Enhancement of the albedo of low stratus marine clouds. : Simulations in a global climate model

Abstract

Anthropogenicgreenhouse gases have become widely accepted as the primary cause of global warming. With increasing emissions rates, warming is set to continue and baring any drastic change in mitigation policies, the rate of warming is also likely to continue to increase. According to the IPCC, the increase in radiative forcing associated with a doubling of pre-industrial CO2 concentrations is estimated to be 3.7 Wm-2.

One method suggested to ameliorate the warming is to increase the effects of short wave cloud forcing (SWCF) via geo-engineering and thus restoring the earth’s radiative equilibrium. The short wave cloud forcing has a cooling effect by reflecting solar radiation back into space. Twomey [1977] suggested that by increasing the cloud condensation nuclei in thin to moderately thick clouds the optical depth, and thus the albedo of the cloud would increase.

In this study an earth system model is used to explore the effects of deliberately increasing the cloud droplet number concentrations in low level marine clouds. The effects of such increase on the short wave cloud forcing are examined as well as on the effective radius of cloud droplets. Three different geo-engineering cases are presented and compared to a control simulation.

The results show that adding CDNC in marine clouds have the desired impact on SWCF. Effects are increasing with with rising CDNC. For a perturbation of 375 the changes in SWCF are large enough to counteract the increase in radiative forcing by greenhouse gases. The lower CDNC perturbation of 50 cm 3 also produce a moderate cooling effect, without completely counteracting the predicted greenhouse gas radiative forcing.