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Climate Change and the Sahara
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In Arabic, sahra signifies a flat, waterless area. In fact, the Sahara, the biggest hot desert in the world, is a combination of immense flat sandy or stony regions scattered with mountain ranges with base rock protrusions or volcanic outcrops. Examples include the Emi Koussi, which culminates at an altitude of 3415 metres, the Toussidé at 3265 metres in the Tibesti Mountains, the Tahat at 2918 metres, the Ilaman at 2760 metres in the Hoggar, and the Greboun at 2000 metres in the Aïr. It also comprises depressions that are below sea level, such as the Qattara in Egypt at -133 metres and the Chott Melhrir in the southern part of the Aures range in northeast Algeria at -31 metres.
The 9.5 million square kilometres of the Sahara receives less than 100 millimetres of rainfall per year, with half of the Sahara receiving less than 20 millimetres. Indeed, the Mediterranean winter rains rarely cross the Atlas Mountains, and the monsoon rains do not exceed farther north than 17 degrees North, the latitude of Timbuktu in Mali, and Khartoum in the Sudan, which mark the limits of present-day Sahara.
The drought is a result of atmospheric circulation. Around 30 degrees latitude, the rotation of the Earth causes the upper part of the atmospheric circulation (the Hadley Cell) to descend, bringing dry air to the ground from the upper layers of the atmosphere. However, another phenomenon partially opposes this seemingly unyielding mechanism: the monsoons that seasonally bring water evaporated from the oceans to the continents.
Neither the past climate nor scenarios of the future can be fully understood without first understanding the monsoon regime. The monsoon regime is largely controlled by:
The West African monsoon is a complex and fragile mechanism because of the multiple interactions amongst the atmosphere, the marine and the continental hydrosphere, the ground and the biosphere. Recent modelling studies underline the important role of the oceans, vegetation cover and topography in the establishment of the monsoon circulation. In addition, human activities modify the rainfall. Thus, the over-exploitation of the land leads to an increase in the albedo, not only in the Sahel but also in the Sudan-Guinea climate zone, with a direct impact on the atmospheric circulation.
The start of the West African monsoon depends on the northward progression of the ITCZ during the spring and the northern summer. The ITCZ is defined by the line of demarcation between the northeasterly trade wind, the Harmattan, which removes the Saharan heat and dust, and the southwesterly monsoon which draws water over the equatorial forest and, above all, over the Atlantic. The migration of the ITCZ develops suddenly, passing from a near-stationary position at five degrees North in May-June to another equilibrium position at ten degrees North in July-August. The Sahel and the southern Sahara then receive the major part of their rainfall. Farther south, the annual amounts are greater and are distributed over two rainy seasons, one in the spring and the other in the autumn.
In this article, we look back over the recent history of climate and the Sahara, then consider what impact humanity may have in years to come.
The Sahara thousands of years ago
Recent research shows that the Sahara dried up quite suddenly about 5500 years BP; four centuries were enough time to complete this desertification. The enormous changes that brought about this climate inversion 4000 years ago were gradual. Why then was this desertification so sudden? Here we have an effect that is somewhat mysterious and which bears testimony to the non-linear character of the climate system. It is possible to surpass the thresholds allowing the system to evolve, sometimes in a major way, under the effect of relatively minor causes.
The most recent climate models show that one of the processes likely to have accelerated the climate inversion is the effect of the vegetation, whose role is crucial. A slight reduction in the vegetation would increase the reflective nature of the soil, thus diminishing the rainfall, which in turn reduces the vegetation, and so on. The heat would then become fiercely overpowering and the desert would spread. This scenario explains the observed evolution as it is presented without any need to resort to the historians' hypothesis according to which agriculture failed because the farmers over-exploited the soil.
As the desertification got worse, over the centuries and millennia, a 'civilization of the desert' developed. For example, founded between the ninth and thirteenth centuries AD, the Mauritanian cities of Chinguetti, Ouadane, Tichitt and Oualata are the surviving witnesses to the prosperity of medieval Mauritania, crossroads of Andalusia, the Arab world and Sahelian Africa.
