The Arctic climate is particularly susceptible to change induced by increases in greenhouse gases, as warming occurs faster than in the lower latitudes. This is because of snow and ice; as these melt, the darker land and ocean surfaces they expose absorb more solar energy. This extra energy then goes into warming the atmosphere. This process may already be underway. Recent years have experienced record minima in Arctic sea-ice extent, while surface air temperatures have increased over much of the Arctic (Fig. 1,2). Additional declines of approximately 10-50% in annual average sea-ice extent are projected to occur by 2100 (Fig 3).

Research at RSMAS aims to increase our understanding of this climatically-important and data-sparse region. One aspect focuses on the role played by clouds. Clouds increase surface warming during the winter through their infrared emissions, but cool the surface during the summer by blocking solar radiation. Their presence or absence has an important influence on both the onset date of snow-melt, and the initiation of surface freezing. Fig. 4 describes an ongoing observational study to understanding the radiative impact of Arctic clouds.

Fig. 1, Right: Sea ice concentration Anomalies for September 2002, 2003, and 2004, along with the 1979-2000 median September ice edge (pink line), derived from passive microwave satellite imagery. These reveal that sea ice extent reached a record minimum in Sept. 2002, followed by two more low-ice years. While sea ice decline can result from natural variability associated with the dynamical Arctic Oscillation (AO), greenhouse warming also favors the AO phase most conducive to warming. Image courtesy of NSIDC, Boulder, CO (http://nsidc.org/)  (Click thumbnail for larger image with caption.)

	Figure 2 (Left) Arctic surface air temperatures have increased in the past 50 years in Alaska and Siberia, with a cooling in Southern Greenland. (Click thumbnail for larger image with caption.)
	Figure 3 (Left) Additional declines of roughly 10-50% in annual average sea-ice extent are projected by 2100 in model simulations. Loss of sea ice is projected to be greater during summer than in the annual average. Top and left Figures provided by The Arctic Climate Impact Assessment (http://www.acia.uaf.edu). (Click thumbnail for larger image with caption.)

Arctic clouds are often complicated to understand because both liquid and ice phases are present at sub-freezing temperatures. Though the direct radiative impact of an Arctic cloud is usually determined by its liquid amount, the ice phase helps regulate the cloud lifecycle. In the example below, a combination of surface-based and aircraft data were necessary to characterize this long-lived super-cooled cloud. An interesting finding was that the variable entrainment of aerosol lying above the boundary layer cloud affected variations in the transition of liquid to ice (note differences in the ice water content for May 3 and 5 in the right-hand panel). In this case a polluted air layer, probably originating from Siberia, lay above the cloudy layer.

Figure 4: The top panel shows the ice water content with the liquid cloud base superimposed. On May 3, the vertical velocity at the top of the boundary layer was upward, and little aerosol from the atmosphere above was entrained. In contrast, on May 5, subsidence was strong. The entrained aerosol acted as contact nuclei for the liquid droplets, converting them into ice.

Figure 5: The lower panel shows the liquid water path.The above panel shows observations gathered during May 1-8 of the Surface Heat Budget of the Arctic experiment. The near-surface temperature was -20 C, and the inversion at -10 C. Note the depletion of the liquid water path when higher clouds seeded ice into the cloud below.