The following changes are typical of desertification.

• Rain-fed farming: In areas of rain-fed farming, desertification often originates on land cleared for cultivation or left fallow. Removal of the original vegetative cover exposes the soil to accelerated wind and water erosion.

• Water and wind erosion: Water and wind erosion work together, as redeposited silts from surfaces stripped by water erosion are particularly vulnerable to wind transport.

Desertification occurs in the tropical drylands fringing the world's deserts where 700 million people support themselves. The most obvious impact in addition to widespread poverty, is the degradation of 3.3 billion hectares of the total area of rangeland.

This constitutes 73 per cent of the rangeland with a low potential for human and animal carrying capacity. There is a decline in soil fertility and soil structure on about 47 per cent of the dryland areas constituting marginal rainfed cropland; and the degradation of irrigated cropland, amounting to 30 per cent of the dryland areas with a high population density and agricultural potential.

There are 5 desert basins:

1. Northern Mexico/Southern USA

2. Atacama Desert in South America

3. Belt from Sahara in Africa through Iran to Pakistan India Mongolia and China

4. Kalahari in Southern Africa

5. Most of Australia.