Africa-wide "Great Green Wall" to Halt Sahara's Spread?

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But the wall has been slow to break ground: Of the 4,350 miles (7,000 kilometers) it needs to cover, only about 326 miles (525 kilometers) have been planted so far, all within Senegal.

In Copenhagen, President Wade emphasized that he has made the wall a priority, and he has already asked scientists working on the project to choose species hardy enough to survive in arid conditions without maintenance.

"One thing the president has insisted is … we have to begin the work now, right now," added Ndiawar Djeng, advisor to the Senegalese environment minister.

"If other international committees follow us, that's OK. If not, we have to do what we have to do," Djeng told National Geographic News.

"It's in the interest of our local people."

Farming Boon

The lush channel through the desert would help farmers already displaced by drought—and may even stem the exodus of "environmental refugees," organizers say.

More than 70 percent of Africa's poor depends on farming, according to the IPCC.

But drought, desertification, and other climate-related disasters are forcing many farmers to abandon their lands, spurring a heavier flow of immigrants out of central and North Africa.

The 9.3-mile-wide (15-kilometer-wide) wall of trees would improve the surrounding, now-degraded soils, allowing farmers to again grow crops and more easily raise livestock in the region.

(Related: "'Fertilizer Tree' May Revive African Farmlands.")

Senegal also plans to dig rainwater reservoirs along its portion of the wall—virtual lifesavers in a region where rain falls only three months out of the year, supporters say.

"France is helping us by bringing its soldiers, who are working with us planting trees and building reservoirs," President Wade added.

The gigantic tree barrier would also trap some atmospheric carbon dioxide, a potent greenhouse gas, and produce a refuge for native animals and plants.

Some of the trees themselves may become valuable crops.

The native acacia senegal tree, which is to be a staple plant in the Great Green Wall, produces gum arabic, a main ingredient in consumer products such as cosmetics and soft drinks.

Farmers could collect the sap and even sustainably harvest some of the wood to make tools or produce charcoal, Senegalese environment advisor Djeng said.

Local Know-How

But Senegal may do more for farmers by simply supporting age-old solutions to desertification, UC Berkeley's Gonzalez noted.

For example the ethnic groups of the Sahel—a swath of semi-arid savanna on the Sahara's southern border—have long been successful at reforesting their land using "natural regeneration."

In this method, farmers plant small native trees from seeds found in the region and raise the trees in agricultural fields protected from nibbling livestock, Gonzalez said.

"The Great Green Wall is less feasible than supporting and reinforcing local farmers and the practice of natural regeneration," he said.

What's more, planting trees alone will not stop the Sahara's spread, according to Matt Brown, senior conservation advisor for the Nature Conservancy's Africa program.

Instead, African governments need to find ways to protect existing vegetation and water sources from overuse, Brown said by email.

Overall, though, the Great Green Wall is an "extremely bold" undertaking, he said, and "sometimes thinking big is what is needed to draw attention to a problem."

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