Desertification is defined as the degradation of the land inarid, semi-arid and sub-humid dry areas caused by
climatic changes
and humanactivities. It is accompanied by a reduction in the natural potential of theland and a depletion in water resources. Itaffects about one-third of theearth's surface, on which one billion people live, namely, one-fifth of theworld population. The vulnerability of land to
desertification is mainly due tothe climate. The state of thesoil (texture, structure and chemical caused by human activities. Droughts occur frequentlyin the areas affected by desertification, and are generally a feature of theirnatural
climatic changes wich are both aconsequence and a cause of desertification. The destruction of the naturalgrass and woody vegetation cover in dry areas affects the topsoil temperatureand the air humidity and consequently influences the movements of atmosphericmasses and rainfall. Furthermore, the drying of the soils and the destructionof soil cover encourage air erosion. In many cases, traditionaland durable rainfed agricultural methods (food crops and alternating fallow)and ancestral pastoral practices are no longer suitable under present-dayconditions. Strong demographic pressure has increased the demand on landresources, and this is aggravated when cash-crop farming spreads to thedetriment of subsistence farming and to the detriment of the rangelands used bynomadic peoples. However, the impact of human societies on natural resourcesdoes not depend solely on the demographic density, and the notions of"load capacity" and "critical threshold" must be handledwith great care. Many examples demonstrate that these criteria can varyenormously, depending upon the strategies and the technologies used by thepeople. The seriousness ofdesertification depends on factors which vary from one region, country or yearto another. These factors include:the severity of the climatic conditions in the period considered (particularly in terms of the annual rainfall population pressure and the standard of living of the people involved; the level of the country's development, and the quality of the preventive measures established there. Lastly, desertification considerably heightens the effects of climatic crises(droughts) and political crises (wars), generally leading to migration, causingsuffering and even death to hundreds of thousands of people worldwide. But desertification alsoleads to a positive change in certain behaviour patterns. These include, inparticular, the attitude of the women who have to cope with the problems causedby the absence of the men, who have to leave to seek work elsewhere. The extraburden of work and the responsibilities which the women have to undertake arehaving two consequences: While thesurvival attitudes caused by desertification have often led to a decline inagricultural know-how, they have conversely encouraged the development oftechnical know-how, particularly relating to the environment and conservation.The micro-undertakings that have been implemented in many places over the pastfifteen years have made it possible to build up a store of know-how to be ableto implement new approaches. In many regions, the perception by the ruralpeople of the importance of their environment and the priority given to abetter relationship with the environment, have also changed. Moreincreasingly, rural people are realising that:Desertificationalso has consequences at the global level, primarily because of the influenceon carbon exchange. A substantial amount of carbon stored in the vegetation inthe dry zones, averaging about 30 tonnes per hectare, declines when thevegetation is depleted or disappears. Furthermore carbon-rich soils, which arefrequently found in the dry zones, store an important amount of this element(practically half the total quantity of carbon is stored in the organic matterin soil, which is more than in the world's vegetation): the destruction ofthese soils has a vpowerful affect on the carbon cycle and boosts thegreenhouse effect as a result of the depletion of carbon. Another consequence ofdesertification at the local and global level is the reduction in biodiversity,since it contributes to the destruction of the habitats of animal and vegetablespecies and micro-organisms. It encourages the genetic erosion of locallivestock and plant varieties and species living in fragile ecosystems. It isextremely difficult to put a figure on this loss because of our inadequatefamiliarity with the features, the siting and the economic importance of thebiodiversity of the dry zones. A substantial part of it is still fairly unknownto scientists, even though the local people are very familiar with it. Reducingthe biodiversity directly affects the food and health of the local people whorely on a large number of different animal and vegetable species. But it isalso a loss to the whole of mankind. Many genetic strains of cultivated plantswhich form the basis of the food and health of the world's population originatefrom the dry zones: their disappearance can affect the possibility of producingplant-based medicines to combat specific diseases or epidemics. Lastly, desertificationdirectly reduces the world's fresh water reserves. It has a direct impact onriver flow rates and the level of groundwater tables. The reduction of riverflow rates and the lowering of groundwater levels leads to the silting up ofestuaries, the encroachment of salt water into water tables, the pollution ofwater by suspended particles and salination, which in turn reduces thebiodiversity of fresh and brackish water and fishing catches, interfering withthe operation of reservoirs and irrigation channels, increasing coastal erosionand adversely affecting human and animal health. Lastly, desertification leadsto an accelerated and often unbridled exploitation of underground fossil waterreserves, and their gradual depletion. There are still many uncertainties at the global, regionaland national level on the causes, the extent and the seriousness ofdesertification. For those who manage natural resources, these uncertaintiesprevent them from planning properly. They also introduce constraints on theoperation of early warning systems with regard to agricultural production anddisasters .