Sahara Desert

The largest hot desert — once green

The Sahara is a desert the size of the USA! But only 25% of it is sand — the rest is rock and mountains. It can go 10 years without rain. Most amazing: 8,000 years ago the Sahara was green — with rivers, lakes, hippos and giraffes! Rock paintings of people swimming have been found. Sahara dust blows all the way to the Amazon and fertilises the forest.

The Sahara covers 9.2 million km² (third after Antarctica and the Arctic, first among hot deserts), stretching from the Atlantic to the Red Sea across 11 countries. A record surface temperature of 58 °C was claimed here; at night it drops to 0 °C. Sand dunes (ergs) reach 180 m, but 70% of the desert is rocky hamada. Because of wobbles in Earth's axis the monsoon shifts every 20,000 years: 11,000–5,000 years ago was the 'Green Sahara' — Lake Chad was the size of the Caspian, and rock art at Tassili shows hippos and swimmers. Sahara dust (180 million tonnes a year) carries phosphorus across the Atlantic to the Amazon. Beneath the desert lies the Nubian Aquifer, the world's largest reserve of fresh groundwater.

Fun facts

• Only 25% of the Sahara is sand — the rest is rock and mountains.
• 8,000 years ago hippos swam in the Sahara — there are rock paintings of them.
• Sahara dust 'feeds' the Amazon rainforest — it flies across the ocean.