Last Glacial Maximum

The Last Glacial Maximum (LGM) was the last period in the Earth's climate history during the last glacial period when ice sheets were at their greatest extension. Growth of the ice sheets reached their maximum positions 26,500 years ago. Deglaciation commenced in the Northern Hemisphere approximately 19,000 years ago, and in Antarctica approximately 14,500 years ago which is consistent with evidence that this was the primary source for an abrupt rise in the sea level 14,500 years ago. At this time, vast ice sheets covered much of North America, northern Europe and Asia. These ice sheets profoundly affected Earth's climate, causing drought, desertification and a dramatic drop in sea levels. It was followed by the Late Glacial Maximum.

Glacial climate

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 preventing continental glaciation in Asia.

Last glacial period

The last glacial period, popularly known as the Ice Age, was the most recent glacial period within the Quaternary glaciation occurring during the last 100,000 years of the Pleistocene, from approximately 110,000 to 12,000 years ago. Scientists consider this "ice age" to be merely the latest glaciation event in a much larger ice age, one that dates back over two million years and has seen multiple glaciations.

During this period, there were several changes between glacier advance and retreat. The Last Glacial Maximum, the maximum extent of glaciation within the last glacial period, was approximately 22,000 years ago. While the general pattern of global cooling and glacier advance was similar, local differences in the development of glacier advance and retreat make it difficult to compare the details from continent to continent (see picture of ice core data below for differences).

From the point of view of human archaeology, it falls in the Paleolithic and Mesolithic periods. When the glaciation event started, Homo sapiens were confined to Africa and used tools comparable to those used by Neanderthals in Europe and the Levant and by Homo erectus in Asia. Near the end of the event, Homo sapiens spread into Europe, Asia, and Australia. Archaeological and genetic data suggest that the source populations of Paleolithic humans survived the last glacial period in sparsely wooded areas and dispersed through areas of high primary productivity while avoiding dense forest cover. The retreat of the glaciers allowed groups of Asians to migrate to the Americas and populate them.

Ice age

An ice age is a period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene—of the ice age that began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist.

Origin of ice age theory

In 1742 Pierre Martel (1706–1767), an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years later he published an account of his journey. He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Later similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin (1767–1858) explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier (1786–1855) in 1834. Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work. Such explanations could also be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra (1806–1878) visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers.