Desertification is a reduction in the ability of the land to sustain plant life. Plants are the key to the process which can lead to desertification. Plants affect the hydrology of the land surface, which in turn affects the climate. Desertification is happening in many places around the world. In Africa, it has meant famine and suffering; in temperate Europe, farms are drying to dust; and in Oklahoma in America, the destructive 'Dust Bowl' of the 1930s threatens to return.

The Sahara, which stretches right across North Africa, is the ultimate desert and the biggest in the world.

Even on this barren land, a few people manage a sustainable nomadic existence. The Bedouin, ancient Arabic tribes who still roam the northern and eastern Sahara, manage to survive by herding goats and sheep.

Another nomadic people, the Tuareg, live on the western Sahara. The Tuareg are the guides who take travellers across the wastes, resting at oases which only they know. But in the 1990s, these all-important oases are shrinking and pasture is disappearing. Three years of drought and no harvest mean that the food is running out. Some Tuareg are being forced to abandon their nomadic ways and move to cities to find food. Irrigation systems have been built to help them survive.

Lake Chad, an important source of water, was once the size of Wales. But for the last twenty years there has hardly been any rain. Lake Chad provides water for over a million people from four countries - Nigeria, Niger, Cameroon and Chad - who live by and around the lake.

The northern part of the lake has disappeared. In fact, the whole lake has shrunk to a tenth of its former size. It is predicted that Lake Chad will dry out altogether within the next five years. Drought is not the only reason why it's shrinking. Nigeria and Cameroon have pumped billions of litres of water from it to try to solve their own water shortages. As deserts spread, so does pressure on people to find ways to survive. Too many people are trying to live off ever-diminishing resources.

Even in Europe, desert is spreading. Just behind the rich resorts of the Costa del Sol, farmland is turning to dust, and there are now large areas of southern Spain which

look like true desert. To reverse this desertification, farmers tried to 'terrace' their fields to retain as much rainwater as possible. They also tried planting drought-resistant crops, and many hectares of forest, but very little rain came.

The factories processing the crops that used to grow here are closing. These are the largest local employers, and are leaving whole districts without work. Traditional farming is now being replaced by acres of polythene greenhouses sheltering crops like tomatoes and strawberries. Costly minerals, fertilisers and chemicals are drip-fed to the crop through miles of hoses and pipes, but the automation creates few jobs.

In the nineteenth century, new American immigrants sought a better life, away from overcrowded Europe. Homesteaders ploughed the land to grow crops like wheat and to graze cattle.

The settlers farmed too intensively, growing only one crop, and in the 1930s came drought. The exhausted soil was just blown away. The area became known as the Dust Bowl. The land had turned to desert. Half a million people were forced to give up everything and migrate west, searching for new land.

It took ten years of drastic action to revive the land, by planting millions of trees and hedges and building lakes to save water. Drought-resistant grasses were developed to bind the topsoil.

Sustainable farming made the Great Plains into the bread-basket that fed the world. But in the 1980s, successive years of cataclysmic drought returned.

People were desperate for rain. All around the world, droughts were happening more often. Meteorologists began to wonder if our climate was changing dramatically. The farming of marginal land, and global warming, are going to cause yet more parched earth. A quarter of the world's population is, for the moment, dependent on that marginal land.