Tibetan Plateau - Tibet During The Ice Age

Tibet During The Ice Age

Today Tibet is the most essential heating surface of the atmosphere. During the Last Ice Age a c. 2,400,000 square kilometres (930,000 sq mi) ice sheet covered the plateau.This glaciation took place in correspondence to a lowering of the snowline by 1,200 metres (3,900 ft). For the Last Glacial Maximum this means a depression of the average annual temperature by 7 to 8 °C (13 to 14 °F) at a minor precipitation compared with that one of today. Owing to this drop in temperature a supposed drier climate has partly been compensated with regard to the glacier feeding by a minor evaporation and an increased relative humidity. Due to its great extension this glaciation in the subtropics was the most important climatically foreign element on earth. With an albedo about 80-90% this ice area of Tibet has reflected an at least 4 times greater global radiation energy per surface into space than the further inland ices at a higher geographical latitude. At that time the most essential heating surface of the atmosphere - which at present, i.e. interglacially, is the Tibetan plateau - was the most important cooling surface. The annual low-pressure area induced by heat above Tibet as a motor of the summer monsoon was lacking. The glaciation thus caused a breaking-off of the summer monsoon with all the global-climatic consequences, e.g. the pluvials in the Sahara, the expansion of the Thar desert, the heavier dust influx into the Arabian Sea etc., and also the downward shifting of the timber line and all forest-belts from the alpine-boreal forests as far down as to the semi-humid mediterranean forest which has replaced the Holocene monsoon-tropical forests on the Indian subcontinent. But also the movements of animals including the Javan Rusa far into South Asia are a consequence of this glaciation. Despite a great evaporation ablation caused by heavy insolation, the discharge of the glaciers into the Inner-Asian basins was sufficient for the creation of meltwater lakes in the Qaidam Basin, the Tarim Basin and the Gobi Desert. The drop in temperature (see above) was in favour of their development. Thus, the clay fraction produced by the ground scouring of the important glaciation was ready to be blown-out. The blow-out of the limnites and the Aeolian long-distance transport were connected to the katabatic winds. Accordingly, the Tibetan glaciation was the actual cause of the enormous loess production and the transport of the material into the Chinese middle- and lowlands continuing to the east. During the Ice Age the katabatic air current - the name 'winter monsoon' is not quite correct - blew all year round. The enormous uplift of Tibet by approx. 10 mm/y measured by triangulations since the 19th century and confirmed by glaciogemorphological findings as well as by seismological investigations equals the uplift of the Himalaya. However, these amounts of uplift are far too important as to a primarily tectonic uplift of the high plateau which only takes place epirogenetically. Actually they can be understood the better by a superimposed glacioisostatic compensation movement of Tibet about 650 metres (2,130 ft).