Deserts are defined as regions that experience 100 millimeters (less than 4 inches) of rain per year, or less. As such, the title of “desert” is expanded to the Arctic basin and the continent of Antarctica as well, despite them not inspiring the typical desert images of sand and heat. Those two regions happen to be the largest deserts in the world, by definition. But the world’s largest warm-weather desert – the Sahara Desert – is quite sizable as well, as it is roughly the size of the continental United States. And according to new research, it’s only getting bigger.
In the first study to assess century-scale changes to the boundaries of the Sahara Desert, researchers from the University of Maryland (UMD) have found that this desert has expanded in size by about 10 percent since 1920. Their findings – published in the Journal of Climate – also suggest that other deserts could be doing the same.
The researchers assessed rainfall data in Africa that spans from 1920 to 2013, and found that the Sahara increased in size by 10 percent during this time. The most significant periods of expansion for the desert occurred in the summer, and resulted in an almost 16 percent increase in the desert’s average seasonal area over the 93-year period covered by the study.
“Our results are specific to the Sahara, but they likely have implications for the world’s other deserts,” says Sumant Nigam, a professor of atmospheric and oceanic science at UMD and senior author of the study.
Their results point to human-caused climate change as one of the main culprits behind this expansion, as well as natural climate cycles such as the Atlantic Multidecadal Oscillation (AMO).
“Deserts generally form in the subtropics because of the Hadley circulation, through which air rises at the equator and descends in the subtropics,” explains Nigam. “Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts. The southward creep of the Sahara however suggests that additional mechanisms are at work as well, including climate cycles such as the AMO.”
The southern border of the desert is adjacent to the Sahel – a semi-arid transition zone that lies between the Sahara and the fertile savannas further south. As the Sahara expands, it pushes back the Sahel, causing a major disruption to the area’s grassland ecosystems and the societies that inhabit them. Lake Chad is also caught in the middle of this desert expansion.
“The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out,” says Nigam. “It’s a very visible footprint of reduced rainfall not just locally, but across the whole region. It’s an integrator of declining water arrivals in the expansive Chad Basin.”
Researchers determined through statistical methods that natural climate cycles are responsible for about two-thirds of their observed expansion of the Sahara, while the remaining one-third they attribute to climate change. However, they do state that longer climate records extending across several climate cycles are needed to reach more definitive conclusions.
There are far-reaching implications for the future of the Sahara, given the results of this study. These implications also expand to other subtropical deserts around the world. Studies such as this are only increasing in their importance. As the world’s population continues to increase, any reduction in arable land and the necessary rainfall to grow crops could mean a future of food scarcity that affects millions.
Image Credit: Mamadou Faye/courtesy Wassila Thiaw, NOAA CPC