The Sahara desert of northern Africa is spreading southward. This is very alarming to the countries on its border, who fear a loss of farm productivity and civil chaos as this kills their agricultural production and leads to swarms of people moving to the nearest city in the hopes of earning enough to sustain themselves.
Discovery News reports that 11 affected nations are cooperating to build a wall of trees that will halt the expansion in place. BBC reports that the trees prevent the desert from expanding by halting erosion, slowing the wind, and changing the ground conditions in a way that encourages water retention.
A big sticking point in this is money. All the nations must contribute land and money and effort if this is to succeed. If even one hole in the wall exists, the desert can push through the hole and around the rest of the wall, rendering it useless. And this region is not particularly famous for being wealthy. Unfortunately, the nations involved are doing this because they must. They must or conditions within will become much worse.
If this works, the affected countries could even push back the desert with additional walls, creating additional farmland and making their country more attractive to both agriculture and industry.