In a study supported by the U.S. National Science Foundation, researchers from the Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland found that the Sahara Desert has grown over 10 percent in the last century.
The study suggests that the rest of the world’s deserts could be expanding too as widespread climate change continues to heat up the planet. Their work was published online in the journal of the American Meteorological Society on March 29, 2018.
“Our results are specific to the Sahara, but they likely have implications for the world’s other deserts,” said Sumant Nigam, a professor of atmospheric and oceanic science at the University of Maryland. The Sahara, the world’s hottest desert, is already roughly the size of China or the continental United States.
However, it has grown about 10 percent larger since 1920. “The trends in Africa of hot summers getting hotter and rainy seasons drying out are linked with factors that include increasing greenhouse gases and aerosols in the atmosphere,” said Ming Cai, a program director in NSF’s Division of Atmospheric and Geospace Sciences.
Loss of the Sahel region
As desertification moves southward, it is impacting on the Sahel region, the semi-arid transition zone that lies south of the Sahara. The study found that as the Sahara expands, the Sahel retreats, disrupting the region’s fragile grassland ecosystems and human societies.
Lake Chad sits at the center of the Sahel and supplies drinking water to about 30 million people in four neighboring countries – Chad, Cameroon, Niger, and Nigeria. The lake has lost 90 percent of its surface area over the last 100 years. The dwindling lake has been labeled an ecological catastrophe by the UN Food and Agriculture Organization.
Because of Lake Chad’s location in the conflicted transition zone, it is used to gauge the severity of the changing conditions in the Sahel. “The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out,” Professor Nigam explained. And the expanding desert and lack of rainfall have had a devastating effect on local economies in the Sahel that depends on water for their crops.
Precipitation levels across Africa have changed
For the study, researchers used annual rainfall data recorded throughout Africa from 1920 to 2013. The observational analysis data was then used to evaluate the state-of-the-art climate simulations from a comparison of the twentieth-century hydroclimate trends.
Deserts are defined as places that receive less than four inches (100 millimeters) of rain per year. After analyzing the rainfall data, the researchers determined that many areas in the Sahara now fall under this threshold. And as was expected, desert expansion was most evident during the summer months, when it resulted in a nearly a 16 percent increase in the desert’s average area over the 93-year span covered by the study.
Professor Nigam added: “Deserts generally form in the subtropics because of the Hadley circulation, through which air rises at the equator and descends in the subtropics. Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts.”
The Hadley circulation, or cell, is a global scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at 10–15 kilometers above the surface, descending in the subtropics, and then returning toward the equator near the surface. This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams.
And the researchers claim the reason for the expansion of the Sahara is a combination of human-influenced factors and natural change that includes the Atlantic Multidecadal Oscillation (AMO), which alters climate patterns in the region, and works on a 50- to 70-year cycle; and the Pacific Decadal Oscillation (PDO), marked by temperature fluctuations in the northern Pacific Ocean on a scale of 40 to 60 years.
Both these circulation patterns play a role in changing temperatures, as we have already found out this past winter when Europe and the Northeastern part of North America were hit by extremely cold weather. While there is some normal variability in weather patterns. the researchers found the Sahara expansion was steady and undeniable.
As EcoWatch puts it, “The African continent is one of the most vulnerable to climate change, yet its inhabitants are the least responsible for it. Truly, one of the great injustices of our time.”
“Our priority was to document long-term trends in rainfall and temperature in the Sahara,” said Natalie Thomas, a researcher at the University of Maryland and lead author of the paper. “Our next step will be to look at what’s driving these trends, for the Sahara and elsewhere.”
