Reply on RC2

Abstract

Using two decades of satellite-based measurements of reflectance of solar radiation at the top-of-atmosphere and a complementary record of cloud properties, it is concluded that the loss of Arctic brightness due to sea ice retreat is not compensated by a pan-Arctic increase in cloudiness, but rather by a systematic change in the thermodynamic phase of cloud and a resultant effect on cloud reflectance. Liquid water content of the clouds has increased resulting in positive trends in susceptible cloud properties. Consequently, a cooling trend by clouds is superimposed on top of the pan-Arctic amplified warming, induced by the anthropogenic release of greenhouse gases, the ice albedo feedback and related effects. Except above the permanent and marginal sea ice zone around the Arctic circle, the rate of surface cooling by clouds has increased, both in spring (−32 % in total radiative forcing for the whole Arctic) and in summer (−14 %). The magnitude of this effect depends on both the underlying surface type and changes in the regional Arctic climate.