La Locomotora minero-energética 2010 - 2014

The Sahel is a region in sub-Saharan West Africa – an area between the Sahara desert at 18°N and the equatorial forests at 15°N. The Sahel covers several African countries including Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, Sudan, Somalia, Ethiopia and Eritrea. It has a population of around 50 million inhabitants and a surface area over 5 million km2_{. The vegetation is diverse and contains trees, shrubs and grasslands. In some ways the}

Sahel acts as a buffer zone from the arid Sahara north of the region. The climate of the area is controlled by the West African monsoon, itself controlled by the seasonal migration of an atmospheric phenomenon known as the Intertropical Convergence Zone(ITCZ). The location of the ITCZ broadly follows the seasonal variations in the receipt of the Sun’s energy, and is located north of the equator in the northern summer and south in winter. As the ITCZ moves northwards it brings a short period of intense rain between late June and mid-September. Rapid vegetation growth results in a lowering of the surface albedo (as bare soil is light, it has a higher albedo than vegetation). Farming in the Sahel is almost entirely dependent on this rainfall. About 95% of the land in the Sahel is devoted to agriculture and 65% of people depend on agriculture for a livelihood (Druyan, 2011). Traditionally, people in the area maintained a subsistence living by moving to accommodate this climatic variability.

In the twentieth century colonial and post-colonial governments encouraged intensive modern food production in the area. This move accelerated in the 1950s and 1960s, which also coincided with a period of unusually high rainfall in the area (Figure 2.11). By the 1970s, however, the Sahel had entered a period of severe drought and there was a 40% decline in BOX 2.3 CONTEMPORARY CHALLENGES

Figure 2.11Precipitation difference from long-term average 1900–2011 in the Sahel region showing the prolonged dry period since the 1970s.

Source: Figure reproduced with kind permission of the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington.

rainfall (Biasutti, 2011). Drought is associated with either a late onset of the rains, an early finish to the rains or weak rain systems (Druyan, 2011). The Sahel was affected by drought from the 1970s to the 1980s, and although rainfall totals have recovered they are still not back to 1950 levels (Figure 2.11). It was thought that such a large drop in rainfall could only be due to the result of human activity. Overgrazing, which led to an increase in albedo and therefore a cooler surface, would mean less evaporation and a decrease in precipitation (Charney, 1975). Note from Figure 2.4the strong land–atmosphere coupling over the Sahel, which means there is potential for strong soil moisture–precipitation feedbacks. This represents a positive feedback reinforcing desertification. The blame was placed upon environmental degradation due to traditional grazing methods, suggesting that the people in the area ought to modernise further.

Whether overgrazing was the dominant cause of precipitation changes has always been open to debate. It is now recognised that the area has a very variable rainfall both spatially and temporally. Rainfall over the Sahel varies year to year and on a decadal timescale. These variations are linked to SST anomalies in the Pacific, Indian and Atlantic ocean basins. In the Pacific, the El Niño phase of ENSO leads to drying and La Niña to positive rainfall anomalies. Indian ocean warming and changes in the cross-equatorial temperature gradient in the Atlantic all affect rainfall in the Sahel (Biasutti, 2011). On a decadal timescale the temperature gradient across the northern and southern hemisphere oceans is important, with decreased rainfall when it is colder in the north than south (Biasutti, 2011). Land surface changes due to vegetation dieback are still thought to play a role (Caminade and Terray, 2010).

Recent work shows that precipitation changes over the Sahel may well have been enhanced through the release of sulphate aerosols from fossil fuel burning in North America and Europe (Ackerley et al., 2011). Sulphate aerosols have both a direct and an indirect effect on the radiation (and thus energy budget) of the Earth. They reflect more solar radiation away from the Earth, thus cooling the Earth’s surface. They also cool the surface by changing the radiative properties of clouds by making them brighter and also increase cloud lifetimes (IPCC AR4). Such cooling will reduce evaporation, leading to less water in the atmosphere and so reducing precipitation. Ackerley et al. (2011) performed ensembleclimate model runs; this is where a climate model is run many times but each time with small changes to the physics parameters within their known ranges, to see how parameter value assumptions affect the results. They found that a combination of increasing greenhouse gases and sulphate aerosols led to a net decrease in rainfall over the Sahel, mainly by affecting the SST gradient.

Now, it is generally acknowledged that the Sahel disaster of the 1970s and 1980s was about the implementation of development policies which paid little attention to the environment of the area. There was a failure in recognising that there is a complex interweaving of human socio-economic systems with environmental systems. It is important that we continue to more fully explore the impact of anthropogenic changes of the climate to such vulnerable areas. It is also important that we look to improve water and land management in the Sahel so that millions of people are not regularly pushed into starvation.

Figure 2.12 Changes in glacial extent for selected glaciers around the world.

Source: From Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Figure 2.18. Cambridge University Press.