Last Glacial Maximum

The Last Glacial Maximum, also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period where ice sheets were at their greatest extent between 26,000 and 20,000 years ago.
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, along with a large drop in sea levels.
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 and 20,000 years ago. 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. 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.
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. The LGM is referred to in Britain as the Dimlington Stadial, dated to between 31,000 and 16,000 years ago.

Glacial climate

The average global temperature about 21,000 years ago was about 6 °C colder than today. According to the United States Geological Survey, permanent summer ice covered about 8% of Earth's surface and 25% of the land area during the last glacial maximum. The USGS also states that sea level was about lower than in present times. When comparing to the present, the average global temperature was for the 2013–2017 period. As of 2012 about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice.
Carbon sequestration in the highly stratified and productive Southern Ocean was essential in producing the LGM. The formation of an ice sheet or ice cap requires both prolonged cold and precipitation. 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. 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 Southern 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. Most of the world's deserts expanded. Exceptions were in what is the present-day Western United States, where changes in the jet stream brought heavy rain to areas that are now desert and large pluvial lakes formed, the best known being Lake Bonneville in Utah. This also occurred in Afghanistan and Iran, where a major lake formed in the Dasht-e Kavir.
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. 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 25,500 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.

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. The massive sheets of ice locked away water, lowering the sea level, exposing continental shelves, joining land masses together, and creating extensive coastal plains. 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. During the LGM, 21,000 years ago, the sea level was about 125 meters lower than it is today. Across most of the globe, the hydrological cycle slowed down, explaining increased aridity in many regions of the world.

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. The Atlantic deep sea sediment core V22-196, extracted off the coast of Senegal, shows a major southward expansion of the Sahara.
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 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 BP.
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. Seasonal freezing of the ground in the Lesotho Highlands might have reached depths of 2 meters or more below the surface. A few small glaciers did however develop during the LGM, in particular in south-facing slopes. 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. Palaeoclimatological proxies indicate the region around Boomplaas Cave was wetter, with increased winter precipitation. The region of the Zambezi River catchment was colder relative to present and the local drop in mean temperature was seasonally uniform.
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.

Asia

Whereas southern Siberia was covered predominantly by the mammoth steppe biome, northern Siberia contained much more mesic and wet habitats, including mires, woodlands, and tundras.
There were ice sheets in modern Tibet 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.
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.

Australasia

The Australian mainland, New Guinea, Tasmania and many smaller islands comprised a single land mass. This continent is now sometimes referred to as Sahul. In the Bonaparte Gulf of northwestern Australia, sea levels were about 125 metres lower than present. Interior Australia saw widespread aridity, evidenced by extensive dune activity and falling lake levels. Eastern Australia experienced two nadirs in temperature. Lacustrine sediments from North Stradbroke Island in coastal Queensland indicated humid conditions. Data from Little Llangothlin Lagoon likewise indicate the persistence of rainforests in eastern Australia at this time. Rivers maintained their sinuous form in southeastern Australia and there was increased aeolian deposition of sediment in compared to today. The Flinders Ranges likewise experienced humid conditions. In southwestern Western Australia, forests disappeared during the LGM.
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.