Last Glacial Maximum

Last Glacial Maximum
Name	Last Glacial Maximum
Caption	Approximate global extent of ice sheets and sea ice during the Last Glacial Maximum.
Date	~26,500 to ~19,000 years ago
Duration	~7,500 years
Cause	Orbital forcing (Milankovitch cycles), low atmospheric carbon dioxide, ice-albedo feedback
Effect	Global cooling, massive ice sheet expansion, lowered sea levels, arid conditions, major shifts in ecosystems and human populations

Last Glacial Maximum. The Last Glacial Maximum represents the most recent period during the Quaternary glaciation when global ice sheets reached their maximum extent. Occurring approximately between 26,500 and 19,000 years ago, this phase was characterized by profoundly colder and drier global climates. The event had a transformative impact on Earth's geography, ecosystems, and the dispersal of early human populations.

Definition and timing

The event is formally defined by the global maximum in ice volume and the corresponding minimum in sea level. This period is primarily dated through radiocarbon dating of organic materials and cosmogenic nuclide dating of glacial landforms. The timing varies slightly by region, with the Laurentide Ice Sheet in North America and the Fennoscandian Ice Sheet over Scandinavia peaking around this time. The chronology is further refined by studies of ice cores from Greenland and Antarctica, which provide high-resolution climate proxies. The onset and termination are closely linked to changes in Earth's orbital parameters, known as Milankovitch cycles.

Global climate and environment

Global average temperatures were estimated to be about 6°C colder than pre-industrial levels, with much greater cooling at high latitudes. Atmospheric concentrations of carbon dioxide were as low as 180 parts per million, as recorded in the EPICA ice core. The Intertropical Convergence Zone shifted southward, and major monsoon systems, like the Asian monsoon, were significantly weakened. The global climate was generally more arid, leading to the expansion of deserts such as the Sahara and the formation of extensive loess deposits in regions like the Chinese Loess Plateau. The Southern Ocean experienced increased sea ice coverage, further amplifying global cooling.

Ice sheets and sea level

Vast continental ice sheets covered much of the Northern Hemisphere. The Laurentide Ice Sheet sprawled over Canada and the northern United States, while the Cordilleran Ice Sheet enveloped the Pacific Northwest. In Eurasia, the Fennoscandian Ice Sheet covered Scandinavia and parts of Northern Europe, and the Siberian Ice Sheet developed over parts of Asia. The Antarctic ice sheet was more extensive than today. The growth of these ice sheets locked up vast quantities of water, causing global sea level to fall approximately 120–130 meters below present levels. This exposed continental shelves, creating land bridges such as Beringia between Siberia and Alaska, and Sundaland in Southeast Asia.

Flora and fauna

Ecosystems were radically rearranged into biomes not seen today, with extensive mammoth steppe covering much of unglaciated Eurasia. This cold, dry grassland supported a diverse Pleistocene megafauna, including the woolly mammoth, woolly rhinoceros, and steppe bison. In North America, fauna included the American mastodon, Smilodon, and giant ground sloth. Refugia, such as those in the Iberian Peninsula and the Balkans, harbored temperate tree species like oak and beech. The Amazon rainforest may have been fragmented into smaller pockets, while Australia experienced increased aridity, affecting species like Diprotodon.

Human adaptation and migration

Human populations, primarily hunter-gatherers of the Upper Paleolithic, adapted to these harsh conditions. In Europe, cultures such as the Gravettian and later the Magdalenian developed sophisticated tools and art, as seen at sites like Lascaux. The lowered sea levels opened migration routes, facilitating the peopling of the Americas via Beringia and the settlement of Sahul (Australia and New Guinea) from Sundaland. Groups in East Asia and Siberia developed specialized technologies for cold environments. The Clovis culture in North America emerged shortly after the deglaciation of the Ice-Free Corridor.

Deglaciation and legacy

The period of deglaciation began after approximately 19,000 years ago, marked by the onset of the Last Glacial Termination. Key warming events include the Bølling-Allerød interstadial and the abrupt cold reversal of the Younger Dryas. Melting of the Laurentide Ice Sheet contributed to major events like Meltwater Pulse 1A and the filling of proglacial lakes such as Lake Agassiz. The resulting sea level rise flooded continental shelves and separated landmasses, defining modern coastlines. The legacy is evident in contemporary landforms like the Great Lakes, Fjords of Norway, and drumlin fields, while the redistribution of water masses also influenced ocean circulation patterns, including the Atlantic meridional overturning circulation.

Category:Quaternary Category:Paleoclimatology Category:Pleistocene Category:Glaciology