Solar radiation modification

This policy briefing presents an independent scientific review of solar radiation modification (SRM) techniques, which work by reflecting additional sunlight back into space to reduce global heating. The briefing considers the potential risks, detrimental impacts and climate benefits of their implementation at scale. 

As global emissions continue to fall short of the reductions needed to limit warming to below 2°C above pre-industrial levels, interest has grown in SRM as a possible supplementary climate intervention. The briefing highlights that SRM techniques could only mask, not fix, the effects of warming caused by greenhouse gas emissions. While SRM would not address the root cause of climate change or all its associated impacts, it could, in principle, be one of several tools used to reduce climate-related risks.

Headline messages from the briefing include:

• Several SRM techniques have been proposed. Two have received particular attention in the scientific literature: Stratospheric Aerosol Injection (SAI) and Marine Cloud Brightening (MCB).
• The influence of SAI on the climate is currently much better understood than MCB, although climate effects of both methods are less well understood than greenhouse gas driven climate change.
• The effects of SRM are primarily informed by climate models, derived from those used in IPCC projections, and are partially supported by observations of real-world analogues, such as volcanic eruptions and sulfur dioxide emissions from shipping.
• If deployed in an informed and globally coordinated way, SRM could ameliorate many, but not all, of the adverse impacts of climate change. However, if deployed without due diligence, SRM could exacerbate regional climate change.
• There is robust evidence that globally coordinated deployment of SRM could reduce global-mean surface temperature, and associated impacts such as sea-level rise, wildfires and extreme precipitation, and so mask part of human-induced climate change. Significant uncertainties remain in how much cooling would be achieved for a given deployment of SRM.
• Other impacts of climate change are likely to respond to SRM in different ways to global temperature. Global precipitation would likely decrease more under SRM than for an equivalent temperature reduction from emission cuts, and SRM would not address ocean acidification caused by rising CO₂ levels.
• There are limits to the extent to which climate models can predict, with confidence, regional climate change, with or without SRM. This is particularly so for SRM given that relatively few models have been used to simulate its effects. SRM could exacerbate rather than ameliorate some regional changes in climate, such as patterns of rainfall change, and it is uncertain which regions would be so affected.
• The duration of SRM deployments required to reduce global temperatures to a given target level would be unknown when any deployment starts. It would depend on future greenhouse gas mitigation measures and uncertain aspects of the climate system, but could be many decades or even centuries.
• The short atmospheric lifetimes of SRM aerosols means that maintaining their cooling effect would require regular replenishment of the aerosols to mask the climate effect of long-lived greenhouse-gas emissions.
• If deployment of SRM were halted, or significantly reduced, the climate would return to close to its non-SRM state in one to two decades. If the SRM-induced cooling was substantial, the resulting rate of change of temperature would likely have strong impacts.

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