TheLast Glacial Maximum (LGM), also referred to as theLast Glacial Coldest Period,[1] was the most recent time during theLast Glacial Period whereice sheets were at their greatest extent between 26,000 and 20,000 years ago.[2] Ice sheets covered much of NorthernNorth America,Northern Europe, andAsia and profoundly affectedEarth'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 viaterrestrial cosmogenic nuclides andradiocarbon 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 WestAntarctica ice sheet occurred between 14,000 and 15,000 years ago, consistent with evidence for another abrupt rise in thesea level about 14,500 years ago.[6][7] Glacier fluctuations around theStrait 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 theDimlingtonStadial, dated to between 31,000 and 16,000 years ago.[10][11]
Temperature proxies for the last 40,000 yearsA map of vegetation patterns during the last glacial maximum
The average global temperature about 21,000 years ago was about 6 °C (11 °F) colder than today.[12][13][14] According to theUnited 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.[15] The USGS also states that sea level was about 125 meters (410 ft) lower than in present times (2012).[15] When comparing to the present, the average global temperature was 15 °C (59 °F) for the 2013–2017 period.[16] As of 2012 about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice.[15]
Carbon sequestration in the highly stratified and productive Southern Ocean was essential in producing the LGM.[17] The formation of an ice sheet orice cap requires both prolonged cold andprecipitation (snow). Hence, despite having temperatures similar to those of glaciated areas inNorth America andEurope,East Asia remained unglaciated except at higher elevations. This difference was because the ice sheets in Europe produced extensiveanticyclones above them. These anticyclones generatedair masses that were so dry on reachingSiberia andManchuria that precipitation sufficient for the formation of glaciers could never occur (except inKamchatka where these westerly winds lifted moisture from theSea of Japan). The relative warmth of thePacific Ocean due to the shutting down of theOyashio Current and the presence of large east–west mountain ranges were secondary factors that prevented the development of continental glaciation inAsia.
All over the world, climates at the Last Glacial Maximum were cooler and almost everywhere drier. In extreme cases, such asSouthern Australia and theSahel, 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 inWest Africa where a few refugia were surrounded by tropicalgrasslands. TheAmazon rainforest was split into two large blocks by extensivesavanna, and the tropical rainforests ofSoutheast Asia probably were similarly affected, with deciduous forests expanding in their place except on the east and west extremities of theSundaland shelf. Only inCentral America and theChocó region ofColombia 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 what is the present-dayWestern United States, where changes in thejet stream brought heavy rain to areas that are now desert and largepluvial lakes formed, the best known beingLake Bonneville inUtah. This also occurred inAfghanistan andIran, where a major lake formed in theDasht-e Kavir.
InAustralia, shifting sand dunes covered half the continent, while theChaco andPampas inSouth America became similarly dry. Present-daysubtropical regions also lost most of their forest cover, notably in eastern Australia, theAtlantic Forest ofBrazil, and southernChina, where openwoodland became dominant due to much drier conditions. In northern China – unglaciated despite its cold climate – a mixture of grassland andtundra prevailed, and even here, thenorthern 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, whereAboriginal occupation is believed to coincide with a wet period between 40,000 and 60,000 yearsBefore Present (BP). InNew Zealand and neighbouring regions of the Pacific, temperatures may have been further depressed during part of the LGM by the world's most recentsupervolcanic eruption, theOruanui eruption, approximately 25,500 years BP.[18]
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.[19]
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 fromthe atmosphere.[20] The massive sheets of ice locked away water, lowering the sea level, exposingcontinental shelves, joining land masses together, and creating extensivecoastal plains.[21] The ice sheets also changed the atmospheric circulation, causing the northern Pacific and Atlantic oceans to cool and produce more clouds, which amplified the global cooling as the clouds reflected even more sunlight.[22] During the LGM, 21,000 years ago, the sea level was about 125 meters (about 410 feet) lower than it is today.[23][24] Across most of the globe, the hydrological cycle slowed down, explaining increased aridity in many regions of the world.[25]
