Cirrus Cloud Thinning and the Energetics of the Hydrological Cycle

From energy budget considerations, the release of latent heat in the atmosphere can be linked directly to the difference between the net radiative balance at the top-of-the-atmosphere and at the surface. This link provides a useful framework for understanding and comparing changes in the hydrological cycle resulting from different types of climate forcings: greenhouse gases, aerosols, or a combination of the two, as in Solar Radiation Modification. We have investigated the influence of geoengineering via cirrus cloud thinning on the atmospheric energy budget and the hydrological cycle in a global climate model. The objective of cirrus cloud thinning is to enhance the outgoing longwave radiation from Earth, thereby cooling the climate. As justified in recent studies, we use ice crystal fall speed enhancement or ice crystal size increase as simple proxies for the full physics of the problem which involves complex microphysics. During the first simulation year, the changes between the engineered and the control climate are distinctly determined by the nature of the forcing. In this time period, we find that cirrus cloud thinning reduces the TOA radiative balance by 1-2 W m-2, while it enhances the surface radiative balance, which in combination leads to a ~2 W m-2 increase in turbulent fluxes from the surface to the atmosphere, ¾ of that coming from latent heat release. This leads to a ~1% global enhancement of precipitation. The longer term climatological mean gives different results, dominated by the response of the climate system to the cooling implied by cirrus cloud thinning. Due to a cancellation of the influence of the cooling and the initial enhancement of the hydrological cycle, the precipitation is almost unchanged in the engineered climate compared to the control climate, despite the 1-2 K cooling that is achieved. Hence, we conclude that cirrus cloud thinning - as opposed to Solar Radiation Management techniques - enhances the global hydrological cycle.