Subtitles section Play video Print subtitles When most people think of the Sahara Desert, they think of this. But it really used to be this. And it could again. The Great Green Wall is an epic project that aims to grow an 8,000-kilometer belt of vegetation across the entire width of the African continent. If completed, it would be three times larger than the Great Barrier Reef, and be the largest living structure on the planet. But can this massive geo-engineering project transform the landscape into the fertile, tropical place it once was? So, here’s the thing: This part of Africa is heating up. Particularly in the Sahel, which sits between the southern edge of the world’s largest hot desert, the Sahara, and humid savannahs to the south. Vegetation is scarce in this semi-arid belt of land, and the UN has identified it as a hotspot for climate change. Temperatures in the Sahel region are increasing 1.5 times faster than the rest of the world, and the Sahara Desert is now 10% larger than it was in 1920. Periodic drought, deforestation, and unsustainable agricultural practices have reduced the productivity of the Sahel, causing desertification. This has led to massive food insecurity and displacement of people in the region. And with temperatures expected to be 3 to 5 degrees Celsius warmer by 2050, these problems are expected to get worse. But not all is lost. Remember when we said the Sahara used to be green? Well, that was about 11,500 years ago; back then, there was grass, lakes, and animals like hippos and antelopes. Dubbed the African humid period, also known as the "Green Sahara", this era was the result of intense West African monsoons, which were stronger and brought more summer rainfall than today. Over thousands of years, the intensity of the monsoon is affected by a slow wobble in the Earth’s orbit known as precession, which strengthens solar radiation in the summer, driving moist air inland. The African humid period ended some 5,000 years ago, though traces of the period still remain. In the 1930s, explorer László Almásy found Neolithic cave paintings in Egypt of people swimming. But an orbital shimmy-shake doesn't tell the whole story; changes to the land can have a big impact on monsoonal rains, too. When the Sahara was greener, its vegetation allowed more water to circulate from the soil into the atmosphere, increasing humidity and rainfall. The greenery also reduced the amount of dust in the atmosphere, allowing more sunlight to reach the ground. Being darker than the light-desert sands, the plants absorbed more solar radiation, which caused more precipitation. And when it rained, it poured! The combined effects intensified monsoonal winds, bringing about 10 times the rainfall than the desert gets today. Like the African humid period, the Great Green Wall could re-green the Sahel. The ambitious multi-national initiative launched in 2007, with the idea of building a tree line on the edge of the desert. But like with most ambitious projects, it’s having a rough time. 80 percent or more of the planted trees have died. Reasons include a lack of proper irrigation and not having management systems in place. There were also other issues: Non-native species didn’t adapt, grazing animals ate the seeds, and a lack of support from local stakeholders. The wall has since evolved into a patchwork of sustainable land-use practices, like the circular drought resistant gardens known as "tolou keur". And for a while now, scientists have been looking at whether this project could actually work. In 2021, preliminary research was presented on how the wall could have a profound effect on climate. They created hi-res computer simulations, both with and without a green wall. They found that if the wall is completed, it could as much as double rainfall in some parts of the Sahel and decrease average summer temperatures around the Great Green Wall. But on the downside, the hottest parts of the desert would get even hotter. The wall may also have unintended consequences that extend far beyond the region. For one, it could reduce the amount of dust that travels across the Atlantic, impacting the fertility of the Amazon. It might even be capable of increasing the intensity of tropical cyclones. But in spite of this, the project is still moving forward, aiming to plant 100 million hectares of trees, shrubs, and grasses by 2030. The project got a boost in January of 2021, receiving more than 14 billion dollars from donors like the World Bank to fast-track the effort. But having community support is really key to making the effort work. If successful, the program could be a game-changer for the continent by restoring degraded landscapes and potentially transforming millions of lives. And who knows? Maybe one day, we’ll see hippos in the Sahara again. If you love hearing about big ideas to combat climate change, the next one is large, shaggy, and has a trunk. Check out this video on how scientists are trying to bring back the woolly mammoth to help with global warming. Know of any other cool geo-engineering projects? Let us know in the comments below. Make sure to subscribe, and thanks for watching.