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How do we know glaciers are shrinking?

Repeat photography and aerial / satellite photo analysis provide evidence of glacier loss in terms of shape and area. The USGS Benchmark Glacier project has collected mass balance data on a network of glaciers in Alaska, Washington, and Montana for decades, quantifying trends of mass loss at all sites. Extensive field data collection at these sites includes twice yearly visits to measure seasonal change of snow/ice at ablation stakes and snow-pit analysis to measure snow density for extrapolating ice gain or loss across the glacier surface.

Emerging technologies, complemented by the availability of satellite imagery, allow USGS scientists to create Digital Elevation Models (DEMs) that provide data on the elevation of glacier ice.  By comparing DEMS from different times, scientists calculate ice volume change on a regional scale, broadening the understanding of ice loss and its link to climate.  

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