Deserts are generally defined by low average annual precipitation – typically 100 millimeters (less than 4 inches) of rain per year or less. The researchers analyzed rainfall data recorded across Africa from 1920 to 2013 and found that the Sahara, which occupies much of the northern part of the continent, expanded by 10 percent during this period in looking at annual trends.

When the authors examined the seasonal trends over the same period, the most notable expansion of the Sahara occurred in summer, resulting in an increase of almost 16% in the average seasonal area of ​​the desert during the period of 1993. years covered by the study.

“Our findings are specific to the Sahara, but they likely have implications for other deserts around the world,” said Sumant Nigam, professor of atmospheric and oceanic sciences at UMD and lead author of the study. Nigam also has a joint position at the Interdisciplinary Center for Earth System Sciences (ESSIC) at UMD.

The results of the study suggest that human-caused climate change, along with natural climate cycles such as the Atlantic Multidecadal Oscillation (AMO), caused the expansion of the desert. The geographic pattern of the expansion varied from season to season, with the most notable differences occurring along the northern and southern borders of the Sahara.

“Deserts typically form in subtropics due to the Hadley circulation, through which air rises at the equator and descends in the subtropics,” Nigam said. “Climate change is likely to widen Hadley’s circulation, causing subtropical deserts to advance north. The south-Saharan slide suggests, however, that additional mechanisms are also at work, including climate cycles such as than AMO. “

The Sahara is the world’s largest warm-weather desert, roughly the same size as the contiguous United States. (The Arctic Basin and the Antarctic Continent – which are each about twice the size of the Sahara – are also considered deserts due to their low rainfall rates.) Like all deserts, the boundaries of the Sahara fluctuate with the seasons. , extending during the dry winter and contracting during the wetter summer.

The southern border of the Sahara adjoins the Sahel, the semi-arid transition zone that lies between the Sahara and the fertile savannas further south. The Sahara expands as the Sahel recedes, disrupting fragile grassland ecosystems and human societies in the region. Lake Chad, which lies at the center of this conflicting climatological transition zone, serves as a barometer for the changing conditions in the Sahel.

“The Chad basin is in the region where the Sahara has slipped south. And the lake is drying up,” said Nigam. “It is a very visible imprint of reduced rainfall not only locally, but across the region. It is an integrator of the drop in water inflows into the vast Chad basin.”

A number of well-known climate cycles can affect rainfall in the Sahara and Sahel. AMO, in which temperatures over a wide swath of the North Atlantic Ocean fluctuate between hot and cold phases over a 50- to 70-year cycle, is one example. The warm phases of AMO are linked to the increase in precipitation in the Sahel, while the reverse is true for the cold phase. For example, the notable drying up of the Sahel from the 1950s to the 1980s has been attributed to one of these cold phases. The Pacific Decadal Oscillation (PDO), marked by temperature fluctuations in the northern Pacific Ocean on a scale of 40 to 60 years, also plays a role.

To distinguish the effects of man-made climate change, the researchers used statistical methods to rule out the effects of AMO and PDO on the variability of precipitation during the period 1920 to 2013. The researchers concluded that these natural climatic cycles accounted for about two-thirds of the total observed expansion of the Sahara. The remaining third can be attributed to climate change, but the authors note that longer climate records that span multiple climate cycles are needed to reach more definitive conclusions.

“Many previous studies have documented precipitation trends in the Sahara and Sahel. But our article is unique, in that we use these trends to infer changes in desert extent on a century scale,” said Natalie Thomas, a graduate student in ocean sciences at UMD and lead author of the research paper.

The results of the study have far-reaching implications for the future of the Sahara, as well as other subtropical deserts around the world. As the world’s population continues to grow, a reduction in arable land with sufficient rainfall to support crops could have devastating consequences.

“Trends in Africa of hot summers getting hotter and rainy seasons drying up are linked to factors that include increasing greenhouse gases and aerosols in the atmosphere,” said Ming Cai, director. program in the Division of Atmospheric and Geospatial Sciences of the National Science Foundation, which funded the research. “These trends are also having a devastating effect on the lives of Africans, who depend on economies based on agriculture.

Thomas and Nigam strive to learn more about the drivers of desert expansion to the Sahara and beyond.

“With this study, our priority was to document long-term trends in precipitation and temperature in the Sahara. Our next step will be to examine what is driving these trends, for the Sahara and elsewhere,” explained Thomas. “We’ve already started looking at seasonal temperature trends in North America, for example. The winters here are getting warmer but the summers are pretty much the same. In Africa, it is the opposite: the winters are maintained but the summers are warming. tensions in Africa are already more severe. “