Non-Technical General Interest

What’s the Least Bad Way to Cool the Planet?

What’s the Least Bad Way to Cool the Planet?

October 1, 2021

New York Times, Guest Essay

What’s the Least Bad Way to Cool the Planet?

By David Keith

Dr. Keith is a professor of applied physics and of public policy at Harvard, where he led the development of the university’s solar engineering research program. He is also a co-host of the podcast “Energy vs Climate” and the founder and a board member of the company Carbon Engineering, which provides technology to capture carbon dioxide from the...

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Matthias Honegger, Steffen Munch, Annette Hirsch, Christoph Beuttler, Thomas Peter, Wil Burns, Oliver Genden, Timo Goeschl, Daniel Gregorowius, David Keith, Markus Lederer, Axel Michaelowa, Janos Pasztor, Stefan Schafer, Sonia Seneviratne, Andrea Stenke, Anthony Patt, and Ivo Wallimann-Helmer. 5/2017. Climate change, negative emissions and solar radiation management: It is time for an open societal conversation. Risk-Dialogue Foundation St.Gallen for the Swiss Federal Office for the Environment.
David Keith, Debra Weisenstein, John Dykema, and Frank Keutsch. 12/12/2016. “Stratospheric Solar Geoengineering without Ozone Loss.” Proceedings of the National Academy of Sciences. Publisher's VersionAbstract

Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.