Why is the Sahara a desert?

The Sahara covers about 9 million square kilometres and lies around 30 degrees north, where air that rose near the equator sinks back down as part of the Hadley circulation. Sinking air warms and dries, preventing clouds and rain. Yet it has not always been a desert: during the African Humid Period, roughly 11,000 to 5,000 years ago, the Sahara was covered with grasslands, lakes and wildlife, as shown by rock art of hippos and cattle. Changes in the tilt and wobble of Earth’s orbit shifted monsoon rains north, and when the rains retreated the region dried out again.