Article

What Is SRM?

Sunlight reflection methods or solar radiation modification (SRM) describes a set of ideas to counteract global warming by reflecting a small fraction of incoming sunlight back to space. Such interventions could play a role, alongside emissions cuts and other climate policies, in managing climate change risks, but they raise serious challenges and difficult questions.

Key takeaways

  • The Earth is warming due to the heat-trapping effect of greenhouse gases; SRM could limit or slow some warming by increasing the amount of sunlight the Earth reflects.
  • Some SRM approaches appear technically feasible and might reduce the overall impacts of climate change if used alongside emissions cuts and other efforts.
  • How SRM is used would be crucial: if it undermines emissions cuts or is deployed unwisely, it might lead to worse outcomes than if it were never deployed.

The Earth’s atmosphere has a natural greenhouse effect – it permits light to pass through, but it traps heat – keeping it relatively warm. During the 10,000 years before the industrial revolution, the energy coming in from the sun was roughly balanced by the heat the Earth emitted to space, so the Earth’s temperature was relatively stable.

Human actions have disturbed this energy balance, but they could restore it. The build-up of carbon dioxide (CO2) and other greenhouse gases in the atmosphere, largely a result of the burning of fossil fuels, is trapping more heat and causing the earth to warm. Eliminating net emissions of CO2 would bring global warming to a halt but achieving this could take several decades. However, global warming could be halted earlier if the amount of light the Earth reflects back to space could be increased.

THE EARTH’S ENERGY BUDGET

When sunlight reaches the earth, it is either reflected to space or absorbed and re-emitted as heat. Emissions of greenhouse gases, like carbon dioxide, trap heat, causing warming.

Heat reabsorbed by the atmosphere

Earth

Sunlight reflected by Earth

SRM

Sunlight reflection methods (SRM) aim to reflect some sunlight to offset that warming.

Source: SRM360

Sunlight reflection methods 

Sunlight reflection methods or solar radiation modification (SRM) describes a set of ideas to increase the amount of sunlight that the Earth reflects to space. This idea goes by other names – solar geoengineering, solar climate engineering, and solar climate intervention. All SRM approaches would have the same fundamental aim: to offset some of the warming effects of greenhouse gases by rebalancing the Earth’s energy budget.

The Earth reflects around 30% of the light that reaches it. If this could be increased by just one percentage point overall, this could offset around 1°C of global warming.1 This corresponds roughly to the difference between the most ambitious goal of the Paris Agreement – limiting warming to 1.5°C – and the projected warming the world will see this century if countries follow through on their current emissions pledges.

Is it possible to make the Earth 1% more reflective? There are a few SRM ideas, but two stand out as having the potential for a large cooling effect and being technically feasible.2 

Stratospheric aerosol injection (SAI) would involve releasing tiny particles into the stratosphere a stable layer of the atmosphere above the tops of most clouds to create a thin, global layer that would reflect a small fraction of incoming light. There is little doubt that adding these particles would reflect light and could lower the global temperature,3 and specifically designed high-altitude jets would offer a practical means of deploying this on a global scale.4 Hundreds of modelling studies have been published on this idea, evaluating how it could change the climate and assessing its potential environmental impacts. 

Marine cloud brightening (MCB) would involve spraying sea water from ships to stimulate the formation of cloud droplets to make marine clouds more reflective. While there are uncertainties around this idea’s effectiveness and its regional climate effects, it might offer a way to produce local, regional or even global cooling if applied over a large enough area.5 In addition to many modelling studies, several teams across the world are looking to develop and test the equipment needed for MCB and initial field experiments have been conducted.6

Could SRM counteract all the effects of rising CO2 concentrations?

Were SRM used to lower the global temperature, it would not replace the need for eliminating net CO2 emissions or other climate policies.

First, CO2 has direct effects on the environment that SRM methods cannot address. Most importantly, the build-up of CO2 is acidifying the ocean, threatening marine species, especially shell-forming creatures and corals.

Second, unlike other pollutants and greenhouse gases, CO2 does not break down in the environment. Instead, most of the atmospheric CO2 will slowly be absorbed by the ocean over the course of hundreds to thousands of years.7 In contrast, the particles released by SAI would persist for only a few years, and those of MCB for only a few days. This means that a significant break in large-scale SRM deployment would cause a rapid and dangerous return of the warming offset by SRM.8

Third, while SRM could halt or even reverse global temperature increase, it could not counteract all the climate effects of greenhouse gases. Notably, no SRM approach could fully counteract changes to rainfall, and some regions are likely to see greater rainfall changes with SRM than they would have under climate change alone.9

Finally, some SRM ideas would have significant side effects. For example, SAI could add to acid rain and delay the recovery of the ozone hole.

An overview of sunlight reflection methods 

Sunlight reflection methods (SRM) are hypothetical approaches to lower global temperatures by increasing the amount of sunlight reflected to space.

Sunlight

Space-based SRM

Reflective material between the earth and sun could scatter light, but delivery would be extremely costly.

Stratospheric aerosol injection (SAI)

Tiny particles released in the stratosphere could reflect a small fraction of sunlight, producing a global cooling.

Cirrus cloud

thinning (CCT)

Seeding might thin cirrus clouds, allowing more heat to escape to space.

Heat

Surface albedo modification

Brighter surfaces could reflect more sunlight, but global cooling potential is limited.

Marine cloud brightening (MCB)

Sea-salt particles could be sprayed from ships to enhance the reflectivity of low-lying clouds.

Space-based SRM

Reflective material between the earth and sun could scatter light, but delivery would be extremely costly.

Sunlight

Stratospheric aerosol injection (SAI)

Tiny particles released in the stratosphere could reflect a small fraction of sunlight, producing a global cooling.

