Volume Changes Of Alaska Glaciers: Contributions To Rising Sea Level And Links To Changing Climate

Abstract

We have used airborne altimetry to measure surface elevations along the central flowline of 86 glaciers in Alaska, Yukon Territory and northwestern British Columbia (northwestern North America). Comparison of these elevations with contours on maps derived from 1950s to 1970s aerial photography yields elevation and volume changes over a 30 to 45 year period. Approximately one-third of glaciers have been re-profiled 3 to 5 years after the earlier profile, providing a measure of short-timescale elevation and volume changes for comparison with the earlier period. We have used these measurements to estimate the total contribution of glaciers in northwestern North America to rising sea level, and to quantify the magnitude of climate changes in these regions. We found that glaciers in northwestern North America have contributed to about 10% of the rate of global sea level rise during the last half-century and that the rate of mass loss has approximately doubled during the past decade. During this time, summer and winter air temperatures at low elevation climate stations increased by 0.2+/-0.1 and 0.4+/-0.2�C (decade)-1 respectively. There was also a weak trend of increasing precipitation and an overall lengthening of the summer melt season. We modeled regional changes in glacier mass balance with climate station data and were able to reproduce altimetry measurements to within reported errors. We conclude that summer temperature increases have been the main driver of the increased rates of glacier mass loss, but winter warming might also be affecting the glaciers through enhanced melt at low elevations and a change in precipitation from snow to rain, especially in maritime regions. Uncertainties in our calculations are large, owing to the inaccuracies of the maps used to provide baseline elevations, the sparsity of accurate climate data, and the complex and dynamic nature of glaciers in these regions. Tidewater, surging, and lake-terminating glaciers have dynamical cycles that are not linked in a simple way to climate variability. We found that regional volume losses can depend on one or several large and dynamic glaciers. These glaciers should be treated separately when extrapolating altimetry data to an entire region.