Desertification does not refer to the expansion of existing deserts. It occurs because dryland ecosystems, which cover over one third of the world's land area, are extremely vulnerable to over-exploitation and inappropriate land use. Poverty, political instability, deforestation, overgrazing and bad irrigation practices can all undermine the productivity of the land.

Over 250 million people are directly affected by desertification, and about one billion people in over one hundred countries are at risk. These people include many of the world's poorest, most marginalized and politically weak citizens.

UNCCD Explanatory Leaflet

The definition of desertification has been a hotly debated question for a number of years. The word conjours up a landscape that is "turning into a desert" and implies irreversible loss of natural resources leading to decreasing productivity and impoverishment of the population. The problem is that the loss of natural resources has many causes, both natural and anthropic and the consequences are equally varied, from rural depopulation to soil salinisation. The many local variations in the appearance of desertification phenomena lead to many different uses of the word. In turn these different uses have meant that it has been difficult to reach agreement on how to combat it.

Desertification has been neatly defined in the text of the United Nations Convention to Combat Desertification (UNCCD) as   "land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities."

This is the definition that is now universally used (although still not everyone entirely agrees with it because it excludes other regions also vulnerable to land degradation).

And what is land degradation?
Again the UNCCD provides a useful answer. It is the loss of biological or economic productivity and complexity of croplands, pastures or forests resulting from land uses or processes including those arising from human activities. These processes include things like soil erosion, deterioration of the quality of the soil and long-term loss of natural vegetation.

Systematic presentation of the relation between desertification and land degradation (after »GIZ, 2011)

What parts of the world are affected by desertification?

But the biggest problem is undoubtedly in Africa where a high proportion of the land and a substantial number of countries have low rainfall and high evapotranspiration rates. These are compounded by widespread poverty, difficult socio-economic conditions and heavy reliance of the populations on natural resources which lead to serious land degradation.