °C (2.7 °F)." However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible, particularly if used in a suboptimal manner.
David W. Keith is a professor in the Department of the Geophysical Sciences at the University of Chicago. He joined the University of Chicago in April 2023. Keith previously served as the Gordon McKay Professor of Applied Physics for Harvard University's Paulson School of Engineering and Applied Sciences (SEAS) and professor of public policy for the Harvard Kennedy School at Harvard University. Early contributions include development of the first atom interferometer and a Fourier-transform spectrometer used by NASA to measure atmospheric temperature and radiation transfer from space.
Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing (RF) of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.
The history of the scientific discovery of climate change began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect was first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change Earth's energy balance and climate. The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier. The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.
References
1 2 3 4 5 Trisos, Christopher H.; Geden, Oliver; Seneviratne, Sonia I.; Sugiyama, Masahiro; van Aalst, Maarten; Bala, Govindasamy; Mach, Katharine J.; Ginzburg, Veronika; de Coninck, Heleen; Patt, Anthony. "Cross-Working Group Box SRM: Solar Radiation Modification"(PDF). Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. pp.221–222. doi:10.1017/9781009325844.004. In Climate Change 2022: Impacts, Adaptation and Vulnerability [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke,V. Möller, A. Okem, B. Rama (eds.)].
1 2 World Meteorological Organization (WMO) (2022). Scientific Assessment of Ozone Depletion: 2022. Geneva: WMO. ISBN978-9914-733-99-0.
↑ Haywood, James; Tilmes, Simone (2022). "Chapter 6: Stratospheric aerosol injection and its potential effect on the stratospheric ozone layer". Scientific assessment of ozone depletion. World Meteorological Organization. pp.325–383.
↑ President’s Science Advisory Committee, Environmental Pollution Panel (1 November 1965). Restoring the Quality of Our Environment. Washington: U.S. Government Printing Office.{{cite book}}: CS1 maint: date and year (link)
↑ Horton, Joshua (2011). "Geoengineering and the myth of unilateralism: pressures and prospects for international cooperation". Stanford J Law Sci Policy (2): 56–69.
1 2 Julia Simon. "Startups want to cool Earth by reflecting sunlight. There are few rules and big risks". NPR. Retrieved 11 June 2024. In the past year, the conversation around solar geoengineering as a climate solution has become more serious, says David Keith ... Suddenly we're getting conversations with senior political leaders and senior people in the environmental world who are starting to think about this and engage with it seriously in a way that just wasn't happening five years ago,
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
