Satellite data from 1982-99 reveal that the Arctic has higher surface temperature and more clouds in spring and summer but lower surface temperature and less clouds in winter (Wang and Key, 2003). Cloudiness in summer, fall, and winter in this region tends to have a net cooling effect at the surface. Evidently if seasonal cloudiness were not changing, surface warming would be more intense than observed.
Reference
Wang, X., and J. R. Key, 2003: Recent trends in Arctic surface, cloud, and radiation properties from space. Science, 299, 1725-1728.