Royal Society assessment says solar radiation modification techniques could reduce global temperatures if done in a globally coordinated way.
Geoengineering to reduce temperatures in one part of the world could worsen droughts and hurricanes elsewhere, scientists have warned.
A report on the current science around techniques to reflect some sunlight back into space – known as solar radiation modification (SRM) – has found that internationally co-ordinated deployment could reduce global temperatures and some of the impacts of climate change.
But the briefing from the Royal Society warned the hypothetical techniques, which are likely decades from deployment, are no substitute for action to cut the greenhouse gas emissions pushing up global temperatures.
And “unilateral” action by individual nations could cause serious impacts for others, the study’s authors said.
Interest in geoengineering techniques to curb climate change has grown as efforts to cut greenhouse emissions to limit global warming to 1.5C or “well below” 2C above pre-industrial levels have faltered.
The latest UN assessment ahead of climate talks in Brazil put the world on track for up to 2.8C of warming on current measures.
But there are serious concerns about the technological feasibility and ethical questions around deploying geoengineering methods to curb climate change.
Reaching out to experts as part of the Royal Society report highlighted the two options for reflecting sunlight from the Earth that are thought to be most effective for inducing a 1C reduction in global temperatures and the relatively lowest technical barriers.
These are stratospheric aerosol injection (SAI), a process which would mimic the natural cooling impacts of volcanic eruptions by injecting sulphur dioxide that rapidly becomes reflective particles in the stratosphere, and marine cloud brightening (MCB) which increases the reflective capacity of clouds.
The SAI technique would require planes to fly higher than most aircraft can currently manage, while the MCB could involve spraying saltwater from ships in a technology that is “embryonic”, report author Prof Keith Shine said.
Other techniques such as mirrors in space are seen as highly technically challenging.
The report found robust evidence that if SRM was deployed globally, it could reduce global temperatures and some impacts such as sea level rises, wildfires and extreme rainfall.
But there are still major uncertainties about how much cooling it could achieve, and if it could make some impacts worse on a regional basis.
For example, deploying SAI in the northern hemisphere only could lead to drought in the Sahel region of Africa, while using it only in the southern hemisphere could increase North Atlantic hurricane frequency and intensity.
Deploying SAI in the tropics could affect ozone levels in the stratosphere and lead to winter droughts in the Mediterranean, while inducing marine cloud brightening in the south-east Atlantic could lead to die-back of the Amazon rainforest.
The study also warned that the geoengineering methods would not stop consequences of greenhouse gas emissions such as making the oceans more acidic, and would only mask other climate impacts.
If SRM was halted, temperatures would rapidly rebound to where they would be without the techniques being used, causing rates of warming that are damaging to ecosystems, the assessment found.
The report’s authors warned the techniques should only be used to complement strategies to cut emissions, “not as the main policy response to climate change”.
Report author Prof Jim Haywood, from the University of Exeter, said: “If policy makers took a decision to deploy SRM, it must be scientifically informed, globally coordinated and internationally agreed upon, ie, judicious.
“Injudicious SRM could worsen rather than ease some climate impacts of global warming, particularly at a regional level,” he warned.
“It has to be globally coordinated, and you would not want this to be done by a single rogue actor who was acting in their own best interest,” he said.
Professor Sheila Rowan, from the University of Glasgow, added: “Unilateral action by individual nations could cause serious regional impacts for others.”
The Royal Society does not take a position on whether research into SRM techniques should be pursued or not.
But Prof Shine said: “We are quite clear in the report that any research into SRM shouldn’t detract from efforts to mitigate emissions”.
And policymakers would have to weigh up the risks of geoengineering measures against the threats posed by insufficient efforts to mitigate climate change.
With costs for measures such as SAI estimated to be in the low tens of billions of dollars a year, the price tag would not be the “showstopper”, with other things to be more worried about than the financial implications, he added.
