Raymond Dixon, of Waikanae, asks :-

Until about 2,500 years ago the Sahara was a green land until the rains suddenly stopped. Given the climate changes over a long period of time since, why has the world weather not resumed a pattern which brings rain again to such a desert area?

James Renwick, a climate scientist at Victoria University of Wellington, responded.

It has to do with slow changes in the orbit of the Earth around the Sun.

The angle of tilt of the Earth’s axis off the vertical changes between about 22 and 25 degrees every 41,000 years – the Earth wobbles on its axis very slowly, like a spinning top. The direction of the Earth’s tilt, relative to the Sun, also sweeps out a circle every 26,000 years or so. This means that the season in which the Earth is closest to the Sun in its elliptical orbit changes gradually. Presently, Earth is closest to the Sun in our summer, so the Southern Hemisphere gets more intense summer sunlight than the Northern Hemisphere does in its summer.

Ten thousand or so years ago, the Northern Hemisphere summer occurred when the Earth was closest to the Sun, and the axis of the Earth was a bit more tilted over than it is today. The result was that the Northern Hemisphere received more intense summer sunlight than it does today. The extra sunlight allowed the seasonal monsoon rains that occur in Africa (and on all the tropical continents) to extend farther north, bringing plenty of rain to the area we know today as the Sahara Desert. This lasted for several thousands of years, until around 5,000 years ago, when the intensity of the summer sunlight over northern Africa dropped away to the point where the monsoon retreated south again.

Because so much dust and sediment either blows off the Sahara, or is carried off by the major rivers, we can look at layers of sediment deposits on the ocean floor to work out how often the ‘greening of the Sahara’ has happened. What we see is that this has happened very regularly every 20,000 years or so, many many times over the past few million years. Right now, we are in the dry phase for the Sahara, but in another 10,000 years, the Sahara should be grassland and forest again.

There are many good resources about this on the web, just search for "humid sahara".