Researchers warn that efforts to reflect sunlight and cool the Earth could have unpredictable, far-reaching impacts.
Credit: SciTechDaily.com
Scientists are questioning whether humanity can truly “dim the Sun” without causing chaos.
A new Columbia University study shows that stratospheric aerosol injection could trigger massive side effects depending on where, when, and what materials are used. From monsoon disruptions to supply-chain limits and uncertain chemistry, the obstacles are enormous.
The Rising Reality of Solar Geoengineering
An idea once dismissed as far-fetched, cooling the planet by spreading sunlight-reflecting particles through the upper atmosphere, has now become a serious topic in climate science. This approach, known as stratospheric aerosol injection (SAI), aims to counter global warming by mimicking the natural cooling that follows volcanic eruptions. Hundreds of studies have modeled how such a system could work in theory. But researchers at Columbia University warn that supporters of the concept overlook just how uncertain, technically challenging, and risky it could be in practice.
“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.”
An illustration of climate geoengineering techniques, including stratospheric aerosol injection (SAI), cirrus cloud thinning (CCT), and marine cloud brightening (MCB), and their proposed delivery systems and potential impacts. Natural stratospheric aerosol release from a volcanic eruption is also shown for context. Surface albedo geoengineering (SAG), which is based on increasing the albedo of various surfaces, is also represented with two examples: installing white roofs on urban buildings and modifying plants and shrubs surface.
Credit: Creative Commons
Reckoning With Real-World Limits
In a study published in Scientific Reports, McNeill and her coauthors explored the physical, political, and economic barriers that could complicate efforts to deploy SAI. They compiled findings from previous research to better understand how different design choices — such as timing, altitude, and injection location — could influence the planet’s climate response. Even small differences in how and where aerosols are released could drastically change the results.
Among the many variables, latitude stands out as one of the most important. For instance, injecting particles over the poles could disrupt tropical monsoon systems, while focusing efforts near the equator might interfere with the jet stream and alter the circulation of heat between hemispheres.
“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.
Lessons From Volcanic Cooling
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 afterwards. 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.
Searching for Safer Sunlight Shields
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.”
The Harsh Economics of Aerosol Alternatives
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.
Uncertain Futures and Risky Trade-Offs
According to the Columbia University researchers, every real-world challenge — from how SAI would be carried out to the types of particles used — adds new layers of uncertainty to an already unpredictable idea. They argue that these complications must be recognized before any serious consideration of deploying stratospheric aerosol injection takes place.
“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 percent of these papers model.”
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