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THE DRYING-UP OF THE SAHARA  ca ~3000

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

撒哈拉的氣候在過去的幾十萬年,在乾與濕之間劇烈變動著。在最終的冰河時期,撒哈拉比現今的範圍還大,從現在的邊界線向南擴展。自公元前80006000年,冰河時期的終結,帶給了撒哈拉更多的雨水,可能是因為低氣壓越過了北邊崩塌的冰原。

一旦冰原消失,北撒哈拉就乾涸掉了。但是在南撒哈拉,季風阻擋了乾燥的蔓延,而帶給北邊比今日還多的雨水。在夏季時,季風加熱地表上的空氣。熱空氣上升,冷空氣下降,濕空氣從海上來,這造成了降雨。如此,雖然看起來違反常態,當撒哈拉接收較多的夏季太陽輻射時,會更加潮濕。這是由於當時地球的軌道軸遠比今日傾斜,並且發生於七月的近日點。

公元前3400年左右,季風向南退到現在的位置,導致撒哈拉逐漸沙漠化。撒哈拉現在如此乾燥,源自於13000年前。這種沙漠化的情形被稱作撒哈拉泵浦理論。

(維基連結:撒哈拉沙漠) 

 

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