Growing near-term risks from climate change create a need for more and faster research on the most promising and rapid form of climate intervention: solar radiation modification (SRM), its methods, efficacy, and side effects.Solar radiation modification, or SRM, is the scientific term used to describe increasing the reflection of sunlight or the release of infrared radiation from Earth, or reducing inbound solar radiation from space, to counteract global warming.
In light of the rapid escalation in harms to date from climate change, sunlight reflection and other climate interventions are a growing topic of global dialogue. The United Nations and the European Union have called for more scientific research in this area, and the US government has issued a congressionally mandated report describing the research required to assess SRM's potential to reduce climate risks.
The world's safety, security, and the basic needs of the most vulnerable people are at risk from the increasingly catastrophic impacts of climate change. All UN Intergovernmental Panel on Climate Change (IPCC) scenarios for emissions reduction project that the climate system will continue to warm over the next several decades, with impacts and risks growing further. Increased suffering and mortality, economic and environmental loss, and instability are expected. Breaching natural systems' tipping points would risk irreversible change.
Society currently lacks sufficient ability to predict and reduce these near-term risks and impacts. This makes reducing uncertainty in their projection and expanding the portfolio of available responses an urgent matter of global safety and security.
An increase in greenhouse gases (GHGs) in the atmosphere is the primary cause of climate change. The only long-term remedy is to reduce their atmospheric concentrations by eliminating emissions and pursuing the withdrawal of gases already emitted. Due to the long lifespan of GHGs and the difficulty of withdrawing them from the atmosphere, the climate is projected to continue to warm for several decades in all scenarios.
According to scientific assessments, a promising approach to rapidly reducing significant heat energy (warming) in the climate system is to slightly alter the amount of incoming or outgoing sunlight (the phenomena that causes Earth to appear luminous, sometimes called solar radiation modification, or SRM). Proposed approaches generally fall into three categories:
This paper examines SRM approaches, research needs, and potential risks and benefits as a near-term climate response or "intervention" and US and international policy options on SRM. It then makes recommendations on responsibly advancing SRM research, international cooperation, and governance.
Due to greenhouse gasses and warming already present in the climate system, global warming is projected to continue rising for several decades. This is the case in every IPCC scenario for emissions reduction, with recent evidence indicating that these projections underestimate the risks. At the current rate, warming will likely cross the long-term average threshold for dangerous climate change of 1.5°C soon (also called "overshoot") and surpass 2.0°C by 2050.
Warming causes increased weather extremes, which cause natural systems to change. This impairs the sustainability of plants and accelerates the loss of species, which in turn further changes the chemical and physical environment. It also pushes some natural systems nearer to tipping points for substantial, abrupt, and often irreversible changes that cause catastrophic damage, make rapid large-scale contributions of GHGs or warming into the system ("feedbacks"), or both. These changes are hard to predict and underrepresented in current climate models -- which leads to underestimation of catastrophic risks. Some, like large-scale forest burn, Amazon "die-back," and changes in the earth's major oceanic circulation system, give indications of being underway already.
Climate-linked weather extremes and ecosystem changes have wide-ranging impacts on people and human systems including: health and welfare impacts of heat exposure; disease vectors; displacement; economic disruption of industries such as agriculture, transportation, and tourism; infrastructure damage such as buckling railways and failing energy grids; and coastal erosion and flooding. Most economic sectors are affected by climate change, including many with substantial climate-related risks. Climate change is happening significantly faster than human systems' ability to adequately adapt, and human systems also face tipping points from economic and societal shocks.
"This Council [on Foreign Relations] Report builds a compelling case [for increased research on SRM] . . . It is sad that such a report is necessary, but it will be even sadder still if we do not make exploring the potential of sunlight reflection an urgent priority."
Council on Foreign Relations President Richard Haass, https://www.cfr.org/report/reflecting-sunlight-reduce-climate-risk.
The United States is currently experiencing deadly, costly, and disruptive climate-linked disasters. They are occurring in the context of ongoing changes in weather conditions that threaten agriculture, livestock, and fisheries; water supplies; real estate; natural parks and protected areas; forest-adjacent and coastal cities and towns; and military infrastructure and operations.
No country is safe from the large-scale impacts of climate change. Countries like Canada that were thought to be advantaged have been devastated by fires and surface heat. Europe has faced a wide array of impacts including extreme weather conditions, which its infrastructure was not designed to support. Still, the United States and many of its allies are comparatively advantaged economically for climate adaptation and disaster recovery.
Climate change has the greatest impact on the world's most vulnerable people and regions. Many of the world's most climate-vulnerable countries are also its poorest and least developed. Poor and vulnerable populations in every country are the most heavily affected by food and water scarcity, cannot easily migrate, and lack resources for protective adaptation.
Climate change amplifies disparities and creates disruptions in vulnerable populations, increasing domestic and international security threats and risks. Current projections suggest that more than one billion people may be displaced by 2050 -- estimates that may be conservative. Billions of people face food and water insecurity, loss of livelihood, and extreme weather exposure. These circumstances increase the risk of violence and instability, fostering heightened political risks and global security threats.
