II. The extent of desertification
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19. The complexity of the causes of desertification and the diversity of its effects make it difficult to evaluate its magnitude with any degree of acuracy, and estimates of the areas affected or threatened by it are a matter of controversy. The different notions of irreversibility in terms of the timescale are also a matter of disagreement among scientists.
20. Despite the modernization of observation methods using satellite imagery and of data analysis using computers, there are still many uncertainties at the global, regional and national level about the causes, the extent, and the seriousness of desertification. For those who manage natural resources, these uncertainties are a handicap to proper planning. They also limit the effectiveness of early warning systems with regard to agricultural production and disasters such as locust infestations.
21. It is above all at the national and local level that information is urgently needed. The results of analyses of high spatial and spectrum resolution satellite imagery, such as Spot or Landsat images, combined with high-frequency low resolution satellite, data such as Meteosat and NOAA data, can be developed by geographic information systems and completed by results obtained from new methods of data collection on the ground using navigation satellites (GPS). These methods, the development of which is supported by FAO, make it possible to observe, evaluate and monitor both the big-physical and the socioeconomic aspects of desertification.