The Root Causes of the Sahel Drought
A new study exploring the root causes of the two-decade drought that ravaged the African Sahel from the 1970s to the 1990s arrives at an interesting conclusion. Historically it had been maintained that the drought—which resulted in the deaths of hundreds of thousands of people and sparked Bob Geldoff’s Band Aid and We Are the World—was rooted in environmental degradation caused by poor land management practices (particularly overgrazing) in the region.
But according to a new study conducted by Yen-Ting Hwang, Dargan Frierson (both of University of Washington), and Sarah Kang (at Ulsan National Institute of Science and Technology in South Korea) and published in Geophysical Research Letters, shifting rainfall patters that facilitated the drought were likely the result of increased aerosols in the Northern Hemisphere.
The paper argues that the widespread use of aerosols led to a slight cooling of the Northern Hemisphere, which caused the tropical rainfall band (known as the Intertropical Convergence Zone) to shift slightly southward. This shift led to a drying out of Central Africa and parts of South America and South Asia, while northeastern Brazil and Africa’s Great Lakes region saw increased rainfall. And equally interestingly, as clean air legislation led to sharp cuts in the most powerful aerosols in the global north, the rainfall band shifted back to the global north, ending the drought in the early 1990s. (See graph below).
As Dargan Frierson, one of the lead authors of the study observed, “t’s pretty clear that in addition to greenhouse gases, air pollution really does affect climate, and not just in one place. These emissions over the U.S. and in Europe affected rainfall over Africa.”