John Dykema

David Keith, Debra Weisenstein, John Dykema, and Frank Keutsch. 12/12/2016. “Stratospheric Solar Geoengineering without Ozone Loss.” PNAS. Publisher's Version Abstract

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.

John Dykema, David Keith, and Frank Keutsch. 7/30/2016. “Improved aerosol radiative properties as a foundation for solar geoengineering risk assessment.” Geophysical Research Letters. Publisher's Version Abstract

Side effects resulting from the deliberate injection of sulfate aerosols intended to partially offset climate change have motivated the investigation of alternatives, including solid aerosol materials. Sulfate aerosols warm the tropical tropopause layer, increasing the flux of water vapor into the stratosphere, accelerating ozone loss, and increasing radiative forcing. The high refractive index of some solid materials may lead to reduction in these risks. We present a new analysis of the scattering efficiency and absorption of a range of candidate solid aerosols. We utilize a comprehensive radiative transfer model driven by updated, physically consistent estimates of optical properties. We compute the potential increase in stratospheric water vapor and associated longwave radiative forcing. We find that the stratospheric heating calculated in this analysis indicates some materials to be substantially riskier than previous work. We also find that there are Earth-abundant materials that may reduce some principal known risks relative to sulfate aerosols.

John Dykema

John Dykema

Project Scientist, Harvard John A. Paulson School of Engineering and Applied Sciences

John Dykema is a Project Scientist in the Harvard John A. Paulson School of Engineering and Applied Sciences. He focuses on the development of instrumentation (particularly infrared remote sensing systems) and validation of long-term data records for atmospheric model testing applications. From this perspective, quantitatively testing model performance in simulating geoengineering impacts provides a new set of observational challenges. Recent work has assessed the measurement requirements necessary to empirically quantify the risk of ozone loss posed by geoengineering through improved understanding of stratospheric chemistry and dynamics. He holds a Ph.D. in Applied Physics from Harvard University.