Frequently Asked Questions

Some entrepreneurs have started selling the release of small quantities of sulphur dioxide in the stratosphere as “cooling credits”, claiming that this will offset the impacts of atmospheric CO₂. There is currently no way to verify the amount of warming offset by such a scheme. More importantly though, unlike carbon credits, such “cooling credits” are not equivalent to a reduction in CO₂ emissions. SRM would not counteract all the climate effects of CO₂, it would have side-effects, and it does not address ocean acidification.

While billionaires would have the resources to deploy stratospheric aerosol injection at small scales, e.g., from balloons or business jets, this would not have a meaningful effect on the climate. Large-scale deployments would require not only vast sums of money, but technical capabilities and political power beyond the reach of even the wealthiest individuals.

It seems unlikely that SRM could be weaponised and used to wage war. However, its use might raise tensions between nations and could conceivably trigger a war. On the other hand, if SRM is effective in reducing climate change impacts it might reduce tensions between nations.

No – SRM alone would not worsen ocean acidification. Ocean acidification is the result of human-caused emissions of CO₂, primarily from the burning of fossil fuels. By lowering temperatures SRM may help to reduce the emissions of CO₂ expected from warmer soils and permafrost in the future, and so modestly reduce ocean acidification. However, if SRM undermines efforts to decarbonise economies these extra CO₂ emissions could worsen ocean acidification.

The idea of placing reflective material in space was an early SRM idea. However, there are substantial cost and technological barriers to placing enough reflective material in orbit. Despite technological breakthroughs and enormous reductions in the costs of lifting material to orbit, this approach is likely still several decades away from implementation.

Several SRM field experiments have been proposed, and while some have been cancelled, others have gone ahead. An ongoing marine cloud brightening (MCB) experiment in Australia (started in 2020) aims to shade the Great Barrier Reef as part of the Reef Restoration and Adaptation Program . The CloudLab experiment in Switzerland (started in 2022) seeded low, heat-trapping clouds over the alps to understand their dynamics and whether they could be thinned to allow heat to escape to space. An experiment to test the instruments that would spray sea salts for MCB began off the coast of Alameda, CA in spring 2024 but was cancelled due to community objections.

Cirrus cloud thinning is a proposal to thin cirrus clouds and allow more heat to escape to space, lowering temperatures. As it doesn’t work by increasing the amount of sunlight reflected it isn’t strictly a sunlight reflection method. However, as it is a proposal to lower global temperatures without addressing the root cause of global warming, it raises similar issues to other SRM proposals.

No – no treaty that applies to SRM prohibits researching or using it. The Convention on Biological Diversity does call on countries to refrain from engaging in SRM activities that might affect biodiversity, with an exception for small-scale research, but this call is not legally binding. International law does, however, advise caution on SRM.

In 2010, parties to the Convention on Biological Diversity called on countries to prevent SRM and other climate interventions that might affect biodiversity, except for small-scale studies, until more knowledge is available and international governance is ready. Because the decision calling for such action is not legally binding, however, it does not constitute a moratorium.

Urban areas are often hotter than surrounding areas because urban surfaces like asphalt and concrete absorb more sunlight than natural surfaces. Brightening urban surfaces can increase the amount of sunlight reflected, leading to a cooling. Doing this could provide relief from heat in the city, but it will not be possible to scale this up to achieve a substantial global cooling effect.

Acid rain occurs when acidic compounds are released into the atmosphere, and it can harm ecosystems by changing the acidity of soils and lakes. Some fossil fuels like coal have sulphur as an impurity and so when they are burned this sulphur reacts to form sulphuric acid. If stratospheric aerosol injection (SAI) were implemented using sulphate aerosols, those aerosols would also add to acid rain. However, the amount of sulphur needed to cool the planet by around 1 °C with SAI is less than one tenth of what’s emitted today by burning fossil fuels.

The stratospheric ozone layer protects earth’s surface from dangerous ultraviolet radiation. In the 1970s, scientists documented a dramatic thinning of the ozone layer over Antarctica due to human activity. International efforts to eliminate industrial chemicals that harm stratospheric ozone have largely succeeded, and the ozone layer is healing. Adding sulphate particles to the stratosphere could potentially delay the recovery of the stratospheric ozone hole by decades. However, research suggests that even for large-scale deployments, the extent of the ozone hole would likely not be as great as it was during the late 20th century.