Although the Sahara has long been the world’s largest desert, a faint wobble in Earth’s orbit and other factors occurring some 12,000 years ago caused Africa’s seasonal monsoons to shift slightly north, bringing new rains to the Sahara. From Egypt in the east to Mauritania in the west, lakes with lush margins dotted the formerly parched landscape, drawing animals, fish and eventually people. Separating these two populations was an arid interval perhaps as long as a millennium that began about 8,000 years ago, when the lake disappeared and the site was abandoned.
http://www.world-science.net/othernews/080814_sahara.htmThe climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years. During the last ice age, the Sahara was larger than it is today, extending south beyond its current boundaries.<2> The end of the ice age brought wetter times to the Sahara, from about 8000 B.C.E. to 6000 B.C.E., perhaps due to low pressure areas over the collapsing ice sheets to the north.<3>
Once the ice sheets were gone, the northern part of the Sahara dried out. However, not long after the end of the ice sheets, the monsoon, which currently brings rain to the Sahel, came farther north and counteracted the drying trend in the southern Sahara. The monsoon in Africa (and elsewhere) is due to heating during the summer. Air over land becomes warmer and rises, pulling in cool wet air from the ocean. This causes rain. Paradoxically, the Sahara was wetter when it received more insolation in the summer. In turn, changes in solar insolation are caused by changes in the earth's orbital parameters.
By around 2500 B.C.E., the monsoon had retreated south to approximately where it is today<4>, leading to the desertification of the Sahara. The Sahara is currently as dry as it was about 13,000 years ago.<5>
During periods of a wet Sahara, the region became a savanna, and African flora and fauna become common. During the following dry arid period, the Sahara reverts to desert conditions. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread retreats northward to the Atlas Mountains, southward into West Africa, or eastward into the Nile Valley and then either southeast to the Ethiopian highlands and Kenya or northwest across the Sinai into Asia. This separated populations of some species in areas with different climates, forcing them to adapt.
http://www.newworldencyclopedia.org/entry/Sahara_DesertThought I would post this in case there were any climate change deniers out there - plenty of proof that it has changed in the past and will again.
Printer-friendly format
Email this thread to a friend
Bookmark this thread
(1000+ posts)
Send PM |
Profile |
Ignore
