Annual mass balance of Blue Glacier, USA: 1955-97

Mass changes of Blue Glacier, USA are calculated from topographic maps made from vertical aerial photography in late summer of 1939, 1952, 1957, and 1 987. along with laser altimetry flown in June 1996. Changes in elevation be tween maps were adjusted for seasonal variations in the snow cover, and to account for the ablation between the date of photography and 1 October. Top ography obtained from the laser altimetry was adjusted for snow thickness a nd glacier motion to estimate topography of 1 October 1995. The mass of Blu e Glacier has changed less than 7 m (water equivalent) during this 56 year period which is minor compared with other glaciers in the region and elsewh ere in the world. Glacier-average annual mass balances, beginning in 1956, have been calculated either from stake measurements and probing of late-sea son snow, or from a regression analysis using late-season measurements of t he equilibrium line altitude. A comparison with the changes derived from su rface maps shows values obtained from field measurements are too positive b y about 0.5 m a(-1). indicating that considerable caution is needed when in terpreting time series of mass balance. Two alternative time series of mess balance consistent with the long-term mass changes are created by making s imple adjustments: (i) a single constant is subtracted from each value so t hat the series is consistent with the 1957-95 mass change; (2) one constant is subtracted from each value over 1957-87 and another is subtracted from each value over 1987-95 so that the series is consistent with both the 1957 -87 and 1987-95 mass changes. The mass balance of Blue Glacier was generally positive until the mid-1970s and negative since. The fluctuations of mass balance closely resemble thos e of snowfall on the glacier as estimated from the joins distribution of te mperature and precipitation. The climate in western Washington was cooler a nd wetter during the decade before the mid-1970s, but the trend since has b een towards warmer and drier conditions.