These cities are unavoidable stopovers on the major trans-Sahara trade routes, by which the products of the North are bartered for those of the South (salt, cloth, gold, fine glassware, and so on) and from which radiates an intense cultural and religious life, as well as numerous scientific and artistic activities. Long periods of drought, epidemics, sometimes even famines have brought irreparable damage to these cities. The advance of the desert reinforces their isolation, which is exacerbated by the appearance of new economic axes turning towards the Atlantic.
The past century
During the last decades, the most marked climate evolution has not affected the heart of the Sahara but its southern fringe, the Sahel. The inhabitants of the Sahel know that throughout their life they will suffer drought and malnutrition that accompany life in the region, and that only their ingenuity or escape can save them. This 'shore' (sahel in Arabic) of the sea of sand is subject to climate fluctuations depending on the more or less marked, more or less lengthy, advance in summer of the rainfall zone associated with the ITCZ.
For the Sudano-Sahel, the periods 1930-31, 1940-41 and 1947-49 had brief but intense droughts. Yet two long and intense episodes marked the twentieth century. The first lasted from 1898 to 1916 with peaks in 1911 and 1914-15. During these twenty years, Lake Chad, considered the 'climatometer' of the Sahel, lost half its quantity of water, during which the tides of the Nile were reduced by a third. When the rainy season arrived, many people no longer had the strength to work the land. As a result, nearly 5000 Fulani died in Nigeria, where the cattle herds dropped from 88,000 head in 1913 to 26,000 in the following year.
The second major period of drought was from 1968 to 1988, with minimal rainfall between 1971 and 1973, in 1981 and 1982, and again in 1987. This drought also affected the humid region of Africa. The drop in the rainfall south of the fourteenth parallel reached 20 per cent with marked regional inequalities. In the Niamey region in Niger, 490 millimetres of rain fell, on average, per year from 1970 to 1990 compared with 690 millimetres for the preceding twenty years. As a direct consequence, the River Niger's low water discharge, in the city of Niamey, dropped from 50 to three cubic metres per second.
The concurrent rise of ten centimetres per year in the water table was, at first glance, something of a paradox. For some researchers, this paradox was explained by changes in the landscape. In response to the population growth, the surfaces cultivated or in short-term fallow rose in the vicinity of Niamey from 10 per cent to 60 per cent. This transformation of the vegetation cover accentuated the run-off of rainwater as a result of the impermeabilization of the ground surface, a reduction in the obstacles to the flow and in the activity of the soil fauna, which became concentrated in the hollows forming temporary stagnant pools that alimented the water table by infiltration.
Although this drought period affected Guinea, Liberia, Sierra Leone, Mali and Burkina Faso, it spared much of the Côte d'Ivoire, Ghana, Nigeria and Cameroon up until the end of the 1970s. In contrast, in the 1980s, the decrease in rainfall was widespread, with a maximum intensity in those regions close to the Sahel as well as in the west near the Atlantic, in Côte d'Ivoire, Liberia and Guinea.
There is no way of knowing whether this climatic breakdown around 1970 is a response to the appearance of a long-term drier climate in the Sahel or to a transitory period of drought such as the region has experienced several times before. There was a rainfall deficit (on average, -180 mm of rain per year) almost continuously from the end of the 1960s to the mid-1990s; this deficit was not limited to the Sahel but extended right to the Gulf of Guinea.
This drought is only one of the indicators of climate variability in West Africa. The deficits in the discharge of the major drainage basins are greater than the rainfall deficit. Thus, the basin discharges in Senegal and Niger have decreased by 50 per cent to 60 per cent, whereas the annual rainfall on their drainage basins decreased by only 20 per cent to 30 per cent.
The Sahara of ‘tomorrow': the role of humanity
Sub-Saharan Africa and, more generally, the whole of West Africa have experienced the greatest known decrease in rainfall in the world during the past fifty years, with sand and dunes invading about one million square kilometres since 1900. What would happen with global warming?
