Cryosphere during the twentieth century

During the 20th century earth's ice bodies underwent substantial changes. This article reviews the present state of the cryosphere and investigates important variations observed in the seasonal snow cover, sea ice and glaciers. The seasonal snow cover area increased slowly until about 1970 when the area rapidly started to recede. The fastest recession was observed from the 1970s to the 1980s with a mean loss of 1.45 x 10(6) km(2). The most significant retreat occurred in late spring, amounting to 3 X 10(6) km(2) in area and shifting the end of the snow cover season by 3 weeks. While the sea ice extent decreased by 2.8%/decade in the north polar region, it increased by 1.3%/decade in the Antarctic during the last quarter century. The sea ice in the Arctic lost 40% of its thickness during the same time. The retreat of the sea ice limit was most significant in the warmer season. Most mountain glaciers and small ice caps showed significant mass loss during the second half of the last century. The mean increase in the equilibrium line altitude of 36 glaciers was 200 in for the second half of the 20th century. The small number of glaciers with a positive balance also showed increasing rates of melting. The global average specific mean mass balance for the last three decades was -265 mm y(-1) or a loss of 135 km(3) y(-1) corresponding to a 0.4 mm y-1 sea level rise. The rate of mass loss accelerated at 12 mm y(-2). The last decade showed an especially strong loss amounting to -475 mm y(-1), -242 km(3) y(-1) or + 0.7 mm y-1 sea level equivalent. The mass balance of Greenland is negative: -50 to -90 km(3) y(-1), or -34 to -52 mm y(-1) in mean specific balance, corresponding to a sea level contribution of 0.15 to 0.25 mm y(-1). The range of uncertainty for the Antarctic Ice sheet is still large. There is a report giving much less annual loss than the last IPCC report. The present work proposes 260 km(3) y(-1) as the annual total discharge of the terrestrial cryosphere for the second half of 20th century, corresponding to +0.7 mm y(-1) sea level equivalent. The most significant change in the Antarctic is the loss of ice shelves. Since the disappearance of an ice shelf tends to accelerate the flow of the background glaciers directly into the ocean, this may become a dominant mechanism in the future by which the Antarctic ice sheet can influence the sea level.