U.S. researchers have proposed a new solar geoengineering method that would inject a material called calcite high into the atmosphere to help fight global warming while simultaneously healing the ozone layer.
Solar geoengineering is the idea that humans might deliberately increase the reflection of sunlight back to space, for example, by injecting light-reflecting sulfate aerosols into the stratosphere.
But these sulfate aerosols carry significant risks, since they can produce sulfuric acid in the stratosphere, which damages ozone, according to the study published in the U.S. journal Proceedings of the National Academy of Sciences.
"In solar geoengineering research, introducing sulfuric acid into the atmosphere has been the only idea that had any serious traction until now," said first author David Keith, professor at the Harvard University.
"This research is a turning point and an important step in analyzing and reducing certain risks of solar geoengineering."
Previous research focused on ways to limit the ozone-damaging reactions produced by nonreactive aerosols. But Keith and colleagues took a completely different approach, targeting aerosols that are highly reactive.
"Anytime you introduce even initially unreactive surfaces into the stratosphere, you get reactions that ultimately result in ozone destruction as they are coated with sulfuric acid," said coauthor Frank Keutsch, professor at the Harvard University.
"Instead of trying to minimize the reactivity of the aerosol, we wanted a material that is highly reactive but in a way that would avoid ozone destruction."
In order to keep aerosols from harming the ozone, the particles would need to neutralize sulfuric, nitric, and hydrochloric acid on their surface.
Through extensive modeling of stratospheric chemistry, the team eventually found that calcite, a constituent of limestone, could counter ozone loss by neutralizing emissions-borne acids in the atmosphere, while also reflecting light and cooling the planet.
"Calcite is one of the most common compounds found in the earth's crust," said Keith. "The amounts that would be used in a solar geoengineering application are small compared to what's found in surface dust."
The researchers have already begun testing calcite in lab experiments that mimic stratospheric conditions.
Keith and Keutsch cautioned that anything introduced into the atmosphere may have unanticipated consequences and that even if all the attendant risks could be reduced to acceptable levels, solar geoengineering is not a solution to climate change.
"Geoengineering is like taking painkillers," said Keutsch. "When things are really bad, painkillers can help but they don't address the cause of a disease and they may cause more harm than good. We really don't know the effects of geoengineering but that is why we're doing this research."
"Although both unpalatable and sub-optimal, I'm afraid the time has come to start looking at these techniques more seriously," Matt Watson of the University of Bristol, who was not involved in the study, said in a statement.
"Recent political events, especially those in the U.S., strongly indicate that we are becoming ever less likely to solve this problem through conventional mitigation alone. That therefore requires us to think about a range of potential solutions no matter how politically, socially or technologically challenging they may be."
