Seminar: Dr. John Dykema

Please join us as we welcome Dr. John Dykema, Project Scientist from the John A. Paulson School of Engineering and Applied Sciences at Harvard University, for a seminar titled "Accurate assessment of large-scale geoengineering impacts starts with accurate aerosol optical properties". Coffee will be served.

Abstract

The deliberate modification of the Earth’s albedo to partially offset the radiative forcing due to rising greenhouse gas concentrations is a type intervention that falls into the category of geoengineering. The risks and feasibility of such an intervention achieved by increasing stratospheric sulphate aerosol optical depth is being studied using ensembles of climate models. These studies indicate that there are a number of physical side effects, aside from sociopolitical risks, which are also serious. Sulphate aerosols significantly absorb infrared radiation, which heats the tropical tropopause layer, increasing the flux of water vapor into the stratosphere, accelerating ozone loss, and increasing radiative forcing. Other side effects include perturbations to the global hydrological cycle and increases in clear-sky diffuse radiation. These side effects have motivated the investigation of alternatives, including solid aerosol materials. The high refractive index of some solid materials may lead to reduction in these risks associated with the radiative properties of aerosols intended to scatter solar radiation back to space. A new analysis of the scattering efficiency and absorption of a range of candidate solid aerosols will be presented. This analysis utilizes a comprehensive radiative transfer model driven by updated, physically consistent estimates of optical properties. As part of the analysis, the potential increase in stratospheric water vapor and associated longwave radiative forcing is computed. It is found that the stratospheric heating calculated in this analysis indicates some materials to be substantially riskier than previous work. Another finding is that there are Earth-abundant materials that may reduce some principal known risks relative to sulphate aerosols. An overview will be provided of laboratory experiments to improve the understanding of the fundamental physical and chemical properties of solid aerosols suggested for albedo modification. The role of laboratory experiments and process studies will be discussed as part of an overall strategy for improving the representation of these aerosols to improve accuracy of risk assessments