Circa 24,000–16,000 BCE; most recent era when ice sheets were at their greatest extent
The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period,[1] was the most recent time during the
Last Glacial Period where
ice sheets were at their greatest extent 26,000 and 20,000 years ago.[2]
Ice sheets covered much of Northern
North America,
Northern Europe, and
Asia and profoundly affected
Earth's climate by causing a major expansion of deserts,[3] along with a large drop in sea levels.[4]
Based on changes in position of ice sheet margins dated via
terrestrial cosmogenic nuclides and
radiocarbon dating, growth of ice sheets in the southern hemisphere commenced 33,000 years ago and maximum coverage has been estimated to have occurred sometime between 26,500 years ago[1] and 20,000 years ago.[5] After this, deglaciation caused an abrupt rise in sea level. Decline of the West
Antarctica ice sheet occurred between 14,000 and 15,000 years ago, consistent with evidence for another abrupt rise in the
sea level about 14,500 years ago.[6][7] Glacier fluctuations around the Strait of Magellan suggest the peak in glacial surface area was constrained to between 25,200 and 23,100 years ago.[8]
There are no agreed dates for the beginning and end of the LGM, and researchers select dates depending on their criteria and the data set consulted. Jennifer French, an archeologist specialising in the European Palaeolithic, dates its onset at 27,500 years ago, with ice sheets at their maximum by around 26,000 years ago and deglaciation commencing between 20,000 and 19,000 years ago.[9] The LGM is referred to in Britain as the Dimlington
Stadial, dated to between 31,000 and 16,000 years ago.[10][11]
Glacial climate
The average global temperature about 21,000 years ago was about 6 °C (11 °F) colder than today.[12][13]
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.[14] The USGS also states that sea level was about 125 meters (410 ft) lower than in present times (2012).[14]
When comparing to the present, the average global temperature was 15 °C (59 °F) for the 2013–2017 period.[15] As of 2012 about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice.[14]
Carbon sequestration in the highly stratified and productive Southern Ocean was essential in producing the LGM.[16] 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 that prevented the development of 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 have been diminished by up to 90% compared to the present, with flora 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.
In
Australia, shifting sand dunes covered half the continent, while 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 much 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 LGM, 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).
In
New Zealand and neighbouring regions of the Pacific, temperatures may have been further depressed during part of the LGM by the world's most recent
supervolcanic eruption, the
Oruanui eruption, approximately 28,500 years BP.
However, it is estimated that during the LGM, low-to-mid latitude land surfaces at low elevation cooled on average by 5.8 °C relative to their present-day temperatures, based on an analysis of noble gases dissolved in groundwater rather than examinations of species abundances that have been used in the past.[17]
World impact
During the Last Glacial Maximum, much of the world was cold, dry, and inhospitable, with frequent storms and a dust-laden atmosphere. The dustiness of the atmosphere is a prominent feature in ice cores; dust levels were as much as 20 to 25 times greater than they are in the present. This was probably due to a number of factors: reduced vegetation, stronger global winds, and less precipitation to clear dust from
the atmosphere.[18] The massive sheets of ice locked away water, lowering the sea level, exposing
continental shelves, joining land masses together, and creating extensive
coastal plains.[19] During the LGM, 21,000 years ago, the sea level was about 125 meters (about 410 feet) lower than it is today.[20][21] Across most of the globe, the hydrological cycle slowed down, explaining increased aridity in many regions of the world.[22]
Africa and the Middle East
In Africa and the Middle East, many smaller mountain glaciers formed, and the
Sahara and other sandy deserts were greatly expanded in extent.[19] The Atlantic deep sea sediment core V22-196, extracted off the coast of Senegal, shows a major southward expansion of the Sahara.[23]
The
Persian Gulf averages about 35 metres in depth and the seabed between
Abu Dhabi and
Qatar is even shallower, being mostly less than 15 metres deep. For thousands of years the
Ur-Shatt (a confluence of the
Tigris-
Euphrates Rivers) provided fresh water to the Gulf, as it flowed through the
Strait of Hormuz into the
Gulf of Oman.
