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| Camels walk on the sand during the “Gallops of Morocco” equestrian race in March in the southern Moroccan Sahara desert. (Fadel Senna/AFP/Getty Images) |
Yet it is not just the spread of the Sahara that is frightening, the researchers say. It’s the timing: It is happening during the African summer, when there is usually more rain. But the precipitation has dried up, allowing the boundaries of the desert to expand.
“If you have a hurricane come suddenly, it gets all the attention from the government and communities galvanize,” said Sumant Nigam, a professor of atmospheric and oceanic science at the University of Maryland and the senior author of the study. “The desert advance over a long period might capture many countries unawares. It’s not announced like a hurricane. It’s sort of creeping up on you.”
The study was published Thursday in the Journal of Climate. The authors said that although their research focused only on the Sahara, it suggests that climate changes also could be causing other hot deserts to expand — with potentially harsh economic and human consequences.
Deserts form in subtropical regions because of a global weather circulation called the Hadley cell. Warm air rises in the tropics near the equator, producing rain and thunderstorms. When the air hits the top of the atmosphere, it spreads north and south toward the poles. It does not sink back down until it is over the subtropics, but as it does, the air warms and dries out, creating deserts and other areas that are nearly devoid of rain.
“Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts,” Nigam said in a statement that announced the study.
At the same time, he said, the Sahara’s southward creep suggests that additional mechanisms are at work. One is probably the natural climate cycle called the Atlantic Multidecadal Oscillation, or AMO, in which temperatures over a large swath of the northern Atlantic Ocean fluctuate between warm and cold phases for 50 years to 70 years. The warm cycles deliver precipitation to subtropical areas, and the cold cycles keep it away. Human-caused climate change can increase the intensity and length of the drier cycle.
Nigam and the study’s lead researcher, Natalie Thomas, a doctoral candidate at the University of Maryland, used data from the Global Precipitation Climatology Center to arrive at their finding. They studied grids and patterns from 1920 to 2013, mixing in satellite data compiled “over the last three decades,” Nigam said.
They determined that the AMO was in a positive phase that delivered more rain to areas near the Sahara from the 1930s to the early 1960s. It then switched to a negative cycle that lasted 40 years. A 1980s drought — “the most intense … of the 20th century” — was attributed to the latter phase and linked to “higher levels of greenhouse gases in the atmosphere.”
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| Railway tracks are covered by sand as a result of desert encroachment in 2013 at Ogrein Railway Station in Sudan. (Mohamed Nureldin Abdallah/Reuters) |
Africa is the continent least responsible for human-caused climate change, but it’s the most vulnerable to its effect because of unique features. It is, for example, a land mass almost evenly divided between the Southern and Northern hemispheres, creating a wide variety of climate zones.
Thomas said she started the research as a way to characterize century-long trends but focused on Africa’s Northern Hemisphere when she noticed “really strong trends over the proximity of the Sahara.”
As the researchers went about their work, downloading satellite data and information from the global climatology center, the evidence became more concerning. “The finding was impressive because it was happening in the summer season, the growing season where Africa receives most of its rainfall, a really important season for agriculture,” Nigam said.
Yet that is when the greatest southward advance of the Sahara occurred, he said. A season of rain was being replaced by the expansion of a desert, without the affected governments, Chad and Sudan mostly, noticing.
The future implications for countries already affected by lack of rain and drought could be dire, Nigam said. “Water resource planning, water use and long-term planning is important.”
Links
- African women are breaking their backs to retrieve water for their families
- Four things you should know about food security in Africa
- Climate change could devastate Africa. It’s already hurting this Kenyan town.
- Indigenous people on the world’s coastline are losing their fisheries — and a way of life
- Scientists say climate change is threatening the lifeblood of Canada’s Native people
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