Advancing Deserts of the World
There is a controversy about the advance of deserts in the world. There is a widespread belief that the Sahara desert is advancing into the Sahel region, for instance. The Sahel is a narrow band of West Africa between 15o N and 18- N, between the Sahara to the north and savannah (grass and open forest) and equatorial forest to the south. It extends from Senegal at the coast at about 15- W, across Mali and Niger, to about 15- E. It receives rainfall during a short but active wet season, from late June to mid September. It is covered by grassland and supports a pasture-based society which traditionally moved meridianally following the rains. Its northern limit may be defined by the 200 mm/a isohyet.
Is the Sahara extending into the Sahel? And if so, is this because of fluctuations of rainfall (total amount, rainfall intensity, duration of wet season) or is it largely the result of human activities, such as overgrazing or the removal of trees for firewood? There are also the questions: Do deserts create droughts? Do droughts create deserts? In other words, is there a positive climate feedback, which accelerates land degradation? A new idea by a researcher in 1975 speculated that overgrazing in the Sahel leads to less vegetation,
which raises the ground's albedo, so that less solar radiation is absorbed and the Earth's surface becomes cooler. It should be noted that the atmosphere above the Sahara experiences continuous radiative cooling, because the dry air, free of clouds, absorbs very little of the long wave radiation upwelling from the ground. This radiative cooling is naturally compensated by subsidence heating and the subsidence sustains the dry air, cloudlessness, and arid surface conditions. According to a study, overgrazing would enhance the radiative loss, which would foster subsidence within the troposphere, leading to drier conditions in the Sahel and therefore less plant growth during the wet season. Less vegetation means a higher albedo. So we have positive feedback and a self-aggravating process, culminating in desertification, a process of land degradation that destroys its productivity.
The problem of overgrazing in the Sahel is as acute now as it was in the 1960s, yet there is no clear rainfall trend in the Sahel. The period 1930-60 was slightly wetter than 1960-90 in most parts of the Sahel. More significant than any trend is the occurrence of dry and wet periods, each lasting several years. The Sahel enjoyed a notably wet decade in the 1950s, which was followed by a drought in the 1970s and 1980s. However, land productivity was fully recovered around 1990.
Satellite-estimated albedo of the Sahel between 1983-88 was about 35 per cent in (dry) January and 31 per cent in (wet) July, a difference of about 4 per cent. The seasonal variation greatly exceeded any overall change during the period. This points to there being no irreversible change towards desert. It is concluded that the formation of desert is not a single self-aggravating process, but is complex, reflecting changes of both climate and human activities.
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