With severe climate impacts becoming more and more apparent, many scientists think we should explore ways to block out solar radiation, but doing so would be risky.
Earlier this month, on the deck of a second world war aircraft carrier docked in San Francisco, a giant fan began spraying sea salt particles into the air.
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| A machine sprays sea salt particles from the flight deck of a decommissioned aircraft carrier in California to test a technique to make clouds brighter New York Times/Redux/eyevine |
Few people, beyond those on the ship and bystanders on the nearby dock, would have taken much notice of the resulting plume of salt spray drifting upwards.
But this fan, and the spray it pumps out, has global significance. It marks one of the first real-world trials of a climate intervention known as marine cloud brightening – essentially an attempt to cool the planet by making clouds more reflective, so that they bounce more of the sun’s energy back into space.
Our chances of limiting global warming to 1.5°C above pre-industrial levels are rapidly slipping away, with a recent analysis suggesting the world will burn through the remaining carbon budget for this temperature goal by 2029 or earlier.
Meanwhile, global temperatures rose to record levels in 2023. And that extreme heat has brought extreme impacts, with widespread coral reef bleaching, severe marine heatwaves and rapid glacier loss just some of the consequences. Time is running out, scientists agree, to avert disaster.
Could geoengineering buy us time to get our house in order?
Solar radiation modification (SRM) is a type of geoengineering that involves modifying the atmosphere to tweak how much of the sun’s radiation makes it to Earth. Essentially, it would involve pumping tiny reflective particles into the atmosphere to bounce more solar radiation back into space.
A 2 per cent reduction in the amount of sunlight absorbed by the planet would be enough to offset the warming from a doubling of CO2, according to climate models.
Scientists stress that SRM isn’t an alternative to cutting emissions – it would merely mask some of the impacts of greenhouse gas pollution, for a little while. And it comes with huge, unpredictable risks. Intervening in Earth’s climate could trigger changes in global rainfall patterns, harm the ozone layer or cause regional accelerations in warming, for example.
It’s therefore a hugely controversial idea, but one that we might have to consider if greenhouse gas emissions continue to rise, says Jim Haywood at the University of Exeter, UK. After all, it’s the only option we know of that could cool the planet in a matter of years – potentially providing temporary relief from the worst impacts of runaway climate change.
“I do think that it does become rather an inevitability that we will be researching this more, because we really need to know whether, in an emergency situation, could SRM be effective? Could it be deployed? And could it have more positive impacts than negative impacts?” says Haywood.
Stratospheric aerosol injection
There are two main types of SRM on the table. The first is stratospheric aerosol injection (SAI), which, as the name suggests, involves shooting reflective aerosol particles into the lower stratosphere, 20 to 25 kilometres above ground level.
These particles would mimic the cooling effect of major volcanic eruptions. The 1991 eruption of Pinatubo, a volcano in the Philippines, pumped 15 million tonnes of sulphur dioxide into the stratosphere, enough to cool the climate by 0.6°C for the following 15 months.
The idea behind SAI is that humans could do something similar by sending aircraft up into the stratosphere to release payloads of reflective aerosols. Modelling suggests cooling could take effect within months.
SAI could reduce global temperatures, and if combined with substantial emissions cuts it could help save polar ice sheets from collapse, according to research New Scientist reported this week.
But it isn’t without risks. Releasing the aerosols in the wrong place, such as only in the northern hemisphere, could disrupt global weather patterns and trigger faster localised warming.
Volcano eruptions are as close as we have got to testing SAI in the real world. A project run by Harvard University planned to use a propelled balloon to inject about 1 kilogram of aerosols 20 kilometres up , but the project was scrapped last month after pushback from environmental campaigners.
Marine cloud brightening
Marine cloud brightening (MCB) is the other option. It involves seeding clouds over the ocean with tiny particles of sea salt to increase their reflectivity. Theoretically, the cooling effect could be evident with days. It could be localised to target specific areas, such as polar ice sheets or vulnerable coral reefs – or it could be deployed on a global scale to lower overall temperatures.
