Snowpack Glaciers and Snowmelt

Worldwide, snow cover is decreasing, although substantial regional variability exists (Lemke et al., 2007; Slaymaker and Kelly, 2007). Since the 1920s, Northern Hemisphere snow cover has steadily declined (Figure 8.2), despite increased precipitation. Between 1966 and 2005, the total area of Northern Hemisphere snow cover shrank by approximately 1.4 percent per decade. In the Southern Hemisphere, there has been no significant trend in South American snow cover, and data are sparse and inconclusive in Australia and New Zealand.

In the United States, snowpack changes in the West currently represent the best-documented hydrological manifestation of climate change (e.g., Barnett et al., 2008; Pierce et al., 2008). About half of the observed decline in western snowpack, and resulting changes in the amount and seasonality of river discharge, can be linked to a warming climate. The largest losses in snowpack are occurring in the lower elevations of the mountains of the Northwest and California, because higher temperatures are causing more precipitation to fall as rain instead of snow. Moreover, snowpack is melting as much as 20 days earlier than the historical average in many areas of the West (Kapnick and Hall, 2009; Kim and Waliser, 2009; Stewart et al., 2005). Snow is expected

FIGURE 8.2 Area of Northern Hemisphere covered by snow in the spring. There is an overall trend toward a decrease in the area covered by snow for the entire period (1922-2005). The black dots correspond to individual years, the smooth black line shows decadal variations, and the yellow area indicates the 5 to 95 percent confidence range associated with decadal variations. SOURCE: Lemke et al. (2007).

FIGURE 8.2 Area of Northern Hemisphere covered by snow in the spring. There is an overall trend toward a decrease in the area covered by snow for the entire period (1922-2005). The black dots correspond to individual years, the smooth black line shows decadal variations, and the yellow area indicates the 5 to 95 percent confidence range associated with decadal variations. SOURCE: Lemke et al. (2007).

1920 1930 1940 195Û 1960 1970 19S0 1990 2000 2010

Year

1920 1930 1940 195Û 1960 1970 19S0 1990 2000 2010

Year

Jan Feo Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month ChnsosnsEn eîbî.

FIGURE 8.3 An example of how the timing and amount of runoff is projected to change following warming in the 21st century. The black line shows the amount of stream flow occurring in the Green River, which is part of the Colorado River Basin. The stream flow of the Green River is dominated by the timing and amount of snowmelt, and peak flows historically have occurred around June. Warming in the twenty-first century would tend to decrease snowfall during the winter and accelerate the timing and pace of snowmelt, leading to earlier peak flows and overall less stream flow (red line). SOURCE: USGCRP (2009a); data from Christensen et al. (2004).

Jan Feo Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month ChnsosnsEn eîbî.

FIGURE 8.3 An example of how the timing and amount of runoff is projected to change following warming in the 21st century. The black line shows the amount of stream flow occurring in the Green River, which is part of the Colorado River Basin. The stream flow of the Green River is dominated by the timing and amount of snowmelt, and peak flows historically have occurred around June. Warming in the twenty-first century would tend to decrease snowfall during the winter and accelerate the timing and pace of snowmelt, leading to earlier peak flows and overall less stream flow (red line). SOURCE: USGCRP (2009a); data from Christensen et al. (2004).

to melt even earlier under projections of future climate change, resulting in reduced later-summer stream flows (Figure 8.3). This change would have major implications for ecosystems, hydropower, urban and agricultural water withdrawals, and requirements for other water uses. In regions where the summer growing season is the dry season, as in much of the western United States, this concentration of runoff in the spring and reduction in summer will stress water supply systems and could lead to summer water shortages (Barnett et al., 2005b; Cayan et al., 2009).

Finally, as discussed in Chapter 7, nearly all of the world's glacier systems are shrinking, and in many cases their rate of ice loss has been accelerating. Disappearing glaciers are ultimately expected to lead to reductions in river flows during dry seasons and lost water resources for the hundreds of millions of people who rely upon glacier-fed rivers worldwide (Barnett et al., 2005b). Changes in glacier-stream flow interactions are also expected to lead to changes in ecosystems and in water quality (Milner et al., 2009).

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