A series of climate simulations, co-led by Dr Camille Contoux of the Bjerknes Center for Climate Research in Bergen, Norway, suggests that the desertification of Sahara started about 7 million years ago, at least four million years earlier than previously thought.
Camel caravan crosses the Sahara desert in Morocco. Image credit: Bachmont / CC BY 2.0.
The Sahara Desert is often cited as the world’s largest desert. This is not entirely true, however, as it is only the third largest desert after Antarctica and the Arctic and the world’s largest hot desert.
At over 9,400,000 km2, it covers nearly 10 percent of the African continent, including Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Sudan and Tunisia.
Geological evidence suggests that the desert is no more than 2 to 3 million years old.
However, a recent discovery of 7-million-year-old sand dune deposits in the northern Chad Basin has hinted at a much older age.
To unravel the Sahara’s mystery, Dr Contoux and his colleagues explored paleoclimate of North Africa over the past 30 million years using the Norwegian Earth System Model and the Community Atmosphere Model.
They identified the Tortonian stage (7-11 million years ago) of the Late Miocene epoch as the pivotal period for triggering North African aridity and creating the desert.
“The region undergoes aridification with the shrinkage of the Tethys – a giant ocean that was the origin of the modern Mediterranean, Black and Caspian Seas – during the Tortonian,” the scientists explained.
“The simulations are the first to show that the Tethys shrinkage has two main consequences for North African climate.”
“First, it weakens the African summer monsoon circulations and dries out North Africa. Second, it enhances the sensitivity of the African summer monsoon and its associated rainfall to orbital forcing.”
“The Tortonian stage thus marks the time when North Africa shifted from a permanently lush, vegetated landscape to a landscape experiencing arid and humid cycles on orbital time scales.”
The results were published on September 18, 2014 in the journal Nature.
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Zhongshi Zhang et al. 2014. Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene. Nature 513, 401–404; doi: 10.1038/nature13705
