Scientific studies, based on evidence from climate models, have consistently shown that some forms of SRM could reduce global warming and many effects of climate change.[3][4][5] 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. SRM would therefore pose environmental risks, as would a warmed world without SRM. Confidence in the current projections of how SRM would affect regional climate and ecosystems is low.[2] Furthermore, a suboptimal implementation of SRM--such as starting or stopping suddenly, or intervening too strongly in the Earth's energy balance--would increase environmental risks.
SRM presents political, social and ethical challenges. A common concern is that attention to it would lessen efforts to reduce greenhouse gas emissions. Because some SRM approaches appear to be technically feasible and have relatively low direct financial costs, some countries could be capable of deploying it on their own, raising questions of international relations.[6] Although some existing applicable governance instruments and institutions are applicable, there is currently no formal international framework designed to regulate SRM. Issues of governance and effectiveness are intertwined, as poorly governed use of SRM might lead to its suboptimal implementation.[7] For these reasons and more, SRM is often a contested topic among environmentalists.
In the face of ongoing global warming and insufficient reductions to greenhouse gas emissions, SRM receives increasing attention. Climate scientists and other experts from around the world research and publish academic articles, while more nongovernmental and intergovernmental organizations, as well as national governments, are examining and developing views.
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, rising global temperatures, and worsening climate impacts. 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.[9] This imbalance, or "radiative forcing," means that the Earth absorbs more energy than it emits, causing global temperatures to rise[10] 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 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.[11][12][13] 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.[14]
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.[15]
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.[16]
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".[2]
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.[21][22] Along with carbon dioxide removal, SRM was discussed jointly as geoengineering in a 1992 climate change report from the US National Academies.[23]
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.[24][25][26] He has also appealed for international standards of governance and oversight for how such research might proceed.[27]
The first modeled results of SRM were published in 2000.[28] 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."[29]
Atmospheric methods
The atmospheric methods for SRM include stratospheric aerosol injection (SAI), marine cloud brightening (MCB) and cirrus cloud thinning (CCT).[1]:348
For stratospheric aerosol injection (SAI) small particles would be injected into the upper atmosphere to cool the planet with both global dimming and increased albedo. Of all the proposed SRM methods, SAI has received the most sustained attention: The IPCC concluded in 2018 that SAI "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5°C."[1]:350 This technique would mimic a cooling phenomenon that occurs naturally by the eruption of volcanoes.[30] 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.[31][32][33][34][35] Delivery by custom aircraft appears most feasible, with artillery and balloons sometimes discussed.[36][37][38]
This technique could give much more than 3.7W/m2 of globally averaged negative forcing,[39] which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.
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."[5]
A potential disadvantage of SAI is its potential to delay the regeneration of the stratospheric ozone layer (dependent on assumptions about which aerosols would be used to do the cooling). [40][41]
Marine cloud brightening (MCB) would involve spraying fine sea water to whiten clouds and thus increase cloud reflectivity. It would work by "seeding to promote nucleation, reducing optical thickness and cloud lifetime, to allow more outgoing longwave radiation to escape into space".[1]:348
The extra condensation nuclei created by the spray would change the size distribution of the drops in existing clouds to make them whiter.[42] 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.[43] 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,[39] which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.
Cirrus cloud thinning (CCT) involves "seeding to promote nucleation, reducing optical thickness and cloud lifetime, to allow more outgoing longwave radiation to escape into space."[1]:348Natural 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.[17]
The IPCC describes ground-based albedo modification as "whitening roofs, changes in land use management (e.g., no-till farming), change of albedo at a larger scale (covering glaciers or deserts with reflective sheeting and changes in ocean albedo)."[1]:348 It is a method of enhancing Earth's albedo, i.e. the ability to reflect the visible, infrared, and ultraviolet wavelengths of the Sun, reducing heat transfer to the surface.
Znamya-2, which was deployed as part of a series of orbital space mirror experiments in the 1990s by Russia.
