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