Lamont-Doherty Earth Observatory, Columbia University,
"Responses of the Climate to Solar Geoengineering As Simulated by Reducing the Solar Constant"
Solar geoengineering, or the artificial cooling of Earth by reflecting sunlight, has recently been a topic of interest in the climate modeling community, leading to the Geoengineering Model Intercomparison Project (GeoMIP). In the GeoMIP G1 experiment, the solar constant was reduced in fully coupled climate models by an amount tuned to maintain preindustrial global mean temperatures under quadrupled CO2, in order to see what residual climate changes exist under opposing solar and greenhouse gas forcings. In this talk I will present an overview of my Ph.D. work on the G1 experiment, in which I studied changes in meridional temperature gradients, tropical precipitation, cloud cover, and the atmospheric radiative balance, and developed a theory explaining the amount of solar constant reduction required for each model. G1 exhibits a pattern of tropical cooling and polar warming, owing to the imposition of a net negative forcing in the tropics and a net positive forcing at the poles. Atmospheric poleward heat transport is reduced, helping to explain why most of the polar amplification of warming in quadrupled-CO2 experiments is canceled in G1. The seasonal migration of the tropical rain belt is damped in G1 due to preferential cooling of the summer hemisphere by the solar reduction, and its annual mean position also shifts in some models. Low cloud cover exhibits a broad, robust decrease, due to reductions in boundary layer inversion strength over the ocean and plant physiological responses to increased CO2 over land, while high cloud fraction increases in the global mean in most most models. The required solar constant reduction to achieve energy balance in G1 varies between 3.2% and 5.0%, depending on the model, and is uncorrelated with the models’ equilibrium climate sensitivity, while a formula from the experiment specifications based on the models’ effective CO2 forcing and planetary albedo is well correlated with but consistently underpredicts the required solar reduction. I showed that the required solar constant reduction in G1 can be explained to within about 6% by the sum of the instantaneous CO2 forcing and the rapid radiative adjustments to the combined CO2/solar forcing. This formula could make future runs of G1 easier through better initial guesses of the solar constant reduction, although tuning is still necessary because rapid adjustments are not known before running the model. In the real world, advance knowledge of the rapid adjustments to the geoengineering forcing could only be obtained through large-scale outdoor field tests, which would raise serious ethical and governance questions. Beyond geoengineering, the G1 experiment raises interesting questions about the linearity of the climate response to solar and CO2 forcings, which will require solar-forcing-only experiments in a multi-model framework to fully explore.