Researchers at Columbia University have published a study warning that stratospheric aerosol injection, a proposed method to cool the planet by dimming the sun, faces significant physical, geopolitical, and economic hurdles not fully captured in climate models. The technique, which mimics volcanic eruptions by injecting sunlight-reflecting particles into the upper atmosphere, could lead to unpredictable outcomes like disrupted monsoons and material shortages. The team emphasizes that real-world implementation would be far messier than simulations suggest.
In a study published in Scientific Reports, a team from Columbia University's Climate School and Engineering examined the complexities of stratospheric aerosol injection (SAI). SAI aims to offset global warming by scattering particles to reflect sunlight, similar to the cooling effect from volcanic eruptions. The researchers reviewed existing studies, focusing on variables like the altitude and latitude of particle release, the time of year, and the total amount of material injected.
Latitude emerged as the most influential factor. Releases near the poles could disrupt tropical monsoons, while those near the equator might alter jet streams and global air circulation. "It isn't just a matter of getting five teragrams of sulfur into the atmosphere. It matters where and when you do it," said V. Faye McNeill, an atmospheric chemist at Columbia.
The 1991 Mount Pinatubo eruption provides a historical parallel, dropping global temperatures by nearly one degree Celsius but also disrupting the Indian monsoon, reducing South Asian rainfall, and contributing to ozone depletion. SAI using sulfate aerosols could produce similar side effects, including acid rain and soil contamination.
Alternatives like calcium carbonate, alpha alumina, rutile and anatase titania, cubic zirconia, and diamond were assessed, but practicality issues abound. Diamond is too scarce and expensive, while cubic zirconia and titania would see skyrocketing production costs. Only calcium carbonate and alpha alumina are abundant enough, yet they clump into larger particles over one micron, reducing scattering efficiency. "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," said lead author Miranda Hack.
Geopolitical realities make coordinated deployment unlikely, widening the range of possible outcomes. "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," McNeill added. Co-authors include Dan Steingart and Gernot Wagner, who noted, "It's all about risk trade-offs when you look at solar geoengineering... it isn't going to happen the way that 99 percent of these papers model." The study underscores the need to acknowledge these uncertainties in policy discussions.