Â鶹ÒùÔº

Once considered a fringe idea, the prospect of offsetting global warming by releasing massive quantities of sunlight-reflecting particles into Earth's atmosphere is now a matter of serious scientific consideration. Hundreds of studies have modeled how this form of solar geoengineering, known as stratospheric aerosol injection (SAI), might work.

There is a real possibility that nations or even individuals seeking a stopgap solution to climate change may try SAI—but the proponents dramatically underestimate just how difficult and complicated it will be, say researchers from Columbia University.

"Even when simulations of SAI in climate models are sophisticated, they're necessarily going to be idealized. Researchers model the perfect particles that are the perfect size. And in the simulation, they put exactly how much of them they want, where they want them. 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," says V. Faye McNeill, an atmospheric chemist and aerosol scientist at Columbia's Climate School and Columbia Engineering.

"There are a range of things that might happen if you try to do this—and we're arguing that the range of possible outcomes is a lot wider than anybody has appreciated until now."

In a paper in Scientific Reports, McNeill and her colleagues reckon with the physical, geopolitical and economic limitations of SAI. They begin by collecting the scattered scientific literature on how SAI's impacts would be shaped by the nuances of its deployment.

Many factors affect how aerosols interact with Earth systems: the altitude and longitude at which they are released, the time of year when this takes place, and, of course, the sheer number of particles involved.

The most significant variable, though, appears to be latitude. For example, SAI concentrated in polar regions would likely disrupt tropical monsoon systems. Releases concentrated in equatorial regions could affect the jet stream and disrupt atmospheric circulation patterns that conduct heat toward Earth's poles.

"It isn't just a matter of getting five teragrams of sulfur into the atmosphere. It matters where and when you do it," says McNeill.

These variabilities suggest that, if SAI takes place, it should be done in a centralized, coordinated fashion. Given geopolitical realities, however, the researchers say that is unlikely.

Model studies to date have focused almost entirely on SAI approaches that would use sulfate-rich gases analogous to those formed when volcanic plumes oxidize and condense in the stratosphere.

Volcanic eruptions have cooled Earth in the past. When Mount Pinatubo erupted in 1991, for example, planetary temperatures dropped by nearly one degree Celsius for several years afterward. That event is often cited as a proof-of-principle for how SAI could work.

Beside cooling at ground level, SAI also poses undesirable consequences, both expected and unexpected. For example, Pinatubo's eruption also disrupted the Indian monsoon system, leading to decreased rainfall across South Asia, and caused warming in the stratosphere and depletion of the ozone layer.

The use of sulfates for SAI could pose similar risks, or additional environmental concerns, including acid rain and soil pollution. These concerns have led to a search for other aerosol ingredients for SAI.

Proposed mineral alternatives include calcium carbonate, alpha alumina, rutile and anatase titania, cubic zirconia and diamond. Consideration of alternatives has focused on their optical qualities, but other factors have been neglected.

"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," says Miranda Hack, an aerosol scientist at Columbia University and the new paper's lead author.

"A lot of the materials that have been proposed are not particularly abundant."

Diamond is optically well-suited to the task, but there simply isn't enough of it. As for cubic zirconia and rutile titania, supply might conceivably meet demand, but the Columbia team's economic modeling suggests that increased demand would strain supply chains and make them much more expensive.

Sufficient supplies of alpha alumina and calcium carbonate exist to absorb demand without driving prices to prohibitive levels—but, along with the other candidates, there are serious technical challenges involved with dispersing them.

At the minuscule, sub-micron particle size necessary for SAI, the mineral alternatives all tend to clump into larger aggregates. According to the researchers' calculations, these aggregates are less effective at reducing sunlight than are particles, and their climate impacts are even less understood.

"Instead of having these perfect optical properties, you have something much worse. In comparison to sulfate, I don't think we would necessarily see the types of climate benefits that have been discussed," says Hack.

All these practical considerations—in deployment strategies, governance, availability and material properties—made SAI even more uncertain than it already is, say the researchers. This should be acknowledged when considering the use of SAI.

"It's all about risk trade-offs when you look at solar geoengineering," says Gernot Wagner, a climate economist at the Columbia Business School and a close collaborator with the Climate School. Given the messy realities of SAI, he says, "it isn't going to happen the way that 99% of these papers model."

The study was coauthored by Daniel Steingart, co-director of the Columbia Electrochemical Energy Center.