Techniques to reflect an additional small portion of sunlight back into space could help cool the planet if deployed globally, but they cannot address the full range of climate impacts or replace emission cuts, according to a Royal Society briefing.
"Solar radiation modification" (SRM), published today, considers the potential impacts of SRM if deployed globally, in a scientifically informed way, across both hemispheres and for long periods of time. It evaluates its potential effectiveness, limitations, risks, and challenges with monitoring and governance. The briefing highlights major uncertainties that remain around SRM, including how much cooling it could achieve and whether it could worsen rather than ease some climate effects, particularly at a regional level.
SRM has attracted growing interest as global efforts to reduce emissions and limit warming to well below 2°C, as required by the United Nations Framework Convention on Climate Change Paris Agreement, appear increasingly unlikely to be successful. Some projections suggest we can expect temperature increases of over 3°C by 2100 under current policies. This would bring severe risks for human and natural systems, including more severe weather events such as monsoons and wildfires.
SRM has been proposed as a potential way to reduce global temperatures by reflecting some sunlight back into space. These 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.
Professor Keith Shine FRS, Regius Professor of Meteorology and Climate Science, University of Reading and Chair of the report's working group, said: "Unless there is a significant shift in our mitigation strategies, we are on track to breach the 1.5°C Paris Agreement warming goal in the near future. Both SRM and unmitigated climate change carry significant risks, and the key challenge is to understand those risks in detail and assess them side-by-side, not in isolation.
"This is not a question of whether SRM is safe, as it is clearly not without risks. However, there may come a point where those risks are seen to be less severe than the risks of insufficiently mitigated climate change.
"If policy makers did take the decision to deploy SRM, a scientifically informed, globally coordinated and internationally agreed upon strategy would be essential both to achieve global cooling and avoid potentially large undesirable regional climate impacts."
The report focuses on two SRM methods that have received the most scientific attention: Stratospheric Aerosol Injection (SAI), which would involve releasing reflective particles into the upper atmosphere, and Marine Cloud Brightening (MCB), which proposes to increase the reflectivity of low-altitude clouds over ocean regions. Of the two proposed techniques, SAI is considered more technically feasible to scale up, and the mechanism by which it impacts the climate is currently better understood.
Professor Jim Haywood, from the University of Exeter, is an expert in the field of Solar Radiation Modification and was a lead author of the Royal Society report. Dr Matthew Henry, a Senior Research Fellow at Exeter, also contributed significantly to the report.
Professor Haywood said: "If deployed in a scientifically informed, globally coordinated and internationally agreed way, SAI 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 from climate modelling studies, as well as real-world analogues such as volcanic eruptions and ship tracks, that globally coordinated, long-term deployment of SRM would lower global average temperatures. However, there is much less confidence in the ability of climate models to reliably predict regional impacts.
Climate responses would depend strongly on how and where SRM was deployed. Concentrating aerosol injection in one hemisphere, or along the equator, could alter rainfall patterns and lead to uneven impacts across different parts of the world. Coordinated global implementation would be needed to minimise these undesirable climate impacts.
If SRM were ever deployed and then stopped abruptly while greenhouse gas concentrations remained high, global temperatures could increase by 1-2°C within a couple of decades. This phenomenon, known as the 'termination effect' would likely have severe impacts on ecosystems and populations unable to adapt quickly. Any consideration of SRM would therefore require long-term international commitments.
