The Sahara Desert, located in West Africa, is the largest hot desert in the world. While deserts are traditionally characterized by a dry climate with low levels of vegetation, the Sahara has been much greener than usual as of late — mainly due to climate change.
From brown to green
Between July and September every year, rainfall north of the equator in Africa increases due to monsoon season. Such stormy weather occurs when the tropical air from near the equator meets the hot, dry air from the north of the continent. This boundary is called the Intertropical Convergence Zone (ITCZ). The ITCZ "shifts north of the equator in the Northern Hemisphere's summer months," and "sags south of the equator during the Southern Hemisphere's warm months," said CNN.
However, the ITCZ "moves farther north the warmer the world gets," Karsten Haustein, a climate researcher at Leipzig University in Germany, said to CNN. "At least, this is what most models suggest." The boundary has pushed farther north this year than is typical, making portions of the Sahara Desert two times to six times wetter than normal. "The Northern Hemisphere, as there is more land than the Southern Hemisphere, tends to warm up more and hence it could push the ITCZ further north," Francesco S.R. Pausata, an atmospheric science professor at the University of Quebec, said to The Washington Post.
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The largest increase in greenness this year was in southern Chad, southern Sudan and Eritrea; parts of Mali, Niger, Chad, Sudan and Eritrea had the biggest increases in rainfall. One sudden wave of lushness followed an extratropical cyclone in the northwestern Sahara on September 7 and 8, which brought a downpour of rain to regions that hardly receive any, according to NASA's Earth Observatory. "What's also fascinating is that normally dry lakes in the Sahara are filling due to this event," Moshe Armon, a senior lecturer at the Institute of Earth Sciences and the Hebrew University of Jerusalem, said in an Earth Observatory release.
Climate consequences
The northward shift of the ITCZ and the greening of the Sahara likely have two catalysts: the transition from El Niño to La Niña and worsening climate change. "El Niño — a natural climate pattern marked by warmer than average ocean temperatures in the equatorial Pacific — typically leads to drier than normal conditions in the wet portions of West and Central Africa," said CNN. " La Niña, or even a budding one, can have the opposite effect." Climate change is also warming the region, forcing the ITCZ boundary higher. In addition, higher CO2 emissions, like those from fossil fuels, may cause the ITCZ to shift more often, according to a study published in the journal Nature.
Not only can climate change affect the ITCZ, but the ITCZ can affect the climate. The recent greening of the Sahara may also be tied to a calmer-than-usual Atlantic hurricane season. "Because tropical waves spawned by the ITCZ are exiting Africa into the Atlantic Ocean farther north than usual, they have been encountering cooler air and waters, which can limit the potential for [hurricane] development," said the Post.
The increased rainfall has so far put 4 million people in 14 countries at flood risk, according to the World Food Programme. Also, with some regions receiving more rain than usual, others have received less, putting them in danger of drought. "Every single event is affected by climate change," Haustein said.
