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The Last Glacial Maximum (LGM) refers to the time of maximum extent of the ice sheets during the last glacial period, approximately 20,000 years ago. This extreme persisted for several thousand years. It is followed by the Late Glacial Maximum.

At this time, ice sheets covered the whole of Icelandmarker and all but the southern extremity of the British Islesmarker. Northern Europe was largely covered, the southern boundary passing through Germanymarker and Polandmarker, but not quite joined to the British ice sheet. This ice extended northward to cover Svalbardmarker and Franz Josef Landmarker and eastward to occupy the northern half of the West Siberian Plain, ending at the Taymyr Peninsulamarker, and damming the Ob and Yeniseimarker rivers forming a West Siberian Glacial Lake. In North America, the ice covered essentially all of Canadamarker and extended roughly to the Missourimarker and Ohio Rivers, and eastward to New York Citymarker. A person could theoretically walk from North America to Europe across the frozen north Atlantic ice sheet.

In the Southern Hemispheremarker, the Patagonian Ice Sheet covered Chilemarker and western Argentinamarker north to about 41 degrees south. Ice sheets also covered Tibet (scientists continue to debate the extent to which the Tibetan Plateau was covered with ice), Baltistan, Ladakhmarker, the Venezuelan Andesmarker and the Andean altiplanomarker. In Africa, the Middle East and Southeast Asia, many smaller mountain glaciers formed, especially in the Atlas, the Bale Mountains, and New Guineamarker.

Permafrost covered Europe south of the ice sheet down to present-day Szegedmarker 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.

The Indonesianmarker islands as far east as Borneomarker and Balimarker were connected to the Asian continent in a landmass called Sundaland. Palawanmarker was also part of Sundaland, while the rest of the Philippine Islandsmarker formed one large island separated from the continent only by the Sibutu Passagemarker and the Mindoro Straitmarker. Australia and New Guineamarker were connected forming Sahullandmarker. Between Sundaland and Sahulland, Wallacea remained islands, though the number and width of water gaps between the two continents were considerably smaller.

Glacial climate

Temperature proxies for the last 40,000 years
Map of vegetation patterns during the last glacial maximum
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 Alaskamarker remained unglaciated except at higher elevations. This difference was caused by the fact that the ice sheets in Europe produced extensive anticyclones above them. These anticyclones generated air masses that were so dry on reaching Siberiamarker and Manchuria that precipitation sufficient for the formation of glaciers could never occur (except in Kamchatkamarker where these westerly winds lifted moisture from the Sea of Japanmarker). The relative warmth of the Pacific Oceanmarker 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 cooler and almost everywhere drier. In extreme cases, such as South Australiamarker and the Sahel, rainfall could be diminished by up to ninety percent from present, 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 Colombiamarker 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 Lake Bonnevillemarker in Utahmarker. This also occurred in Afghanistanmarker and Iranmarker where a major lake formed in the Dasht-e Kavirmarker. In Australia, shifting sand dunes covered half the continent, whilst the Chaco and Pampasmarker 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 Brazilmarker, and southern Chinamarker, 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).

See also



Notes

  1. Sathiamurthy, E. and Voris, H. K. 2006. Pleistocene Sea Level Maps for the Sunda Shelf. The Field Museum, Chicago, Illinois.


Further reading

  • Ehlers, J., and P.L. Gibbard, 2004a, Quaternary Glaciations: Extent and Chronology 2: Part II North America. Elsevier, Amsterdam. ISBN 0-444-51462-7
  • Ehlers, J., and P L. Gibbard, 2004b, Quaternary Glaciations: Extent and Chronology 3: Part III: South America, Asia, Africa, Australia, Antarctica. ISBN 0-444-51593-3
  • Gillespie, A.R., S.C. Porter, and B.F. Atwater, 2004, The Quaternary Period in the United States. Developments in Quaternary Science no. 1. Elsevier, Amsterdam. ISBN 978-0-444-51471-4
  • Mangerud, J., J. Ehlers, and P. Gibbard, 2004, Quaternary Glaciations : Extent and Chronology 1: Part I Europe. Elsevier, Amsterdam. ISBN 0-444-51462-7
  • Sibrava, V., Bowen, D.Q, and Richmond, G.M., 1986, Quaternary Glaciations in the Northern Hemisphere, Quaternary Science Reviews. vol. 5, pp. 1-514.


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