Heat

Cirrus cloud

thinning (CCT)

Seeding might thin cirrus clouds, allowing more heat to escape to space.

Surface albedo modification

Brighter surfaces could reflect more sunlight, but global cooling potential is limited.

Marine cloud brightening (MCB)

Sea-salt particles could be sprayed from ships to enhance the reflectivity of low-lying clouds.

Sunlight

Heat

Marine cloud brightening (MCB)

Sea-salt particles could be sprayed from ships to enhance the reflectivity of low-lying clouds.

Space-based SRM

Reflective material between the earth and sun could scatter light, but delivery would be extremely costly.

Surface albedo modification

Brighter surfaces could reflect more sunlight, but global cooling potential is limited.

Cirrus cloud

thinning (CCT)

Seeding might thin cirrus clouds, allowing more heat to escape to space.

Stratospheric aerosol injection (SAI)

Tiny particles released in the stratosphere could reflect a small fraction of sunlight, producing a global cooling.

Sunlight

Heat

Marine cloud brightening (MCB)

Sea-salt particles could be sprayed from ships to enhance the reflectivity of low-lying clouds.

Space-based SRM

Reflective material between the earth and sun could scatter light, but delivery would be extremely costly.

Surface albedo modification

Brighter surfaces could reflect more sunlight, but global cooling potential is limited.

Cirrus cloud

thinning (CCT)

Seeding might thin cirrus clouds, allowing more heat to escape to space.

Stratospheric aerosol injection (SAI)

Tiny particles released in the stratosphere could reflect a small fraction of sunlight, producing a global cooling.

Source: SRM360.org

A risky response to the risks of climate change 

How the risks of a world with SRM would compare to the risks of climate change would depend on how it is implemented and would differ from one region to another. SRM could not offset all the effects of climate change, and it could have substantial side effects, but many climate hazards are closely tied to temperature and so extreme heat, extreme rain and the melting of ice would all be reduced.10 Used wisely, SRM may be able to substantially reduce climate risks overall, though it may also raise some risks in some places.10

However, would it be used wisely? Some nations would be able to develop and deploy SRM on their own. Might they deploy it further their own interests at the expense of others? Might some nations or corporations promote SRM as a cheap solution to climate change, undermining efforts to cut CO2 emissions? Could SRM be maintained for the decades or centuries needed to keep the warming it offset at bay?

At present, international law has little to say about SRM. It is rarely on the agenda in international discussions, and there have been only limited international efforts to assess the science of SRM. However, with global temperatures rapidly rising and research into this topic rapidly growing, international decision makers may soon need to start making decisions on this difficult issue.

Should SRM be integrated into climate policy?

Human emissions of greenhouse gases have pushed the Earth’s energy balance far out of equilibrium, ending 10,000 years of relative climate stability and ushering in an era of rapid warming. Eliminating these emissions would bring global warming to a halt, but not before great harm is done to societies and ecosystems around the world. 

SRM offers the potential to end this warming quickly and – if used wisely and as a complement to emissions cuts – it might greatly reduce these harms. However, SRM brings its own additional risks and challenges, and it raises some very difficult questions. To reach a point where informed decisions can be made about SRM at the international level, more research will be needed as well as more open discussions of the difficult questions it raises. 

Open questions

  • How would the risks of a world with SRM compare to a world without?
  • Could SRM undermine efforts to cut emissions, or could it be a useful complement to these efforts?
  • Could SRM help limit climate-related disruptions to international relations, or could nations come into conflict over SRM?

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Endnotes

  1. Visioni D, MacMartin DG, Kravitz B, et al. (2021). Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations. Atmospheric Chemistry and Physics. 21(13):10039–63. https://doi.org/10.5194/acp-21-10039-2021
  2. Space-based SRM could produce a large cooling but would be enormously costly and may be many decades away. Cirrus cloud thinning may have potential, but there are large uncertainties and no concrete suggestions for how to achieve it in practice. Surface albedo modification ideas generally operate at too small a scale to have much of a global impact.
  3. Kravitz B, MacMartin (2020). Uncertainty and the basis for confidence in solar geoengineering research. Nature Reviews Earth & Environment. 1(1):64-75. https://doi.org/10.1038/s43017-019-0004-7
  4. Smith W. (2020). The cost of stratospheric aerosol injection through 2100. Environ Res Lett. 15(11):114004. https://doi.org/10.1088/1748-9326/aba7e7
  5. Feingold G, Ghate VP, Russell LM, et al. (2024) Physical science research needed to evaluate the viability and risks of marine cloud brightening. Science Advances. 10(12):eadi https://doi.org/10.1126/sciadv
  6. For example, projects involving the University of Washington and Southern Cross University.
  7. Archer D, Eby M, Brovkin V, et al. (2009) Atmospheric lifetime of fossil fuel carbon dioxide. Annual review of earth and planetary sciences. 37(1):117-34. https://doi.org/10.1146/annurev.earth.031208.100206
  8. Parker A, Irvine P. (2018) The risk of termination shock from solar geoengineering. Earth’s Future. 6(3):456-67. https://doi.org/10.1002/2017EF000735
  9. Ricke K, Wan JS, Saenger M, et al. (2023). Hydrological Consequences of Solar Geoengineering. Annual Review of Earth and Planetary Sciences. 51(Volume 51, 2023):447–70. https://doi.org/10.1146/annurev-earth-031920-083456
  10. Irvine P, Emanuel K, He J, et al. (2019) Halving warming with idealized solar geoengineering moderates key climate hazards. Nat Clim Chang. 9(4):295–9. https://doi.org/10.1038/s41558-019-0398-8

Citation

Pete Irvine, Kimberly Samuels-Crow (2024) - "What Is SRM?" Published online at SRM360.org. Retrieved from: 'http://srm360.org/article/what-is-srm/' [Online Resource]

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