this decrease in volume is larger than the decrease in sea-ice concentration, indicating thinning of the sea ice. Substantial sea-ice loss occurs at all longitudes especially in the west Antarctic. In summer the greatest loss is east of the peninsula. Ice loss at these levels suggests a reduction in the seasonal cycle of sea-ice concentration and volume and means a reduction of brine rejection at the margins of Antarctica. This has implications for the future formation of Antarctic deep water and therefore for global ocean heat transport (Sen Gupta et al., 2009).

Snow Cover

Snow cover depends on both temperature and precipitation and is strongly negatively correlated with air temperature. Due to its high albedo, much less solar energy is absorbed by snow-covered areas as contrasted with snow-free areas, and surface temperatures tend to be lower, especially during periods of snow melt. Snow also plays an important role in storing moisture that is released when temperatures rise above freezing. It is a significant variable in the water balance of snow-covered regions and plays an important role in water management, especially in the western United States, where much of the region’s runoff originates in snow that accumulates in mountainous areas in winter. Along with snowfall amounts and snow-covered area, snow water equivalent (SWE) and snow cover duration (SCD) are variables that can be used to characterize snow cover.

Current State of Snow Cover

In the Northern Hemisphere the snow-cover area (SCA) in March has exhibited a negative trend since about 1970, and this decrease has been associated with increased winter temperatures. This correspondence between March SCA and winter temperature appears to have strengthened toward the end of the 20th century (McCabe and Wolock, 2010). Since 1950, SWE has displayed a negative trend in the Pacific Northwest, particularly at lower elevation (Mote, 2003). A shift in the Pacific Decadal Oscillation (PDO) may have contributed to this trend but Mote et al. (2005), in an analysis expanded to include all of the western United States, argued that the PDO alone could not explain the trend in SWE. These conclusions are supported by Hamlet et al. (2005) who attributed the downward trend in SWE to increases in temperature rather than a decrease in precipitation. Figure 4.18 illustrates the observed trends in SWE as well as the trends simulated by the Variable Infiltration Capacity (VIC) hydrologic model. Although some positive trends



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