Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming.

The authors attribute significantly increased Greenland summer warmth and Greenland Ice Sheet melt and runoff since 1990 to global warming. Southern Greenland coastal and Northern Hemisphere summer temperatures were uncorrelated between the 1960s and early 1990s but were significantly positively correlated thereafter. This relationship appears to have been modulated by the North Atlantic Oscillation, whose summer index was significantly (negatively) correlated with southern Greenland summer temperatures until the early 1990s but not thereafter. Significant warming in southern Greenland since ϳ1990, as also evidenced from Swiss Camp on the west flank of the ice sheet, therefore reflects general Northern Hemisphere and global warming. Summer 2003 was the warmest since at least 1958 in coastal southern Greenland. The second warmest coastal summer 2005 had the most extensive anomalously warm conditions over the ablation zone of the ice sheet, which caused a record melt extent. The year 2006 was the third warmest in coastal southern Greenland and had the third-highest modeled runoff in the last 49 yr from the ice sheet; five of the nine highest runoff years occurred since 2001 inclusive. Significantly rising runoff since 1958 was largely compensated by increased precipitation and snow accumulation. Also, as observed since 1987 in a single composite record at Summit, summer temperatures near the top of the ice sheet have declined slightly but not significantly, suggesting the overall ice sheet is experiencing a dichotomous response to the recent general warming: possible reasons include the ice sheet's high thermal inertia, higher atmospheric cooling, or changes in regional wind, cloud, and/or radiation patterns.

Abstract. The Greenland climate network has currently 18 automatic weather stations (AWS) distributed in most climate regions of the ice sheet. The present network captures well the regional climates and their differences in the accumulation region of the ice sheet. An annual mean latitudinal temperature gradient of-0.78øC / 1 ø latitude was derived from the AWS data for the western slope of the ice sheet, and an annual mean latitudinal temperature gradient of-0.82øC / 1 ø latitude was derived for the eastern slope. The mean annual lapse rate along the surface slope is 0.7 IøC / 100 m, with monthly mean lapse rates varying between 0.4øC / 100 rn in summer and 1.0øC / 100 rn in winter. The annual range of monthly mean temperatures is between 23.5øC and 30.3øC for the western slope of the ice sheet, with increasing ranges from south to north and with increase in elevation. The annual mean air temperature was found to be 2øC warmer for the central part of Greenland for the time period 1995-1999, as compared to the standard decade 1951-1960. This annual mean temperature change decreased to approximately iøC for the elevation 1000-2000 m, whereas at lower elevations, no AWS data are available with sufficient spatial and temporal coverage to verify any temperature trend. Firn temperatures (10-m depth) at highelevation sites were found to be colder than the mean annual air temperature of the preceding year for the central part and northern Greenland by as much as 2.5øC. In the percolation zone and at the equilibrium line altitude the firn and ice temperatures at 10 rn were consistently warmer than the annual mean air temperature because of percolation of meltwater and the isolation effect of the snow cover. The wind speed and direction are affected by the katabatic outflow of the cold air along the slope of the ice sheet, whereas at higher elevations the large-scale synoptic condition is the dominant factor that governs the wind field. The surface height change at high elevations (accumulation minus sublimation) can be approximated with a linear model over an annual cycle using AWS data, whereas in the ablation region and along the equilibrium line altitude the surface height change shows a strong annual cycle.