Occupying roughly 9.4 million square kilometers (3.6 million square miles) of northern Africa, the Sahara is the largest hot desert in the world. And it’s expanding.
Researchers at the University of Maryland used historical records collected throughout Africa and climate simulations to analyze how the great desert has responded to both natural fluctuations and climate change during the 20th century. Their findings, published in the Journal of Climate, reveal that reduced rainfall along the Sahara’s southern edge is pushing the desert into the bordering grassland ecosystem, the Sahel, in Nigeria, Chad, and Sudan.
Annual rainfall is the criteria that defines a habitat as desert, and using this metric, the authors determined that the Sahara grew 10 percent between 1920 and 2013. Yet the boundary area is known to fluctuate seasonally, temporarily expanding outward during the dry winters then contracting during wet summers. Due to this variation, the local communities rely heavily on the summer rains to raise and harvest crops.
When examining seasonal rainfall patterns over the 93-year period, the authors found that the crucial summer precipitation has reduced more dramatically than rain at other times of the year. As a result, the Sahara’s summer boundaries have grown 16 percent.
According to the analysis, the Chad Lake area has seen the most extreme shifts – further corroborating a known climate crisis in the region as food and water shortages caused by years of drought and desertification have led to violent political instability.
"The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out," Sumant Nigam said in a statement. "It's a very visible footprint of reduced rainfall not just locally, but across the whole region.”
To determine what proportion of the shifting Sahara and Sahel can be attributed to climate change, Niagim and his colleagues needed to factor in the potential effects of two long climate cycles: The Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO).
Both the AMO and PDO produce alternating, multiple decade-long periods of global warmth or cooling. These 40- to 70-year cycles, driven by varying sea surface temperatures, alter rainfall patterns when they interact with local weather systems.
After painstakingly correcting their climate model to remove the influence of the natural cycles, the team concludes that about one-third of the reduced rainfall can be attributed to climate change.
Due to the lack of reliable records prior to 1920, however, combined with the fact that only one to two of AMO and PDO cycles are experienced per century, the authors warn that their Sahara expansion percentages are only approximations.
“Our next step will be to look at what is driving these trends, for the Sahara and elsewhere," said lead author Natalie Thomas. "We have already started looking at seasonal temperature trends over North America, for example. Here, winters are getting warmer but summers are about the same. In Africa, it's the opposite – winters are holding steady but summers are getting warmer.”