The Great Barrier – Sandstone Plateau
Orbit: Earth’s Extraordinary Journey
It’s extraordinary to think that the moon is both evidence of what caused Earth’s 23° tilt and the celestial object that helps maintain it. But the stabilisation that the moon provides isn’t perfect. And the smallest of variations in the angle of tilt can have profound consequences.
Remarkable evidence for this can be found in the Egyptian desert. This is the Sahara. Hidden in this apparently lifeless landscape is proof that the Earth’s tilt has changed, and in the recent past. And that change has transformed climate and history. With me is geographer Nick Drake. He’s a veteran explorer of this region. We travelled through the desert for 600 km to reach our destination. This is the Gilf Kebir, the Great Barrier, a plateau over 7,000 km in length. For hundreds of years, explorers have come here in search of the lost world. A decade ago, one group succeeded.
In 2002, a couple of Italians were exploring this Gilf Kebir region when they spotted a cave. I don’t think even they could have hoped for something as spectacular as of this.
The extraordinary paintings in the Cave of Beasts, are around 8,000 years old, more than 3,000 years older than the pyramids.
When you start to look at the figures on the wall, this seemed to be a very athletic population of people. They all seem to be running or jumping or throwing things. But you’ve also got wonderful pictures of antelope here, and images of giraffe. But there are some figures that are in a very strange position indeed. There is a theory, that they could be swimming.
So where did the waters that sustained those people and animals come from? A day’s travel away is the Valley of Wadi Bakht. Here, there are clues that have helped to resolve this mystery. Nick Drake studies the ancient geology of the African deserts. The pattern of highly seasonal rainfall, that Nick predicted, can mean only one thing, this now barren desert, once received a monsoon. The geology of this site tells us that the rains fell in this area between 5,000 and 10,000 years ago, transforming the landscape of Wadi Bakht and creating a lake.
Nick Drake’s research has revealed that the ancient African monsoon helped feed a verdant Sahara, a place criss-crossed by many rivers, with huge lakes, one was 20% bigger than the UK. The mystery, then, is what could have brought these rains here to Gilf Kebir?
We know from the Indian monsoon that when the land is hottest in the summer months, it creates a low pressure system which draws in cold, moist air. So the irony is, this part of the Sahara must have been receiving more of the sun’s energy, it must have been hotter back then 5,000 years ago, than it is today. And that’s what allowed the monsoon rains to cover this area with water. What’s remarkable is that higher temperatures that drove the Saharan monsoon with the consequences of a tiny change in the angle of the Earth’s tilt.
Although the gravitational pull of the moon and sun together have stabilised our tilt, they don’t do it perfectly. Today, the angle of tilt is 23.4°, but over regular, 41,000 year cycles, the angle swings between 22° and 24.5°. Back when the Sahara was green, the Earth’s tilt was close to its maximum angle. Together with small cyclical changes in the direction of the tilt and the shape of our orbit, the result was the sun shone more intensely over the northern hemisphere, powering a monsoon in the Sahara.
About 5,000 years ago, the monsoons failed here, and very quickly, the vegetation started to disappear. Within a few hundred years or so, this area had gone from Savannah to Desert. And people who settled this once verdant land were forced to move north and east to a still fertile river valley, of the Nile. It’s rather wonderful to think that because the changes in our tilt and orbit are cyclical, there may come a day when the Sahara will be green once again. But, not for another 15,000 years.