In Africa and the Middle East, many smaller mountain glaciers formed, and theSahara and other sandy deserts were greatly expanded in extent.[21] The Atlantic deep sea sediment core V22-196, extracted off the coast of Senegal, shows a major southward expansion of the Sahara.[26]
ThePersian Gulf averages about 35 metres in depth and the seabed betweenAbu Dhabi andQatar is even shallower, being mostly less than 15 metres deep. For thousands of years theUr-Shatt (a confluence of theTigris-Euphrates Rivers) provided fresh water to the Gulf, as it flowed through theStrait of Hormuz into theGulf 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 asLake 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 BP.[27]
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 widespreadglaciation orpermafrost in theDrakensberg Mountains or theLesotho Highlands.[28] Seasonal freezing of the ground in the Lesotho Highlands might have reached depths of 2 meters or more below the surface.[29] A few small glaciers did however develop during the LGM, in particular in south-facing slopes.[28] In theHex River Mountains, in theWestern Cape, block streams and terraces found near the summit ofMatroosberg evidences pastperiglacial activity which likely occurred during the LGM.[30] Palaeoclimatological proxies indicate the region aroundBoomplaas Cave was wetter, with increased winter precipitation.[31] The region of theZambezi River catchment was colder relative to present and the local drop in mean temperature was seasonally uniform.[32]
On the island ofMauritius in theMascarenhas Archipelago, open wet forest vegetation dominated, contrasting with the dominantly closed-stratified-tall-forest state of Holocene Mauritian forests.[33]
A map showing the probable extent of land and water at the time of the last glacial maximum, 20,000 years ago and when thesea level was likely more than 110 metres lower than it is today
Whereas southern Siberia was covered predominantly by the mammoth steppe biome, northern Siberia contained much more mesic and wet habitats, includingmires, woodlands, and tundras.[34]
There were ice sheets in modernTibet (although scientists continue to debate the extent to which theTibetan Plateau was covered with ice) as well as inBaltistan andLadakh. InSoutheast Asia, many smaller mountain glaciers formed, and permafrost covered Asia as far south asBeijing. Because of lowered sea levels, many of today's islands were joined to the continents: the Indonesian islands as far east asBorneo andBali were connected to the Asian continent in a landmass calledSundaland.Palawan was also part of Sundaland, while the rest of thePhilippine Islands formed one large island separated from the continent only by theSibutu Passage and theMindoro Strait.[35]
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 theSouth China Sea being about 100 metres lower than the present day.[36]
The Australian mainland,New Guinea,Tasmania and many smaller islands comprised a single land mass. This continent is now sometimes referred to asSahul. In the Bonaparte Gulf of northwestern Australia, sea levels were about 125 metres lower than present.[37] Interior Australia saw widespread aridity, evidenced by extensive dune activity and falling lake levels.[38] Eastern Australia experienced two nadirs in temperature.[39] Lacustrine sediments from North Stradbroke Island in coastal Queensland indicated humid conditions.[40] Data from Little Llangothlin Lagoon likewise indicate the persistence of rainforests in eastern Australia at this time.[41] Rivers maintained their sinuous form in southeastern Australia and there was increased aeolian deposition of sediment in compared to today.[42] TheFlinders Ranges likewise experienced humid conditions.[43] In southwestern Western Australia, forests disappeared during the LGM.[44]
Between Sahul andSundaland – a peninsula of South East Asia that comprised present-day Malaysia and western and northern Indonesia – there remained an archipelago of islands known asWallacea. 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 theSouthern Alps were under permanent ice cover, with alpine glaciers extending from them into much of the surroundinghigh country.[45]
