Last Glacial Maximum
The period known as the Last Glacial Maximum occurred between 26,000 and 20,000 years ago. This era marked the time when Earth's ice sheets reached their greatest extent in recent history. Scientists use terrestrial cosmogenic nuclides to date the position of these ancient ice margins. Radiocarbon dating provides another method for establishing the timeline of glacial growth. Growth of ice sheets in the southern hemisphere began approximately 33,000 years ago. Researchers estimate maximum coverage happened sometime between 26,500 and 20,000 years ago. Jennifer French, an archeologist specializing in European Palaeolithic, dates the onset at 27,500 years ago. She notes that ice sheets were at their maximum by around 26,000 years ago. Deglaciation commenced between 20,000 and 19,000 years ago according to her research. There are no agreed dates for the beginning and end of this period among all scientists. Different researchers select dates depending on their specific criteria and data sets consulted. In Britain, the event is referred to as the Dimlington Stadial. This regional name covers a span from 31,000 to 16,000 years ago. The decline of the West Antarctica ice sheet occurred between 14,000 and 15,000 years ago. Glacier fluctuations around the Strait of Magellan suggest peak surface area was constrained to between 25,200 and 23,100 years ago.
The average global temperature about 21,000 years ago was approximately six degrees Celsius colder than today. Permanent summer ice covered about eight percent of Earth's surface during this time. Land area coverage reached twenty-five percent under these frozen conditions. Carbon sequestration in the Southern Ocean played an essential role in producing the Last Glacial Maximum. Formation of an ice sheet requires both prolonged cold and precipitation in the form of snow. East Asia remained unglaciated except at higher elevations despite having temperatures similar to North America. Ice sheets in Europe produced extensive anticyclones that generated dry air masses reaching Siberia. These winds were so dry that precipitation sufficient for glacier formation could never occur over most of the region. Only Kamchatka received moisture lifted by westerly winds from the Sea of Japan. The relative warmth of the Pacific Ocean due to the shutting down of the Oyashio Current prevented continental glaciation in Asia. Large east-west mountain ranges acted as secondary factors blocking glacial development across the continent. All over the world climates were cooler and almost everywhere drier during this period. Rainfall in extreme cases like Southern Australia and the Sahel diminished by up to ninety percent compared to present levels. 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. A few refugia surrounded by tropical grasslands existed within the African landscape. The Amazon rainforest split into two large blocks separated by extensive savanna. Tropical rainforests of Southeast Asia probably suffered similar effects with deciduous forests expanding in their place.
Sea level was about 125 meters lower than it is today when the Last Glacial Maximum peaked. Massive sheets of ice locked away water exposing continental shelves and joining land masses together. Extensive coastal plains formed where oceans once covered the land. Ice sheets changed atmospheric circulation causing northern Pacific and Atlantic oceans to cool. These cooled waters produced more clouds which amplified global cooling by reflecting even more sunlight. Across most of the globe the hydrological cycle slowed down explaining increased aridity in many regions. Sea surface temperatures in the western subtropical gyre of the North Atlantic were around five degrees Celsius colder compared to today. Atlantic meridional overturning circulation or AMOC was weaker and more shallow during this era. Intermediate depth waters of the North Atlantic were better ventilated by Glacial North Atlantic Intermediate Water relative to its present-day ventilation. 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. Due to immense ice sheets in Europe and North America continental weathering flux into the North Atlantic was reduced. Increased proportion of radiogenic isotopes in neodymium isotope ratios measured this reduction. Evidence from sediment cores in the Scotia Sea suggests Antarctic Circumpolar Current was weaker during the LGM. The Antarctic Polar Front was located much farther north than its present-day location. Studies suggest it could have been placed as far north as 43 degrees south.
Whereas southern Siberia was covered predominantly by the mammoth steppe biome northern Siberia contained much more mesic habitats. Mires woodlands and tundras populated these wetter areas of the continent. There were ice sheets in modern Tibet although scientists continue to debate the extent of coverage. Permafrost covered Asia as far south as Beijing during this period. Lowered sea levels joined many islands to continents creating vast landmasses like Sundaland. Indonesian islands as far east as Borneo and Bali connected to the Asian continent. Palawan was also part of Sundaland while Philippine Islands formed one large island separated only by straits. Northern Europe was largely covered by ice with the southern boundary passing through Germany and Poland. This ice extended northward to cover Svalbard and Franz Josef Land. Northeastward expansion occupied the Barents Sea Kara Sea and Novaya Zemlya ending at the Taymyr Peninsula. Fennoscandian ice sheet reached its maximum extent approximately 17,000 years ago in northwestern Russia. Lobes originated as result of ice following shallow topographic depressions filled with soft sediment substrate. The northern Ural region was covered in periglacial steppes. 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 along with Ireland and roughly the northern half of the British Isles.
