10-28-2004, 12:47 AM
ICE AGE THE BIG FREEZE
As the Palaeolithic clock rolled on towards 20,000 years ago, however, events in the Earthâs spin axis and influences on its orbit hundreds of millions of kilometres away from our planet took tighter hold. Three great heavenly cycles of the solar system moved into a conjunction that ensured a minimum of the Sunâs heat reached the northern hemisphere during summer. The weather became colder, and the recurrent brief warm periods, or interstadials, which had characterized the period of 30,000â50,000 years ago, just stopped. It was these warm periods and their summer sunshine that had helped to melt the accumulated northern ice and prevent the ice caps from advancing across Scandinavia into Northern Europe. Now, the ice caps were able to expand in the north. The sea level started to fall again, eventually by 130 metres (430 feet). In short, the Earth was approaching its most recent ice age, or glaciation. (There had been quite a bit of ice on and off for the previous 100,000 years, and archaeologists tend to call the height of the Big Freeze the Last Glacial Maximum, or LGM for short, rather than an ice age.)
The LGM and its aftermath saw far more dramatic disruption and movement of northern human populations than at any time since. A glance at the world climate map around 18,000 years ago begins to give us the reason why. Huge areas of land became totally uninhabitable. For a start, the ice caps, some of them 5 km (3 miles) thick, clearly prevented the land they covered from being occupied. These white sheets were not laid evenly across the northern hemisphere. In Europe they mainly affected the central and northwestern regions. The British Isles, then part of the European mainland, were frozen down as far as Oxford in the south. Scandinavia will for ever bear the scars of the glaciers in its lakes and fjords and in the crustal depression now known as the Baltic Sea. Northern Germany, Poland, and the Baltic states bore the southern edge of the ice sheet, which extended north-east around the Arctic Circle across Finland and Karelia, into Archangel, and as far as the northern Urals. Farther south in Europe, mountainous regions such as the Pyrenees, the Massif Central, the Alps, and the Carpathians were ice-bound. As we shall see, however, Eastern Europe came off rather more lightly than the west. Asia fared rather better than Europe. Most of North and Central Asia remained ice-free. Just to the eastern side of the Urals, a large cap covered the Tamyr Peninsula and spread some way to the south. The other part of the continent which could have sported an ice cap was the huge Tibetan Plateau much farther south, its great elevation making it a very cold place. Even here there is some doubt of the extent of ice cover, since surprising forensic evidence of the presence of humans in Tibet dates back to the LGM.
North America was particularly severely affected, with Canada, the Great Lakes, and the north-eastern states â in other words the entire northern two-thirds of the continent â weighed down by two massive ice sheets that connected on the east with the Greenland ice cap. Alaska, on the other hand, was then connected to Siberia by a huge ice-less bridge of now submerged land, Beringia, and to some extent shared Asiaâs freedom from ice. The largest of the two American ice sheets, the Laurentide in the east, left its vast imprint as a deep dent in the Earthâs crust in the form of the great inland sea now known as Hudson Bay.
In some places, in both Eurasia and America, huge lakes (known as periglacial lakes) surrounded the ice caps. The best-known remnants of these lakes are the Great Lakes of North America. The ice sheets themselves were not static, but flowed like the glaciers they were. Not only did these frozen rivers grind out new valleys and fjords, they also obliterated much evidence that humans had ever lived in the north.