The interplay between natural system changes and human responses is likely to produce large-scale unanticipated shocks. These could be economic, as extreme storms drive insurers from a market, government backstops are exhausted, and a large property market bottoms, setting off a cascade effect in financial systems and markets. They also could be nature-induced, where the collapse of a major ice sheet creates substantial, rapid sea level rise, devastating large parts of numerous coastal cities across the globe.
As these ongoing processes and escalating pressures and shocks continue, human systems will grow more taxed, less resilient, and less able to respond to subsequent developments and disasters.
"Sunlight reflection . . . could -- in light of current warming trends and risk projections -- make what is likely to be a lengthy transition to a decarbonized world tolerable. . . . It would be vastly preferable for the world to make progress on the science of sunlight reflection today, so that policymakers are prepared to make informed decisions tomorrow, rather than being forced to act out of ignorance on the fly when all other options have failed."
Council on Foreign Relations Special Report No. 93, April 2022, https://cdn.cfr.org/sites/default/files/report_pdf/Patrick-CSR93-web.pdf.
As conditions become increasingly hazardous for human and natural systems, society faces a critical gap in the knowledge and options to evaluate and respond to near-term climate risks. Addressing this gap is now a matter of considerable urgency.
The present climate response portfolio, reducing GHG emissions, is itself jeopardized by climate-linked changes in human and natural systems that worsen the problem. Weather extremes dramatically increase demand for energy for heating and cooling. Food and water scarcity and economic disruption induce other unsustainable practices.
The current policy portfolio lacks viable means for substantially reducing warming in the climate system within a few decades, leaving enormous risk exposure and a substantial portfolio gap that is now critical to fill.
In assessments by scientific bodies in the United Kingdom, United States, and the UN, scientists evaluated promising approaches to countering climate change more rapidly than is possible through emissions reductions alone. The broadest categories of these are:
GGR has many proposed variations, from mechanical filtration (direct-air capture, DAC) to massive-scale cultivation of plants and ocean flora to absorb carbon. GGR could directly reduce the negative effects of climate change by reducing its cause (increased GHG concentrations). However, it comes with significant technological uncertainty (for DAC), currently very high costs, and scaling uncertainties; for approaches that involve leveraging natural systems, there are very likely to be trade-offs. While likely an important climate portfolio element in this century, GGR is not projected to slow warming significantly prior to 2050 in any of the scenarios considered by the IPCC. It is not safe, or even reasonable, to assume that GGR could meaningfully reduce the catastrophic near-term risks of climate change.
In 2017, Wallace Broecker, the oceanographer who coined the phrase "global warming," concluded -- in his last statements on the subject before his death -- that, given the seriousness of the warming, the present response of cutting emissions alone was insufficient. He called for focusing also on SRM research.
All SRM approaches share the benefit of rapidly reducing heat energy or warming in the climate system -- within months or years, once developed and implemented at appropriate scale. Research suggests that stratospheric aerosol injection (SAI) might be the most feasible method in the near-term, as will be seen below, but all methods need further research.
SRM approaches do not address the GHG cause of climate change or offset all of the damage caused by excess GHG in the atmosphere. They do not directly reduce the increase in ocean acidity caused by the increased uptake of CO from the air. However, SRM would address the cause of many of the impacts of climate change: excess heat energy. By directly reducing this heat energy, SRM could significantly reduce climate extremes and impacts, including GHG release from natural systems into the air and ocean, thereby reducing future climate and acidity risk.
Approaches for increasing the reflection of sunlight from the Earth's surface include planting lighter colored crops, painting infrastructure white or using special reflective coating, covering melting snowcaps and ice sheets with reflecting beads or material, and generating foam on the ocean's surface. All surface-based approaches face similar drawbacks: implementing at a scale sufficient to offset significant global warming is not generally considered to be feasible and involves substantial environmental side effects associated with disrupting natural surface-atmosphere interactions.
Approaches for altering sunlight reaching Earth from space include placing a large surface area (billions of square meters) of light-filtering surfaces at the L Lagrange point between the Earth and the Sun and the generation of a filtering layer of dust from the moon. These methods are theoretically estimated to be effective at reducing incoming heat energy but entail engineering in space at a massive scale that is not achievable with today's technology. Even if such techniques were practically feasible, little is known about the engineering challenges associated with this approach, and it is not likely to be something that could be implemented in the near-term (e.g., within thirty years).
Scientific assessments indicate that the most promising approaches for rapidly reducing climate warming are those that increase the reflection of sunlight from the atmosphere by dispersing particles (aerosols) to reflect sunlight or alter the properties of clouds. This phenomenon already is naturally occurring: Earth is cooled by the effects of particles emitted from ocean spray, dust, and ecosystems into the atmosphere. It is cooled in a less continuous way when large volcanoes emit material into the atmosphere.
