Thinking through what the Great Green Wall will do.

Science News looks at the long-term implications of the “Great Green Wall,” a proposed belt of tree-plantings intended to block the southward spread of the Sahara Desert. The project, if completed, has implications not just for countries to the south, but for North Africa, the Mediterranean, and the rest of the world:

By 2030, the project aims to plant 100 million hectares of trees along the Sahel, the semiarid zone lining the desert’s southern edge. That completed tree line could as much as double rainfall within the Sahel and would also decrease average summer temperatures throughout much of northern Africa and into the Mediterranean, according to the simulations, presented December 14 during the American Geophysical Union’s fall meeting. But, the study found, temperatures in the hottest parts of the desert would become even hotter.

Previous studies have shown that a “green Sahara” is linked to changes in the intensity and location of the West African monsoon. That major wind system blows hot, dry air southwestward across northern Africa during the cooler months and brings slightly wetter conditions northeastward during the hotter months.

… these effects lead to more heat and more humidity over the land relative to the ocean, creating a larger difference in atmospheric pressure. And that means stronger, more intense monsoon winds will blow.

Led by the African Union, Africa’s Great Green Wall project launched in 2007 and is now roughly 15 percent complete. Proponents hope the completed tree line, which will extend from Senegal to Djibouti, will not only hold back the desert from expanding southward, but also bring improved food security and millions of jobs to the region.

What effect the finished greening might ultimately have on the local, regional and global climate has been little studied — and it needs to be, [Université du Québec à Montréal climate dynamicist Francesco] Pausata says. The initiative is, essentially, a geoengineering project, he says, and when people want to do any type of geoengineering, they should study these possible impacts.

To investigate those possible impacts, Pausata created high-resolution computer simulations of future global warming, both with and without a simulated wall of plants along the Sahel. Against the backdrop of global warming, the Great Green Wall would decrease average summertime temperatures in most of the Sahel by as much as 1.5 degrees Celsius.

But the Sahel’s hottest areas would get even hotter, with average temperatures increasing by as much as 1.5 degrees C. The greening would also increase rainfall across the entire region, even doubling it in some places, the research suggests.