| CLIMATE - Extreme climatic tensions
A worrying weakening… year by year variations capable of doubling… The West African monsoon, the main source of rain on the continent, particularly in the Sahel strip, is suffering from difficulties that have now been clearly identified. There is a long way, however, between spotting them and making a convincing diagnosis, identifying the causes and mechanisms at play. It is precisely this gap that the AMMA project intends to close, by means of an experimental plan of action of unprecedented size.
What of climate changes?
Starting on the largest scale, scientists wonder about the past and future influence of global climatic changes on the African monsoon. The shortage of rainfall in West Africa dates back to the 1970s - almost perfectly in tune with the increase in global temperatures.
Simple coincidence? Perhaps. But the signs can only cause concern, as everyone knows warming has only just begun. Unfortunately, global climatic models available to the international community provide contradictory projections on the future of the zone. “The Sahel is one of the regions with the greatest heterogeneity of all models in the world,” states Jan Polcher, coordinator of AMMA-Europe. Some simulations forecast an increase in rainfall, others a moderate fall, and others a catastrophic drought. Global phenomena that seem to weigh heavily on the African monsoon – and are themselves susceptible to modification by climate changes – are therefore thoroughly examined.
Climatologists, for example, have noticed that the El Niño phenomenon, which originates in the distant Pacific, has clearly had an impact on Africa and its monsoon. In the years when it is strong, the rains are weak in Africa – and we know that the majority of models forecast an intensification of El Niño for the coming century. In the same way, the Indian monsoon seems to exert an influence. When it is intense, precipitation drops in West Africa. More recently, work has shown that the temperature of the Mediterranean also has an impact. When it is higher, the intensity of the rains increases. But it is primarily the temperature of the Atlantic, and in particular that of the Gulf of Guinea, which seems to play a defining role in this, and undoubtedly also in the West African monsoon.
The effects of demography
On a more local scale, one of the most important issues relates to the extent of human responsibility. Over a few decades, the impact of the rapidly expanding human population has clearly disrupted Africa (its vegetation, hydrology and atmosphere) to a point that is difficult to perceive unless we go into the remotest areas. “In the 1930s, West Africa must have had 8 million inhabitants,” estimates the hydrologist Thierry Lebel. “Today, there are at least 200 million.”
A thousand kilometres further north, in the heart of the Niger Sahel, the changes - just as radical - take completely different forms. Jean-Louis Rajot, Director of research at CNRS and a specialist in aerosols, comments on a landscape that has turned orange, arid and burning, where the few bushes are short of leaves. A Fulani family, a group of nomadic farmers, have put up some temporary huts. Further away, on the horizon, around twenty solid houses can barely be distinguished from the soil that provided the clay for the buildings. Nevertheless, we can easily pick out the village in the mini oasis of large trees which, visible from afar, cover it in shade. They are the only trees for several dozen kilometres in every direction not to have been cut down.
Some distance away, surprisingly, a dense green grove stands out against the arid backdrop. “This is a zone that the scientists placed under protection twelve years ago and to which neither man nor animal has access,” explains the researcher. “This abundance in a quasi-desert tells us a lot about the extent of the human impact.”
As far as rapidly growing populations in these regions are concerned, they are almost completely deprived of financial revenue. Wood is the sole source of energy, and is used for cooking and most craftsmen’s activities. Along the roads and tracks, piles of branches are for sale. Niger, one of the countries with the highest demographic growth in the world, currently has 95% cultivated or fallow land (periods of growing and leaving fallow alternate so that the soil can be restored), against just 5% at the start of the century.
The canker of deforestation
These disruptions can impact the climate, especially precipitation, in many ways. The destruction of forests, for example, even when they are replaced by cultivation, increases the soil albedo, in other words its capacity to reflect light rather than absorb it. As a result, a larger share of solar energy is sent back towards the atmosphere, and may then cause faults in usual meteorological processes such as the triggering of storms.
Deforestation reduces, moreover, the capacity of the soil to store water. Trees slow down the flow of water, both mechanically (water descending into soil along the roots) and chemically (the humus traps water). On land that has been stripped bare, water reaches the rivers more quickly, and is effectively lost to the vegetation. At the same time, deforestation also reduces the quantity of water retained by the soil after rain. The trees effectively operate as pumps. Their roots search for the precious liquid in the depths, bring it back to the surface through their vessels, before it evaporates into the atmosphere via the leaves. Cultivated land is clearly less capable of playing this role - its leafy surface is weaker and the surface roots capture much less water. Water vapour restored by vegetation can sometimes trigger rain by saturating air.
The dramatic increase in cultivated surfaces in Africa may also influence the climate through other mechanisms. For instance, we can observe, especially in the Sahel, effects of wind erosion, which is indicated by the suspension of colossal quantities of mineral particles (sand, etc.) and organic particles (soot produced by the burning of vegetation). It is believed that this continent is the main global source of atmospheric dust. Such particles could have a great influence on rainfall by providing (or not providing) water vapour with condensation nuclei, either making possible or hindering the formation of drops.