Desertification occurs when the natural vegetation cover is reduced and the topsoil becomes susceptible to erosion. This initiates a number of other problems including increase surface runoff and stream discharge, reduction of water infiltration and groundwater recharge, change in surface microclimate and reduction in native plants.
Climatic change over a very long time scale can, by itself, result in the gradual depletion of vegetation structure and cover, species diversity, and the humus, mineral content and structure of the soil, which eventually lead to the formation of deserts. But the human actions and activity can accelerate this process, producing man-made deserts. Overgrazing, overcultivation, deforestation, bad irrigation and soil erosion all tend, where the climate is favourable, to produce desert and semi-desert conditions.
Once initiated, the spread of deserts is contagious. Sand not fixed in position by plants will spread in sandstorms; sand dunes, in a short period, can advance - engulfing roads, villages, crops and previously fertile land. The dynamics of erosion are such that land already eroded by wind and rain is more susceptible to further erosion. Successive removals eventually create soil conditions wherein plant growth is minimized and erodibility greatly increased. Control becomes more and more difficult. In the extreme, the sands begin to drift and form unstable dunes which encroach on better surrounding lands. Moreover, there are adverse climatic consequences of erosion, for much rain is lost to the sea or to underground deposits instead of being retained by the soil and evaporated. Ruined land is thus a source of hot, dry air which increases the aridity of conditions on neighbouring land.
Population growth and agricultural mismanagement have exacerbated desertification problems in regions worldwide. Extensive clearance of natural vegetation, grazing of livestock, agricultural expansion, and soil salinization have contributed to land degradation, reduced water supplies and limited agricultural production in desert areas. Water resources become a critical natural resource on which the livelihoods depend.
Although it is probable that existing deserts will remain where they are over the next 100 years, human misuse of the arid lands could accelerate natural processes of desertification to the point where most arid lands could be converted into deserts over that period. This would have significant implications for regional and global climate change.
Desertification contributes to global climate change by increasing exposed land surface, hence albedo; increasing the potential and decreasing the actual evapotranspiration rate; changing the ground surface energy budget and adjoining air temperature; and adding dust and carbon dioxide to the atmosphere. Also, by contributing to the planet's loss of biomass and bioproductivity and the exhaustion of the global humus reserve, desertification disrupts normal global biogeochemical turnover and reduces the global carbon dioxide sink in particular.
One major consequence of desertification is the development crisis affecting many dryland countries. Drylands still provide much of the world's grain and livestock, and form the habitat that supports the last remaining big game animals. The human population of the drylands lives in increasing insecurity as productive land per capita diminishes. Social effects of desertification are economic instability and political unrest in areas affected, and those neighbouring, as people struggle to survive with scarce land and water resources. The human costs often include malnutrition, threat of famine, and dislocation of people who must abandon their lands.
By 2050, an estimated 4 billion people will live in drylands. By then it is likely that land degradation, together with the closely related problems of climate change, will have forced 50-700 million people to migrate. Decreasing land productivity also makes societies more vulnerable to social instability – particularly in dryland areas, where years with extremely low rainfall have been associated with an increase of up to 45% in violent conflict.
Desertification is a significant threat to the arid, semiarid and dry sub-humid areas of the globe - the 'susceptible drylands' which cover 40 per cent of the Earth's land surface. Soil degradation in the drylands affects or puts at risk the livelihoods of more than 1,000 million people who are directly dependent on the land for their habitat and source of livelihood.
Some 1,035 million hectares, or 20 per cent of the world's susceptible drylands, are affected by human-induced soil degradation. Of this total, 45 per cent is affected by water erosion, 42 per cent by wind erosion, 10 per cent by chemical deterioration and 3 per cent by physical deterioration of the soil structure. Water erosion is the dominant form of degradation in semi-arid areas (51 per cent of total degradation) and dry sub-humid regions (also 51 per cent), and wind erosion is dominant in the arid zone (60 per cent). An estimated 35% of the world's land is now threatened by the advance of deserts.
More than a billion hectares of arid lands are already degraded worldwide, an area the size of China. Hundreds of millions of people suffer the consequences, forced migration and economic ruin. Globally every year an additional 200,000 square kilometres - an area larger than Senegal - are reduced by desertification to the point of yielding nothing. The process is accelerating: some 3.6 billion hectares of dryland (almost 70% of the total, comprising irrigated drylands, rainfed croplands and rangelands) UNEP calculated that between $150 and $600 billion would need to be spent over a 20 years period to halt further desertification of the world's 5.2 billion hectares of drylands. However, it has estimated that the direct annual income foregone due to desertification at $42.3 billion. The annual cost of preventative, corrective and rehabilitation measures combined are between 10.0 and 22.4 billion, less than half the costs (which do not include indirect and social costs difficult to calculate, such as the destruction of human potential or the loss of biodiversity). In 1993 UNEP indicated that $10-12 billion was required annually to assist persons affected by desertification, although total expenditures on desertification control had amounted to less than $1 billion in 1991.
Nearly two-thirds of African land is arid or semi-arid. The continent is the most seriously affected by desertification which threatens more than one-third of Africa's land area, particularly in Mediterranean Africa, the Sudano-Sahelian region and Southern Africa. In Northern Africa alone, more than 432 million hectares (57 per cent of total land) are threatened by desertification. Although overgrazing has long been considered the primary cause of desertification in Africa, it is now thought that rainfall variability and long-term droughts are more important determinants.
Attempts at reforestation in Spain, Italy and Greece would certainly have been more successful had the opposite shores of the Mediterranean still been covered with a wide belt of fertile land, as they once were. But the desert has already reached the shore of the Mediterranean on a wide front and sends out its drying winds to the European countries. Although approximately 100 countries are affected by desertification, the process is most serious in sub-Saharan Africa (particularly the Sudano-Sahelian zone), northwestern Asia, and the Middle East. 80% of sub-Saharan Africa's drylands and rangelands are affected -- some 1.5 billion hectares -- and the wellbeing of 900 million people is now threatened. By 1993, one country, Tunisia, was beyond the "water barrier" -- less that the bare minimum 500 cubic metres of water per person per year. By the end of the century it is predicted that 11 African countries with a total population of 25 million people will be in this state, and by 2025 this could rise to 1.1 billion people (two-thirds of Africa's projected population).