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Sahel Climate

Between the vast sands of the Sahara Desert and the dense foliage of the Congo Rainforest stretches a band of semi-arid grassland. Known as the Sahel, this hardy landscape is one of Africas most productive crop regions. Despite its productivity, the Sahel has a dark history of famine tied to highly erratic rainfall. Starting in the early 1970s, a string of dry years drove millions to starvation across the Sahel from Ethiopia in the east to Mauritania in the west. The dry period lasted until the mid-1990s, leaving many to wonder if the Sahara was creeping south, swallowing the arable land in the Sahel.

The Sahel occupies a transitional zone between the Sahara to the north and tropical rain forests to the south. As for many such transitional zones adjacent to tropical rain belts, climate is highly variable due to the sharp gradients in rainfall. Drought is a recurring phenomenon in semiarid West Africa, where average rainfall conditions seldom prevail, and rainfall is skewed to dryness, i.e., a few heavy rainfall years are balanced out by a larger number of below-average rainfall years. From the late 1960s through the 1980s, the Sahel zone experienced droughts of unprecedented spatial extent and duration. These droughts followed a period of more favorable rainfall in the 1950s and early 1960s, which had encouraged government planners and farmers to expand agriculture northward.

Pollen records show that vegetation in North Africa has two stable states. Since 5,000 years ago, this area has been in a "desert" state, with little to no vegetation in the Sahara and mixed steppe and savanna to the south in the Sahel. Following the last glacial period and the Younger Dryas, climate warmed around much of the world and human settlements expanded. In Africa, the monsoon rains grew stronger and spread northward into the Sahara. Instead of being the sandy desert we now know, the Sahara was a steppe, covered in grasses and shrubs. In this inviting environment, hunters domesticated buffalo and goats and developed an early system of symbolic art.

In the 1970s and 1980s, the region experienced a profound drought, with over a 30% decrease in rainfall over most of the Sahel as compared to the 1950s arguably the most dramatic drought in any region of this large an extent observed in the 20th century. Given this potential for large rainfall variations, and the economic, political, and social challenges facing the region, the possibility that increasing greenhouse gases could alter Sahelian rainfall is of great concern.

It has been understood since the 1980s that changes in ocean temperature played a significant role in producing the late 20th century Sahel drought. The observation that the rainfall reaches a maximum in mid-century, and that this maximum can be simulated, given the observed ocean surface temperatures, is evidence that the drying from the 1950s to the 1980s is unlikely to be primarily forced by increasing greenhouse gases in the atmosphere (assuming that these early century rainfall and ocean temperature records are robust).

A variety of modeling studies have pointed to changes in the inter-hemispheric temperature gradient, within the tropical oceans but also globally, as being of key importance for Sahel rainfall, with drought occurring when the Northern Hemisphere oceans are relatively cold as compared to the Southern Hemisphere oceans. During the early part of the 20th century, the Northern Hemisphere warmed more rapidly than the Southern Hemisphere, but from mid-century till 1980, the Northern Hemisphere cooled on average, while the Southern Hemisphere continued to warm. Since 1980, the Northern Hemisphere has once again warmed more rapidly. The maximum warmth of the Southern relative to the Northern Hemisphere was achieved during the most severe period of the Sahel drought.

The picture that emerges seems to be relatively simple, even though the underlying dynamics may be quite complex: the tropical rain-belts are attracted to the relatively warm hemisphere, with the Sahel, the northernmost extension of these rain-belts in Northern summer, suffering drought when the Southern Hemisphere is relatively warm. This effect is thought to be relevant more generally throughout the tropics, and not just for Africa.

An alternative hypothesis, popular during the 1970-1980s drought itself, is modification of the land surface due to human activity, usually termed desertification. Climate models do suggest that one can reduce Sahel rainfall by increasing the surface albedo (the reflectivity of the surface to solar radiation), a result expected from the loss of vegetation, but models have not produced responses of adequate magnitude by this mechanism, and it is difficult to explain a mid-century maximum in rainfall through land alteration. This major shift in perspective from one in which land alteration is directly responsible for the drought to one in which oceanic changes unrelated to any land modifications are deemed to be the dominant cause. The dominance of oceanic forcing in the past does not deny the possibility that major land modifications could be a factor in the future, or that the feedback of land vegetation to the changes in rainfall could be important for the observed variations.

Accepting that inter-hemispheric gradients in ocean temperature are a key proximal cause of multi-decadal Sahel rainfall variations, the search for the ultimate cause becomes the search for an explanation for these ocean temperature variations. A substantial part of these changes in temperature gradient is likely due to internal variability, the source of this variability most likely being in the meridional overturning circulation in the Atlantic Ocean. Manipulating the northward heat flux by the Atlantic ocean in a climate model can alter Sahel rainfall.

But aerosol forcing has also been implicated as a potential candidate for the drying tendency since mid-century, with increasing sulphate pollution in the Northern Hemisphere causing the Northern Hemisphere to cool relative to the south. The recent warming in the north is, in this interpretation, caused by cleansing of the Northern Hemisphere atmosphere. The evolution of aerosol forcing is large enough to explain a large fraction of the Sahel drought, if one includes relatively large indirect aerosol effects associated with cloud/aerosol interactions. There is a small forced component to the drying trend over the 20th century in most of these models. This trend does not continue into the future in the multi-model average, implying that the dominant forcing in the models is not greenhouse gases but rather aerosols. Greater confidence in our ability to estimate past variations in aerosol forcing will be needed to make further progress on this aspect of the problem. But, once again, to the extent that the precipitation did indeed maximize in mid-century, the more plausible explanation for the bulk of the observed variability remains internal variability.

Slow variations in the Earth's orbit caused a gradual decrease in summer solar radiation in the tropics from the early to the mid Holocene. This decreased the amount of summer monsoon rainfall in Africa and other parts of the tropics. Scientists hypothesize that as monsoon precipitation gradually decreased, at some point conditions became too dry for plants and a rapid transition to dusty, desert conditions resulted. It is not possible to reject the hypothesis that, superposed on large multi-decadal internal climate variations, and on aerosol induced changes, there is also a suggestion of a greenhouse induced drying trend that will emerge clearly over the present century.



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Page last modified: 26-12-2019 18:24:18 ZULU