China built its famous Great Wall to keep out marauders. Now, millennia later, a "Great Green Wall" may rise in Africa to deter another, equally relentless invader: sand.

The proposed wall of trees would stretch from Senegal to Djibouti as part of a plan to thwart the southward spread of the Sahara, Senegalese officials said earlier this month at the UN's Copenhagen climate conference.

The trees are meant "to stop the advancement of the desert," Senegalese president and project leader Abdoulaye Wade told National Geographic News in Copenhagen.

In many central and West African countries surrounding the Sahara, climate change has slowed rainfall to a trickle, according to the UN's Intergovernmental Panel on Climate Change (IPCC).

Crops have died and soils have eroded—crippling local agriculture. If the trend continues, the UN forecasts that two-thirds of Africa's farmland may be swallowed by Saharan sands by 2025 (explore an interactive Sahara map).

Trees are almost always formidable foes against encroaching deserts, said Patrick Gonzalez of the University of California, Berkeley's Center for Forestry.

That's because stands of trees act as natural windbreaks against sandstorms, and their roots improve soil health—especially by preventing erosion.

But choosing the right tree species to populate the wall will be crucial to the project's success, Gonzalez said via email.

Similar tree-planting efforts by outside agencies have failed, he said, in part because they planted foreign species that soon perished in the harsh desert.

"We Have to Do What We Have to Do"

Nigerian President Olusegun Obasanjo first proposed the idea of a desert-blocking wall in 2005, and it was approved by the African Union in 2007.

All 11 countries that would house the Great Green Wall have pledged to help fund the project.

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.

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.