An unintentional form of atmospheric SRM is also produced by particulates from pollution (e.g., SO from coal burning and vehicle exhaust); these currently cool the climate to a significant but uncertain degree, estimated to be between 0.2℃ and 1.0℃. Reducing these emissions has been a priority for human health, but is also exposing us to the increased warming caused by GHGs.
Proposed approaches to SRM in the atmosphere include:
The approaches differ in terms of duration and localization, their projected efficacy and potential side effects, and the current level of uncertainty in their projection.
SAI involves dispersing particles or gases that turn into particles into one of the outer layers of the atmosphere, the stratosphere, to slightly increase the reflection of sunlight (about 1 to 2 percent). Particles in the lower atmosphere (troposphere) fall to Earth within days, but particles that reach the stratosphere, such as emissions from forceful volcanoes, become entrained and remain for a year or more before falling back to Earth. Observation of the cooling effects of volcanic emissions prompted consideration of SAI.
In 1991, the eruption of Mount Pinatubo released sulfate particles into the stratosphere that measurably cooled the climate by over 0.3℃ for over a year. This cooling was associated with a substantial recovery in Arctic ice mass in the year following the eruption. Such observations have led scientists to express confidence in the effectiveness of SAI for reducing warming and in its environmental safety over short periods of time at levels comparable to that 1991 eruption.
While this confidence has made SAI the primary candidate for rapid climate intervention, what is less certain -- and critical to evaluate -- are its possible negative side effects over longer periods or at higher levels. The most significant side effects include damage to the ozone layer, warming of the stratosphere that changes the dynamics and circulation of the atmosphere, and changes in the insulating cirrus cloud layer that lies just below the stratosphere. Other potential side effects include changes in weather patterns in particular regions and changes in incoming light and solar energy that affect plants. Relatively low concentrations of sulfates in the lower atmosphere makes impacts on human health from direct exposure unlikely. While not considered a major risk, SRM could, however, slightly worsen acid rain in places that already have high concentrations of sulfate.Many of the unintended side effects of SAI with sulfate occur because of its light-absorbing properties and because it becomes acidic when combined with water. Other materials with properties with potential for minimizing these side effects have been proposed for SAI, including calcium carbonate and diamonds. With no history of presence in the stratosphere, however, uncertainties in projecting their effects are challenging to resolve.
Implementing SAI in a way that optimizes benefits and minimizes risks requires understanding and projecting relevant atmospheric processes before dispersing quantities of optimally sized particles (generally small and relatively uniform) into the stratosphere in the manner suggested by these projections and monitoring the atmosphere closely. Delivering large volumes of material to the stratosphere is challenging. While less scientifically informed efforts have undertaken demonstration efforts using balloons, the more prominently considered delivery mechanism is high-altitude aircraft, which may require a new generation of aircraft capable of flying large payloads to the stratosphere. No technologies currently exist for generating optimally small particles in narrow size ranges at the scales required.
Effective evaluation, implementation, and regulatory governance of SAI would require substantially expanded atmospheric observing systems and improved climate and atmosphere models. This implies an investment of billions of dollars in both general climate and specific SAI research to assess whether approaches are viable. Costs would be in the tens of billions of dollars annually for implementation (though it may be cheaper and easier to do it poorly). This level of cost relies on existing infrastructure for stratospheric research and high-performance computing, which together limit effective execution to a small number of nations and other actors with access to advanced aerospace and computing technology.
It is not possible to adequately detect or measure SAI with the current portfolio of satellite and air observations or project its effects, leaving a considerable governance and security gap. However, platforms carrying material are observable, so security concerns are less likely to lie in identifying clandestine activity by rogue actors than in the lack of information on potential climate effects to inform an appropriate response to observed activity by a nation or other actors seeking to address catastrophic climate impacts.
Marine cloud brightening (MCB) would involve dispersing sea salt particles from seawater into targeted regions of low-lying clouds over the ocean to slightly increase the reflection of sunlight from the clouds.The slight increase is approximately 5 to 7 percent. The concept is based on observations of the bright streaks in clouds created by pollution particles in exhaust from ships passing below. Ships' pollution particles attract moisture from clouds, creating more and smaller cloud droplets and increasing the overall surface area of water in the clouds, making them brighter. In some cases, this also causes the clouds to last longer. The initial effect is localized to the area where the particle plume mixes into the clouds, with the effects on clouds continuing for about two to three days, as the affected air is transported downwind. The effect is strongest in marine stratocumulus clouds and under favorable meteorological conditions; in other conditions, effects can be weak or even counterproductive.
The effect of particles (aerosols) on clouds is one of the key drivers of present-day climate change. A cooling effect from cloud brightening is produced accidentally today by pollution emissions (mostly sulfates) from coal-fired power plants, transportation exhaust, smoke, and other sources. Collectively, this aerosol "shield" is estimated to cool the climate by between 0.2°C and 1.0°C, a significant level of cooling with a very high degree of uncertainty. Because aerosols only last for a few days to a week in the lower atmosphere, aerosol cooling stops shortly after aerosol emissions are removed. Clouds are extraordinarily complex, and these effects are one of the greatest areas of uncertainty in climate science.
