Desertification

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 sub-humid areas, of the biological or economic productivity and complexity of rainfed 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, because of 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) 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): 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 in north Africa and west Asia is pressured by overgrazing, combined with an extension of cropped area, and by salinization 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, and the Caribbean 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 areas (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.”

Link to Climate Change

Climate-simulation models indicate substantial future increases in soil erosion. Desertification will be exacerbated by reductions in average annual rainfall and increased evapotranspiration, especially in soils that have low levels of biological activity, organic matter, and aggregate stability. Desertification and climate can form a feedback loop, with the loss of vegetation caused by desertification reducing carbon sinks and increasing emissions from biodegrading plants.

Soil degradation begins with removal of vegetation. Unprotected, dry soil surfaces are readily eroded by rain and wind, leaving infertile lower soil layers that bake in the sun and become an unproductive hardpan. Sand dunes may form where the blown surface material accumulates. Water is a defining constraint of the drylands. Drought avoidance and coping strategies are imperative, such as choosing drought-tolerant crops, low plant densities, water conservation, and water harvesting. While water shortage is a constant concern, much of the water that is available is not efficiently used. In the Sahel, degraded soils often exhibit impeded water infiltration, so much is lost as runoff.

Given the uncertainty in the models, some models still project other outcomes, such as the regreening of the Sahel zone. Nevertheless, a loss of vegetative productivity can lead to long-term declines in agricultural yields, livestock yields, plant standing biomass and plant biodiversity—changes that reduce the ability of the land to support people. Before, even in adverse areas, people in drylands were able to cope with the cycles of droughts without depleting soils. Now, with the extension of droughts because of climate change, these systems are vulnerable to breakdown, often sparking an exodus of rural people to urban areas. Breaking the strong connection of people to the land produces profound changes in social structure, cultural identity, and political stability.

Desertification also impacts remote areas, and has silted-up rivers and retreated lake surfaces far away from its origin. Since the early 1950s, 670,000 ha. of arable land and 2.35 million ha. of rangeland, steppe, and grassland were invaded by shifting sands. Dust storms created as a result of destroyed vegetative cover led to air quality problems and acid rain elsewhere.