Reconstructions of the LGM climate in the WesternPalearctic, based on rodent associations, suggest that cooling was significantly greater in winter than in summer,[47] consistent with findings from other proxies like pollen.[48] The rodent-based model estimated an average mean annual temperature anomaly of −5.3 °C compared with present-day temperatures, with the mean temperature of the coldest month exhibiting a change of almost −7 °C, while the mean temperature of the warmest month anomaly reached only −3.4 °C. Spatial analysis of these reconstructions indicates a pronounced latitudinal gradient, alongside regional differences, suggesting that temperature anomalies were higher in northwestern Europe but smaller in southwestern and eastern Europe compared to results from somegeneral circulation models.[47]
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 coverSvalbard andFranz Josef Land and northeastward to occupy theBarents Sea, theKara Sea, andNovaya Zemlya, ending at theTaymyr Peninsula in what is now northwestern Siberia.[49] 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.[50]
In northwesternRussia, theFennoscandian ice sheet reached its LGM extent approximately 17,000 years ago, about five thousand years later than in Denmark, Germany and Western Poland. Outside theBaltic 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 theDvina,Vologda andRybinsk basins respectively. Lobes originated as result of ice following shallow topographic depressions filled with asoft sediment substrate.[51] The northern Ural region was covered in periglacial steppes.[52]
Permafrost covered Europe south of the ice sheet down to as far south as present-daySzeged in Southern Hungary. Ice covered the whole ofIceland.[53] In addition, ice coveredIreland along with roughly the northern half of theBritish 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 ofDoggerland toDenmark.[54] Central Europe had isolated pockets of relative warmth corresponding to hydrothermally active areas, which served as refugia for taxa not adapted to extremely cold climates.[55]
In theCantabrian Mountains of the northwestern corner of theIberian 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.[56]
In northeasternItaly, in the region aroundLake 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.[57]
Northern hemisphere glaciation during the lastice ages during which three to four kilometer-thick ice sheets caused asea level lowering of about 120 m
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, whereasAtlantic meridional overturning circulation (AMOC) variability was the dominant influence on southern Greenland's climate.[62]Illorsuit Island was exclusively covered by cold-based glaciers.[63]
Eastern Beringia was extremely cold and dry.[64] July air temperatures in northern Alaska and Yukon were about 2–3 °C lower compared to today.[65] Equilibrium line altitudes in Alaska suggest summer temperatures were 2–5 °C compared to preindustrial.[66] Sediment core analysis from Lone Spruce Pond in southwestern Alaska show it was a pocket of relative warmth.[67]
Following a preceding period of relative retreat from 52,000 to 40,000 years ago,[68] theLaurentide 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 theMissouri andOhio Rivers, and eastward toManhattan,[69][70][71] reaching a total maximum volume of around 26.5 to 37 million cubic kilometres.[72][73][74] 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.[75] In addition to the large Cordilleran Ice Sheet in Canada andMontana,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 theearly human populations who had originally migrated from northeast Siberia intorefugia, reshaping theirgenetic variation bymutation anddrift. This phenomenon established the olderhaplogroups found amongNative Americans, and later migrations are responsible for northern North American haplogroups.[76]
In southeastern North America, between the southern Appalachian Mountains and the Atlantic Ocean, there was an enclave of unusually warm climate.[77]
In the Southern Hemisphere, thePatagonian 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 the41 degrees south atChacao Channel.[citation needed] The western coast ofPatagonia 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 ofSeno Skyring,Seno Otway,Inútil Bay, andBeagle Channel. On the Straits of Magellan, ice reached as far asSegunda Angostura.[78]
A map of the world during the Last Glacial Maximum
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 largepiedmont glacier lobes. Glaciers extended about 7 km west of the modernLlanquihue Lake, but not more than 2 to 3 km south of it.Nahuel Huapi Lake in Argentina was also glaciated by the same time.[79] Over most of theChiloé Archipelago, glacier advance peaked 26,000 years ago, forming a long north–southmoraine system along the eastern coast ofChiloé Island (41.5–43° S). By that time the glaciation at the latitude of Chiloé was ofice sheet type contrasting to the valley glaciation found further north in Chile.[80]