Meanwhile, that shield is affected by successful actions to decrease particulate pollution -- a drive to stem its negative impacts on health and the environment, e.g., acid rain -- which, in turn, could spur a rapid increase in warming in the coming decade or two. MCB can be thought of as a cleaner, more managed replacement for the cooling currently being provided by pollution particles.
The candidate material for MCB is the tiny salt particle from seawater.The size proposed is 20 to 200 nanometers (nm), or about one one-thousandth the width of a human hair. Salt attracts water, making it an optimum material for forming cloud droplets. It is readily available and already present in the atmosphere in that environment, acting as one of the particles on which cloud droplets form. Sulfates from ship emissions have been evaluated less favorably, but no other candidate material has been seriously considered. Aerosolizing salt (or any solid material) at the tiny size and massive scale required is a significant engineering problem. Some progress on this problem has been made over the last decade in the United States.
In the primary candidate approach to MCB, regions of susceptible (marine stratocumulus) clouds would be targeted for brightening. Early studies suggest that brightening clouds in these areas equivalent to 3 percent or 5 percent of the ocean's surface could offset 2°C or more of global warming. Initial simulations of MCB are consistent in showing that this would produce significant cooling over nearly all of the globe. Technical and field research are needed to determine the number of vessels required to deliver the required material, with estimates varying by an order of magnitude from several thousand to tens of thousands. Autonomous, clean-energy vessels are likely to be most efficient and would require a major manufacturing effort. Aircraft dispersal may be possible, but it is estimated to be far more costly and less efficient for continuous delivery.
Recently, scientists have begun to leverage AI techniques to explore the potential for climate cooling effects from aerosols in other types of clouds or by scattering sunlight in cloudless conditions ("marine sky brightening," or MSB). Early findings point to potential promise for existing ships to contribute as platforms for marine atmosphere SRM.
The localized nature of MCB has led to proposals to explore its potential to reduce climate impacts such as coral bleaching and extreme storms by cooling ocean waters or to restore sea ice by cooling polar regions. A research effort and small demonstration project in Australia is exploring MCB to protect the Great Barrier Reef, but there is scant research on local applications. In general, localized approaches to cooling the ocean and atmosphere are limited by the interconnected nature of these systems: their efficient transport of heat energy circulates more heat into localized areas of cooling, offsetting MCB effects. For this reason, efforts to counter Arctic warming solely through localized MCB may be limited in effectiveness while producing unusual climate anomalies with uncertain risks.Local cooling has difficulty maintaining pace with surrounding warming influences that work to balance heat energy by pushing more heat into the cooler local region. This is particularly problematic for proposals to brighten clouds to cool the Arctic, because most of its warming derives from heat circulating from the equator via ocean currents, and this heat transport would be accelerated if the equator-to-pole temperature gradient were increased by cooling only the Arctic.
Because air and water circulate, changing the properties of clouds and producing cooling in large, isolated areas can have large effects on distant regions, which is why MCB, while implemented locally, could produce cooling globally. However, unlike SAI, this cooling would not be uniform everywhere. MCB is expected to produce variations in cooling that more significantly affect circulation patterns and weather. The greatest side effect risks are associated with these "teleconnection" effects on other regions and the difficulty in predicting and minimizing them. This includes the possibility of significantly reduced precipitation in some areas and other changes in weather patterns.
MCB is a challenging approach to SRM because of uncertainties in the magnitude of its cooling effect, the need to target favorable meteorological conditions, the potential for undesirable weather patterns, and the scale of continuous delivery. However, pollution aerosols produce an effect similar to MCB at global scales today, and research on the processes underlying MCB can help us evaluate the risks of removing this cooling influence -- one of the greatest uncertainties in our projections of near-term climate. Research on cloud-aerosol effects and MCB is therefore an urgent priority.
This uncertain cooling from pollution has come to widespread attention recently with the coincidence of the enforcement of new regulations reducing sulfate emissions from ships and record global and ocean surface temperatures, elevated by an analysis by prominent climate scientist James Hansen substantially raising estimates of near-term climate risk, partly due to accelerated warming from the reduction of pollution aerosols.
Cirrus cloud thinning (CCT) would entail dispersing aerosols to induce precipitation in high-altitude clouds that block some infrared energy from leaving the atmosphere. Under the right conditions, such precipitation could "thin" the cloud, letting more infrared energy out to space.
Conversely, emissions from aircraft can, in the right conditions, produce contrails, some of which can evolve to become cirrus clouds. The warming effect on climate from the influence of these aircraft aerosols is estimated to be equivalent to the warming produced by their GHG (CO) emissions. Researchers have proposed optimizing the altitude and flight path of aircraft to minimize the production of "contrail cirrus" to reduce air traffic warming.
Aerosol effects on cirrus clouds are highly uncertain, with substantial research required for projecting and optimizing these effects. Overall, 3.5 percent of climate warming is attributed to airline emissions, making the potential impact of these changes modest overall but potentially substantial for the industry.
