Some meteorologists have insisted that the severity of the drought in sub-Saharan West Africa and its long duration (nearly 40 years to date) must be a sign of a long-term alteration in climate. Among the theories proposed to explain this change, one hypothesis that has gained widespread attention attributes the drought to a cooling of the Northern Hemisphere. This hypothesis is based on the fact that, between 1945 and the early 1970s, the average annual air temperatures over the landmasses of the Northern Hemisphere decreased by about half a degree Fahrenheit (approximately one quarter of a degree Celsius—a small but significant amount). Several meteorologists have suggested that this cooling was caused by an increase in atmospheric dust emanating from volcanic eruptions and from urban and industrial pollution; the dust reflected incoming sunlight, causing the ground to receive less solar radiation and to transfer less heat to the atmosphere. The cooling seemed to be more pronounced in the middle and high latitudes than in the tropics, an observation that is consistent with the fact that the Sun’s rays enter the atmosphere at a greater angle farther north, and so have to pass through more dust-laden atmosphere on the way to the Earth.
Since winds are set in motion by differences in air pressure caused by unequal heating of the atmosphere, supporters of the cooling hypothesis have argued that a growing temperature differential between the unusually cool middle and high latitudes and the warm tropical latitudes is causing a southward expansion of the circumpolar vortex—the high-altitude westerly winds that circle the Northern Hemisphere at middle latitudes. According to this hypothesis, as the circumpolar vortex expands, it forces south other components of large-scale atmospheric circulation and, in effect, displaces the northward-moving monsoon that ordinarily brings sub-Saharan rain. Proponents have further argued that this change in atmospheric circulation might be long-term since cooling in the Northern Hemisphere could be perpetuated by increases in ice and snow coverage there, which would lead to reflection of more sunlight away from the Earth, to further cooling, and, indirectly, to further drought in sub-Saharan West Africa.
Despite these dire predictions, and even though the current African drought has lasted longer than any other in this century, the notion that the drought is caused by cooling of the Northern Hemisphere is, in fact, not well supported. Contrary to the predictions of the cooling hypothesis, during one period of rapid Northern Hemisphere cooling in the early 1950s, the sub-Sahara was unusually rainy. Moreover, in the early 1980s, when the drought was particularly severe, Northern Hemisphere lands actually warmed slightly. And further doubt has been cast on the hypothesis by recent analyses suggesting that, when surface temperatures of water as well as land are taken into account, the Northern Hemisphere may not have cooled at all.