Despite glacier advances much of the area west of Llanquihue Lake was still ice-free during the Last Glacial Maximum. 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 byNothofagus species.[81] Within this parkland vegetationMagellanic moorland alternated withNothofagus forest, and as warming progressed even warm-climate trees began to grow in the area. It is estimated that thetree 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.[82] OnIsla Grande de Chiloé, Magellanic moorland and closed-canopyNothofagus forests were both present during the LGM, but the former disappeared by the late LGM.[83]
Little is known about the extent of glaciers during Last Glacial Maximum north of theChilean Lake District. To the north, in thedry Andes ofCentral and the Last Glacial Maximum is associated with increased humidity and the verified advance of at least some mountain glaciers.[84] Montane glaciers in the northern Andes reached their peak extent approximately 27,000 years ago.[85] In northwestern Argentina, pollen deposits record the altitudinal descent of the treeline during the LGM.[86]
Amazonia was much drier than in the present.[87]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.[88] Eastern Brazil was also affected; the site of Guanambi inBahia was much drier than today.[89]
AMOC was weaker and more shallow during the LGM.[90] 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.[91]
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.[92]
There is controversy whether upwelling off the Moroccan coast was stronger during the LGM compared to today. Though coccolith size increases inCalcidiscus leptoporus suggest stronger trade winds during the LGM caused there to be increased coastal upwelling of the northwestern coast of Africa,[93] 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.[94]
In the westernSouth Atlantic, whereAntarctic 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.[95]
On theIsland of Hawaii, geologists have long recognized deposits formed by glaciers onMauna 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 onMauna Loa, they have long since been buried by younger lava flows.[96]
Lowsea surface temperature (SST) and sea surface salinity (SSS) in theEast China Sea during the LGM suggests theKuroshio Current was reduced in strength relative to the present.[97] 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.[98] The El Niño–Southern Oscillation (ENSO) was strong during the LGM.[99] 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.[100]
In theGreat Barrier Reef along the coast ofQueensland, 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.[102] Microbial carbonate deposition in the Great Barrier Reef was enhanced due to low atmospheric CO2 levels.[103]
The deep waters of theIndian Ocean were significantly less oxygenated during the LGM compared to the Middle Holocene.[104] The deep South Indian Ocean in particular was an enormous carbon sink, partially explaining the very lowpCO2 of the LGM.[105] The intermediate waters of the southeasternArabian Sea were poorly ventilated relative to today because of the weakened thermohaline circulation.[106]
Evidence from sediment cores in theScotia Sea suggests theAntarctic Circumpolar Current was weaker during the LGM than during the Holocene.[107] 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.[108]
^French, Jennifer (2021).Palaeolithic Europe: A Demographic and Social Prehistory. Cambridge, UK: Cambridge University Press. p. 226.ISBN978-1-108-49206-5.
^Pettitt, Paul; White, Mark (2012).The British Palaeolithic: hominin societies at the edge of the Pleistocene world. London: Routledge. pp. 424–426.ISBN978-0-415-67454-6.
^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. 3 April 2008.Archived from the original on 28 September 2013. Retrieved21 September 2012.), and Claquin et al., "Radiative Forcing of Climate by Ice-Age Atmospheric Dust", Climate Dynamics (2003) 20: 193–202. (www.rem.sfu.ca/COPElab/Claquinetal2003_CD_glacialdustRF.pdf)
^"Glaciers and Sea Level".U.S. Geological Survey. U.S. Geological Survey, U.S. Department of the Interior. 30 May 2012. Archived fromthe original on 4 January 2017. Retrieved4 January 2017.
^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)."Deglaciation of North America".Geological Survey of Canada Open File, 1574. (32 digital maps at 1:7 000 000 scale with accompanying digital chronological database and one poster (two sheets) with full map series.)
