Writing Skills: Note Making

The Sahara is the world’s largest desert, but beneath it lie vast aquifers of fresh fossil water. Around 6,000 years ago it was green and could support life, as shown by rock art of hippopotamuses. This green phase was caused by a shift in the paleo-monsoon, which moved north when Earth’s tilt and orbit increased summer sunlight over North Africa. About 5,000 years ago the monsoon shifted south again, the land dried, vegetation declined, and a runaway drying effect turned the Sahara into a desert by 4,000 years ago. Scientists are still unsure how future climate change may affect it.