INVITED ABSTRACT
Geoengineering as an Optimization Problem

GEORGE BAN-WEISS (1), Ken Caldeira (1)

(1) Carnegie Institution, Dept of Global Ecology, Stanford

     Abstract Number: 424
     Last modified: May 12, 2010

Abstract
There is increasing evidence that Earth’s climate is currently warming due to emissions of greenhouse gases and aerosols from human activities, and Earth is projected to continue warming throughout this century. Scientists have begun to think about geoengineering options for reducing surface temperatures and whether such options could possibly contribute to environmental risk reduction. Perhaps the most widely discussed method is deliberately increasing aerosol concentrations in the stratosphere, which would scatter sunlight to space. Previous modeling studies have attempted to predict the climate consequences of hypothetical aerosol additions to the stratosphere. These studies have shown that this method could reduce surface temperatures, but could not recreate a low-CO2 climate in a high-CO2 world. In this study, we attempt to predict the latitudinal distribution of stratospheric aerosols that would most closely recreate a low-CO2 climate in a high-CO2 world. We find, using the NCAR CAM3.1 general circulation model, that to more closely reproduce low-CO2 temperature distributions, stratospheric aerosol concentrations would need to be greater in polar regions than tropical regions. However, such polar weighting tends to degrade the degree to which land runoff can be restored, and thus polar weighting of stratospheric aerosols does not markedly contribute to improved recovery of a low-CO2 climate. In the model, latitudinally varying aerosol distributions can diminish the r.m.s. zonal mean land temperature change from a doubling of CO2 by 94% and the r.m.s. zonal mean land runoff change by 74%.