<i>Ice was not the only barrier to human occupation during the big freeze. The worldâs deserts expanded to an even greater extent. Around the ice caps, huge regions of North Eurasia and America became polar desert in which only the hardiest of plants and animals could survive. In Europe, the polar desert stretched east from the southern edge of England and due east across northern Germany, to the south of the Finno-Scandinavian ice sheet. The whole of the region from the Levant and the Red Sea to Pakistan, normally pretty dry, became a continuous extreme desert. Southern Central Asia, from Turkmenistan and Uzbekistan by the Caspian Sea in the west, through Xinjiang (north of Tibet), to Inner Mongolia in the east became continuous desert either side of the 40th parallel. This desert, which effectively replaced Guthrieâs Mammoth Steppe heartland, also split North and northern Central Asia from the whole of East and Southeast Asia.</i>
As the Palaeolithic clock rolled on towards 20,000 years ago, however, events in the Earthâs spin axis and influences on its orbit hundreds of millions of kilometres away from our planet took tighter hold. Three great heavenly cycles of the solar system moved into a conjunction that ensured a minimum of the Sunâs heat reached the northern hemisphere during summer. The weather became colder, and the recurrent brief warm periods, or interstadials, which had characterized the period of 30,000â50,000 years ago, just stopped. It was these warm periods and their summer sunshine that had helped to melt the accumulated northern ice and prevent the ice caps from advancing across Scandinavia into Northern Europe. Now, the ice caps were able to expand in the north. The sea level started to fall again, eventually by 130 metres (430 feet). In short, the Earth was approaching its most recent ice age, or glaciation. (There had been quite a bit of ice on and off for the previous 100,000 years, and archaeologists tend to call the height of the Big Freeze the Last Glacial Maximum, or LGM for short, rather than an ice age.)
The LGM and its aftermath saw far more dramatic disruption and movement of northern human populations than at any time since. A glance at the world climate map around 18,000 years ago begins to give us the reason why. Huge areas of land became totally uninhabitable. For a start, the ice caps, some of them 5 km (3 miles) thick, clearly prevented the land they covered from being occupied. These white sheets were not laid evenly across the northern hemisphere. In Europe they mainly affected the central and northwestern regions. The British Isles, then part of the European mainland, were frozen down as far as Oxford in the south. Scandinavia will for ever bear the scars of the glaciers in its lakes and fjords and in the crustal depression now known as the Baltic Sea. Northern Germany, Poland, and the Baltic states bore the southern edge of the ice sheet, which extended north-east around the Arctic Circle across Finland and Karelia, into Archangel, and as far as the northern Urals. Farther south in Europe, mountainous regions such as the Pyrenees, the Massif Central, the Alps, and the Carpathians were ice-bound. As we shall see, however, Eastern Europe came off rather more lightly than the west. Asia fared rather better than Europe. Most of North and Central Asia remained ice-free. Just to the eastern side of the Urals, a large cap covered the Tamyr Peninsula and spread some way to the south. The other part of the continent which could have sported an ice cap was the huge Tibetan Plateau much farther south, its great elevation making it a very cold place. Even here there is some doubt of the extent of ice cover, since surprising forensic evidence of the presence of humans in Tibet dates back to the LGM.
North America was particularly severely affected, with Canada, the Great Lakes, and the north-eastern states â in other words the entire northern two-thirds of the continent â weighed down by two massive ice sheets that connected on the east with the Greenland ice cap. Alaska, on the other hand, was then connected to Siberia by a huge ice-less bridge of now submerged land, Beringia, and to some extent shared Asiaâs freedom from ice. The largest of the two American ice sheets, the Laurentide in the east, left its vast imprint as a deep dent in the Earthâs crust in the form of the great inland sea now known as Hudson Bay.
In some places, in both Eurasia and America, huge lakes (known as periglacial lakes) surrounded the ice caps. The best-known remnants of these lakes are the Great Lakes of North America. The ice sheets themselves were not static, but flowed like the glaciers they were. Not only did these frozen rivers grind out new valleys and fjords, they also obliterated much evidence that humans had ever lived in the north.
<i>Ice was not the only barrier to human occupation during the big freeze. The worldâs deserts expanded to an even greater extent. Around the ice caps, huge regions of North Eurasia and America became polar desert in which only the hardiest of plants and animals could survive. In Europe, the polar desert stretched east from the southern edge of England and due east across northern Germany, to the south of the Finno-Scandinavian ice sheet. The whole of the region from the Levant and the Red Sea to Pakistan, normally pretty dry, became a continuous extreme desert. Southern Central Asia, from Turkmenistan and Uzbekistan by the Caspian Sea in the west, through Xinjiang (north of Tibet), to Inner Mongolia in the east became continuous desert either side of the 40th parallel. This desert, which effectively replaced Guthrieâs Mammoth Steppe heartland, also split North and northern Central Asia from the whole of East and Southeast Asia.</i>