Space-based approaches could be advantageous compared to stratospheric aerosol injection because they do not interfere directly with the biosphere and ecosystems.[44] However, space-based approaches would cost about 1000 times more than their terrestrial alternatives.[45] In 2022, the IPCC Sixth Assessment Report discussed SAI, MCB, CCT and even attempts to alter albedo on the ground or in the ocean but did not address space-based approaches.[2]
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.[32] 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.[46] and this was followed by other books in 1929, 1957 and 1978.[47][48][49] 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.[15] 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.[15]
When summarizing these spaced-based 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.[50][15]
A study in 2020 looked at the cost of SAI through to the year 2100. It found that relative to other climate interventions and solutions, SAI remains inexpensive. However, at about $18 billion per year per degree Celsius of warming avoided (in 2020 USD), a solar geoengineering program with substantial climate impact would lie well beyond the financial reach of individuals, small states, or other non-state potential rogue actors.[51] The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at $5–10billion US dollars.[51]
SAI is expected to have low direct financial costs of implementation,[52] relative to the expected costs of both unabated climate change and aggressive mitigation.
Aspects of regional scales and seasonal timescales
Modelling studies have consistently concluded that moderate SRM use would significantly reduce many of the impacts of global warming —for example, average and extreme temperature, water availability, and cyclone intensity [53] 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]
However, SRM has significant potential risks and uncertainties. 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."[54]:69[3]
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."[4]:14 The report also said: "An operational SRM deployment would introduce new risks to people and ecosystems".[4]: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 might be able to compensate for some of this heterogeneity by changing and optimizing injection rates by latitude and season.[55][56]
Impacts on precipitation
Models indicate that SRM would reverse warming-induced changes to precipitation more rapidly than changes to temperature.[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.[57] Likewise, the intensity of tropical monsoons is increased by climate change and decreased by SRM.[58]
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 misleading claims that SRM "would disrupt the Asian and African summer monsoons", something that has been repeatedly challenged by climate scientists who study SRM. Ultimately the impact would depend on the particular implementation regime.[citation needed]
Deployment length
A modeling study in 2023 showed that the range of possible deployment timescales is vast even if pathways start at a similar point at the beginning of SRM deployment.[59] This is because the evolution of mitigation under SRM, the availability of carbon removal technologies and the effects of climate reversibility are not precisely known. Since these effects will be mostly uncertain at the time of SRM initialization, a precedent prediction of deployment length seems unlikely, with possibilities ranging from decades to multiple centuries. This is a knowledge gap that must be considered before any SRM proposal is seriously considered.[59]
For all realizations that follow current NDC (nationally determined contributions) median 2100 warming projections (2.4 ∘C), none deploy SRM for a shorter period than 100 years.[59]
The direct climatic effects of SRM are reversible within short timescales.[16] Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained.[60]
Termination shock
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.[61] 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 risk might be manageable because it would be in states' interest to resume any terminated deployment, and maintaining back-up SRM infrastructure would increase the resilience of an SRM system.[62][63]
Failure to reduce ocean acidification
Change in sea surface pH caused by anthropogenicCO2 between the 1700s and the 1990s. This ocean acidification will still be a major problem unless atmospheric CO2 is reduced, and subsurface acidification will persist even afterwards.
SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification.[64] 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[65] and solar energy.[66] 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.[67] As a result, deployment of atmospheric SRM would affect the growth rates of phytoplankton, trees, and crops [68] between now and the end of the century.[69] Uniformly reduced net shortwave radiation would affect solar photovoltaics, but the real-world impact is complex and is affected by temperature and cloud fraction, and interacts with demand-side factors (especially heating and cooling load).
Uncertainty regarding effects
Much uncertainty remains about SRM's likely effects.[64] 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.[70] Volcanoes are an imperfect analogue as they release the material in the stratosphere in a single pulse, as opposed to sustained injection.[71]
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 (in combination with raised carbon dioxide levels) has been predicted by some studies due to the combination of more diffuse light and carbon dioxide's fertilization effect.[72] Other studies suggest that SRM would have little net effect on agriculture.[73]
There have also been proposals to focus SRM at the poles, in order to combat sea level rise[74] 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.[2]
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.[75] 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.[76] 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.