Uncertainties in contrail effects and CCT include the effects of aerosol sizes and composition, the processes controlling cirrus cloud formation and dissipation, and the influence of climate change on these processes. Because CCT and contrail reduction have the potential to rapidly reduce a fraction of climate warming, research in this area -- the most underinvested SRM approach -- is warranted.
"Multiple scientific assessments have concluded that, as a complement to greenhouse gas emissions reductions and CDR, the most rapid way to potentially counter some near-term climate warming is through an important class of [SRM] techniques . . . Given the above findings, we believe that scientific research should be conducted to support the assessment of:
Open letter from physical scientists, including James Hansen, regarding SRM research, February 2022, https://climate-intervention-research-letter.org.
Limited research on SRM approaches and insufficient societal investment in climate basic research, notably in aerosol influences on clouds and climate, have left significant uncertainties about the underlying atmospheric processes tied to the feasibility and risks of SRM. Substantial research is required to inform decision-making, to help design mechanisms for governance, to understand how to best monitor and report on any potential SRM activity, and to identify optimum approaches for maximizing benefits and minimizing risks.
Governance and decision-making on large-scale environmental influences are generally informed by expert scientific assessments that review and report on available science. Such assessments of SRM were separately recommended in 2023 by the UN, the EU, and the US government. Given the limited research to date on SRM, making a robust assessment in a period of five years would require substantial, concerted research efforts.
Driven by a bipartisan direction from the US Congress, the US government's interagency report described the research required to inform scientific assessment of the potential for SRM to reduce near-term climate risks. Needs include direct research on specific SRM approaches via computer modeling, observations, and, importantly, small-scale experimental releases of aerosols to inform models of their impacts at larger scales. Needs also include significant advances in observations, models, and climate basic science in general, including key climate drivers such as clouds' aerosols effects, feedbacks, and tipping points -- areas that have had flat or declining funding for several decades. These research needs were quantified in a recent report that recommended an investment of US$13 billion over five years in climate basic science and observations. The majority of these investments would be dual-purpose: improving the ability both to observe and project the near-term climate and to predict aerosol influences on clouds and climate.
Even incremental steps in SRM research have provided large benefits to understanding the nature of the problem and real versus perceived areas of concern. New analysis is helping to clarify the limitations of proposals to use SRM locally to restore Arctic sea ice. And in 2021, major modeling centers began the first high-fidelity global model simulations of the median pathway for GHG emissions with and without stratospheric SRM (SAI). These simulations indicated that, rather than increasing climate extremes, as many have highlighted as a concern, SRM reduced extremes in most of the world, and significantly reduced disparities in climate impacts.
Recent research indicates that SRM research and implementation are capital-intensive tasks and require advanced technology (e.g., supercomputing, stratospheric aircraft, aerosol generation expertise, and observational systems). SRM research is also surfacing gaps and limitations in climate modeling and risk analysis capabilities and flaws in interpreting their outputs.
SRM research needs to compare real-world impacts of projected climate change against those under various SRM approaches. However, projections of climate change today insufficiently represent important factors, including feedbacks and tipping points. Climate model design has affected such limitations. Also, climate change has yet to equivalently benefit from the analytical techniques used in other complex fields such as portfolio and risk analysis.
No significant technology yet exists for aerosol generation with the characteristics and scale required for SRM, and no platforms have been developed for the efficient delivery of aerosols in the stratosphere or marine environment. While critics fear a rapid "slippery slope" from research to scaled implementation, the greater risk is the delays imposed by the inherent, substantial technology barriers and costs. For example, the first research-grade spray system, which achieved scale and particle requirements, had high energy consumption and other features unsuitable for scaled use, costing several million dollars to develop a single unit. A single small-scale release study (equivalent to one ship's plume) is estimated to cost in the low millions of dollars. Many such studies would be required to evaluate the localized effects of particles on clouds under different conditions. Technology for scaled use would require tens of millions of dollars or more in research and development and billions for the development of platform systems.
Globally, direct investment in SRM physical sciences and technology research amounts to less than $50 million annually and is growing only modestly. Investment in cloud-aerosol influences on climate, feedbacks, and tipping points remains relatively flat against 1990 levels.
This low level of investment is not justifiable. The US Planetary Defense program, for instance, receives $200 million annually to protect against the risk of an approximately 1/20,000 probability catastrophic asteroid strike within fifty years, while the odds of breaching at least one major climate tipping point in that period range from 10 percent to 50 percent. Society's investment in research on the most promising rapid climate interventions needs to be a more reasonably proportioned fraction of the major ongoing investments in energy transition, emissions reductions, and disaster response -- which are each three orders of magnitude (1,000 times) greater.
Improvements in knowledge, data, and capabilities would reduce uncertainties in analysis of whether or not SRM might reduce the worst effects of climate change and under what circumstances SRM approaches are safe. It would greatly reduce the very real risk of the inability to respond to a "ready or not" implementation amid escalating climate crises. It would support evaluation of the high near-term climate and environmental risks of changes in anthropogenic aerosol emissions. In these ways, investment in research on the climate effects of atmospheric aerosols and SRM could be among the highest return-on-investment areas in society.