It might be tempting to extrapolate from thousands of years of history and conclude, somewhat hastily, that, in a warmer world due to the increase in greenhouse gases, the climate of the Sahara would be similar to that of the warm climatic optimum of the early Holocene Epoch and, hence, more humid. Unfortunately, things are never so simple and the same climate modelling methods that describe very well the more humid conditions of the early Holocene Epoch indicate, on the contrary, a significant risk of drought for the coming centuries.
Even if climate models are imprecise on a local scale, they give a reliable indication of the geographical distribution of warming on the global scale. In all the future climate simulations, the warming of the terrestrial surface by greenhouse gases is maximal towards the poles because it remains close to the surface, whereas in the tropical or inter-tropical regions it is mitigated by the effects of convection that mixes the air up to an altitude of more than ten kilometres. For the same reasons, warming is more marked in winter.
These conditions are very different from those of the early Holocene - we no longer find strong warming of the continental surfaces in summer at low latitudes which draw the monsoons northwards. They do not favour an extension of the rainfall regime but rather an amplification of the intensity of the existing regime. In other words, more rain in those regions where it rains already, and less rain in the arid regions.
If the models have got it right, the Sahara and the Sahel will not experience a decrease in their aridity, but on the contrary, an increased fragility of the semi-arid zones.
So there is warming and warming, the regional consequences depending on the mechanism that generates climate variation. The climate optimum of the Holocene was truly a golden age for the Sahara. By contrast, the moderate and symmetrical warming of the two hemispheres associated with the increase in the greenhouse effect should bring about a strengthening of the climate contrasts.
We note that the last millennium provides an example of the complexity of such effects. The Sahara had rather abundant rainfall during the periods of cooling in Europe (the Little Ice Age between the sixteenth and nineteenth centuries), whereas the Sahara-Sahel region was much drier than today between 900 and 1270 AD, which was the medieval climate optimum in Europe as proved by the recent dating of the levels of Lake Chad, then occupying a surface of 350,000 square kilometres. If these rapid fluctuations were linked to the variations in solar activity, as certain indicators suggest, we should have a coherence with the results of the models.
West Africa is an example of a region in which the natural variations of the climate have important social repercussions. The increases in greenhouse gases could invariably lead to greater and more serious consequences. Moreover, the seriousness of the drought affecting the Sahel and West Africa during a quarter of a century was due to the interaction of two factors: the climatic hazards themselves and the degree of fragility or vulnerability of the infrastructure and its economic and social organization.
The marked impact of the 1968-88 drought was, paradoxically, due to the favourable climate conditions of the preceding twenty years. The international community financed development programmes at that time such that the cattle herds increased from 18 to 25 million heads, whereas that optimum number of cattle with respect to the climatic conditions is closer to 15 million. As the rainfall decreased, the cattle could no longer find enough food and the grazing land thus transformed back into desert.
Following this hypothesis, the expansion of the desert would be less a direct consequence of the climate than the impact of overgrazing. It is quite likely that the phenomenon was self-perpetuating, such that the disappearance of vegetation increased the albedo, which in turn decreased the soil temperature thereby limiting evaporation and accentuating the airborne inputs.
Human actions may further complicate the situation. On the one hand. the persistence of economic structures left over from more prosperous times means that, during periods of crisis, the situation can be made worse. Thus, the Sahel, during the period of famine, continued to export cereal for cattle in Europe. On the other hand, the images of the 1972 famine served as a wake-up call for the international community; non-governmental organizations were, thus, better prepared to deal with the consequences of the 1982-83 episode, even though it was equally severe from a climatic point of view.
The assessment of the consequences of changes in the climate calls into play considerations that go beyond the strictly environmental problem: it is the ability of a society to adapt itself that is at stake, and the more fragile the society - poverty, civil war, high dependence on sophisticated technology - the greater the climatic hazard.
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