IN THE united Nation's Convention to Combat Desertification (UNCCD), desertification is defined as "land degradation in arid, semi-arid and dry subhumid areas resulting from various factors, including climatic variations and human activities." This land degradation is defined as a:

reduction or loss in arid, semi-arid and dry subhumid areas, of the biological or economic productivity and complexity of rain-fed cropland, irrigated cropland, or range, pasture, forest and woodlands resulting from land uses or from a process or combination of processes including processes arising from human activities and habitation patterns, such as soil erosion caused by wind and/or water, deterioration of the physical, chemical and biological or economic properties of soil, and long-term loss of natural vegetation.

In a more complex understanding, desertification also involves land-use change in pastoral and agricultural dryland systems, due to environmental pressures.

Many assessments have been conducted on desertification, each as varied as the indicators that they have been based upon. The most recent one is the Land Degradation Assessment in Drylands (LADA assessment) of the Food and Agriculture Organization of the United Nations (FAO). In the World Atlas of Desertification, based on the Global Assessment of Human-induced Soil Degradation (GLASOD) Assessment on human-induced soil degradation: drylands are characterized as the zones having a ratio of average annual precipitation to potential evapotranspiration (P/ETp) between 0.005 and 0.65, which includes semiarid and arid areas. Hyper-arid zones are not part of the United Nations Convention to Combat Desertification (UNCCD) desertification definition because they are presumed to be so dry that human degradation is severely limited unless irrigation is practiced.

Arable land resources of north Africa and west Asia are being pressured by overgrazing, combined with an extension of the cropped area, and by salini-zation caused by dam and irrigation systems such as in the Nile Delta. Dryland degradation is widespread in sub-Saharan Africa, northern China, Australia, northeastern Brazil, the Carribean islands; many other dryland areas have experienced damage from deforestation and overgrazing in the drylands. Also, in Europe, degradation is significant in the southern Mediterranean zone.

The complexity of definitions make mapping desertification difficult, but according to 2003 data from the UNCCD and WDA, 70 percent of all cultivated drylands are affected by desertification, and 17 percent are already desertified. 24 billion tons of topsoil is lost every year to desertification, which affects the livelihoods of 250 million people. Only 22 percent of global drylands have degraded soils, this figure increases to 70 percent when vegetation degradation is added. The dryland subtype that is most degraded is the arid subtype, whereas in regions with smaller overall dryland area, the semiarid drylands (the Americas), or the dry subhumid ones (Europe), are the most degraded, respectively. However, within these data it is also difficult to distinguish between states and processes. Africa is particularly vulnerable, with around 60 percent of its total area covered by desert or drylands. The United Nations Environment Programme (UNEP) notes that the extent of desertification is increasing worldwide: "desertification currently affects approximately 25 to 30 percent of the world's land surface area. About 1.2 billion people in at least 100 states are at risk."

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