The searing Sahara Desert is getting even hotter, at a rate two to four times greater than the rest of the tropics, say scientists in a new study.
That puts it on par with the Arctic which is also exceeding the global warming average. But whereas the widely studied Arctic "amplification" melts sea ice and permafrost, the Sahara warming could be reducing the huge outflow of dust that blows off Africa and be causing big changes to regional weather - and local people.
"A lot of people live there – three million or so," said researcher Kerry Cook of the University of Texas at Austin. "And it's adjacent to the Sahel region, which has many more people."
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The Sahara is the world's largest non-polar desert, covering 6 percent of Earth's land surface. At 3.6 million square miles, it is larger than Australia, or the lower 48 United States.
Cook and her coauthor Edward Vizy looked at five separate temperature data sets to glean what happened in the Sahara from 1979 to 2012. They found that whereas the warming in the Earth's tropics is about a quarter of a degree Celsius, and global warming for the periods was a half degree Celsius, the warming in the Sahara Desert was more than 1 degree Celsius.
They published their results in a paper in the August issue of Journal of Climate.
Just why it is warming faster than other regions is not at all clear, said Cook. One possibility is that the hot arid land simply can't transfer heat up and away, as other, moister lands do.
"In most places evaporated water cools the surface and water goes up to upper troposphere and heats the air there," Cook said. The water vapor serves as a sort of heat elevator, in other words. But that doesn't happen in the Sahara because it's very dry. So the heat just stays at the surface.
A similar thing happens in the Arctic. There global warming is melting the ice, revealing the darker ground or dark open waters which absorb the sunlight and warm up up the air near the surface (as opposed to reflecting that sunlight off snow and ice). The warmth stays trapped near the ground, like a smog layer in a city, because of the very stable layers of air above the Arctic.
"That's why [the Sahara heating] is analogous to polar amplification," said Cook.
But unlike the Arctic the Sahara has millions of people who are vulnerable to heat stress and dehydration as their oases and reservoirs dry up. The higher temperatures are also increasing the differences between the Sahara and the more slowly warming areas around it. It's these sorts of temperature differences, or gradients, between the Sahara and the Atlantic that help draw in rains to the Sahel region.
"As you go north from the equator it gets hotter in the summer," said Cook. That's only true in the Sahara, not in any other part of the tropics. "That increases the flow of moisture to the Sahel from North Atlantic and is driven by the gradients."
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In the winter, the high pressure that normally sits atop the Sahara has been weakening over the decades, which could affect the amount of dust worldwide.
"The Sahara and Sahel are primary sources of mineral dust for the global atmosphere when surface material is suspended by the northeasterly surface winds in winter," Cook explained. "Amplified Sahara warming in winter would weaken this high and the northeasterly flow, and reduce the suspension of mineral dust."
If that happens it could be bad news to the Amazon, Caribbean, the equatorial Atlantic, Europe, and the Mediterranean Sea all of which are fertilized by the mineral dust. But it's not yet clear that is the case.
"The paper shows an interesting analysis and discusses possible explanations for the heating seen over the Saharan desert," comments Sahara dust researcher Stephanie Fiedler of the University of Leeds in the U.K.
"Regarding the impact of Saharan heating on winds and dust emission, one would need to do further investigations to understand the processes at play. Dust emission depends on high and extreme wind speeds, (that is,) the occurrence of a storm, rather than mean winds shown in the paper."