What Is SRM?

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.

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.

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.