African leaders have convened to discuss the planting of a proposed "green wall" intended to halt the advance of the Sahara Desert through a process known as desertification(砂漠化). If current trends of soil degradation continue in Africa, the UN forecasts that two-thirds of the continent's farmland may be overtaken by the Sahara by 2025. Advocates of the Great Green Wall project believe that planting a strip of forest about nine miles (15 kilometers) wide and more than 4,400 miles (7,100 kilometers) long will protect these lands by slowing soil erosion, slowing wind speeds, and helping rainwater filter into the ground.