Last Glacial Maximum

• 1. Introduction

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  The Last Glacial Maximum gripped Earth roughly 26,000 to 20,000 years ago, and its fingerprints are everywhere once you know how to read the land. Global average temperatures ran about 6 degrees Celsius colder than today. Sea levels sat roughly 125 meters below their present mark, and ice sheets buried a quarter of all dry land. The questions worth asking are not just how cold it got, but what that cold actually did: to the oceans, to the forests, to the deserts, to the humans trying to survive inside it. And perhaps most unexpectedly, what can a pile of dust trapped in ancient ice, a bloom of coral off Queensland, or the bones of mammoths on the Eurasian steppe tell us about a world that vanished before any written record began?

• 2. Ice and the Shape of Land

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  Growth of the southern hemisphere ice sheets commenced around 33,000 years ago, well before the global maximum. In North America, the Laurentide Ice Sheet eventually covered essentially all of Canada east of the Rocky Mountains and pushed south roughly to the Missouri and Ohio Rivers, and eastward as far as Manhattan. At its thickest, it reached about 3.2 kilometers in height around Keewatin Dome. The total maximum volume of the Laurentide alone has been estimated at somewhere between 26.5 and 37 million cubic kilometers of ice. In Europe, the ice sheet's southern boundary ran through Germany and Poland, extended north to cover Svalbard and Franz Josef Land, and swung northeast to occupy the Barents and Kara seas, finally ending at the Taymyr Peninsula in what is now northwestern Siberia. Ice also covered the whole of Iceland, all of Ireland, and roughly the northern half of the British Isles, with the southern margin running from the south of Wales across the submerged terrain of Doggerland toward Denmark. All that locked-away water lowered sea levels enough to connect landmasses that are now separated by ocean: the Indonesian islands as far east as Borneo and Bali joined the Asian continent in a single landmass called Sundaland, while the Australian mainland, New Guinea, and Tasmania merged into a single continent now sometimes called Sahul. In the Bonaparte Gulf of northwestern Australia, the sea floor sat about 125 meters below its present position.

• 3. A World Gone Drier

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  Cold alone does not tell the full story. Climates at the Last Glacial Maximum were, almost without exception, both cooler and drier than today. In extreme cases, such as Southern Australia and the Sahel, rainfall may have been reduced by as much as 90 percent. The Amazon rainforest was split into two large blocks separated by extensive savanna. Rainforest cover in West Africa shrank to a few refugia surrounded by tropical grassland. Even the tropical forests of Southeast Asia likely contracted, with deciduous forests spreading across the Sundaland shelf in their place. Most of the world's deserts expanded. In Australia, shifting sand dunes covered roughly half the continent. The Chaco and Pampas regions of South America became similarly arid. In northern China, despite the absence of glaciers, a mixture of grassland and tundra replaced the forests, and the northern limit of tree growth sat at least 20 degrees farther south than it does today. The atmosphere itself was thick with dust; ice cores record dust levels as much as 20 to 25 times greater than in the present. Reduced vegetation, stronger global winds, and less precipitation to wash particles from the sky all contributed. The notable exceptions were a handful of regions where jet stream shifts brought paradoxically heavy rain. In what is now the western United States, altered atmospheric circulation created large pluvial lakes, the best known being Lake Bonneville in Utah. In central Iran, geomorphological evidence from the Masileh Depression suggests that several basins merged into a single integrated lake covering roughly 6,190 square kilometers.

• 4. Oceans Under Pressure

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  The Atlantic Meridional Overturning Circulation, known as AMOC, was weaker and more shallow during the Last Glacial Maximum than it is today. Sea surface temperatures in the western subtropical gyre of the North Atlantic ran around 5 degrees Celsius colder compared to the present. A different water mass called Glacial North Atlantic Intermediate Water, or GNAIW, ventilated the intermediate depths of the North Atlantic. Unlike today's upper North Atlantic Deep Water, GNAIW was nutrient poor. Below it, nutrient-rich southern source bottom water filled the deep basin. In the Pacific, the Kuroshio Current was weaker, indicated by low sea surface temperature and salinity readings in the East China Sea. The El Nino-Southern Oscillation was strong during the LGM. Along the coast of Queensland, reef development in the Great Barrier Reef shifted seaward as sea levels dropped, reaching maximum distance from the present coastline around 20,700 to 20,500 years ago; enhanced microbial carbonate deposition there was linked to low atmospheric CO2 levels. The deep South Indian Ocean acted as an enormous carbon sink, and the Antarctic Polar Front shifted far to the north, potentially as far as 43 degrees south, well into the southern Indian Ocean. Carbon sequestration in the highly stratified Southern Ocean was itself considered essential to producing the conditions of the LGM.

• 5. People on the Edge

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  During the Last Glacial Maximum, Europe experienced a significant reduction in human population, with some estimates suggesting a decline of up to 60 percent. Megafaunal abundance in Europe, by contrast, actually peaked around 27,000 and 21,000 years before present; the cold stadial climate supported large herds, even as it forced human communities to the margins. In North America, glaciers pushed the early human populations who had originally migrated from northeast Siberia into refugia, reshaping their genetic variation through mutation and drift. That process established the older haplogroups found among Native Americans today, while later migrations account for the northern North American haplogroups. Not every region was equally hostile. In southeastern North America, between the southern Appalachians and the Atlantic, an enclave of unusually warm climate persisted. Sediment core analysis from Lone Spruce Pond in southwestern Alaska shows it too was a pocket of relative warmth amid the cold, dry conditions that kept July air temperatures in northern Alaska and the Yukon about 2 to 3 degrees Celsius below their present values. In the period before the LGM, areas that would later become desert were wetter; in southern Australia, Aboriginal occupation is believed to coincide with a wet period between 40,000 and 60,000 years before present.

• 6. Deglaciation and Aftermath

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  Warming commenced in northern latitudes around 20,000 years ago, but it was limited at first. Significant warming in northern Europe did not arrive until around 14,600 years ago. The West Antarctica ice sheet declined between 14,000 and 15,000 years ago, consistent with evidence for an abrupt rise in sea level around 14,500 years ago. The Fennoscandian ice sheet reached its LGM maximum in northwestern Russia approximately 17,000 years ago, about five thousand years later than in Denmark, Germany, and western Poland. In South America, the tree line around Llanquihue Lake had been depressed roughly 1,000 meters relative to present elevations during the coldest phase, but it rose gradually until 19,300 years ago, when a cold reversal replaced much of the forest with Magellanic moorland and alpine species before warming resumed. Glacier fluctuations around the Strait of Magellan suggest the peak in glacial surface area in that region was constrained to between 25,200 and 23,100 years ago. As of 2012, year-round ice covered only about 3.1 percent of Earth's surface and 10.7 percent of its land, compared to 8 percent of the surface and 25 percent of the land during the LGM, a contrast that defines just how thoroughly the planet has changed since ice was the dominant force shaping every coastline, river, forest, and human migration route on Earth.