Some political scientists have argued that the current international political system is inadequate for the fair and inclusive governance of SRM deployment on a global scale.[77] Other researchers have suggested that building a global agreement on SRM deployment would be very difficult, and speculated whether power blocs might emerge.[78] However, there may be significant incentives for states to cooperate in choosing a specific SRM policy, which make unilateral deployment unlikely.[79]
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 generally simulate 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.[80] Likewise, blame for actual or perceived local negative impacts from SRM could be a source of international tensions.[81]
There is a risk that countries may start using SRM without proper research and evaluation. 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.[82] 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".[83] Others argue that states will insist on maintaining control of SRM.[84]
A common concern is that the use of SRM, or even the idea, might reduce the political and social impetus for climate change mitigation.[85] This has often been called a potential "moral hazard", although such language is not precise. However, some engagement work has suggested that SRM may in fact increase the likelihood of emissions reduction because the pursuit of such a risky approach underlines the seriousness of global warming.[86][87][88][89]
Support for SRM research
Support for SRM research has come from scientists, NGOs, international organisations, and governments. The leading argument in support of SRM research is that there are large and immediate risks from climate change, and SRM is the only known way to quickly stop (or reverse) warming. 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.[90][91]
Scientific and other large organizations that have called for further research on SRM include:
In the European Union: the Group of Chief Scientific Advisors[100] (the report from 2024 specifically examines "how the EU can address the risks and opportunities associated with research on solar radiation modification and with its potential deployment".)
Two sign-on letters in 2023 from scientists and other experts have called for expanded "responsible SRM research". One wants to "objectively evaluate the potential for SRM to reduce climate risks and impacts, to understand and minimize the risks of SRM approaches, and to identify the information required for governance". It was endorsed by "more than 110 physical and biological scientists studying climate and climate impacts about the role of physical sciences research."[101] Another called for "balance in research and assessment of solar radiation modification" and was endorsed by about 150 experts, mostly scientists.[102]
Some nongovernmental organizations actively support SRM research and governance dialogues. The Degrees Initiative is a UK registered charity, established to build capacity in developing countries to evaluate SRM.[103] It works toward "changing the global environment in which SRM is evaluated, ensuring informed and confident representation from developing countries."[103]
Operaatio Arktis is a Finnish youth climate organisation that supports research into solar radiation modification alongside mitigation and carbon sequestration as a potential means to preserve polar ice caps and prevent tipping points.[104]
SilverLining is an American organization that advances SRM research as part of "climate interventions to reduce near-term climate risks and impacts."[105] It is funded by "philanthropic foundations and individual donors focused on climate change".[105][106] One of their funders is Quadrature Climate Foundation which "plans to provide $40 million for work in this field over the next three years" (as of 2024).[107]
The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM, in particular by engaging civil society organisations in the Global South and supporting a broader conversation on SRM governance.[108] The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal,[109] although it ended operations in 2023.
The Climate Overshoot Commission is a group of global, eminent, and independent figures.[110] It investigated and developed a comprehensive strategy to reduce climate risks. The Commission recommended additional research on SRM alongside a moratorium on deployment and large-scale outdoor experiments. It also concluded that "governance of SRM research should be expanded".[111]:15
Campaigners have claimed that the fossil fuels lobby advocates for SRM research.[112][113] However, researchers have pointed out the lack of evidence in support of this claim.[114]
Opposition to deployment and research
Opposition to SRM has come from various academics and NGOs.[115] Common concerns are that SRM could lessen climate change mitigation efforts, that SRM is ultimately ungovernable, and that SRM would cause tensions, or even conflict, between nations. 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.[116]
In 2021, researchers at Harvard put plans for an SRM-related field experiment on hold after Indigenous Sámi people objected to the test taking place in their homeland.[120][121] 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."
In 2022, a scientific journal Wiley Interdisciplinary Reviews: Climate Change published "Solar geoengineering: The case for an international non-use agreement". The authors argued that geoengineering cannot be used in a responsible manner under the current system of international relations, so the only option is for as many governments as possible to make a commitment they would neither deploy such technologies, nor fund research into them, grant intellectual property rights or host such experiments when conducted by third parties.[115] In 2024, the same journal had published a commentary from a different group of scientists, which criticized the proposed non-use agreement and argued for a more permissive research framework.[122]. The academic paper launched a campaign which, as of December 2024, has been supported by nearly 540 academics[123] and 60 advocacy organizations[124] have endorsed the proposal.
