THE SAHARA and Arabia were transformed abruptly from fertile land covered with shrubs and grasses into a parched desert in a "brutal" period of climatic change lasting 400 years, scientists have found.

For once, humans were not to blame: the cause was not farming or overgrazing, as is usually thought. Scientists reckon that the Sahara began to turn into a desert after the Earth underwent one of its periodic changes in orientation, starting 9,000 and finishing about 6,000 years ago. Its tilt lessened from 24.14 degrees off vertical to its present 23.45 degrees, while the time when the planet is closest to the Sun shifted gradually from July to January.

Nobody knows what triggered the changes, but geologists think they were probably caused by shifts of material deep inside the Earth's molten core. They altered the pattern of sunshine on the Earth, with profound effects on many weather systems. The discovery has implications today, as many climate researchers are worried that gradual changes in the Earth's temperature caused by global warming will have marked effects on ocean currents - particularly those that warm Britain.

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Before the change, the northern hemisphere received more summer sunlight, which amplified summer monsoons. But once the change was over, the new conditions created a vicious feedback loop between vegetation and climate. As the African monsoon lessened, plants began dying. As they stopped retaining water and releasing it back into the atmosphere, the rains lessened further, until rivers and streams dried up. The Sahara Desert now covers 3.5 million square miles.

The discovery emerged from a new computerised climate model made by a team of scientists at the Potsdam Institute for Climate Change.

"Our simulations show how interactions between vegetation, atmosphere and ocean current can lead to relatively abrupt climate changes - a process that might influence climate in future, too," said Martin Claussen, the team's leader.

"It was very severe, ruining ancient civilisations and socio-economic systems," he added. In the Sahara, "we find an abrupt decrease in vegetation, from a green Sahara to a desert shrubland within a few hundred years".

The ancient civilisations that had lived there - and left rock paintings - may have been forced to migrate to the more fertile Nile valley and other river valleys, where great civilisations developed.

Professor Claussen noted that changes in the Earth's orbit and tilt will continue to occur. As to their effects, he said: "What will happen in the future, frankly, we can only speculate."