- Solar Radiation Management (SRM) uses sulfur dioxide to reflect solar radiation back into space
- SRM more immediate effect than emissions cuts
- SRM may reduce rainfall in some countries
- Risks of geopolitical conflict over control over SRM
Volcanic eruptions like that of Mt. Pinutabo in 1991 launch large amounts of sulfur dioxide into the upper atmosphere where it forms tiny particles or droplets, known as ‘aerosols’. These partially reflect solar radiation back into space, thereby cooling the climate.
David Keith, Professor of Applied Physics at Harvard University, proposes to mimic this process by deliberately releasing sulfur dioxide into the air. Climate modelling suggests SRM has the potential to “substantially offset a global temperature rise,” states the latest IPCC report on climate science.
SRM must be tested, he argues, because even if we halted all emissions tomorrow, the climate would continue warming. Those most vulnerable to climate change impacts cannot wait.
“You cannot achieve the same effect by cutting emissions. This can be done quickly and cheaply, costing in the order of $1 billion a year,” he says.
SRM is a temporary fix. Sulfate aerosols don’t prevent long-term build up of CO2. They evaporate or fall to the lower atmosphere after a couple of years, potentially leading to side effects like ozone depletion and air pollution.
David Keith insists it is a fix worth trying. He proposes a coordinated research effort to confirm scientific feasibility. If successful, gradually increasing amounts of sulfur dioxide would then be released, probably from aircraft, for about 70 years.
The goal would be to reduce about half the rate of increase in global warming. SRM would work in tandem with emissions cuts enforced by a high carbon price, and then be phased out. “100 years from now you wouldn’t be doing SRM. It would have reduced climate risk and given more time for emissions cuts to take effect,” David Keith explains.
Why not reduce temperatures back to pre-industrial levels? First, SRM would affect the global water cycle, potentially reducing rainfall levels. Second, over-reliance on SRM could lead to backsliding on emissions cuts. “You leave a very strong incentive to keep cutting emissions. I propose ramping up very slowly, watching for side effects, and then ramping down to zero,” says David Keith.
SRM poses unacceptable geophysical and geopolitical risks, counters Mike Hulme, Professor of Climate and Culture at King’s College London.
It may well cool temperatures on a global scale, but the side effects on local weather are unknown. This uncertainty could lead to international disputes and even conflict. “How would you get 190 nations to agree what temperature the climate should be regulated to?” he asks. He believes that SRM should be banned.
He argues that David Keith’s proposal presumes too much knowledge.
“It assumes a precise delineation between weather risks attributable to GHGs and dangers that are a consequence of sulfur,” he says. “But we can’t do that. We don’t understand enough of the regional dynamics of climate to be confident SRM will not further disrupt local weather.”
Because climate change and SRM risks are so uncertain, Mike Hulme maintains there is little ethical or political justification for deliberate climate intervention. He is therefore skeptical that nations could agree on incremental SRM as David Keith envisages, or what a successful outcome would look like.
“It is very hard to imagine an equitable, representative and sustainable political regime that would manage this process. Imagine if after two years of sulfur injections there is a major drought in northeast Brazil. Brazilians are going to ask whether the drought is occurring because of sulfur injections. But climate models will not give a precise answer. Suspicion will fall on those who are funding and managing SRM.”
David Keith agrees that governance and control are the biggest hurdles for SRM, but instead of shutting it down he says we should open it up to wider discussion. Governments will then be more likely to cooperate and act wisely.
“Many in the scientific community are deliberately underplaying the extent to which SRM reduces climate risk,” he says. “I believe at some level that is immoral. It is scientists’ job to state what the facts are. It is the job of the public and politicians to make value judgements and decisions. We are committed to major risk because of the CO2 now in the air. The question is whether we should apply an imperfect, ugly Band-Aid that may partially reduce that risk.”
Mike Hulme adopts the precautionary principle and concludes that, on the balance of risks, SRM should be rejected. Instead, the world should adapt to climate change, ramp up the energy revolution, and slash emissions of greenhouse gases like methane and nitrous oxide.
“Adaptation is a public good. Reducing air pollution is a public good. Seeding the energy revolution is a public good,” he says. “Those three pillars can stand entirely on their own without the need for sulfur injections.”
A promising solution that should be further explored, or a dangerous distraction that ought to be prohibited?
This article was based on interviews with David Keith and Mike Hulme. You can watch the full debate at the Oxford Martin School here.