A number of societal factors influence incentives for research on climate interventions. These are likely to grow as the climate warms.
In physical terms, the potential for SRM to improve human and environmental outcomes and the uncertain climate effects of its pollution analogue warrant its thorough investigation as a science problem and policy matter. Because SRM would not fully offset climate change and may have harmful side effects, extensive research is required to understand SRM in the context of near-term climate change well enough to support effective decision-making.
For human welfare, SRM has the potential to reduce mortality by alleviating warming linked to climate extremes. It also has the potential to reduce direct harms from surface heat and climate-linked disasters and indirect harms that include water and food scarcity, increased tropical disease, displacement, and instability. Understanding the potential for SRM approaches to reduce these effects, and how they would be distributed, requires concerted research.
SRM could also help reduce climate-linked economic losses in many sectors of the global economy. It has the potential to prevent future economic shocks associated with large-scale disruptions and improve global economic welfare relative to a world under currently projected warming.
Rising economic and human costs from climate change increase the potential for political instability and conflict that could be reduced by SRM. For example, reduced warming could decrease natural disasters, displacement, food scarcity, and other factors that contribute to political instability.
SRM also could reduce the political costs of decarbonization policies, such as the 2018 Yellow Vest protests of gas taxes in a wealthy country like France. The risk of this kind of backlash against policy costs can be even greater in countries of the Global South, where there is not the cushion of wealth that France has. Similarly, SRM might smooth the transition in countries with heavy debt loads that "stand to lose economically" amid climate change. As the world experiences more climate extremes, energy demand rises to meet demand for cooling, heating, and other adaptive responses -- and available transition resources fall due to the escalating costs of climate impacts. Where the political costs of carbon mitigation grow higher in a warming world, reducing climate extremes through SRM could reduce political turmoil and resistance to national decarbonization efforts.
Concerns persist about SRM research acting as a disincentive to reducing emissions -- a "moral hazard." This type of concern has been the primary objection and barrier to SRM research. The same concern, until relatively recently, also inhibited research on adaptation, carbon removal, and methane mitigation, delaying research, limiting evidence for decision-making, and even inducing overly optimistic estimates of their potential effectiveness.
These posited moral hazard effects are not well-supported by the evidence to date.In "Presenting Balanced Geoengineering Information Has Little Effect on Mitigation Engagement," Christine Merk and Gernot Wagner also reference earlier studies with varying but mostly similar results, and find that research is as yet sufficient for drawing strong conclusions about the effect of discussion of SRM. In fact, some studies indicate that SRM research may actually motivate support for emissions reduction by sending a disaster signal to society. Evidence generated by SRM research may also reduce magical thinking about its effectiveness in consideration of its very real limitations and risks.
Misaligned financial incentives create a different concern. A handful of early-stage companies have marketed "cooling credits" based on small releases of pollution aerosols into the atmosphere. Unchecked profit incentives could motivate releases at ever-increasing scales -- irrespective of the benefits or significant risks, making such credits as yet a form of fraud. Substantially more research and evidence are needed for governance and regulation of SRM and alignment of economic incentives for its use.
Lack of information can also create counterincentives or societal hazards: supporting the retention of a fixed view, free from pressure to alter it in conjunction with new evidence, reducing incentives to move away from positions that diverge from reality and that increase physical risks for others (i.e., those most at risk from climate change), and delaying research and actions that evidence suggests could help reduce harm. Given the overwhelming value and time-sensitivity of evidence with the potential to help reduce climate change impacts and risks, a new category of societal hazard is needed: "temporal hazard."
Rapidly escalating risks from climate change create major imperatives for increasing scientific understanding of the potential for SRM and other rapid climate interventions to reduce catastrophic impacts to people and natural systems in the next few decades. A proactive, urgent approach to climate intervention research will put the world in a stronger position for reducing climate risk, potentially add new options to the policy portfolio, and promote improved understanding of the benefits and risks of intervention approaches.
In recent years, both opponents and some supporters of research have recommended extensive consultation processes and nonscience influence on research, including many of the kind that are currently associated with long delays in clean energy development. Creating burdens or incentives that slow, delay, or bias SRM research, even where not overtly rejecting it, puts the world in a weak and dangerous position with respect to escalating climate risks: a temporal hazard.
Ignorance regarding climate interventions leaves national policymakers and the international community in a weak position for responding to unilateral use of SRM for climate risk.
Objections to research and related depression of funding have inhibited the field for several decades. This limits our understanding today. As risk impacts grow, the cost of additional delay rises. Projections of the costs of climate change over the next fifty years are in the tens of trillions of dollars, even without full accounting for potential tipping points and systemic shocks."Unchecked climate change would be a major impediment to economic growth during the next 50 years, costing an estimated $178 trillion in net present value terms during the 2021-2070 period." See The Turning Point: A New Economic Climate in the United States, Deloitte Economic Institute, 2022, https://www2.deloitte.com/us/en/pages/about-deloitte/articles/economic-cost-climate-change-turning-point.html. Unknown and hard-to-model factors will likely drive real costs higher than projected, as we are already seeing in the costs of unexpectedly severe and frequent climate-linked disasters.
