The Sahara Desert has expanded by about 10 per cent since 1920, according to a new study published in the Journal of Climate.
The research by University of Maryland (UMD) scientists is the first to assess century-scale changes to the boundaries of the world’s largest desert and suggests that other deserts could be expanding as well.
The researchers analysed rainfall data recorded throughout Africa from 1920 to 2013 and found that the Sahara, which occupies much of the northern part of the continent, expanded by 10 per cent during this period when looking at annual trends.
The study results suggest that human-caused climate change, as well as natural climate cycles such as the Atlantic Multidecadal Oscillation (AMO) — variation of currents in the North Atlantic Ocean, leading to changes in sea temperatures — caused the desert’s expansion.
The geographical pattern of expansion varied from season to season, with the most notable differences occurring along the Sahara’s northern and southern boundaries.
When the researchers looked at seasonal trends the most notable expansion of the Sahara occurred in summer, resulting in a nearly 16 per cent increase in the desert’s average seasonal area over the 93-year span covered by the study.
“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 UMD, and the senior author of the study.
“Deserts generally form in the subtropics because of the Hadley circulation, through which air rises at the equator and descends in the subtropics,” Prof Nigam said.
“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 Sahara is the world’s largest warm-weather desert. Like all deserts, the boundaries of the Sahara fluctuate with the seasons, expanding in the dry winter and contracting during the wetter summer.
Reduced rainfall
“The Chad Basin falls in the region where the Sahara has crept southward. And the lake is drying out,” Nigam explained. “It’s a very visible footprint of reduced rainfall not just locally, but across the whole region.”
The study’s results 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 adequate rainfall to support crops could have devastating consequences.
“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 programme director in the National Science Foundation’s Division of Atmospheric and Geospace Sciences, which funded the research.
“These trends also have a devastating effect on the lives of African people, who depend on agriculture-based economies.”