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The climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years. During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries. The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps due to low pressure areas over the collapsing ice sheets to the north.

Once the ice sheets were gone, northern Sahara dried out. But in southern Sahara, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today. The monsoon is due to heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more solar insolation in the summer. This was caused by a stronger tilt in Earth's axis of orbit than today, and perihelion occurred at the end of July.

By around 3400 BC, the monsoon retreated south to approximately where it is today, leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago. These conditions are responsible for what has been called the Sahara pump theory. (click to see all the article on Wikipedia) 

撒哈拉的沙漠化  ~西元前3千年







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