The Sahara is the world largest desert, and getting larger. It threatens to creep ever further to the south and turn arable land in desert wasteland. The nations in its path have an idea, though: We'll build a fence. Of trees. The "Great Green Wall" would be a tree band that spans the breadth of northern Africa, 9 miles wide and nearly 5,000 miles long, from Senegal at the western edge near the Atlantic to Djibouti on the eastern edge near the pirate-infested Gulf of Aden. It may sound too dreamy or crazy to ever go forward, but this week at a meeting in Chad about desertification, the Global Environment Facility backed the belt idea with $119 million. Chad's minister of environment, Hassan Térap, says it can be achieved: