“For decades, I have heard this phrase repeated: 'the desert is advancing'. Africa should be nothing more than an immense desert as I speak to you.” At the opening, Monday, December 2 in Saudi Arabia, of the 16th UN Conference on Desertification (COP16), paleoecologist and research director at the CNRS Anne-Marie Lézine underlines, not without irony, that we cannot deliver not lightly the title of “desert”. “A desert is a biome – the combination of weather conditions, plants, animals, etc. – You don’t just replace one biome with another,” summarizes the specialist, who studies the interactions between plants, climate and humans, in the driest regions of the planet.
The United Nations Convention to Combat Desertification (CCUNCD), which oversees the current UN summit in Riyadh, admits that it is not in fact a question of fighting against any “expansion of existing deserts”, but to limit “land degradation in arid, semi-arid and dry sub-humid zones”. Because behind this catch-all term hides an existential threat: that of seeing cultivable spaces shrink like nothing under the effect of human activities.
So that a territory deprived of water becomes truly desert“time scales are not what you imagine”, warns Anne-Marie Lézine. But if the use of the word “desertification” here contravenes the geomorphological and climatic logics which gave birth and then shaped deserts over several million years, the term reflects the transformation of millions of hectares each year. Deterioration observable with the naked eye, from China to Spain via South Africa, to the rhythm of episodes of drought. More intense and frequent – particularly on the Mediterranean rim, in West Asia, in several areas of South America, in a large part of Africa and North-East Asia, according to the IPCC report on desertification (link a PDF) –, these episodes are not just a passing scourge. They are “a new standard”.
“A prolonged drought, over two or three years for example, can have irreversible effects on the plant cover,” explains physicist Mehrez Zribi, research director at CNRS and director of the Center for Space Studies of the Biosphere. When they are “prolonged and repetitive”, they “imply a high risk of transition to desertification”, he continues, pointing out negative effects on “resilience [des sols] in the face of these extreme phenomena.
In the areas concerned, “soil fertility decreases, water reserves decrease, biodiversity decreases”, list Jean-Luc Chotte, research director at the Research Institute for Development (IRD) and president of the French Scientific Committee on Desertification. Under vegetation conducive to going up in smoke, “soils become more fragile in the face of the proliferation of invasive species and water shocks”.
Brief, “we produce less and of lower quality, [ce qui entraîne] a risk of food and nutritional insecurity. Human health is affected”, continues the specialist, who describes a vicious circle. “The services that soils provide, particularly by sequestering carbon, are weakened, contributing to an increase in greenhouse gas emissions.” This fuels global warming, the primary vector of new droughts.
This “aridification of landscapes, however, combines a set of factors”, completes Anne-Marie Lézine. Thus, the dramatic drought that hit the Sahel in the 1970s did not turn it into a desert. But “when you couple drought with intensive land use, you allow the beginning of an evolution towards desert, even if, of course, this is not going to happen in ten years,” she tempers. The scientist therefore insists on the need to ensure the health of this vulnerable area, sandwiched between the savannah and the Sahara. “That was the whole idea of the ‘great green wall’ project,” continues Anne-Marie Lézine: “strengthen the quality of the existing plant cover in this sensitive and populated area, so that it is able to cope with episodes of drought.”
Because in the Sahel, she recalls, nomadic populations depend on their herds, which feed on vegetation.
However, overgrazing “bares the soil and makes it more susceptible to erosion”continues Marc-André Selosse, professor at the National Museum of Natural History, biologist and mycologist. Plowing, uprooting hedges or even maintaining bare soil, agricultural practices widespread throughout the world, degrade soils, which become depleted in organic matter, store less water, erode and produce less plants in return, he explains.
But sometimes, it is the artificial maintenance of plants on a territory which threatens to condemn the soil. In Chile, intensive avocado cultivation is drying up waterways. In Spain, industrial production of fruit and vegetables in greenhouses and by irrigation is worsening drought and attacking the water table. “When we irrigate a lot, we end up making the downstream area thirsty,” remarks Marc-André Selosse, who cites the tragic case of the Aral Sea, which once stretched between Kazakhstan and Uzbekistan.
Starting in the 1960s, water from the rivers that crossed this arid steppe landscape was diverted to quench the thirst of cotton crops. Sixty years later, the sea has lost 90% of its volume and all the forms of life that once populated it, with the exception of artemia, a small crustacean barely larger than a grain of rice. The sea has given way to sand as far as the eye can see and, therefore, to salt.
Driven by the wind, the salt spreads and sterilizes the soil, preventing the regrowth of any vegetation, as if the desert did not need truly desert conditions to progress. Marc-André Selosse also describes “salinization” lakes and streams of “entry point to desertification”.
“There is desertification linked to intensive use of soil and water bodies”, continues Mehrez Zribi. “We find these situations particularly in semi-arid zones with intensive exploitation for agriculture. There are cases of agricultural zones based solely on the exploitation of aquifers.” However, when the latter are dried up, the territories “are abandoned”. But to go where?
In the Mekong Delta or parts of Indonesia, it is the evaporation of surface water caused by heat that makes the water unfit for consumption, ruined by its salt content. Without the supply of rainwater, the population is then forced to draw on groundwater, which in turn deteriorates. Another vicious circle favoring desertification.
