Using a new computer simulation of the Earth's climate, German scientists say that the Sahara underwent a brutal climate change about 4,000 years ago.

Over a very short time scale - possibly as short as 300 years - it went from grasslands with low shrubs to the desert we are familiar with today. Summer temperatures increased rapidly and rainfall almost ceased. The change devastated many ancient cultures and caused those that did survive to migrate elsewhere.

Scientists at the Potsdam Institute for Climate Research say that the desertification of the Sahara was one of the most dramatic changes in climate over the past 11,000 years.

The loss of agricultural land to the desert may have been one of the reasons why early civilisations developed along the valleys of the Nile, the Tigris and the Euphrates.

Feedback mechanism

Slight climate alterations caused by subtle changes in the Earth's orbit around the Sun were amplified by a climatic feedback mechanism.

Some 9,000 years ago the tilt of the Earth's axis was 24.14 degrees; today it is 23.45 degrees. Today, the Earth is closest to the Sun in January. Nine thousand years ago, our planet was closest to the Sun at the end of July.

The changes in the tilt of the Earth occur gradually. However, the interplay of atmosphere, ocean and landmass can react to these changes in abrupt and severe ways.

The climate model suggests that land use by man was not an important factor in the creation of the Sahara.