How would the world react if one country launched a solar geoengineering project to cool the planet without the permission of others? Would the sky change colour based on the type of particles injected into the upper atmosphere? Even if global average temperatures cooled in response, how would rainfall change over Africa? These are just some of the questions about solar geoengineering scientists grappled with at a December meeting of the American Geophysical Union (AGU) in Washington DC, and they came away with more questions still.
“The more we research these things, the more we learn how complex they are,” said at the University of Washington in Seattle.
Solar geoengineering – any large-scale attempt to lower global temperatures by reflecting sunlight away from the planet – is controversial. Detractors say it may offer an excuse to continue emissions as usual. But with little political will to rein in emissions, a growing number of researchers are studying the potential risks and benefits of such interventions.
Advertisement
There are two main approaches. One would use high-flying planes or balloons to inject sulphur dioxide or other aerosols into the stratosphere, drawing a reflective curtain around the planet. The other would brighten low-lying marine clouds that form over the ocean.
Real-world geoengineering tests have raised alarms about its possible effects and the lack of regulation. Earlier this year, a California city shut down a small-scale outdoor experiment focused on marine cloud brightening, with locals. Lawmakers in Mexico, the European Union and a few US states are considering legislation prohibiting such tests. A common concern is that research on solar geoengineering is a slippery slope to actual deployment of experiments large enough to impact the climate, with risks ranging from altering weather patterns to the potential for countries to use it for geopolitical leverage.
What little research we do have makes clear that there are still many questions to be answered. “We know we could cool the climate with stratospheric aerosol injection,” says at Colorado State University, referring to what is probably the most-studied method of solar geoengineering, often called SAI. “This would work. The question is: would you create unintended dangers? And would there be winners and losers?”
Some of the uncertainty is down to imprecision in climate models. For instance, at the National Center for Atmospheric Research in Colorado found that variations in the way such models represent aerosol injection lead to significantly different predictions. Change the rate at which aerosols are injected in each hemisphere, for instance, and you either halt the slowdown of Atlantic Ocean currents or get no effect. “If you want to cool the planet relatively evenly, which hemisphere do you inject more in?” said Lee. According to him, this is an unanswered question.
Zoom in on a more regional scale, and effects are even harder to predict. at the University of Cape Town in South Africa modelled an SAI scenario that held the rise in global average temperatures to the Paris Agreement target of 1.5°C above the pre-industrial baseline. He found this would boost rainfall in southern Africa, even as it led to drier weather for farms in Central Africa. However, effects on a key atmospheric driver of drought in southern African were still unclear.
One area ripe for more research is the current state of the stratosphere. This is the layer of the upper atmosphere that would serve to rapidly disperse any injected aerosols. But it’s unclear what aerosols are currently there and how they behave. “The stratosphere is critically under sampled,” said at the US National Oceanic and Atmospheric Administration, which has started launching more weather balloons to learn about the upper atmosphere. “Where is the aerosol, what’s it made out of and how is it going to be different tomorrow?”
Other unknowns involve people. at Cornell University in New York used a simple model to test how the climate might respond to a worst-case “uncoordinated” scenario where countries race to respond to a rogue group that unilaterally deploys geoengineering to cool the Arctic. He found that counteractive injections into the stratosphere elsewhere could balance the cooling effect, but it looks like chaos, as multiple actors with different aims scramble to react with the climate at stake. “It’s one world,” said Farley. “Every choice made by every actor affects the others.”
