Or rather the precession continued its stately progress and gradually took away the extremely favourable conditions for Saharan rains

Or rather, the precession continued its stately progress and gradually took away the extremely favourable conditions for Saharan rains. The climate system had crossed a threshold that triggered massive change Why?First, the sun moved. In the space of a century, the Saharan rivers emptied, swamps dried up and the monsoon rain clouds were replaced by clouds of wind-blown sand. The continually moistened winds took rain to the heart of the Sahara.But the African Humid Period came to an end – known as the Fall – suddenly. Meanwhile, the monsoon rains were recycled by the rich vegetation across North Africa. Rather as in the Amazon today, the rain nurtured lush vegetation that ensured that much of it evaporated back into the air. As the warm air rose, wet air was drawn in from over the Atlantic to replace it.

The process was the same one that creates today’s monsoon rain system in Asia. It coincided with a time when the Earth’s tilt known as the precession ensured that the sun was blazing down on the Sahara with full intensity in summer The land cooked and convective air currents were strong. Colonel Gaddafi has constructed pumps and a huge pipeline network to take this water from beneath southern Libya to his coastal farmers. He calls the network his Great Man-made River, though it is a feeble imitation of the real rivers that once ran here.The wet Sahara and the era known more generally as the African Humid Period began around 13,000 years ago as the ice age abated, and, except for the Younger Dryas hiatus, lasted to the end of the “golden age” of the African Humid Period.

They were mostly filled from leaking wadis in the early Holocene Some desert settlements today tap these waters at oases. Rocks beneath the Sahara contain the largest underground reservoir of fresh water in the world. Paintings in caves deep in the desert depict the lives of the inhabitants of the verdant Sahara of the Holocene.There are other, more practical remains. If there was a vestige of true desert at the heart of North Africa, it was very much smaller than the desert is today.And there were people: shepherds and fishers and hunters, and some of the earliest known fields of grains. Archaeologists digging in the sands of northern Chad, the dustiest place on Earth, have found human settlements around the shore of the ancient Lake Megachad.

And in waterless southern Libya, archaeologists are finding the bones of crocodiles and hippos, elephants and antelope. Along their banks were swamps and forests.Beneath the Algerian desert, archaeologists have found the remains of wadis that once drained 1,000 kilometres from the Ahaggar Mountains into the Mediterranean. From Senegal on the shores of the Atlantic to the Horn of Africa in the east, and from the shores of the Mediterranean in the north to the threshold of the central African rainforest, vast rivers flowed for thousands of kilometres. The whole of North Africa was watered by a monsoon system rather like the one that keeps much of Asia wet today Rain-bearing winds penetrated deep into the interior. Today the lake evaporates in the desert sun, but then it overflowed its inland basin and, at different times, drained south via Nigeria into the Atlantic or east down a vast wadi to the Nile.The difference is that back then the Sahara had assured rains. Where seas of sand now shimmer, there were once vast lakes, swamps and rivers.

Lake Chad, which today covers a paltry few hundred square kilometres, was then a vast inland sea, dubbed by scientists Lake Megachad. It was the size of France, Spain, Germany and the UK put together. Optimists suggest that such conditions might be what awaits us in a greenhouse world. There are celestial reasons why that might not happen, but that era, and its abrupt ending, may still offer important lessons about our future climate in the 21st century.
No place on Earth exemplifies the fall from this climatically blessed state better than the Sahara The world’s largest desert was not always so arid. From around 8,000 to 5,500 years ago, the world was as warm as it is today, but there appear to have been few strong hurricanes and few disruptive El Ni? and it was a world in which the regions occupied today by great deserts in Asia, the Americas and Africa were much wetter than they are now.