IMPACTS OF CLIMATE ENGINEERING USING STRATOSPHERIC AEROSOLS

'Geoengineering' is the idea of taking purposeful action to offset global greenhouse warming, in this case by artificially increasing Earth's reflectivity and thus reducing the solar energy available to the climate system. This project uses climate models, particularly the Community Earth System Model (CESM) to examine the potential consequences of injecting sulfate into the stratosphere to produce reflecting aerosols, mimicking the effect of large volcanic eruptions. The award is a continuation of previous work by the PI under AGS-1157525.Previous work by the PI and others raised concerns that stratospheric geoengineering would result in a reduction in monsoon rainfall, with negative impacts on tropical agriculture. Here the PI proposes that the negative impacts could be mitigated using a geoengineering technique that preferentially cools the Southern Hemisphere ocean. The idea is based on research suggesting that cooling the Southern Hemisphere pushes the intertropical convergence zone northward, thereby causing preferentially greater amounts of rainfall in Northern Hemisphere monsoon regions. Numerical experiments to test this idea would be conducted by artificially altering ocean surface albedo in CESM simulations.A second topic pursued here is the effect of stratospheric sulfate injection on terrestrial vegetation and agriculture. Previous work using vegetation models embedded in climate models suggests a small reduction in photosynthesis due to stratospheric aerosol injection, in part because scattering of sunlight by stratospheric aerosols increases diffuse surface insolation at the expense of direct sunlight (in addition to reducing total sunlight reaching the ground). But other effects could be important, in particular the increase in ultraviolet (UV) sunlight due to the reduction of stratospheric ozone as a consequences of aerosol injection (an effect seen following volcanic eruptions). The impact of increased UV on agricultural production is studied through the use of the Community Land Model (CLM), the land surface component model of CESM, using the CLM-Crop version which allows representation of agricultural crops. Some of the effort is devoted to developing CLM-Crop to include more crops and represent processes relevant to the UV impact.In addition to the work with CESM, the project also supports the Geoengineering Model Intercomparison Project (GeoMIP), a coordinated international project in which multiple climate modeling centers perform a specified set of simulations, so that the range of possible outcomes can be assessed. The project supports annual workshops for GeoMIP participants, and is also launching an intercomparison project for crop model simulations of the agricultural impacts of geoengineering. The crop model intercomparison effort is organized under the existing Global Gridded Crop Model Intercomparison (GGCMI) initiative.The broader impacts of this project stem from the need for studies such as this one to inform policy makers and the public regarding the potential costs and benefits of geoengineering schemes. In addition, the proposed developments to CLM-Crop will provide an enhanced resource for the research community. The project support a graduate student and a postdoc, thereby providing for the future climate science workforce.