The CCD defines desertification as land degradation in the drylands ("'Desertification' means land
degradation in arid, semi-arid and dry sub-humid areas."), yet the two terms are often used as if they
are distinct (e.g., "Land degradation and desertification in desert margins" by Reich et al. 2000). The
CCD also defines "land" by its primary productivity service ("'land' means the terrestrial bio
productive system.") and "land degradation" as an implicit loss of provision of this service ("'land
degradation' means reduction or loss of the biological or economic productivity .'). The
definition of biological productivity and economic benefit depends on users' priorities - transforming
woodland to cropland may decrease biological productivity, degrade the economic benefit of
firewood production but increase the economic benefit of food production. With respect to the
mechanisms of land degradation - changes in the properties of the land (soil, water, vegetation) do not
correspond linearly to changes in productivity. Loss of productivity can also be attributed to non14
human-induced factors such as rainfall variability and human factors such as low labor input. Thus, a
range of interacting variables that affect productivity should be addressed in order to assess
objectively and unambiguously land degradation.
Commonly considered degradation processes are vegetation degradation, water and wind erosion,
salinization, soil compaction and crusting, and soil nutrient depletion. Pollution, acidification,
alkalization, and water logging are often important locally (Oldeman, 1994; Lal, 2001; Dregne,
2002). Field experiments, field measurements, field observations, remote sensing, and computer
modeling are carried out to study these processes. The higher the aggregation level in each of these
study approaches, the more problematic each of the methods becomes, either because of upscaling
issues or because of questionable extrapolations and generalizations (Stocking, 1987; Scoones and
Toulmin, 1998; Matthews, 2000; Mazzucato and Niemeijer, 2000; Lal, 2001; Warren et al., 2001;
Dregne, 2002).
Source: Millennium Ecosystem Assessment, Current State and Trends, Chapter 22: Dryland Systems (details...)
Drylands are characterized by lack of water, which constrains their two major interlinked services - primary production and nutrient cycling. Over the long-term, natural moisture inputs (i.e., precipitation) are counterbalanced by moisture losses through evaporation from surfaces and transpiration by plants (i.e., evapo-transpiration). This potential water deficit affects both natural and managed ecosystems, which constrains the production of crops, forage and other plants and has great impacts on livestock and humans. However, drylands are not uniform. Rather, they differ in the degree of water limitation they experience. Following the UN CCD approach, four dryland subtypes are recognised in this assessment - dry subhumid, semiarid, arid and hyperarid subtypes - based on an increasing level of aridity or moisture deficit. The level of aridity typical for each of these dryland subtypes is given by the ratio of its mean annual precipitation to its mean annual evaporative demand, expressed as potential evapotranspiration. The long-term mean of this ratio is termed the "Aridity Index" (or AI). This chapter follows the World Atlas of Desertification (WAD, Middleton and Thomas 1997) and defines drylands as areas with an AI value of less than 0.652. Using AI values, the four dryland subtypes can be positioned along a gradient of moisture deficit. Together, these cover more than 6 billion ha, or 41.3% of the Earth's land surface (Table 22.1). Though the classification of an area as a dryland subtype is determined by its AI, which relates to the mean values of precipitation, it is important to remember that these areas do experience large between-year variability in precipitation.