Last Glacial Maximum

A map of sea surface temperature changes and glacial extent during the last glacial maximum according to Climate: Long range Investigation, Mapping, and Prediction.

The Last Glacial Maximum (LGM) was the most recent time during the Last Glacial Period when ice sheets were at their greatest extent. Vast ice sheets covered much of North America, northern Europe, and Asia. The ice sheets profoundly affected Earth's climate by causing drought, desertification, and a large drop in sea levels.[1] The ice sheets reached their maximum coverage about 26,500 years ago (26.5 ka BP). Deglaciation commenced in the Northern Hemisphere at approximately 20 ka and in Antarctica approximately at 14.5 ka, consistent with evidence for an abrupt rise in the sea level at about 14.5 ka.[2]

The LGM is referred to in Britain as the Dimlington Stadial, dated by Nick Ashton to between 31 and 16 ka.[3][clarification needed][4]In the archaeology of Paleolithic Europe, the LGM spans the Gravettian, Solutrean, Magdalenian and Périgordian.

The LGM was followed by the Late Glacial.

Glacial climate

Temperature proxies for the last 40,000 years.

According to Blue Marble 3000 (a video by the Zurich University of Applied Sciences), the average global temperature around 19,000 BC (about 21,000 years ago) was 9.0 °C (48.2 °F).[5] This is about 6.0 °C (10.8°F) colder than the 2013-2017 average.

The figures given by the Intergovernmental Panel On Climate Change (IPCC) estimate a slightly lower global temperature than the figures given by the Zurich University of Applied Sciences. However, these figures aren’t exact figures and are open more to interpretation. According to the IPCC, average global temperatures increased by 5.5 ± 1.5 °C (9.9 ± 2.7 °F) since the last glacial maximum, and the rate of warming was about 10 times slower than that of the 20th Century.[6] It appears that they are defining the present as sometime in the 19th Century for this case, but they don’t specify exact years, or give a temperature for the present.

Berkeley Earth puts out a list of average global temperatures by year. If you average all of the years from 1850 to 1899, the average temperature comes out to 13.8 °C (56.9°F).[7] When subtracting 5.5 ± 1.5 °C (9.9 ± 2.7 °F) from the 1850-1899 average, the average temperature for the last glacial maximum comes out to 8.3 ± 1.5 °C (47.0 ± 2.7 °F). This is about 6.7 ± 1.5 °C (12.1 ± 2.7 °F) colder than the 2013-2017 average. This figure is open to interpretation because the IPCC does not specify 1850-1899 as being the present, or give any exact set of years as being the present. It also does not state whether or not they agree with the figures given by Berkeley Earth.

According to the United States Geological Survey (USGS), permanent summer ice covered about 8% of Earth's surface and 25% of the land area during the last glacial maximum.[8] The USGS also states that sea level was about 125 meters (410 feet) lower than in present times (2012).[8]

When comparing to the present, the average global temperature was 15.0 °C (58.9 °F) for the 2013-2017 period.[7] Currently (as of 2012), about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice.[8]

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 remained unglaciated except at higher elevations. This difference was because 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.

All over 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 90% from present, with florae 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 grasslands.

The Amazon rainforest was split into two large blocks by extensive savanna, and the tropical rainforests of Southeast Asia probably 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.

A map of vegetation patterns during the last glacial maximum.

Most of the world's deserts expanded. Exceptions were in what is now the western United States, 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 20° farther south than today.

In the period 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 Before Present (BP, a formal measurement of uncalibrated radiocarbon years, counted from 1950 CE).

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