Geoscience Reference
In-Depth Information
h
e Disappearing Snowpack
h e American West depends on snow-bearing winter storms for its water
supply. In fact, the region is unique in the contiguous United States in its
dependence on the winter snowpack in the high mountains for a natural
water reservoir. h e snow begins melting in the late spring and continues into
the summer, i lling streams, lakes, and reservoirs that sustain ecosystems and
humans throughout the dry summer months. h e snowpack has supported
growth in cities and irrigated agriculture. In California, agriculture is an
industry that produces 55 percent of the nation's produce, making this semi-
arid region an important source of food for the nation. Snowpack provides
up to 80 percent of the annual water supply in this region.
A team of climatologists at the Scripps Institution of Oceanography,
located near San Diego, predicts that global warming will af ect this critical
snowpack in several ways. For example, less precipitation will fall as snow as
the region warms. For each degree of warming, the snowline moves up the
mountainsides by 500 feet. Considering the climate-warming projection for
the year 2100 of 5.4°F, the change means snow would fall 1,500 feet higher
on the mountainsides; the lower elevations would receive rain. Regions like
Northern California that have relatively low mountain elevations in the
Sierra Nevada (averaging 7,000 to 8,000 feet) would lose half of their area
of snowpack.
Studies have shown that the amount of the snowpack in California has
declined by 10 percent during the twentieth century. Climate-modeling pre-
dictions suggest that, by the end of the twenty-i rst century, the snowpack
will be reduced by at least 40 percent and perhaps as much as 80 percent.
(h e amount ultimately depends on just how much warming the region
experiences in the future.)
In addition to changes in the volume and location of snow, there will be
a change in the timing of snowmelt. h e snow that does fall will melt earlier
in the spring, rather than during the late spring and early summer when it is
so critically needed. h e decrease in summer snowmelt will only exacerbate
the future drying that will come with warmer conditions during the summer
months.
In the 1990s, the U. S. Geolog ica l Sur vey, Department of Water Resources,
and other California agencies began continuous monitoring of streaml ows,
temperature, and chemistry of the Tuolumne and Merced rivers in Yosemite
National Park. Changes in stream chemistry provide information about
