Is the Sahara really spreading, and can a Great Green Wall stop it?

Deserts are not simply marching outward across the map. Desertification is the gradual degradation of fertile land into arid desert because of natural processes and human activities, and the immediate cause is the loss of most vegetation. Drought, climate shifts, ploughing, overgrazing and cutting trees for fuel can all strip the plants away, and then dry soil blows or washes off, leaving infertile hardpan. Drylands cover about 40 percent of Earth's land and are home to more than 2 billion people. The old picture of the desert's edge advancing like a tide has been rejected as the idea evolved.

The Sahel, the belt south of the Sahara, shows how tangled the causes are. A long drought from 1968 to 1974 made grazing unsustainable, stripped the land and brought famine. Early studies put much of the blame on overgrazing. But scientists now think Sahel drought was principally caused by shifting rainfall linked to sea surface temperatures, and since the 1970s and 1980s the Sahel has had more rain, a partial re-greening.

Against this backdrop, the African Union adopted the Great Green Wall in 2007: a planned line of trees across the Sahel from Djibouti to Dakar. The idea became a mosaic of water harvesting, land protection and local land-use methods, aiming to restore 100 million hectares by 2030.

The results are hard to read. A report commissioned by the UN Convention to Combat Desertification and published in 2020 found the wall covering only 4 percent of its planned area. Later figures count something different, land restored against the 100-million-hectare target: about 18 percent as of 2023 and about 30 percent as of 2024. Because the measures differ, they are not a clean trend, and promised funding has often failed to arrive.