With help from high resolution satellite imagery and some advanced artificial intelligence techniques, European scientists have been counting the trees in a parched African desert. They pored over 1.3 million square kilometres of the waterless western Sahara and the arid lands of the Sahel to the south, to identify what is in effect an unknown forest. This region a stretch of dunes and dryland larger than Angola, or Peru, or Niger proved to be home to 1.8 billion trees and shrubs with crowns larger than three square metres. "We were very surprised to see that quite a few trees actually grow in the Sahara Desert because up till now, most people thought that virtually none existed. We counted hundreds of millions of trees in the desert alone," said Martin Brandt, a geographer at the University of Copenhagen in Denmark, who led the research.