Impact of geoengineering schemes on the global hydrological cycle

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 105; no. 22; pp. 7664 - 7669
• Main Authors: Bala, G, Duffy, P.B, Taylor, K.E
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 03-06-2008; National Acad Sciences
• Subjects: Carbon dioxide; Carbon Dioxide - chemistry; Climate change; Climate cycles; Climate models; Computer Simulation; Earth; Engineering - methods; Environment Design; Environmental impact; Geoengineering; Geology - methods; Global climate models; Greenhouse Effect; Hydrology; Latent heat; Physical Sciences; Precipitation; Radiative forcing; Solar Activity; Surface temperature; Temperature; Water - chemistry; Water cycle
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0711648105

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% K⁻¹ for solar forcing, but only 1.5% K⁻¹ 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.