Using 4 years of observations between 2000 and 2003 from Barrow,Alaska,it is shown that clouds have a net warming effect on the Arctic surface during all months except June and July.Haze aerosols strengthen the cloud thermal emission by increasing the cloud thermal emissivity,an effect that is offset in summer when higher shortwave albedos dominate.Approximately one half of the net effect is caused by changes in cloud droplet effective radius and the remainder is caused by unknown atmospheric feedbacks that affect cloud liquid water path.The yearly averaged increase in surface cloud radiative forcing by haze pollution under cloudy skies is ~ 5.2 Wm-2.Assuming a low-level cloud occurrence frequency of 60% and that half of clouds are polluted,haze pollutants warm the surface by an estimated 1.6 Wm-2 (or 0.4 K in temperature) yearly averaged.