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.
Topic
Theories about the cause of the long-term drought in sub-Saharan West Africa, focusing on the "Northern Hemisphere cooling" hypothesis
Framework
Challenge Position
Main Point
Although some meteorologists attribute the sub-Saharan drought to a cooling of the Northern Hemisphere, the evidence actually does not support this cooling hypothesis. The Most Valuable Sentence: Appears in the first sentence of the last paragraph:
P1: Introduction of the Cooling Hypothesis
Some meteorologists think the persistent drought in sub-Saharan West Africa is due to long-term climate change, specifically the cooling of the Northern Hemisphere. This theory links the cooling trend from 1945 to the early 1970s to increased atmospheric dust reflecting sunlight. The cooling was strongest in the higher latitudes, fitting with how sunlight passes through more atmosphere the farther north you go.
P2: Mechanism of the Hypothesis
Proponents of the cooling theory say that cooler temperatures in the north create new pressure differences, pushing the circumpolar vortex (a band of high-altitude winds) farther south, which in turn pushes the monsoon rains away from sub-Saharan Africa. They also worry that this change could become self-reinforcing due to increased ice and snow reflecting more sunlight.
P3: Evidence Against the Hypothesis
But, actually, the evidence for the cooling hypothesis is weak. For example, there was a rainy period in the sub-Sahara during one of the biggest Northern Hemisphere cool-downs in the 1950s, and by the 1980s, when the drought was severe, the Northern Hemisphere was actually warming a bit. Plus, when considering water temperatures, recent analyses cast even more doubt on whether the cooling happened as claimed.