Last Glacial Maximum
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The Last Glacial Maximum refers to the time of maximum extent of the ice sheets during the last glaciation (the Würm or Wisconsin glaciation), approximately 20,000 years ago. The conditions of the Last Glacial Maximum persisted for probably two thousand years. At this time, all of Northern Europe, almost all of Canada and the northern half of the West Siberian Plain were covered by huge ice sheets extending roughly to the southern boundary of the Great Lakes in North America and to a line from the mouth of the Rhine River through Kraków, Moscow up to the mouth of the Anabar River in Russia.
Ice sheets covered the whole of Iceland and all but the southern extremity of the British Isles, whilst the Patagonian Ice Sheet covered southern Chile down to about 41 degrees south. Ice sheets also covered Tibet (scientists worldwide continue to debate the extent to which the Tibetan Plateau was covered with ice), Baltistan, Ladakh and the Andean altiplano. In Africa, the Middle East and Southeast Asia, many smaller mountain glaciers formed, especially in the Atlas, the Bale Mountains, and New Guinea.
The Ob and Yenisei Rivers had their flows stopped by the vast ice sheets, creating huge lakes.
Permafrost covered Europe south of the ice sheet down to present-day Szeged and Asia down to Beijing. In North America, latitudinal gradients were so sharp that permafrost did not reach far south of the ice sheets except at high elevations.
[edit] Glacial climate
The formation of an ice sheet or ice cap requires both prolonged cold and precipitation (snow). Hence, despite having temperatures similar to those of glaciated areas in North America and Europe, East Asia and parts of Alaska remained unglaciated except at the highest elevations. This apparent anomaly was caused by the fact that the ice sheets in Europe produced extensive anticyclones above them. These anticyclones generated westerly winds that were so dry on reaching Siberia and Manchuria that precipitation sufficient for the formation of glaciers could never occur (except in Kamchatka where these westerly winds lifted moisture from the Sea of Japan). The relative warmth of the Pacific Ocean due to the shutting down of the Oyashio Current and the presence of large east-west mountain ranges were secondary factors preventing continental glaciation in Asia.
In warmer regions of the world, climates at the Last Glacial Maximum were extremely dry and generally cold. In extreme cases, such as South Australia and the Sahel, rainfall could be diminished by up to ninety percent, with floras diminished to almost the same degree as in glaciated areas of Europe and North America. Even in less affected regions, rainforest cover was greatly diminished, especially in West Africa where a few refugia were surrounded by tropical grassland. The Amazon rainforest was split into two large blocks by extensive savanna, and it is probable that the tropical rainforests of Southeast Asia were similarly affected, with deciduous forests expanding in their place except on the east and west extremities of the Sundaland shelf. Only in Central America and the Chocó region of Colombia did tropical rainforests remain substantially intact - probably due to the extraordinarily heavy rainfall of these regions.
Most of the world's deserts expanded. Exceptions were in the American West, where changes in the jet stream brought heavy rain to areas that are now desert and large pluvial lakes formed, the best known being Pluvial Lake Bonneville. This also occurred in Afghanistan and Iran where a major lake formed in the Dasht-e Kavir (possibly also in North Africa but this remains unclear). In Australia, shifting sand dunes covered fifty percent of the continent, whilst the Chaco and Pampas in South America became similarly dry. Present-day subtropical regions also lost most of their forest cover, notably in eastern Australia, the Atlantic Forest of Brazil, and southern China, where open woodland became dominant due to drier conditions. In northern China - unglaciated despite its cold climate - a mixture of grassland and tundra prevailed, and even here, the northern limit of tree growth was at least twenty degrees further south than today.
In the period immediately before the Last Glacial Maximum, many areas that became completely barren desert were wetter than they are today, notably in southern Australia where Aboriginal occupation is believed to coincide with a wet period between 40,000 and 60,000 years BP (Before Present, a formal measurement of uncalibrated radiocarbon years, counted from 1950 AD).
