Why ‘dimming the sun’ could be the riskiest climate fix yet

(CN) — For decades, scientists have debated whether humans could cool the planet by releasing sunlight-reflecting particles high into the atmosphere.

A new study from Columbia University scientists says the idea of cooling the planet by blocking sunlight is far trickier and less predictable than many of its advocates suggest.

The research, published Tuesday in Scientific Reports, examines how stratospheric aerosol injection, or SAI, might work in practice and why real-world limitations could make it nearly impossible to control.

“Even when simulations of SAI in climate models are sophisticated, they’re necessarily going to be idealized,” said V. Faye McNeill, an atmospheric chemist and aerosol scientist at Columbia’s Climate School and Columbia Engineering, in a press release. “Researchers model the perfect particles that are the perfect size. But when you start to consider where we actually are, compared to that idealized situation, it reveals a lot of the uncertainty in those predictions.”

SAI aims to cool Earth by injecting reflective particles, usually sulfur compounds, into the stratosphere to mimic the cooling effect of large volcanic eruptions. When Mount Pinatubo erupted in 1991, for example, global temperatures briefly dropped by nearly a full degree Celsius.

McNeill and her colleagues reviewed decades of data to highlight how difficult it would be to reproduce such effects intentionally. The results, they say, show that “dimming the sun” isn’t as simple as loading a few aircraft with sulfur and turning them loose.

“It isn’t just a matter of getting five teragrams of sulfur into the atmosphere. It matters where and when you do it,” McNeill said.

Latitude and timing make a huge difference, say researchers. Releasing aerosols near the poles could disrupt tropical monsoons, while equatorial releases might alter the jet stream or shift global rainfall patterns.

The team argues that SAI, if ever attempted, would need centralized global coordination — something they say is politically unlikely.

Besides logistical challenges, the chemistry of the particles themselves poses problems. Sulfate aerosols, the standard option in models, can deplete ozone and cause acid rain. That’s why researchers have proposed mineral alternatives such as calcium carbonate, alumina or titanium dioxide.

However, those substitutes come with trade-offs. Many are scarce, expensive or difficult to disperse in the tiny, sub-micron size required to stay aloft. Once airborne, they tend to clump together, reducing their effectiveness and adding new uncertainties.

“Scientists have discussed the use of aerosol candidates with little consideration of how practical limitations might limit your ability to actually inject massive amounts of them yearly,” said Miranda Hack, an aerosol scientist at Columbia and the study’s lead author, in the press release. “A lot of the materials that have been proposed are not particularly abundant.”

Hack said even the best candidates, such as calcium carbonate, would face major engineering hurdles. Larger aggregates would scatter light differently, weakening the intended cooling effect and creating unpredictable consequences.

“In comparison to sulfate, I don’t think we would necessarily see the types of climate benefits that have been discussed,” she said.

The researchers also examined the geopolitical and economic realities. Unlike controlled computer simulations, real-world deployment would be messy, influenced by uneven global interests, material shortages and uneven governance.

“It’s all about risk trade-offs when you look at solar geoengineering,” said Gernot Wagner, a climate economist at Columbia Business School and co-author of the study, in the press release. Given those complications, he said, “it isn’t going to happen the way that 99 percent of these papers model.”

The study doesn’t rule the idea out entirely, but researchers say there’s still a lot we don’t know — from how the particles would actually behave in the atmosphere to who would be responsible for using them.

“There are a range of things that might happen if you try to do this,” McNeill said. “And we’re arguing that the range of possible outcomes is a lot wider than anybody has appreciated until now.”