Bathymetric data suggests there were two palaeo-basins in the Persian Gulf. The central basin may have approached an area of 20,000 km2, comparable at its fullest extent to lakes such as
Lake Malawi in Africa. Between 12,000 and 9,000 years ago much of the Gulf's floor was not covered by water, only being flooded by the sea after 8,000 years ago.[24]
It is estimated that annual average temperatures in Southern Africa were 6 °C lower than at present during the Last Glacial Maximum. This temperature drop alone would however not have been enough to generate widespread
glaciation or
permafrost in the
Drakensberg Mountains or the
Lesotho Highlands.[25] Seasonal freezing of the ground in the Lesotho Highlands might have reached depths of 2 meter or more below the surface.[26] A few small glaciers did however develop during the LGM, in particular in south-facing slopes.[25] In the
Hex River Mountains, in the
Western Cape, block streams and terraces found near the summit of Matroosberg evidences past
periglacial activity which likely occurred during the LGM.[27] Palaeoclimatological proxies indicate the region around Boomplaas Cave was wetter, with increased winter precipitation.[28] The region of the
Zambezi River catchment was colder relative to present and the local drop in mean temperature was seasonally uniform.[29]
On the island of
Mauritius in the
Mascarenhas Archipelago, open wet forest vegetation dominated, contrasting with the dominantly closed-stratified-tall-forest state of Holocene Mauritian forests.[30]
Asia
There were ice sheets in modern
Tibet (although scientists continue to debate the extent to which the
Tibetan Plateau was covered with ice) as well as in
Baltistan and
Ladakh. In
Southeast Asia, many smaller mountain glaciers formed, and permafrost covered Asia as far south as
Beijing. Because of lowered sea levels, many of today's islands were joined to the continents: 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.[31]
The environment along the coast of South China was not very different from that of the present day, featuring moist subtropical evergreen forests, despite sea levels in the
South China Sea being about 100 metres lower than the present day.[32]
Australasia
The Australian mainland,
New Guinea,
Tasmania and many smaller islands comprised a single land mass. This continent is now referred to sometimes as
Sahul.
Between Sahul and
Sundaland – a peninsula of South East Asia that comprised present-day Malaysia and western and northern Indonesia – there remained an archipelago of islands known as
Wallacea. The water gaps between these islands, Sahul and Sundaland were considerably narrower and fewer in number than in the present day.
The two main islands of New Zealand, along with associated smaller islands, were joined as one landmass. Virtually all of the
Southern Alps were under permanent ice cover, with alpine glaciers extending from them into much of the surrounding
high country.[33]
Northern Europe was largely covered by ice, with the southern boundary of the ice sheets passing through Germany and Poland. This ice extended northward to cover
Svalbard and
Franz Josef Land and northeastward to occupy the
Barents Sea, the
Kara Sea, and
Novaya Zemlya, ending at the
Taymyr Peninsula in what is now northwestern Siberia.[35] Warming commenced in northern latitudes around 20,000 years ago, but it was limited and considerable warming did not take place until around 14,600 year ago.[36]
In northwestern
Russia, the
Fennoscandian ice sheet reached its LGM extent approximately 17,000 years ago, about five thousand years later than in Denmark, Germany and Western Poland. Outside the
Baltic Shield, and in Russia in particular, the LGM ice margin of the Fennoscandian Ice Sheet was highly lobate. The main LGM lobes of Russia followed the
Dvina,
Vologda and
Rybinsk basins respectively. Lobes originated as result of ice following shallow topographic depressions filled with a
soft sediment substrate.[37]
Permafrost covered Europe south of the ice sheet down to as far south as present-day
Szeged in Southern Hungary. Ice covered the whole of
