Nowhere is global warming more evident than in the Arctic. The high sensitivity of the Arctic climate is determined by the strong amplification of perturbations in radiative forcing by sea ice. Clouds interfere with ice effects on both the shortwave and longwave radiative fluxes. The high reflectivity of low stratus clouds prevailing in the Arctic reduces the magnitude of the ice-albedo feedback during summer. However, clouds in the Arctic also warm the surface during almost the entire year by increasing the downwelling longwave flux. This thesis examines seasonal changes in clouds and sea ice, their effects on shortwave and longwave radiation, and the role of clouds in the recent Arctic sea ice decline.

Analysis of satellite observations shows that, for average cloud conditions, replacing sea ice with open ocean in the Arctic decreases the top-of-atmosphere albedo by about 0.2. This small but robust value causes a notable reduction in the reflected shortwave flux during the summer insolation maximum. In coupled models different cloud phase parameterizations and simulation of sea ice properties cause large differences in the surface net shortwave flux, partly explaining the wide range in predicted 21st Century sea ice area changes. The role of cloud longwave forcing on melt onset and duration is examined using ground-based observations from the Surface Heat Budget of the Arctic campaign and the TOVS Polar Pathfinder satellite data. The timing of melt onset is determined by the increase in cloud temperatures and high cloud liquid water content at the end of spring contributing 52 W m^-2 to the downwelling longwave flux. The tendency towards a longer melt period observed in the Arctic Pacific sector in the beginning of the 21st Century is similarly associated with larger downwelling longwave flux at the end of summer/early fall due to increased cloudiness and warmer cloud temperatures. Experiments with a sea ice thermodynamic model show that increased longwave cloud forcing can explain nearly 70% of the sea ice thinning in the NCAR CCSM3 model during the 21st Century. This thesis work undermines the traditional view of clouds as the “umbrella” protecting the Arctic Ocean surface from increased solar flux absorption. On the contrary, it shows that clouds actively contribute to the presently occurring sea ice decline.