This month’s San Francisco trials are focused on MCB. Researchers at the University of Washington want to find out whether the particles leaving the aerosol-spraying machine remain the correct size as they deal with wind and humidity.
Similar research is under way by researchers at Australia’s Reef Restoration and Adaptation Program. Field trials were conducted last year along the Great Barrier Reef to see if spraying sea salt could create low-lying clouds to shade the reef during hot weather.
Experiments like this are essential if we are ever to know whether MCB could be deployed in the real world, says David Fahey at the National Oceanic and Atmospheric Administration in the US. “Theoretical analyses will only take you so far,” he says.
MCB would be easier to deploy than SAI – no stratospheric jets required, for starters. But it is arguably the riskier option. There’s a huge amount of uncertainty involved in tweaking marine clouds. Make the droplets too large, and you could cause warming rather than cooling. Seeding clouds in the wrong place could have adverse global impacts: one recent study led by Haywood found that doing so could trigger a strong, permanent response equivalent to a La Niña climate phase, raising sea levels in Australia and disrupting rainfall patterns across South America, the US, India and Australia.
Another major risk with deploying MCB or SAI is what happens if the cooling effect is turned off. If the world is pumping huge amounts of reflective particles into the sky each year, that could hold the warming effects of climate change at bay. But turn those nozzles off and, within weeks or months, the world suddenly gets dramatically hotter.
“If an SAI deployment was to be suddenly halted, the previously masked warming would manifest within a few years,” a UN report warned last year. “If the deployment were of sufficient scale, this could produce severe adverse effects on ecosystems and biodiversity, increasing risks of extinction for thousands of species.”
Extinguishing a global MCB programme overnight could have even more dramatic impacts, says Fahey. “If you really were cooling the planet at scale… all the aerosols would be out of the atmosphere within a matter of days to weeks,” he says.
One approach to minimise this risk is to use SRM for “peak shaving”, in other words, to mitigate the worst impacts of climate change as the world plays catch up to reduce emissions. Then, once emissions are brought down to net zero, the use of SRM could gradually taper off.
Who decides if we should start geoengineering?
So, where do we go from here?
Firstly, more research is required, to answer the countless questions about the risks, benefits and feasibility of deploying any kind of SRM for real. That will need to include some limited, real-world experiments.
But beyond further research, there is also a need for some global system of assessment and decision-making, says Fahey. “The attention to these topics is growing around the world,” he says. “But it’s not very well organised, nationally or internationally, and not well funded.”
A system like the Montreal Protocol, an international treaty to reduce the use of ozone-harming chemicals, could be established, says Fahey. Scientists would provide regular assessments on the state of SRM knowledge and nations would use these to inform their decisions on whether to pursue climate interventions and how they would do so.
“In the case of the Montreal Protocol, these scientific assessments guide the decision-making by the 197 nations that participate. So it really stabilises the decision-making,” says Fahey. When it comes to geoengineering, there’s a “distinct gap and absence of scientific guidance”, he says.
Without a system of governance, there is a greater risk of rash decision-making, potentially by a rogue state that decides to push ahead with SRM without global consensus. The relatively low cost of SRM technologies – estimated at about $20 billion per year per 1°C of cooling, according to one 2020 study – puts it within the reach of most countries.
As climate impacts escalate, that rogue-state risk is growing, says Fahey. “In a world that’s not paying attention, where there is no assessment, where there is no policy body, there’s still 197 nations and one or more of them could say, ‘We are tired of waiting, my country needs cooling, my country needs release from climate impacts and this is the only way,’” he says.
Like the idea or loathe it, the question of whether to pursue SRM isn’t going away, concludes Fahey. “This is the only thing that people are going to be talking about when the suffering of the planet is reaching a particular magnitude,” he says. “The cat is out of the bag. It’s not like we can simply ignore this.”
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