"Geoengineering" is the idea of taking purposeful action to mitigate the effects of inadvertent global greenhouse warming. This project uses climate models to simulate the effects of "Solar Radiation Management" (SRM) schemes in which sulfate (SO2) is injected into the stratosphere to produce reflecting aerosols, or marine clouds are brightened through artificial enhancements in the number concentration of cloud condensation nuclei in regions of persistent marine stratus clouds. In both cases the goal is to cool the earth by reflecting sunlight back into space to counteract the global warming produced by anthropogenic greenhouse gases. The goal of the project is to understand both the potential effectiveness of and the unintended consequences of SRM schemes. Previous work, including the PI's work under the previous NSF award, suggests that SRM through stratospheric aerosol generation could result in reductions in summertime precipitation, particularly in regions dependent on summer monsoon rainfall for food production. The work conducted under this award addresses four questions:
1) How would stratospheric geoengineering affect the planet's hydrological cycle, including the rainfall of the African and Asian summer monsoons?
2) How would stratospheric geoengineering affect the planet's ocean and long-term climate? Long-term changes of climate forced by stratospheric geoengineering would be controlled to a large extent by the responses of ocean temperature and circulation. How will the frequencies of El Niño and La Niña change during geoengineering? Will a cooling signal propagate deep into the ocean, as has been observed and modeled after volcanic eruptions, and how will this affect the long-term climate response to geoengineering?
3) Are there strategies of sulfur injection into the stratosphere with particular latitudinal and seasonal patterns that can produce desirable climate changes while avoiding undesirable ones? For example, could spring and summer high-latitude injections maximize the cooling effect at high latitudes, while avoiding lower latitude precipitation impacts and wasteful winter injection when there is little sunlight?
4) How would the changes in climate that result from stratospheric geoengineering affect food production? For example, if there is a reduction of summer precipitation over India and China, accompanied by cooling and less insolation, what would be the net effect on crop production?
These questions are addressed using in-house climate model experiments using models capable of representing the emission and subsequent transport and chemical processes through which aerosols form, grow, circulate and interact chemically and radiatively until they are removed from the atmosphere. In addition to the in-house modeling effort, the research includes analysis of model output from the Geoengineering Model Intercomparison Project (GeoMIP), which the researcher organized under a previous NSF award. GeoMIP consists of a set of geoengineering experiments conducted at over a dozen modeling centers worldwide. The experiments differ in the amount and timing of assumed greenhouse gas increase and counteracting SRM effort. The GeoMIP activity will serve to determine whether the reductions in monsoon precipitation found in previous studies are a robust behavior that can be reproduced across a variety of climate models with different formulations of physics, dynamics, and atmospheric chemistry. The potential reduction in food production due to reduced monsoon rainfall will be addressed through crop modeling simulations using the Decision Support System for Agrotechnology Transfer (DSSAT) model, which will be driven by output from climate models.
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.
