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. ...was once meant to be one tiny brick in one of the world's most ambitious environmental projects: Africa's Great Green Wall. ... This multi-billion dollar project was launched by the African Union in 2007. The plan: to plant a "wall" of trees spanning the entire width of Africa — 4,350 miles long and 10 miles wide — to fight desertification in the Sahel, the arid region to the south of the Sahara desert....