DIVECHA CENTRE FOR CLIMATE CHANGE
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The rapidly rising CO2 level in the atmosphere 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 article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show 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% K1 for solar forcing, but only 1.5% K1 for CO2 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.