How was Sahara desert formed?
Upvote:9
I have in the past seen claims that human activity, specifically overgrazing by domestic goats and salinification of the water table by repeated irrigation, has been responsible for the advance of the desert into previously habitable areas of North Africa. That's a far cry from your book's reported claim, but its at least a nod in that direction.
It appears that a temporary change in the monsoon rain pattern at that latitude caused the Sahara to temporarily become habitable from about 10,000 to 5,000 BP. Scientists have given it the rather uninspiring name Neolithic Subpluvial. Supposedly some inscriptions in Egypt and the Sudan depict this. Here's an article in Live Science from 2006 that discusses the matter. However, before that time the Sahara was actually larger than it is now. So we are still better off that where we started before the monsoon aberration, and can probably expect it to go back toward that larger size, human activity or no.
There are theories that these changes just happen periodically.
I found an article in The Globalist from 2008 that makes the slightly less extravagant claim that human activity accelerated the re-desertification process when the rains went away. So this is an idea that is floating around. I don't know how widely-accepted it is though. Coincidentally, I also heard a story on NPR this morning about "carnivorists", or scientists who study carnivores. They do make the claim that taking carnivores out of an ecosystem can lead to large changes in the vegetation.
Still, we find deserts naturally worldwide in regions where rainfall is less than about 250mm a year. At its current northern limit, the Sahara receives about 100mm a year, and 150 at it southern. So I think it is fair to say that it would be a desert, regardless of what the animals around it are doing.
Upvote:15
Best evidence to date suggests that the Sahara dried up about 5,000 years ago, possibly in as little as 300 years, due to climate changes resulting from the precession and rotation of the Earth's rotation axis.
... around 8,000 years ago, the Earth's orbit was slightly different to how it is today. The tilt changed from around 24.1 degrees to the present-day 23.5 degrees.
...
Given the very strong dependence of vegetation on water availability, the end of the 'Green Sahara' came about quite suddenly around 5,500 years ago," Schmidt said. "Thus, a very slow change in the orbit (led) to an abrupt collapse in that ecosystem.
Update:
As regards the claim that human hunting of large carnivores resulted in over-grazing by large herbivores:
I find it an incredulous suggestion that stone-age humans were preferentially hunting large carnivores in an eco-system in which large herbivores were numerous.
Update #2:
The desertification of the Sahara, by all accounts, certainly occurred at least 2,000 years prior to the Founding of Rome in 753 BC.
Update #3
However, the previous update should not be interpreted as meaning that all of North Africa was exactly as arid 2,000 years ago as it is now. There is considerable evidence that the northern extent of the Sahara is somewhat more arid, and of modestly greater extent, than it was 2,000 years ago. The relationship of these changes to other climate changes, both global and regional, over the past 11,000 years ago remains undetermined.
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