From the ecological point of view, desertification is defined as being the conjunction of two phenomena: the occurrence of the prolonged droughts and the excessive pressure of the man and his animals on unstable fragile ecosystems or little cancelling. (Le HOUEROU, 1979/1987).
In the broad sense, desertification can mean environmental crisis which produces conditions or nearby landscapes of those of a desert (Encyclopedia of Environ-Sciences, 1999).
We appoint by desertification, the ecological consequences of an aridification of the climate (Ramade, 2002).
In the sense of the United Nations convention, desertification is the degradation of land in arid and arid sub-humid areas. It occurs when soils are fragile, the vegetable cover is reduced and the particularly harsh climate ( 07-06-94 ).
There are more than 130 definitions of the word desertification in the literature according to Mainguet 1998.
The desertification knew numerous definitions which were the object of intellectual controversies. Beyond the political compromises, a consensual definition of the process was proposed by the Convention on the fight against the
Desertification: ” the desertification is the degradation of land in arid and arid sub-humid areas due to various factors: including climatic variations and human activities “. The desertification thus concerns a process of degradation of lands linked to natural factors aggravated by the action of the man.
Indeed in arid regions, when the degradation of grounds accelerates ceaselessly, reducing the reserves of the productive grounds, it creates an environment similar to that of the deserts: we speak then about desertification. The desertification does not content with destroying the base of the productive resources, it also provokes the loss of the genetic resources, and it increases the atmospheric dust, disrupts the process of natural recycling of waters and disrupts the economy of a country pulling movements of populations. It is synonymic of loss of biological and economic productivity of arable, pastures and woody lands.
In view of the various examined definitions, the climate countered as a determining factor.
What is the climate determining?
The process of desertification appears, generally in the bioclimatic floors characterized by a pluviometer from 100 to 400 mm / year.
Besides, the irregularity and the level of precipitations, the strong temperatures, the drying winds loaded with particles of sand and the intensity of the ETP are so many factors deteriorating especially when the human activities are transplanted there.
The figure 3 illustrates the action combined by the natural and anthropological factors involved in the process of desertification.
The climate has diverse, direct and indirect incidences on ecosystems:
On water resources (weakness of the rainfall, the irregularity);
- Frequency and continuation of arid period;
- Aridity of streams and brooks;
On the type of vegetation, its distribution and its density
On grounds by reducing their rate of organic matter and their power of keeping back to water with increase of the risk of Salinization
In other words, by acting on these essential parameters, the climate shapes the ecosystems.
The anthropological action is also important. It falls on behind the step in the climatic action by amplifying the process.
How is the process of desertification made?
The desertification is distinguished from risks with shock effect (devastating floods, earthquakes, forest fires) by the involved mechanisms, by the mode of expression and by its spatiotemporal evolution.
Two factors seem essential so that the desertification occurs:
- Conditions of natural physical weakness (physical and biological);
- A strong human pressure exceeding the acceptable threshold.
As the vegetation declines grounds are subjected more and more to the degradation hazards (erosion, Salinization,) being able to lay bare the source rock.
When the vegetation disappears, the desertification accelerates leaving cleaned grounds where the hydrous and wind erosion causes important damages.
If this phenomenon occurred in an insidious and slow way even in the geologic scale, nowadays, the conjugation of several factors (natural and anthropological) seems to look to it a speed of perceptible acceleration and a faster extension.
For illustration:
In Sudan, the desert encroachment is from 90 to 100 km between 1956 and 1975;
In the Chad, the plant cover setting degraded of 32 % between 1954 and 1974;
In Tunisia, on a sample of 20 000 ha, 7500 ha are become depopulated or 347 % between 1965 and 1974. (The HOUEROU, 1979)
In Algeria, on a sample of 13 000ha, 500 000ha of the steppe become totally depopulated or 41 % (KARA 2000) with an effect more marked at the level of the steppe West where we also note a considerable regression of let us tax climatic (white Artemisia and Alfa), during these last years (SALAMANI and HIRCHE, 2007; AYDOND, on 2009)
In Spain, 31 % of lands would be threatened with desertification. The degradation of the socioeconomic conditions is in the heart of the problems of desertification:
destruction of the bases of production, social system in danger, impoverishment of the populations …
The scale of drought and desertification differs according to the zones of impact in the world: the phenomenon affects more than a country and more than a continent and tends to increase these last decades. Meadows of the third party of the appeared lands is threatened.
So, the vulnerable zones represented in figure 4 are situated in arid zones and semi arid in particular in the suburb of the climatic deserts.
The process seems to reach continents of which we thought formerly under cover such as Europe in particular in its south bank: the current situation of the South of Spain is worrisome (Figure 6).
The scale of desertification at the world level:
Meadows the third party (1/3) of the appeared lands is threatened
1 billion persons are concerned
A reduction of the farmlands of: 2/3 for Africa, 1/3 for Asia and 1/5 for the Latin America and the Caribbean (Caribbean islands) [Source: the UNO, the FAO and the UNESCO]
24 billion tons of fertile soils disappear every year.