Iceland.[38] In addition, ice covered
Ireland along with roughly the northern half of the
British Isles with the southern boundary of the ice sheet running approximately from the south of Wales to the north east of England, and then across the now submerged land of
Doggerland to
Denmark.[39]
In the
Cantabrian Mountains of the northwestern corner of the
Iberian Peninsula, which in the present day have no permanent glaciers, the LGM led to a local glacial recession as a result of increased aridity caused by the growth of other ice sheets farther to the east and north, which drastically limited annual snowfall over the mountains of northwestern Spain. The Cantabrian alpine glaciers had previously expanded between approximately 60,000 and 40,000 years ago during a local glacial maximum in the region.[40]
In northeastern
Italy, in the region around
Lake Fimon, Artemisia-dominated semideserts, steppes, and meadow-steppes replaced open boreal forests at the start of the LGM, specifically during Heinrich Stadial 3. The overall climate of the region became both drier and colder.[41]
In Greenland, the difference between LGM temperatures and present temperatures was twice as great during winter as during summer. Greenhouse gas and insolation forcings dominated temperature changes in northern Greenland, whereas
Atlantic meridional overturning circulation (AMOC) variability was the dominant influence on southern Greenland's climate.[45]Illorsuit Island was exclusively covered by cold-based glaciers.[46]
Following a preceding period of relative retreat from 52,000 to 40,000 years ago,[47] the
Laurentide Ice Sheet grew rapidly at the onset of the LGM until it covered essentially all of Canada east of the Rocky Mountains and extended roughly to the
Missouri and
Ohio Rivers, and eastward to
Manhattan,[48][49][50] reaching a total maximum volume of around 26.5 to 37 million cubic kilometres.[51][52][53] At its peak, the Laurentide Ice Sheet reached 3.2 km in height around Keewatin Dome and about 1.7-2.1 km along the Plains divide.[54] In addition to the large Cordilleran Ice Sheet in Canada and
Montana,
alpine glaciers advanced and (in some locations) ice caps covered much of the Rocky and Sierra Nevada Mountains further south. Latitudinal gradients were so sharp that permafrost did not reach far south of the ice sheets except at high elevations. Glaciers forced the
early human populations who had originally migrated from northeast Siberia into
refugia, reshaping their
genetic variation by
mutation and
drift. This phenomenon established the older
haplogroups found among
Native Americans, and later migrations are responsible for northern North American haplogroups.[55]
On the
Island of Hawaii, geologists have long recognized deposits formed by glaciers on
Mauna Kea during recent ice ages. The latest work indicates that deposits of three glacial episodes since 150,000 to 200,000 years ago are preserved on the volcano. Glacial moraines on the volcano formed about 70,000 years ago and from about 40,000 to 13,000 years ago. If glacial deposits were formed on
Mauna Loa, they have long since been buried by younger lava flows.[56]
In the Southern Hemisphere, the
Patagonian Ice Sheet covered the whole southern third of Chile and adjacent areas of Argentina. On the western side of the Andes the ice sheet reached sea level as far north as in the
41 degrees south at
Chacao Channel.[citation needed] The western coast of
Patagonia was largely glaciated, but some authors have pointed out the possible existence of ice-free refugia for some plant species. On the eastern side of the Andes, glacier lobes occupied the depressions of
Seno Skyring,
Seno Otway,
Inútil Bay, and
Beagle Channel. On the Straits of Magellan, ice reached as far as
Segunda Angostura.[57]
During the LGM,
valley glaciers in the southern Andes (38–43° S) merged and descended from the Andes occupying lacustrine and marine basins where they spread out forming large
piedmont glacier lobes. Glaciers extended about 7 km west of the modern
Llanquihue Lake, but not more than 2 to 3 km south of it.