During the Last Glacial Maximum Europe experienced a significant reduction in human population with estimates suggesting a decline of up to sixty percent. Glaciers forced early human populations who had originally migrated from northeast Siberia into refugia. These isolated pockets reshaped genetic variation by mutation and drift over thousands of years. This phenomenon established older haplogroups found among Native Americans today. Later migrations are responsible for northern North American haplogroups observed in modern populations. Megafaunal abundance in Europe peaked around 27,000 and 21,000 BP due to cold stadial climate. Eastern Beringia was extremely cold and dry during this era. July air temperatures in northern Alaska and Yukon were about two to three degrees Celsius lower compared to today. Equilibrium line altitudes in Alaska suggest summer temperatures were two to five degrees Celsius compared to preindustrial levels. Sediment core analysis from Lone Spruce Pond in southwestern Alaska show it was a pocket of relative warmth. Laurentide Ice Sheet grew rapidly at the onset until it covered essentially all of Canada east of Rocky Mountains. It extended roughly to Missouri and Ohio Rivers and eastward to Manhattan reaching maximum volume of around 26.5 to 37 million cubic kilometers.
In the Southern Hemisphere Patagonian Ice Sheet covered the whole southern third of Chile and adjacent areas of Argentina. On western side of Andes ice sheet reached sea level as far north as 41 degrees south at Chacao Channel. Western coast of Patagonia was largely glaciated but some authors point out possible existence of ice-free refugia for plant species. Glacier lobes occupied depressions of Seno Skyring Seno Otway Inútil Bay and Beagle Channel on eastern side. Valley glaciers in southern Andes merged and descended occupying lacustrine and marine basins forming large piedmont glacier lobes. Glaciers extended about seven kilometers west of modern Llanquihue Lake but not more than two to three kilometers south of it. Over most of Chiloé Archipelago glacier advance peaked 26,000 years ago forming long north-south moraine system along eastern coast. Despite glacier advances much area west of Llanquihue Lake was still ice-free during Last Glacial Maximum. Vegetation at this location was dominated by Alpine herbs in wide open surfaces during coldest period. Global warming that followed caused slow change toward sparsely distributed vegetation dominated by Nothofagus species. Tree line was depressed about one thousand meters relative to present day elevations during coldest period.
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Common questions
When did the Last Glacial Maximum occur?
The period known as the Last Glacial Maximum occurred between 26,000 and 20,000 years ago. Scientists use terrestrial cosmogenic nuclides to date the position of these ancient ice margins. Radiocarbon dating provides another method for establishing the timeline of glacial growth.
How much land was covered by ice during the Last Glacial Maximum?
Permanent summer ice covered about eight percent of Earth's surface during this time. Land area coverage reached twenty-five percent under these frozen conditions. Massive sheets of ice locked away water exposing continental shelves and joining land masses together.
What were global temperatures like during the Last Glacial Maximum?
The average global temperature about 21,000 years ago was approximately six degrees Celsius colder than today. Sea surface temperatures in the western subtropical gyre of the North Atlantic were around five degrees Celsius colder compared to today. All over the world climates were cooler and almost everywhere drier during this period.
Where were the largest ice sheets located during the Last Glacial Maximum?
Ice sheets in Europe produced extensive anticyclones that generated dry air masses reaching Siberia. The Laurentide Ice Sheet grew rapidly at the onset until it covered essentially all of Canada east of Rocky Mountains. In the Southern Hemisphere Patagonian Ice Sheet covered the whole southern third of Chile and adjacent areas of Argentina.
How did sea levels change during the Last Glacial Maximum?
Sea level was about 125 meters lower than it is today when the Last Glacial Maximum peaked. Lowered sea levels joined many islands to continents creating vast landmasses like Sundaland. Extensive coastal plains formed where oceans once covered the land.