Impact of geoengineering schemes on the global hydrological cycle

Background/Question/Methods Rapidly rising atmospheric CO2 has led to proposals of climate stabilization by “geoengineering” schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earth's surface. In this talk I address the impact of these climate stabilization schemes on the global hydrological cycle. We performed and analyzed equilibrium simulations using a global climate model consisting of an atmospheric general circulation model, version 3 of the Community Climate Model (CCM3) developed at the National Center for Atmospheric Research, coupled to a terrestrial biosphere model, the Integrated Biosphere Simulator or IBIS. The horizontal resolution is 2.8° in latitude by 2.8° in longitude. CCM3 was coupled to a slab ocean-thermodynamic sea ice model, which allows for simplified representation of the interactions with the ocean and sea ice components of the climate system. To ensure realistic sea surface temperatures and ice distributions for the present climate, the slab ocean model employs a spatially and temporally varying prescribed ocean heat transport and spatially varying mixed-layer depth.

Results/Conclusions We found that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO2 forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4% K-1 for solar forcing, but only 1.5% K-1 for CO forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.