Nahuel Huapi Lake in Argentina was also glaciated by the same time.[58] Over most of the
Chiloé Archipelago, glacier advance peaked 26,000 years ago, forming a long north–south
moraine system along the eastern coast of
Chiloé Island (41.5–43° S). By that time the glaciation at the latitude of Chiloé was of
ice sheet type contrasting to the valley glaciation found further north in Chile.[59]
Despite glacier advances much of the area west of Llanquihue Lake was still ice-free during the Last Glacial Maximum.[60][61] During the coldest period of the Last Glacial Maximum vegetation at this location was dominated by Alpine herbs in wide open surfaces. The global warming that followed caused a slow change in vegetation towards a sparsely distributed vegetation dominated by Nothofagus species.[60][61] Within this parkland vegetation
Magellanic moorland alternated with Nothofagus forest, and as warming progressed even warm-climate trees began to grow in the area. It is estimated that the
tree line was depressed about 1,000 m relative to present day elevations during the coldest period, but it rose gradually until 19,300 years ago. At that time a cold reversal caused a replacement of much of the arboreal vegetation with Magellanic moorland and Alpine species.[61]
Little is known about the extent of glaciers during Last Glacial Maximum north of the
Chilean Lake District. To the north, in the
dry Andes of
Central and the Last Glacial Maximum is associated with increased humidity and the verified advance of at least some mountain glaciers.[62] In northwestern Argentina, pollen deposits record the altitudinal descent of the treeline during the LGM.[63]
Amazonia was much drier than in the present.[64]δD values from plant waxes from the LGM are significantly more enriched than those in the present and those dating back to MIS 3, evidencing this increased aridity.[65] Eastern Brazil was also affected; the site of Guanambi in
Bahia was much drier than today.[66]
Atlantic Ocean
AMOC was weaker and more shallow during the LGM.[67] Sea surface temperatures in the western subtropical gyre of the North Atlantic were around 5 °C colder compared to today. Intermediate depth waters of the North Atlantic were better ventilated during the LGM by Glacial North Atlantic Intermediate Water (GNAIW) relative to its present-day ventilation by upper North Atlantic Deep Water (NADW). GNAIW was nutrient poor compared to present day upper NADW. Below GNAIW, southern source bottom water that was very rich in nutrients filled the deep North Atlantic.[68]
Due to the presence of immense ice sheets in Europe and North America, continental weathering flux into the North Atlantic was reduced, as measured by the increased proportion of radiogenic isotopes in neodymium isotope ratios.[69]
There is controversy whether upwelling off the Moroccan coast was stronger during the LGM compared to today. Though coccolith size increases in Calcidiscus leptoporus suggest stronger trade winds during the LGM caused there to be increased coastal upwelling of the northwestern coast of Africa,[70] planktonic foraminiferal δ13C records show upwelling and primary productivity were not enhanced during the LGM except in transient intervals around 23,200 and 22,300 BP.[71]
In the western
South Atlantic, where
Antarctic Intermediate Water forms, sinking particle flux was heightened as a result of increased dust flux during the LGM and sustained export productivity. The increased sinking particle flux removed neodymium from shallow waters, producing an isotopic ratio change.[72]
Pacific Ocean
Low
sea surface temperature (SST) and sea surface salinity (SSS) in the
East China Sea during the LGM suggests the
Kuroshio Current was reduced in strength relative to the present.[73] Abyssal Pacific overturning was weaker during the LGM than in the present day, although it was temporarily stronger during some intervals of ice sheet retreat.[74] The El Niño–Southern Oscillation (ENSO) was strong during the LGM.[75] Evidence suggests that the Peruvian Oxygen Minimum Zone in the eastern Pacific was weaker than it is in the present day, likely as a result of increased oxygen concentrations in seawater permitted by cooler ocean water temperatures, though it was similar in spatial extent.[76]
In the
Great Barrier Reef along the coast of
Queensland, reef development shifted seaward due to the precipitous drop in sea levels, reaching a maximum distance from the present coastline as sea levels approached their lowest levels around 20,700-20,500 years ago.[78]
Indian Ocean
The deep waters of the
Indian Ocean were significantly less oxygenated during the LGM compared to the Middle Holocene.[79] The deep South Indian Ocean in particular was an enormous carbon sink, partially explaining the very low pCO2 of the LGM.[80] The intermediate waters of the southeastern
Arabian Sea were poorly ventilated relative to today because of the weakened thermohaline circulation.[81]
Southern Ocean
Evidence from sediment cores in the
Scotia Sea suggests the
Antarctic Circumpolar Current was weaker during the LGM than during the Holocene.[82] The Antarctic Polar Front (APF) was located much farther to the north compared to its present-day location. Studies suggest it could have been placed as far north as 43°S, reaching into the southern Indian Ocean.[83]
Late Glacial Period
The Late Glacial Period followed the LGM and preceded the
Holocene, which started around 11,700 years ago.[84]
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^Cowen, Robert C. "Dust Plays a Huge Role in Climate Change" Christian Science Monitor 3 April 2008 (
"Dust plays huge role in climate change". Christian Science Monitor. 2008-04-03.
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^Harrison, Stephan (2004). "The Pleistocene glaciations of Chile". In Ehlers, J.; Gibbard, P.L. (eds.). Quaternary Glaciations – Extent and Chronology: Part III: South America, Asia, Africa, Australasia, Antarctica. pp. 91–97.
Dyke, A.S.; Moore, A.; Robertson, L. (2003).
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