The idea was striking in its ambition: African countries aimed to plant trees in a nearly 5,000-mile line spanning the entire continent, creating a natural barrier to hold back the Sahara Desert as climate change swept the sands south.

The project called the Great Green Wall began in 2007 with a vision for the trees to extend like a belt across the vast Sahel region, from Senegal in the west to Djibouti in the east, by 2030. But as temperatures rose and rainfall diminished, millions of the planted trees died. Efforts to rein in the desert continue in Senegal on a smaller scale.