Climate Through Human History

The End of the Ice Age World

Quicktime Video Animation: Animations of melting of the North American continental glacier.

Patterns of circulation and melting of glacial ice. From Geological Survey of Canada. Animation begins 14,000 years before present.

Bering Land Bridge Movie

The most recent North American glaciers reached their maximum thickness and extent abou 18,000-22,000 years ago. At that time, the sea level was perhaps 120 m (395 ft) lower than it is now. The lower sea level exposed vast areas of land, such as the Bering land bridge (a strip of land that connected Siberia to Alaska), which allowed human and animal migration from Asia to North America.

Widespread deglaciation began rather abruptly approximately 14,000 years ago. The Cordilleran Ice Sheet meleted rapidly and was gone by approximately 10,000 years ago. The Scandinavian Ice Sheet lasted only slightly longer. Deglaciation was later in North America and the melting of the Laurentice Ice Sheet lagged by approximately 2000 years.

By 8500 years ago the conditions in Europe had reached their present state while this situation was achieved in North America about 7000 years ago.

By 9,000 BP summer temperatures and precipitation had risen above present-day values in much of the land area of the Northern Hemisphere. These warm conditions--the so-called hypsithermal interval (or, in older usage, climatic optimum)--remained in place until about 6,000 BP, after which temperatures fell slowly to their present values (with minor irregularities discussed later) some centuries before the Christian era.

This period of warmer climate occurred earlier in western North America than in the east, where the shrinking Laurentide ice kept temperatures cool. It also made an early appearance in parts of the Southern Hemisphere. In New Zealand, for example (where the glacial maximum is believed to have preceded that of the Northern Hemisphere by 6,000 to 7,000 years), warmest conditions appear to have occurred between 10,000 and 8,000 BP.

Great changes to the landscape:

• melting of large amounts of ice
• gradual disappearance of many lakes
• rise of sea level
• prolonged rise of those land areas that had been weighed down by the masses of ice (270-300 meters in Scandinavia)
• advances of forests to regions with tundra and grassy plains

Drastic changes for the people and animals whose way of life was adjusted to the ice-age world.

Early populations in the Mediterranean, North Sea, Australia lived near the sea in other to catch fish and get salt to preserve their food.

• sea-level rise may have greatly reduced the total numbers of mankind
• event might have started the legends of a great flood in ancient times

The Rising Sea Level and its Effects

Sea level rise began 15,000 B.C., with the most rapid phases between 8,000-5,000 B.C. These changes were effectively over by about 2,000 B.C., when sea levels were about 1-2 meters higher that they are today.

Mean sea-level rise was slow, probably at a rate of 1-5 meters per century. However, the recession of the coast took place in sudden advances of the sea at times of storms and had a great impact to coastal populations.

Early Holocene climatic trends

In the mid-latitudes and the tropics, the end of the last glacial period was marked by a tremendous increase in rainfall. The increased precipitation toward the end of the Pleistocene was marked by a vast proliferation of pluvial lakes in the Great Basin of western North America, notably Lake Bonneville and Lake Lahontan (enormous ancestors of present-day Great Salt Lake and Pyramid Lake). Two peaks of lake levels were reached at about 12,000 BP (the beginning of the Allerød Warm stage) and approximately 9000 (the early Boreal Warm stage). At Lake Balaton (in Hungary) high terrace levels also mark the Allerød and early Boreal Warm stages. Lake Victoria (in East Africa) exhibits the identical twin oscillation in its terrace levels.

In equatorial regions the same evidence of high solar radiation and high rainfall at the end of the Pleistocene and during the early Holocene is apparent in the record of the Nile sediments. The Nile, like the other great rivers of Africa (notably, the Congo, Niger, and Sénégal), became very reduced, if not totally blocked, by silt and desert sand during the low-precipitation, arid phases of the Pleistocene. An erroneous correlation between glacial phases and pluvial phases in the tropics has been widely accepted in the past, although cold ocean water means less precipitation, not more. The pluvial phases correspond to the high solar radiation states, the last maximum being about 10,000 years ago. Thus tremendous increases of Nile discharge are determined, by radiocarbon dating, to have occurred around 12,000 and 9,000 years ago, separated and followed by alluviation, indicating reduced runoff in the headwaters.

The expansion of monsoonal rains during the early Holocene in the tropical latitudes permitted an extensive spread of moist savanna-type vegetation over the Sahara in North Africa and the Kalahari in South Africa and in broad areas of Brazil, India, and Australia. Most of these areas had been dry savanna or arid during the last glacial period. Signs of late Paleolithic and Neolithic people can be seen throughout the Sahara today, and art is representative of the life and hunting scenes of the time. Lake deposits have been dated as young as 5000-6000 BP. Lake Chad covered a vast area in the very late Pleistocene and up to 5000 BP. The Dead Sea throughout the early Holocene shows a record of sedimentation from humid headwaters; there was a Neolithic settlement at Jericho around 9000-10,000 BP.

Human Migrations

As vegetation zones began to move, plains were replaced by forests:

• mankind and animals had to adapt to their changing world
• many animal extinctions took place during this period (horse and mammoth in North America)

Human populations started domesticating animals, specially sheep and goat.

Other regions not covered by permanent ice during the glacial period also experienced changes to their environment.

The Sahara region was much wetter that today during the last Ice Age. Drying of the region occurred from 3500-2800 B.C.

There is a suggestion that the rise of Egypt may have been a response to the great contraction of the habitable terrain in North Africa. Therefore, a concentration of the population along the Nile River valley might have occurred.