Three mighty rivers once flowed through the Sahara
http://io9.com/three-mighty-rivers-once-flowed-through-the-sahara-1316516244
The world’s most famous desert isn’t always quite so dry as it is now—thanks to the Earth shifting on its axis, the Sahara’s climate gets gradually greener over tens of thousands of years. But you’d have to go back a long ways to find real, perennial rivers flowing through the Sahara outside of the Nile. But 100,000 years ago, the Sahara was awash with rivers that might well have led humanity’s ancestors to the Mediterranean.
That’s a possibility raised by new work from an international team of researchers, whose computer simulations suggest the presence of massive river systems in the Sahara between 130,000 and 100,000 years ago.
Milankovitch Cycles
http://en.wikipedia.org/wiki/Milankovitch_cycles
Milankovitch theory describes the collective effects of changes in the Earth’s movements upon its climate, named after Serbian geophysicist and astronomer Milutin Milankovich, who worked on it during his internment as a First World War prisoner of war (POW). Milankovich mathematically theorized that variations in eccentricity, axial tilt, and precession of the Earth’s orbit determined climatic patterns on Earth through orbital forcing.
The Earth’s orbit is an ellipse. The eccentricity is a measure of the departure of this ellipse from circularity. The shape of the Earth’s orbit varies in time between nearly circular (low eccentricity of 0.000055) and mildly elliptical (high eccentricity of 0.0679)[3] with the mean eccentricity of 0.0019 as geometric or logarithmic mean and 0.034 as arithmetic mean, the latter useless. The major component of these variations occurs on a period of 413,000 years (eccentricity variation of ±0.012). A number of other terms vary between components 95,000 and 125,000 years (with a beat period 400,000 years), and loosely combine into a 100,000-year cycle (variation of -0.03 to +0.02). The present eccentricity is 0.017.