Despite the global importance of desertification the available data on the extent of land degradation in drylands are limited. To date, there are only two studies with global coverage and both have considerable weakness, but in the absence of anything better have been widely used as a basis for national, regional and global environmental assessments. The best known study is the Global Assessment of Soil Degradation (GLASOD, Oldeman et al. 1991). Intended as an exploratory study it did not include any remote sensing or field measurements and was based on expert opinion only. A more detailed assessment - Soil Degradation in South and Southeast Asia (ASSOD) also relies heavily on expert opinion (Middleton and Thomas 1997). A more thorough, measurement-based global follow-up has not been conducted. Additionally, these studies only considered soil degradation and placed a strong emphasis on erosion, which is extremely hard to measure. These studies also formed the basis of the data and maps presented in the Word Atlas of Desertification (Middleton and Thomas 1997).

Recognizing the lack of adequate data on land degradation the MA commissioned a desk study (Lepers 2003) that compiled more detailed (and sometimes overlapping) regional data sets derived from literature review, erosion models, field assessments and remote sensing. This study found less alarming levels of land degradation (soil plus vegetation) in the drylands (including hyperarid regions). Achieving only partial coverage, and in some areas relying on a single data-set, it estimated that only 10% of global drylands were degraded.