Abstract. Two decades of measurements of spectral reflectance of solar radiation at
the top of the atmosphere and a complementary record of cloud properties
from satellite passive remote sensing have been analyzed for their
pan-Arctic, regional, and seasonal changes. The pan-Arctic loss of
brightness, which is explained by the retreat of sea ice during the current
warming period, is not compensated by a corresponding increase in cloud
cover. A systematic change in the thermodynamic phase of clouds has taken
place, shifting towards the liquid phase at the expense of the ice phase.
Without significantly changing the total cloud optical thickness or the
mass of condensed water in the atmosphere, liquid water content has
increased, resulting in positive trends in liquid cloud optical thickness
and albedo. This leads to a cooling trend by clouds being 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 over the permanent and parts of the 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.