Twenty-one African countries have come together in an attempt to stop the Sahara desert from encroaching further south. Their mission: plant a 4,750-mile-long wall of trees.

When completed, the Great Green Wall will extend from sea to sea and reclaim 247 million acres. It'll stretch from Senegal in the west to Djibouti in the east, and will be three times the size of the Great Barrier Reef. The massive reforestation project will sequester 250 million tons of carbon.

The countries hope to have the wall completed by 2030.

The Sahara in northern Africa is the world's largest desert (as large as the U.S.) and renowned for it's extreme temperatures. A peer-reviewed study by the journal Climate found that between 1920 and 2013, the desert has expanded southward by 10%. The study says its expansion has been caused by man-made climate change as well as natural climate cycles such as the Atlantic Multidecadal Oscillation.

If the situation is allowed to continue, the desert is likely to expand into the more fertile parts of Africa.