By 2024, U.S. government agencies were allegedly 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]
Research funding
As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually.[126] Almost all research into SRM has to date consisted of computer modeling or laboratory tests,[127] and there are calls for more research funding as the science is poorly understood.[128][17]:17
A study from 2022 investigated where the funding for SRM research came from globally. The authors concluded "the primary funders of [SRM] research do not emanate from fossil capital" and that there are "close ties to mostly US financial and technological capital as well as a number of billionaire philanthropists".[129]
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)".[98]
Government funding
Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom[130] and Germany,[131]: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".[131]:58
Some countries, such as the U.S., U.K., Argentina, Germany, China, Finland, Norway, and Japan, as well as the European Union, have funded SRM research.[132]NOAA in the United States has spent $22 million USD from 2019 to 2022, with only a few outdoor tests carried out to date.[133] As of 2024, NOAA provides about $11 million USD a year through their solar geoengineering research program.[107]
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.[17]:17
In late 2024, the Advanced Research and Invention Agency, a British funding agency, announced that research funds totaling 57 million pounds (about $75 million USD) will be made available to support projects which explore "Climate Cooling".[134] This includes outdoor experiments: "This programme aims to answer fundamental questions as to the practicality, measurability, controllability and possible (side-)effects of such approaches through indoor and (where necessary) small, controlled, outdoor experiments."[135] Successful applicants will be announced in 2025.[136]
Non-profits and philanthropic support for research
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."[137] 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)".[138] Funding comes from the LAD Climate Fund.[139][140]David Keith, a long-term proponent of SRM research,[24][25][26] is one of the members of the advisory board.[141]
Another example is Reflective, which is "a philanthropically-funded initiative focused on sunlight reflection research and technology development".[142] 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.[142]
Deployment activities
Make Sunsets
At least one startup in the private sector has tried to sell "cooling credits" for SRM activities. Make Sunsets[143] launches balloons containing helium and sulfur dioxide. The company sells cooling credits, making the contested claim that each US$10 credit would offset the warming effect of one ton of carbon dioxide warming for a year.[144] Based in California, Make Sunsets conducted some of its first 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, although it is unclear whether this became actual policy.[145] Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking.[146]
Society and culture
Studies into opinions about SRM have found low levels of awareness, uneasiness with the implementation of SRM, cautious support of research, and a preference for greenhouse gas emissions reduction.[147][148] 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.[149][150][151]
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.[152]
The scientific community has been investigating the causes of climate change for decades. After thousands of studies, the scientific consensus is that 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 scientific principle underlying current climate change is the greenhouse effect, which provides that greenhouse gases pass sunlight that heats the earth, but trap some of the resulting heat that radiates from the planet's surface. Large amounts of greenhouse gases such as carbon dioxide and methane have been released into the atmosphere through burning of fossil fuels since the industrial revolution. Indirect emissions from land use change, emissions of other greenhouse gases such as nitrous oxide, and increased concentrations of water vapor in the atmosphere, also contribute to climate change.
Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide (CO2). It is expressed as a multiple of warming caused by the same mass of carbon dioxide (CO2). Therefore, by definition CO2 has a GWP of 1. For other gases it depends on how strongly the gas absorbs thermal radiation, how quickly the gas leaves the atmosphere, and the time frame considered.
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.
Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth’s climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The current rise in global temperatures is driven by human activities, especially fossil fuel burning since the Industrial Revolution. Fossil fuel use, deforestation, and some agricultural and industrial practices release greenhouse gases. These gases absorb some of the heat that the Earth radiates after it warms from sunlight, warming the lower atmosphere. Carbon dioxide, the primary gas driving global warming, has increased in concentration by about 50% since the pre-industrial era to levels not seen for millions of years.
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 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, 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.
The Special Report on Global Warming of 1.5°C (SR15) was published by the Intergovernmental Panel on Climate Change (IPCC) on 8 October 2018. The report, approved in Incheon, South Korea, includes over 6,000 scientific references, and was prepared by 91 authors from 40 countries. In December 2015, the 2015 United Nations Climate Change Conference called for the report. The report was delivered at the United Nations' 48th session of the IPCC to "deliver the authoritative, scientific guide for governments" to deal with climate change. Its key finding is that meeting a 1.5 °C (2.7 °F) target is possible but would require "deep emissions reductions" and "rapid, far-reaching and unprecedented changes in all aspects of society". Furthermore, the report finds that "limiting global warming to 1.5 °C compared with 2 °C would reduce challenging impacts on ecosystems, human health and well-being" and that a 2 °C temperature increase would exacerbate extreme weather, rising sea levels and diminishing Arctic sea ice, coral bleaching, and loss of ecosystems, among other impacts.
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.)].
↑ 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)
↑ 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.
↑ Horton, Joshua (2011). "Geoengineering and the myth of unilateralism: pressures and prospects for international cooperation". Stanford J Law Sci Policy (2): 56–69.
↑ European Commission. Directorate General for Research and Innovation.; European Commission. Group of Chief Scientific Advisors. (2024). Solar radiation modification. LU: Publications Office. doi:10.2777/391614.
↑ 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,
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