The links between
global warming
and desertification
are not obvious and
work both ways

uman civilization may owe much to the interplay between climate change and desertification. Some experts believe that the great cultures of Egypt and Mesopotamia began because the Arabian Peninsula and the Sahara rapidly turned to desert thousands of years ago, forcing people to settle along the rivers Nile, Tigris and Euphrates. The desertification is believed to have been brought about by a complex series of changes caused by alterations in the Earth's tilt and orbit.

Nothing as dramatic is happening today. But the world's top group of experts on global warming, the Intergovernmental Panel on Climate Change, says that 'climate change and desertification remain inextricably linked'. The trouble is that, although we know that the two inter-react, we do not know enough about how - except that it is complicated and subtle.

Everyone expects global warming to bring more droughts to most of the world's driest places, but these don't cause desertification in themselves. Overexploiting the land and cutting down trees are the main causes. But when these are already happening, declining rainfall can be the trigger that sets off desertification.

This is what has happened over the last decades in Africa's Sahel, on the fringes of the Sahara, where rainfall has diminished by up to 40 per cent, and the land has been severely degraded. Climatologists have linked the drier weather to global warming, and projections suggest that rainfall may diminish to the same degree in North Africa and southern Spain.

The link works the other way too. Desertification can help change the climate - though, again, this is a complicated process and far from the main cause. As grass and trees disappear the soil dries out and this may increase air temperatures. And the loss of vegetation removes one of the main buffers against climate change since, as plants grow, they remove carbon dioxide - the main cause of global warming - from the air. But burning coal, gas and oil - and cutting down and burning rainforests - are much bigger causes of the rising levels of the gas in the atmosphere.

It is not straightforward. Global warming, as its name implies,
is a worldwide phenomenon; desertification, essentially, is a collection of local processes. But when they work together they can create havoc for people and societies, whatever may have happened thousands of years ago.