Oct 8 (Reuters) - About 1.2 billion people are at risk from desertification as deserts expand and degraded dry lands cover close to a third of the world's land surface area, the United Nations estimates. Here are five facts about the phenomenon of encroaching desert lands.

* Desertification is not new. The Sumerian and Babylonian empires are among several ancient civilisations thought to have declined more rapidly after their agricultural output fell because of prolonged desiccation and water scarcity.

* Deserts expand naturally, but "desertification" is a different process where land in arid, semi-dry areas becomes degraded, soil loses its productivity and vegetation thins because of human activities and/or prolonged droughts/floods.

* The destruction wrought by spreading deserts grabbed global attention in 1968, nine years before the United Nations held its first conference on the issue. Some 250,000 people and millions of domestic animals died over a six-year period of severe drought in west Africa's sub-Saharan Sahel region, that hit Mauritania, Senegal, Mali, Burkina Faso, Niger, and Chad.

* Globally, the rate of desertification is speeding up, the UN Environment Programme (UNEP) says. Africa is the worst affected continent; with two-thirds of its land either desert or drylands. Almost a third of land in the U.S. is affected by desertification; and one quarter of Latin America and the Caribbean, and one fifth of Spain.

* Desertification is mainly a problem of sustainable development. Its causes include over-cropping, over-grazing, improper irrigation practices, and deforestation. Poor land management practices such as these often stem from the socioeconomic conditions in which the farmers live, and can be prevented.

Sources: Reuters, United Nations Convention to Combat Desertification (www.unccd.int)

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