Summarising 'Green Sahara'

The book's practice passage, 'Green Sahara', makes a good summary exercise. Its main points:

• The Sahara is the world's largest and driest desert. Yet ancient fresh water lies deep beneath it.
• About 6,000 years ago, the Sahara was green. Rock art from that time shows animals such as hippos, which need plenty of water.
• Slow cycles in the earth's tilt, its wobble, and the time of year when it comes closest to the sun move the African monsoon north and south. The monsoon moved north about 10,000 years ago, bringing rain, and moved back south about 5,000 years ago.
• Once the drying began, it fed on itself: fewer plants meant soil that held less water, which meant fewer clouds and less rain. By about 4,000 years ago, the desert was as dry as it is today.

A good summary keeps this chain of causes and the key dates, and leaves out the quotations from scientists and the passage's joke about bottled water.

The method works for any passage: find the main ideas and how they connect, then tell them briefly, in your own words.