Map of sea surface temperature changes and glacial extent during the last glacial maximum according to the
CLIMAP project.
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
Iceland and all but the southern extremity of the
British Isles.
Northern Europe was largely covered, the southern boundary passing through
Germany and
Poland, but not quite joined to the British ice sheet. This ice extended northward to cover
Svalbard and
Franz Josef Land and eastward to occupy the northern half of the
West Siberian Plain, ending at the
Taymyr Peninsula, and damming the
Ob and
Yenisei rivers forming a
West Siberian Glacial Lake. In
North America, the ice covered essentially all of
Canada and extended roughly to the
Missouri and
Ohio Rivers, and eastward to
New York City. A person could theoretically walk from North America to Europe across the frozen north Atlantic ice sheet.
In the
Southern Hemisphere, the
Patagonian Ice Sheet covered
Chile and western
Argentina 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,
Ladakh, the
Venezuelan Andes 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.
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.
The
Indonesian islands as far east as
Borneo and
Bali were connected to the Asian continent in a landmass called
Sundaland.
Palawan was also part of Sundaland, while the rest of the
Philippine Islands formed one large island separated from the continent only by the
Sibutu Passage and the
Mindoro Strait.
[ Sathiamurthy, E. and Voris, H. K. 2006. The Field Museum, Chicago, Illinois.] Australia and
New Guinea were connected forming
Sahulland. 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
Alaska 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
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 cooler and almost everywhere drier. In extreme cases, such as
South Australia 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
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
Lake Bonneville in
Utah. This also occurred in
Afghanistan and
Iran where a major lake formed in the
Dasht-e Kavir. In
Australia, shifting sand dunes covered half 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).
See also
