Sahara made slow transition from green to desert

A picture taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASAs Terra satelliteon shows dust blowing northward out of the Sahara Desert and over the Mediterranean Sea. The Sahara became the world's biggest hot desert some 2,700 years ago after a very slow fade from green, according to a new study which clashes with the theory that desertification came abruptly.