Tassili N’Ajjer plateau, Algeria.
A once fertile savannah with lakes and rivers.
A once fertile savannah with lakes and rivers.
Having visited the Sahara Desert in April about 10 years ago, I can assure readers that it is not the hot, dry place of repute but can be cold and wet, at least in the Tunisian part. It was so cold with a fine drizzle, that, shivering in a t-shirt, I offered to buy the thick, hooded duffle coat a local troglodyte guide was wearing, but he quoted me 5000 dinars (about £400) with a knowing twinkle in his eye. I elected to shiver until I got back on the coach and the driver turned the heating on. Yes, there are troglodytes in Tunisia!
There have been times in the past when rain in the Sahara was not just freak weather in Spring to annoy tourists, but the norm in much of the year, so much so that the Sahara was mixed savannah and scrub with lakes and rivers, especially the western part.
Readers may recall how I mentioned the periodic greening of the Sahara in my article about the evolution of rock doves and feral pigeons. Briefly, a species of dove resident in West Africa crossed the Sahara during one such period when there was forest, grassland and water in place of sand. Then when the Sahara became desert the two populations diverged and the one which had made it as far as the Middle East hybridized with a resident related dove. This hybrid quickly became the normal form of the rock dove north and east of the Sahara and diversified further into several subspecies, one of which was domesticated and selectively bred to produce lots of different varieties. Some of these eventually reverted to a feral existence and became the ubiquitous town pigeon with a very different lifestyle and habitat to the original rock dove.
This process of African species moving into and across the Sahara during these periods of greening, and then becoming isolated from the African population, is known as the Saharah Pump and accounts for some of the sub-Saharan African species having a closely related counterpart in North Africa and Eurasia.
And this process has been going on since about 8 million years before creationists think Earth was created and may account for the migration of humans out of Africa some 40-50,000 years ago. More recently, however, there was certainly a population of humans living in a green and fertile Sahara up to at least 11,000 years ago (i.e., at least 1,000 years before 'Creation Week'. We know this because they left a record in rock carvings at Tassili N’Ajjer plateau in present-day Algeria, which show us some of the animals that lived there too.
These periods of greening have occurred approximately every 21,000 years and now two geoscientists from Helsinki University, Finland together with colleagues at Birmingham and Bristol universities, UK., have developed a climate model that explains how the climate changed so regularly and so radically. They have published their findings, open access, in Nature Communications. One of them, Edward Armstrong of Helsinki University, has also written an article about their research in The Conversation. His article is reprinted here under a Creative Commons licence, reformatted for stylistic consistency:
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