The Great Green Wall's one of the world's most ambitious eco-projects. Is it working? In an arid valley near the village of Kourtimale in southern Djibouti, a tattered chain link fence marks the boundaries of what was once Abdi Guelleh's farm. Within it, there's not a speck of greenery in sight. Broken lengths of irrigation piping lie scattered in the dust. A derelict weather station stands in a c...