Changes of Glaciers and Climate in Northwestern North America during the Late Twentieth Century

About 75% of 46 glaciers measured using repeat airborne altimetry in Alaska and northwestern Canada have been losing mass at an increasing rate from the mid-1990s to the middle of the first decade of the twenty-first century, relative to an earlier period beginning in the 1950s-70s. The remaining glaciers have been either gaining mass during the more recent period or continuing to lose mass, but at a decreasing rate. Temperature and precipitation data at 67 climate stations were examined to explain these changes. Nearly all significant changes in winter (October-April) and summer (May-September) air temperatures were positive (2.0 degrees +/- 0.8 degrees and 1.0 degrees +/- 0.4 degrees C) between 1950 and 2002, and all seasonally averaged values of freezing level heights (FLH) increased during the same time period. A small increase in precipitation was observed, but these changes were significant at only 17% of the stations. Regional glacier changes, modeled using mass balance sensitivities and climate station temperature and precipitation changes, agreed with observations to within the limits of reported errors. Seasonal variations in accumulation resulted in large uncertainties in the recent period mass variations. In nearly all regions, increasing summer temperatures accounted for most of the glacier mass losses. FLH variations show that the maritime glacier systems are more sensitive to variations in the mean position of the winter FLH than interior regions, suggesting that strong winter warming has affected these regions in addition to the summer changes. These measurements augment the increasingly strong evidence of late twentieth-century climate change in northwestern North America.