Glaciar Upsala, a freshwater calving glacier in southern Patagonia, has been retreating since 1978, and after a drastic recession of about 700 m a−1 in 1994 the retreat seems to have stopped in 1995. A large ice-thinning rate of 11 m a−1 was obtained between 1990 and 1993, by surveying surface elevations near the terminus of Glaciar Upsala. In 1993–94, the thinning was estimated at about 20 m a−1 near the lateral margin. Some possible causes of the thinning behavior are considered.
In the ablation area of Glaciar Perito Moreno, 50 km south of Glaciar Upsala, ablation rates were measured during 110 d in summer 1993–94, and air temperature was continuously recorded throughout 1994. Using a degree-day method with temperature data at the nearest meteorological station, Calafate, annual ablation during the last 30 years was estimated to fluctuate from about 12 ± 2 to 16 ± 2 m a−1 in ice thickness, with a mean of 14 ± 2 m a−1. Thus, the temperature anomaly alone cannot elucidate the thinning of 11 m a−1 at Glaciar Upsala. As a possible mechanism of the ice-thinning, it is suggested that the considerable retreat due to calving may have resulted in reduction of longitudinal compressive stress exerted from bedrock rises and islands near the glacier front, causing a considerable decrease in the emergence flow. Thus, the ice may have thinned at a rate close to the annual ablation rate.