Curiosity 7 of 10
7. The Sahara was once green
The world's largest hot desert once had lakes, rivers and savannas. Until about 5,000 years ago, the Sahara was much wetter, with hippos, giraffes and people living by hunting and herding cattle.
A cycle that follows Earth's orbit
This period, which scientists call the African Humid Period, lasted thousands of years and ended between 6,000 and 5,000 years ago. Rock paintings in the desert, such as those on the Tassili n'Ajjer plateau in Algeria, show cattle, giraffes and other animals that could not survive there today.
The cause is a slow wobble in Earth's axis, which changes the amount of sunlight each hemisphere receives in summer. When the northern summer becomes stronger, the African monsoons move deeper into the continent and bring rain to the Sahara. This cycle repeats roughly every 21,000 years.
When the rains retreated, the vegetation disappeared, the lakes dried up and the desert returned. Lake Chad, now much smaller, is what remains of a huge ancient lake.
An ancient lakebed, the Bodélé Depression in Chad, has become one of the planet's largest sources of dust. Winds carry this dust far away, across the Atlantic Ocean.
Você sabia? Parte dessa poeira chega à Amazônia. Um estudo com dados de satélite da NASA estimou que cerca de 27,7 milhões de toneladas de poeira do Saara caem por ano sobre a bacia amazônica, levando fósforo, um nutriente que ajuda a adubar a floresta.
