Hi. This month should be a bit of a moment for solar geoengineering, the pioneering and often controversial umbrella term for ways of limiting the amount of the sun’s energy reaching Earth’s surface. The method at hand is stratospheric aerosol injection (SAI), the idea of creating a “solar shield” by pumping tiny particles into the high atmosphere that reflect some incoming energy back into space to cool the planet.
To date, almost all our understanding of this concept has come from computer modelling. But this month an independent panel is due to rule on whether a landmark field experiment can go ahead. The first step will be a test flight sending instruments on a balloon into the stratosphere. If that goes well, it will be followed at a later date by a release of aerosols into the stratosphere - a Rubicon that has not been crossed before. This month should also see a major scientific report on such approaches to global warming.
What could the experiment show us? How safe is it? Where does solar geoengineering fit in with other action on climate change? And why do so many people love to hate it?
The launch of a stratospheric balloon at Esrange in Sweden. Photo: SSC
Tell me more about solar shields We know volcanoes loft sulphate aerosols into the stratosphere where they have a cooling effect. The idea of artificially recreating this by using planes to release particles has been mooted for years as a way to buy time to decarbonise our economies and adapt to warmer temperatures. Research into the idea has mushroomed in the past decade, but field trials have failed to get off the ground. Advocates say we need real tests of this in case governments decide to deploy the idea at scale, and note existing climate action is inadequate. Opponents argue the technology would be used as an excuse for inaction on cutting emissions, experiments would be a slippery slope to bigger trials, there is no international governance in place, we don’t know about the side effects on weather systems and the “shield” would need to be continually replenished.
And what’s the excitement now? A Harvard University team is finally poised to take the first step towards testing the idea in the real world. Plans for the Stratospheric Controlled Perturbation Experiment (SCoPEx) began to take shape between 2013 and 2015. On 15 February, an independent advisory committee will decide whether it can go ahead. Frank Keutsch at Harvard, who is leading the project, is optimistic the panel will at least give a qualified yes. “We really need to do the research because I’m really worried where we’re going with climate change as action is just not fast enough,” he says. Provided the committee says yes, a test balloon will take off from Esrange Space Center in Kiruna, Sweden, in June to test instruments in the stratosphere. Later, another flight will release a few hundred grams of calcium carbonate, a mineral dust, and measure the plume’s behaviour and impact. By coincidence, February will also see the release of a study by the US National Academies of Science on strategies for reflecting sunlight to cool the Earth.
What’s the wider context here? Well, the past six years were the warmest on record. The record-breaking fall in greenhouse gas emissions last year as a result of pandemic restrictions will need to be repeated every year for the next decade if we are to keep below 1.5°C of warming. Even with Joe Biden as US president driving momentum, most countries’ carbon-curbing plans are inadequate. You get the picture. As Keutsch points out, stratospheric solar geoengineering – like halting methane escape from oil and gas wells – is one of the few things that could be done quickly. “I think it is on scientists to provide some facts about the risks of solar geoengineering and I think this has not been done nearly enough. One of the things I have issues with is that nearly everything has been modelling,” he says.
What does SCoPEx hope to achieve? We haven’t injected calcium carbonate into the stratosphere before, so we don’t know exactly how it will behave up there, in terms of chemical interactions and how it moves around. “I have hypothesised a good material is calcium carbonate – based on known optical qualities it should not heat up the stratosphere, and based on lab experiments it looks promising on [avoiding] ozone destruction,” says Keutsch. But he admits it may turn out to be a “terrible idea” too. The point is to find out, and releasing such a small amount of material will have no negative impact, he says. The SCoPEx balloon will have propellers to move around, create a plume and then turn around to move back through the plume to study it. One thing computers are bad at modelling is turbulence and mixing of the material in the stratosphere, and observations will improve that. Most assumptions are based on how volcanoes inject aerosols that high up, but a plume from an aircraft or balloon is very different, as it comes from a pinpoint. The trial may shed light on what particle size to use.
An illustration by the SCoPEx team of their experiment.
What challenges does the experiment face? Public opposition could be an issue in environmentally-conscious Sweden. One Swedish think tank has voiced its objections, though there is no sign yet of widespread public concern. Keutsch says SCoPEx will be a “zero impact experiment”. “The biggest physical risk is that the balloon crashes, which is true for any stratospheric balloon. The likelihood is very low, but that’s why this is done in uninhabited areas.” Hundreds of similar balloons go up into the stratosphere each year. Technical problems during the flight, or delays due to covid-19, may also get in the way. In terms of a second flight, to release the material, co-collaborator David Keith at Harvard University says: “I’m always optimistic, so I’d love to get there this year. But my guess is next year.” If the independent panel outright rejects the first flight – which is thought unlikely – “then we’re done”, says Keith.
What do others think? Shaun Fitzgerald at the Centre for Climate Repair at Cambridge University is in favour of the experiment. “There are sensitivities associated with larger-scale deployment of techniques, but that is not what is being proposed here. We need to increase our understanding of solar geoengineering and the resulting effects. Detailed modelling can take us only so far – at some point small scale experiments are needed,” he says.
Ina Möller at Wageningen University and Research in the Netherlands takes a different tack. She agrees with Keutsch that the initial flight will have “very little direct impact on the environment”. But she thinks the problem is the symbolic value of the milestone when material is released. “SCoPEx would be the first instance in which an outdoor experimental set-up linked to stratospheric aerosol injection is executed. It would therefore cross a currently existing natural threshold – moving from indoor to outdoor experimentation – and open the door to further SAI experiments.” If the independent committee doesn’t impose limits, she fears “a real risk of inducing a slippery slope where one experiment will lead to another”.
What’s next? The first thing is the committee’s decision, officially due on 15 February, but which may slip until later this month. Meanwhile, Keutsch is seeking more international partners. A Swedish group is already involved, and he is pursuing German and Swiss instruments. He hopes researchers from the “global south” will get involved with the analysis of data if everything goes ahead.
Keutsch is acutely aware that, as his experiment will be the first to put particles in the stratosphere intentionally, there is a lot riding on its success or failure.“It doesn’t get much more symbolic than some balloon you’re launching up there, it symbolises ‘oh shit, where has humanity got itself to’. If we do this wrong, then I can’t do more. But it also could make it much harder for other scientists to do it in the right way, and that would be terrible,” he says.
Drax power station in the UK, once western Europe’s single biggest emitter of carbon dioxide, will end commercial coal power generation on 31 March, it said this week. Coal has now almost been entirely squeezed out of the UK electricity grid.
5. A Paris court yesterday ruled that the French state has failed to make policy on cutting carbon emissions quickly enough, after a legal case brought by Oxfam France. It's the latest in a growing wave of climate litigation against countries and companies - check out this London School of Economics report for a broader overview.
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