Catastrophic risks are increased by lack of information on options to reduce actual catastrophic impacts. The scale of the risks underscores the value and urgency of research to reduce them. Should conditions deteriorate to where SRM is required to forestall tipping events, is demanded to counter impacts, or is used in desperation by some countries, the cost of incremental information foregone through delay will have proved enormously high. As nations and communities face ever-growing costs from climate change, the likelihood grows that some actor will, in desperation, try whatever means are currently available to deliver aerosols into the atmosphere -- a very risky proposition with today's level of knowledge and capability.
This is a substantial further temporal hazard that underscores the enormous value of any research undertaken in advance of reactive attempts at SRM and the cost of procedural or other delays to the research needed to inform responses. It increases the value of facilitating rather than inhibiting open research of the kind undertaken by academic and public institutions in democratic countries and made available through intergovernmental bodies. In fact, only thorough research could provide sufficient evidence to rule out SRM approaches or to inform sufficient mechanisms to govern any use of them.
Delay in research also has political costs. Yielding to political or ideological pressures to limit open scientific research into SRM would enhance the potential for misinformation, which stokes fear and increases polarization. It inhibits trust among stakeholders and, more broadly, the public's trust in the integrity of public and scientific comment on SRM and climate policy. Scientific research efforts can themselves reduce the politicization and enhance the ability to address the issue constructively.
"We need to know the risks which [SRM] would bring, and compare them with the risks brought by climate change."
Professor Inés Camilloni of the University of Buenos Aires, December 7, 2022, Tedx Río de la Plata, https://www.youtube.com/watch?v=kVCsFLALGsg.
International cooperation and effective governance mechanisms are vital for cooperatively mitigating the risks of escalating climate change and responding to the possible use of climate interventions by independent actors. For this, it is crucial to ensure that relevant mechanisms facilitate scientific research and assessment and do not impose undue process delay or political influence that would inhibit objective scientific assessment. Mexico's nonspecific ban on SRM activity and the contents of a public "non-use" letter from academics are recent examples of policies likely to inhibit governance-enabling research.
Great power competition adds a troubling dimension to the work of scheduling the research and developing global governance. For example, should the United States move slowly on SRM research, China could decide to fill the void, potentially leading to its dominance over forward use of SRM and to exploiting its advanced position to win considerable global soft power.
With respect to military security, SAI is not easily weaponizable. It is slow-acting, diffuse, and difficult to aim locally. It is a transboundary climate security issue -- but not a significant military one. Other, localized forms of SRM, such as interventions in the lower atmosphere (troposphere), can, however, produce differentiated effects with negative outcomes for some. Today, there is substantial and escalating cloud-seeding activity around the world and interest in cloud brightening for local and regional applications. These activities can produce real and perceived effects far from where they are undertaken and their growth in the world is likely to present a challenging strategic security concern.
Global SRM, particularly diffuse SRM/SAI in the stratosphere, is likely to produce far fewer differential impacts than either localized SRM/MCB or the projected warming. This gives SRM a potential to reduce security tensions relative to other forms of climate action.
Climate intervention is relevant for, and potentially largely governable within, a number of intergovernmental organizations and treaties: the UNFCCC, the Montreal Protocol operating under the Vienna Convention, the World Meteorological Organization (WMO), the UN Environment Programme, the UN Convention on Biological Diversity, and the London Convention that functions through the International Maritime Organization (IMO). Many of these bodies are actively taking steps on scientific assessment, international cooperation, and governance considerations for SRM -- some constructive, and some more obstructive.
Elements of effective governance and decision-making on SRM include:
Internationally, scientific cooperation is advancing with the announcement of a new Lighthouse Activity on Climate Intervention in the World Climate Research Programme, the framework under WMO, the International Science Council (ISC), and the Intergovernmental Oceanographic Commission (IOC) of UNESCO for international cooperation on climate research. Steps toward scientific assessment have advanced in the Montreal Protocol, limited to the stratosphere and focused on atmospheric effects, but advancing rigorous review of the science. UN reports have called for scientific assessment, and language and actions have been brought forward in many UN bodies requesting consideration of SRM.
Some have also called for a moratorium on large-scale experimentation or use of SRM. However, an impactful scale of SRM is not likely to be feasible for many years given the technical barriers, and focusing efforts on negotiating a formal moratorium would tend to inhibit the research and policy efforts needed to develop more effective governance mechanisms.
At the national level, various scientific cooperation structures and steps are visible. US bipartisan efforts on scientific research related to SRM are ongoing, although the subject remains politically delicate. The UK recently announced a program in its new science agency ARIA, and European universities have a variety of efforts in this field.
