Mitigating the risk of geoengineering

THE HARVARD GAZETTE - Dec 12, 2016 -

Aerosols could cool the planet without ozone damage

The planet is warming at an unprecedented rate, and reducing emissions of greenhouse gases alone is not enough to remove the risk.

Last year’s historic Paris climate agreement set the goal of keeping global temperatures no higher than 1.5 degrees Celsius above preindustrial levels. Emission reductions will be central to achieving that goal, but supplemental efforts can further reduce risks.

One drastic idea is solar geoengineering — injecting light-reflecting sulfate aerosols into the stratosphere to cool the planet. Researchers know that large amounts of aerosols can significantly cool the planet; the effect has been observed after large volcanic eruptions. But these sulfate aerosols also carry significant risks. The biggest known risk is that they produce sulfuric acid in the stratosphere, which damages ozone. Since the ozone layer absorbs ultraviolet light from the sun, its depletion can lead to increased rates of skin cancer, eye damage, and other adverse consequences.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have identified an aerosol for solar geoengineering that may be able to cool the planet while simultaneously repairing ozone damage.

The research is published in the 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 David Keith, the Gordon McKay Professor of Applied Physics at SEAS and professor of public policy at the Harvard Kennedy School, the first author of the paper. “This research is a turning point and an important step in analyzing and reducing certain risks of solar geoengineering.”

This research fundamentally rethinks what kinds of particles should be used for solar geoengineering, said Frank Keutsch, the Stonington Professor of Engineering and Atmospheric Science at SEAS and professor of chemistry and chemical biology, a co-author of the paper.

Previous research focused on ways to limit the ozone-damaging reactions produced by nonreactive aerosols. But Keutsch and Keith, along with co-authors Debra Weisenstein and John Dykema, 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 Keutsch. “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.”


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