For several decades, scientists have suggested ways to “geoengineer” the climate. Several proposals call for injecting microscopic particles, called aerosols, into the stratosphere, the quiet region of the atmosphere above the troposphere about 18 kilometers up from the equator. There they reflect sunlight back into space, mimicking the influence of large volcanic eruptions that have temporarily cooled the planet in the past.
Such proposals often involve sulfates, particles that form in the stratosphere from sulfur dioxide ejected by volcanoes, or other molecules with high reflectivity, such as diamond dust or alumina (aluminum oxide). But all these approaches have drawbacks, says Robert Nelson, a senior scientist at the Planetary Science Institute who is based in Pasadena, California. Sulfur dioxide, for example, could eat away at the ozone layer or cause acid rain.
Alumina could be even worse, Nelson says. Although it is extremely reflective, it could embed in the lungs if inhaled and cause chronic disease similar to silicosis. “I was raised in Pittsburgh, [Pennsylvania,] and I remember as a child seeing black lung victims struggling to get down the street.” Still, given the limited amount of alumina that could be required, it’s far from certain such a health risk would be a genuine concern.
So Nelson continued to look for other reflective compounds that might be less hazardous to human health. In 2015, he was studying evaporated salts on the surface of other solar system bodies such as the dwarf planet Ceres. He soon realized that simple table salt is more reflective than alumina, while also harmless to humans. Just as important, Nelson believes that salt, when ground into small enough particles of the right shape and dispersed randomly, would not block outgoing infrared heat released by Earth, adding to its cooling effect.
Nelson is not the first to consider salt, says Matthew Watson, a volcanologist at the University of Bristol in the United Kingdom. Watson led a geoengineering experiment, called the Stratospheric Particle Injection for Climate Engineering project, that was canceled in 2012. His group briefly considered salt for stratospheric injection, he says, but problems popped up.
First, there’s a lot of chlorine in salt, and chlorine can contribute to destroying ozone. That alone could be enough to kill salt as a candidate, Watson says. Few would likely welcome injecting a particle that could reopen the ozone hole. “[This] could be a big problem,” agrees David Keith, an energy and climate scientist focused on geoengineering at Harvard University. Salt is also highly attracted to water, and water is scant enough in the stratosphere that injecting even limited amounts of salt could potentially alter, for example, the formation of the realm’s wispy clouds, to unknown effects.
Nelson hopes these concerns could be addressed by injecting salt in the high troposphere, above the clouds but below the stratosphere. He also plans to look more closely at salt’s properties; if he can resolve some of these questions, he’d like to see a test of the particles above a region forecasted to experience life-threatening extreme temperatures. This would test the science while potentially benefiting society in the short term, he says. Such a research effort could only come after thorough engagement with the public, Nelson adds.
But like nearly all scientists interested in geoengineering, Nelson stresses that the strategy is no substitute for action to curb carbon emissions. No type of solar radiation management, for example, would prevent rising carbon dioxide from acidifying the oceans. This research should only be done so the world can potentially buy itself some time, Nelson says. “This would be a palliative, not a [long-term] solution.”