desertification

Ask most people to name the world's largest desert and they'll say the Sahara. They're wrong — and the reason why reveals what a desert actually is. A desert is defined by how little water falls from the sky, not by sand or heat. By that definition the single largest desert on Earth is Antarctica: a 14-million-square-kilometre sheet of ice that receives less precipitation than parts of the Sahara. The biggest deserts on Earth are frozen.

Russia has the most desert and dryland by total area. But Australia has the highest percentage of desert territory at 35%. The Sahara covers 11 countries. Desertification is advancing at 12 million hectares per year — an area the size of England lost annually to spreading desert. Here's the full ranking and the crisis of expanding deserts.