Posted May 1996
See also: Environment Specials
Desertification is a world-wide phenomenon which causes the earth's ecosystems to deteriorate. It affects about two-thirds of the countries of the world, and one-third of the earth's surface, on which one billion people live, namely, one-fifth of the world population. The vulnerability of land to desertification is mainly due to the climate, the relief, the state of the soil and the natural vegetation, and the ways in which these two resources are used. Climate affects soil erosion and the chemical and biological deterioration of the soil. The state of the soil (texture, structure and chemical and biological properties) is a major factor, particularly in the sub-humid zones where the influence of climatic factors is less marked. It plays an essential role in causing vulnerability to desertification caused by human activities.
The same applies to the status of the natural and cultivated vegetation. Trees and bushes in particular, due to their long life and their capacity to develop powerful root systems, guarantee effective protection against soil degradation. Their disappearance considerably increases the vulnerability of the land to desertification. Lastly, even under the same conditions in terms of climate, relief, soils and vegetation, and with the same population density, the vulnerability of the land to desertification will vary widely depending on the way in which the natural resources are used by the human communities and their livestock.
Droughts occur frequently in the areas affected by desertification, and are generally a feature of their natural climate. The relations between desertification and drought on the one hand, and human influence on the other, are complex. Occasional droughts (due to seasonal or inter-year variations in rainfall) and long-term droughts covering wide areas are both caused or aggravated by the influence of man on the environment (the reduction in vegetation cover, the change in the Albedo effect, changes in the local climate, the greenhouse effect, etc.). Human influence can also hasten desertification and aggravate the negative consequences on man. But the degradation of land due to desertification has a serious compounding effect on drought, and thereby reduces the chances of the local people to cope with difficult periods.
Climatic changes are both a consequence and a cause of desertification. The destruction of the natural grass and woody vegetation cover in dry areas affects the topsoil temperature and the air humidity and consequently influences the movements of atmospheric masses and rainfall. Furthermore, the drying of the soils and the destruction of soil cover encourage air erosion.
Even though the cycles of drought years and climatic changes can contribute to the advance of desertification, it is mainly caused by changes in the ways man uses the natural resources, mainly by over-grazing, land clearance, over-cropping cultivated land and wood formations and more generally using land in a way that is inappropriate for the local conditions. Human activities connected with agriculture, livestock and forestry production vary widely from one country and from one type of society to another, as do the strategies for land-use and the technologies employed.
In many cases, traditional and durable rainfed agricultural methods (food crops and alternating fallow) and ancestral pastoral practices are no longer suitable under present-day conditions. Strong demographic pressure has increased the demand on land resources, and this is aggravated when cash-crop farming spreads to the detriment of subsistence farming and to the detriment of the rangelands used by nomadic peoples. However, the impact of human societies on natural resources does not depend solely on the demographic density, and the notions of "load capacity" and "critical threshold" must be handled with great care. Many examples demonstrate that these criteria can vary enormously, depending upon the strategies and the technologies used by the people.
The seriousness of desertification depends on factors which vary from one region, country or year to another. These factors include:
These consequences, in turn, weaken the economies of the developing countries affected by desertification, particularly when they have no other resources than their agriculture. This is particularly true in the African countries in the dry zones: their economy is unable to offset the increasingly serious effects of desertification, and they have to deal with emergency situations created by drought and desertification despite the increasing debt burden that is reducing their possibility of making productive investment in order to break the spiral of underdevelopment.
But desertification also leads to a positive change in certain behaviour patterns. These include, in particular, the attitude of the women who have to cope with the problems caused by the absence of the men, who have to leave to seek work elsewhere. The extra burden of work and the responsibilities which the women have to undertake are having two consequences:
Another consequence of desertification at the local and global level is the reduction in biodiversity, since it contributes to the destruction of the habitats of animal and vegetable species and micro-organisms. It encourages the genetic erosion of local livestock and plant varieties and species living in fragile ecosystems. It is extremely difficult to put a figure on this loss because of our inadequate familiarity with the features, the siting and the economic importance of the biodiversity of the dry zones. A substantial part of it is still fairly unknown to scientists, even though the local people are very familiar with it. Reducing the biodiversity directly affects the food and health of the local people who rely on a large number of different animal and vegetable species. But it is also a loss to the whole of mankind. Many genetic strains of cultivated plants which form the basis of the food and health of the world's population originate from the dry zones: their disappearance can affect the possibility of producing plant-based medicines to combat specific diseases or epidemics.
Lastly, desertification directly reduces the world's fresh water reserves. It has a direct impact on river flow rates and the level of groundwater tables. The reduction of river flow rates and the lowering of groundwater levels leads to the silting up of estuaries, the encroachment of salt water into water tables, the pollution of water by suspended particles and salination, which in turn reduces the biodiversity of fresh and brackish water and fishing catches, interfering with the operation of reservoirs and irrigation channels, increasing coastal erosion and adversely affecting human and animal health. Lastly, desertification leads to an accelerated and often unbridled exploitation of underground fossil water reserves, and their gradual depletion.
Despite the modernisation of observation facilities by the use of satellite imagery and computers to analyse the data, there are still many uncertainties at the global, regional and national level on the causes, the extent and the seriousness of desertification. For those who manage natural resources, these uncertainties prevent them from planning properly. They also introduce constraints on the operation of early warning systems with regard to agricultural production and disasters such as grasshopper infestations.
It is above all at the national and local level that information is urgently necessary. The results of high spatial and spectrum resolution satellite images such as SPOT or Landsat images, combined with high-frequency low resolution satellite data such as Meteosat and NOAA data, can be used by the geographic information systems and completed by the results obtained from the new methods of collecting soil data using navigation satellites (GPS). These methods, whose development is supported by FAO, is making it possible to observe, evaluate and monitor both the bio-physical and the socio-economic aspects of desertification.