Zhang Qingsong
The Antarctic ice cap at a volume of 24.5 million cubic kilometres, occupying 98 percent of Antarctic area is the largest water resource of the world. The average thickness of the ice cap is about 2,000 m and the maximum reaches 4,800 m. Its formation and evolution play an extremely important role in the generation of climate of the world. The Antarctic ice cap is composed of continental ice sheet, ice shelves and sea ice.This paper mainly deals with their form, thickness, temperature, density, flow, transformation, accumulation and melt, as well as the history of the continental ice sheet.The continental ice sheet began to form around 20 million years (Miocene)ago. By about 5 million years (middle pliocene) ago the ice sheet had grown to be something approaching its present volume. The present Antarctic ice cap is a remnent of the last ice age (120,000 to 18,000 years ago)which was much larger and thicker than what is today with its northern limit extenting to 40 N. Therefore, the Antarctic ice cap had retreated obviously in the last 18,000 years.As known to all, the Antarctica is the coldest and the driest continent of the world. Mean annual temperature is about -25℃ all over the continent, -55℃ in inland plateau, and -171℃ in coast region. Average annual precipitation is about 200-300 mm at the coast and less than 50 mm in the inland. The precipitation is almost all in snowfall except Antarctic penisula and the supplement for the continental ice sheet all depends on atmospheric precipitation. Though snowfall in the inland plateau is very few, as the humidity of air could become saturated with 5-10 per cent of moisture under tremendous freezed condition, the fresh snow soon change to firn which does not melt even in summer. Hence evaporation is extremely weak, the loss of ice would be very few. The ice layer formed each year averages 10 cm in VOSTOK (USSR) and 40-80 cm in CASEY(Australia).Owing to the huge compression of the ice cap itself,ice flows move from the centre of the plateau to the coast. The velocity of ice flow is 1-10 m/y in inland and 20-60 m/y at coast, while for glaciers, the maximum velocity could reach 300-400 m/y or even 700 m/y.The temperature of upper layer of ice sheet (0-10 m deep) roughly coincides with the mean annual temperature elsewhere and it rises with the increase of depth. Measurement reveals that it could reach as high as round 0-- 1℃ at the underground surface. That is why the ice cap moves downward under very high pressure.Ice shelves including Ross ice shelf, Filchner ice shelf and many others totaling more than 1.4 million square kilometres are extremely great moving ice. There are still different ideas about the formation of the ice shelves. But generally, ice shelves near the Antarctic cycle, such as the Amery ice shelf, are the extension of glacial front into the sea,and most oi them are suplied by glaciers. Besides glaciers,the Ross and Filchner ice shelves (located in higher latitude area), are mostly fed by accumulated snow. Ice shelves are more active than continental ice sheet, and the rate of their movement could reach 1,200-1,400 m/y.The icebergs derived from ice shelves and glaciers are very impor-ant water resource. In general, they appeared at the south of 60°S, only a few biggest ones can cross the Antarctic convergence reaching 40°-50° S. The wish that the water resoure of the icebergs could be used, would become true in next 10-20 years.
