Distance travelled ~ 450'240'000 km: day 175

Image and notes courtesy of NASA

Image and notes courtesy of NASA

(On August 25, 2000, the Moderate-resolution Imaging Spectroradiometer (MODIS) acquired this spectacular image of a region in Africa's Sahara Desert, including the southern part of the border between Algeria and Libya. Three large rock massifs appear to be pushing up from beneath red sand dunes: from left to right are the Tassili, Tadrart-Acacus, and Amsak massifs. Different rock types account for rock colors varying from dark brown (Acacus) to the pale tone of Amsak eastern portion (Amsak Mellet means Pale Amsak in the local Tuareg dialect) The dendritic structures of ancient riverbeds are clearly visible in the Acacus-Amsak region.)



How did the Sahara once have a monsoon? Well it's all down to the amount of sunlight hitting the region. But not quite in the way you might think. It seems obvious that a hotter Sahara would have less rain, because it's the opposite. Monsoons are not created by less solar energy, they are created by more. To get a Monsoon you need lots of solar energy that heats up the land creating a region of low pressure to suck cool moist air from the oceans towards it. Strange as it may seem 8000 years ago when the cave paintings in Wadi Sora were made the Sahara was getting more sunlight than it is now. And that extra heat helped bring the monsoon rains to this desert. But how did the Sahara get more solar energy?

It's all down to our planets orientation to the Sun and how that changes over time. This orientation, which dictates the amount of sunlight we receive, is controlled by three main factors, Tilt, Precession and Orbit. 8000 years ago when the Sahara was green, these factors were different to what they are today. The science of this is very complex but here's a simple summary of what was going on.



Currently the earth tilts at an angle of 23.4 degrees. But over a 41,000 year period it changes, wobbling between 22.1 and 24.5 degrees. Back when the Sahara was green, the tilt was close to its largest possible angle, 24.2 degrees. Which meant that 8000 years ago the Sun shone more directly, more intensely over the Northern hemisphere.



Precession is even more important. This is not a change in the degree of the tilt, but a sort of lateral wobble, which changes the direction of the tilt. The best way of explaining it is by looking at the stars. Some of you may know about the North Star or Pole Star called Polaris. While other stars move across the sky, Polaris stays fixed just above celestial north. That's certainly true now but when the people painted themselves in the cave of swimmers, Polaris wasn't close to north in fact it was over to the east. Then 8000 years ago Thubon was the North Star. And in 12,000 years, a new star, Vega, will be pointing out due north. The North Star changes because precession makes the earth wobble a bit like a spinning top slowing down and starting to wobble back and forth. This precessional wobble takes 23000 years to complete one cycle, so it will be 23000 years before Polaris will come back round to be our northern star again.



There is one final factor involved. Our annual orbit around the Sun is not a perfect circle - it's an ellipse. Also the Sun does not sit at the centre, it's offset to one side. So there are times when it is closer to the Sun than others. Just like Tilt and Precession the shape of the orbit also changes slowly over time becoming more or less elliptical moving the earth closer or further from the sun. When the Sahara was green, all these orbital factors were in alignment, so summers in the northern hemisphere were hotter than they are now, the Sahara received more sunlight which pulled the monsoon band to northward.



The changes in our orientation to the Sun change all the time and gradually the orbit, tilt and precession changed so that the amount of solar energy hitting the Sahara eventually decreased and with it the monsoons.

Around 6000 years ago, the monsoons failed completely, the rains stopped, the rivers dried up and the land began to turn to desert. But even as you read this the same orbital factors are slowly changing and at some point in the future they will align again and the monsoons should once again return to the Sahara.



But that's not the end of the story because scientists have discovered that the wet period 8000 years ago wasn't a one off, in fact it had happened many times before. Perhaps the most significant greening of the Sahara occurred 120,000 years ago at a particularly important moment in human history.



120,000 years ago homo sapiens, modern humans, emerged from Africa. For thousands of years the Sahara had been an impassable barrier, a bit like it is now. But 120000 years ago the evidence suggests the Sahara was green and criss-crossed with rivers and lakes, and scientists believe that it allowed passage for our ancestors north. They crossed the Sahara travelling along rivers and settled in North Africa, and eventually, Europe and Asia.



It really is incredible to think that such critical moment in our history could have been triggered by changes in our tilt and orbit.