The picture is less clear elsewhere, though SRM meetings or workshops have occurred in India, Jamaica, Ghana, Pakistan, and Thailand, among other places. China has hosted modest SRM modeling efforts. Scientists in India have previously undertaken modeling research and participated in assessments. In general, SRM research around the world is limited by funding and technical constraints on climate research broadly, an acute problem in low-income and developing countries.
The position of developing and climate-vulnerable countries is complicated, as they stand to gain from successful climate intervention but face intense resource constraints and other acute areas of public concern. Positions vary, from opposition to climate intervention research and activity to requests from highly vulnerable countries such as Micronesia for more scientific information and assessment in treaty bodies on potential climate interventions.
Climate policies have often faced political tension between the global public good they seek to provide and public resistance to their increased costs. SRM, if viable, could be expected to reduce this tension, serving both the global climate policy interest and national socioeconomic interests. It is worth stressing this, in order to support national political will.
The United States, other Western countries, and more recently China have contributed disproportionately to the total GHG concentrations inducing climate change while benefiting from related technological progress that affords them greater capabilities to address it. They arguably bear a greater responsibility to address the climate problem faced by the world.This concept was included in official language by the UNFCCC, which states that developed countries would "take the lead in combating climate change and the adverse effects thereof."
With its substantial climate research capacity and open science policies, the United States and its allies can help to evaluate the potential for climate interventions to reduce climate change-related damages -- for themselves and for the world's most vulnerable peoples. This exploration of alternatives could promote mutuality and reduce animosities and disputes over compensation, fostering healthier global relationships.
For the United States, such a program, undertaken openly and supportively of other countries' interests, could also enhance its diplomatic influence and soft power. The West currently supplies the majority of data and science that informs international assessment and governance of the climate and environment. Government and academic research efforts in these countries promote open, transparent access by stakeholders around the world. By leading research on near-term climate risks and SRM, it can support open science, international scientific cooperation, international scientific assessment, and equitable, science-based international governance in forums like the Montreal Protocol, where all countries of the world have a standing. Without this leadership role, research and development will move forward in closed communities -- such as within defense and commercial sectors or autocratic regimes -- with less transparency and equitability and heightened risks.
The interests of vulnerable and developing countries warrant special consideration. They are often on the front line of climate change impacts, despite their limited contribution to the increased concentration of GHGs in the atmosphere. The severe near-term repercussions of global warming, such as droughts, floods, and cyclones, disproportionately affect these regions. Climate interventions -- and notably SRM -- might be the most promising way, or only way, to provide near-term relief, avoid projected regional instability, and prevent overwhelming population displacements in coming decades.
"While many hazards may now be inevitable, for communities living in already vulnerable conditions these interventions as suggested will help traverse the breakthroughs needed in the regions of Africa and Asia where there's rising concerns of food security and migration, and displacement of people."
Joshua Amponsem, founder, Green Africa Youth Organization; current strategy director, Youth Climate Justice Fund; and former climate lead, UN Youth Envoy's Office, "Climate Intervention: An Option for Global South to Reduce Near-term Climate Risk?" World Economic Forum, November 2022, https://www.weforum.org/agenda/2022/11/will-climate-intervention-sustain-the-global-monopoly-order-or-define-moment-for-global-south-s-ascendancy/.
Climate interventions could promote improved well-being and economic security in the developing world. They could provide intergenerational benefits, promoting safety, welfare, and prosperity and preserving natural systems for youth and upcoming generations. By slowing down the adverse effects of climate change, climate interventions could offer humanity precious time: time for reducing emissions, for technology investments to accelerate decarbonization, for building adaptive capacity, and for pursuing other vital societal goals, from human rights to sustainable development, without disrupting the climate program.
Overall, consideration of climate interventions as part of the climate portfolio of responses provides an avenue to manage the potentially catastrophic risks of climate change more successfully and comprehensively. Research on climate intervention, as it grows, has indicated that it may actually help manage these risks more equitably, peacefully, and securely. National and global interests would align.
The immense security and welfare risks of near-term climate change warrant concerted research on climate interventions and development of international scientific assessment, cooperation, governance, and decision-making mechanisms for them. Immediate steps should include:
Climate change poses enormous threats to global welfare and security. These threats will grow rapidly in the coming decades -- but society's ability to respond is inadequate, leaving the world exposed to globally catastrophic risks. The present climate policy portfolio leaves a critical gap: it does not mitigate near- and mid-term risks and impacts, including potentially devastating tipping events in human and natural systems.
Research on the potential for climate interventions, including SRM, to reduce climate risks as well as substantial investments in climate observations and basic science are now a critical priority for the world. Global cooperation on this research would support equitable and effective decision-making, reduce tensions, and promote the common good. Applying the ensuing knowledge, cooperatively and equitably, to promote the safety of the world's people and natural systems is now essential for us all.
Kelly Wanser is the executive director of SilverLining.
Ira Straus is a senior advisor at the Atlantic Council's Scowcroft Center for Strategy and Security and a councilor of the Atlantic Council.
Image: Earth view from a balloon in the stratosphere on 19 June 2013. Photo: Patrick Cullis, NOAA / CIRES