What made the climate of northern Africa change from tropical to grasslands, and then to desert?
April 9th, 2010 | 
Author: What made the climate of northern Africa change from tropical to grasslands, and then to desert?
tilt of the earth’s axis
the Nile River
Ice Ages
overpopulation of the region
Please only answer if you know FOR SURE.
Posted in Green Q&A | 
Tags: axis, Climate, Earth, grasslands, nile river, northern africa, overpopulation, river ice, tilt
There are two prevailing theories.
The first is the change in the tilt of the Earth which put more direct sunlight onto Northern Africa.
The other is that when India slammed into Asia, the Himalayan Mts rose and it redirected warm winds that would eventually dump onto Northern Africa. But the story is told that Northern Africa was tropical just a little over 15,000 years ago (and maybe even more recently). It is hard to assume that either of those theories could have that type of impact in that time frame. But it is just another example of why we know so little about Earth’s history, we only have guesses.
The end of the last great ice age about 11,000 years ago brought on my a massive comet strike mostly on North America. This raised sea levels almost 450 feet and completely disrupted and altered the currents in the oceans and atmosphere. This is very well known in geology circles but in few others.
These guys don’t claim to know for sure.
"…The green Sahara episodes correspond with the changing direction of the Earth’s rotational axis that regulates the solar energy in the tropical Atlantic Ocean. Periods of maximum solar energy increased the moisture production while pushing the African monsoon further north and increasing precipitation in the Sahara…. "
They go into extra details as well. I just added an excerpt to give a feel for it.
http://www.mpimet.mpg.de/en/presse/pressemitteilungen/bluehende-landschaften-in-der-wueste.html
I remember reading about the Sahara expanding in the 1970s in a process they called desertification. People would bring cattle into very sparsely vegetated regions and that would kill off all the vegetation and the desert would expand. Less vegetation meant less moisture and the desert expanded. It has recently recovered some so that theory sounds incomplete.
A sure answer is unlikely but there seems to be a number of ideas.
The Nile is the least likely reason, It runs in a gorge so never emptied out into the Sahara. Around 5,000 b.c.e to 3,000 b.c.e there was enough vegetation to support human hunters, they even built stone circles. The beginning of Egyptian civilisation around 3,000 b.c.e marked a dry period and hence the reason the hunters settled down in the Nile Valley.
It could have been caused by rebounding weather patterns that followed the end of the Ice-Age [8,000 bce]. Ice core data and patchy dendrocronology data [study of tree rings] indicate a downturn in weather around 3100 b.c.e.
Wet and dry periods occured following the Ice-Age, perhaps caused by volcanoes, perhaps asteriods or simply because the effects of the end of the ice-age caused weather systems to bounce about until relative stability in around 3000 b.c.e.
It was a good thing, the invention of agriculture seems to be linked to these climate changes..
As far I am aware the Earth’s tilt has been stable and around for about 5 billion years.
Domesticated cattle and over grazing of the grass lands finished it off.