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.
The major difficulty that scientists face is undoubtedly the “multi-layered” character of the climatic processes at work in Africa. The monsoon is both a continental event, governed by large global phenomena, and the sum of a group of circumstantial contingencies, influenced by factors that are sometimes very local (relief, vegetation, etc.). The timescales are also very different. The large phenomena generally have long cycles (in the order of a year, or even a decade), while large storms, which are in some way the “elementary units” of the monsoon, last a few hours or at most a few days. Understanding how all these forces interact is a particularly ambitious challenge. “Modern science still finds it very difficult to deal with these problems of changes in scale, and this is not only true of climatology,” one of the founders of AMMA, Jean-Luc Redelsperger, points out.
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.”
Invisible to the tourist, the revolution to which Africa is subject becomes tangible when scientists point out the effects. Travelling by car through kilometres of orchards, interspersed with banana and other fruit plantations, Christophe Peugeot, Operational Manager of AMMA in Benin, points to a small group of trees overlooking an expanse of arable land around 40 metres long like a fortress of vegetation. “We are talking about the remnants of a primary forest, no doubt preserved because it is sacred. This primary forest, just 20 years ago, occupied the whole of this area. There were just a few clearings exploited by man.”
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.
The tropics constitute the zone with the highest levels of solar
radiation on the planet. They represent the thermal “motor”
for the world’s climate, i.e. the main source of energy, which is
then distributed around the world via sea currents like the
Gulf Stream, or through various atmospheric phenomena...
At the heart of a worldwide puzzle
The tropics constitute the zone with the highest levels of solar radiation on the planet. They represent the thermal “motor” for the world’s climate, i.e. the main source of energy, which is then distributed around the world via sea currents like the Gulf Stream, or through various atmospheric phenomena.
Africa, which has the largest tropical continental zone, influences the entire atmospheric flow of the planet in this area. Moreover, it is the largest worldwide source of atmospheric aerosols and injects huge quantities of ozone (or ozone precursors) into the lower atmosphere. All this makes it a strategic crux for the world climate, meaning it needs much more committed study than the current models can provide, giving a very weak reproduction of what is happening. Upon conclusion of the AMMA project, the quantity of data collected should, according to researchers, provide the climatology community with work for over a decade. These data are intended in time to be freely available on the Web.