The Sahel, a semi-arid region, forms the southernmost limit of the Sahara (shown here in Mali). To halt desertification, the “Great Green Wall” project plans to grow trees across the Sahel. New research suggests that these trees may also have a significant impact on the climate of the region.
There are plans for an 8,000-kilometer “Great Green Wall” in Africa to keep the Sahara from spreading further south. This greening might have a significant impact on the climate of northern Africa and possibly further afield, according to new climate simulations that look at both the region’s past and future.
An ambitious initiative is underway to plant 100 million hectares of trees in the semiarid zone that stretches from the desert’s southward edge to the Sahel region. According to the models presented December 14 at the American Geophysical Union’s autumn meeting, that completed tree line could quadruple rainfall in the Sahel and reduce average summer temperatures throughout most of northern Africa and into the Mediterranean. Temperatures are expected to rise much more in the desert’s hot spots, according to the research.
A “green Sahara” has been related to changes in the intensity and location of the West African monsoon, according to previous research. Over northern Africa, that primary wind system brings dry, warm air to the south in the cooler months while bringing a little more rain to the north in summer.
There was a green Sahara time between 11,000 and 5,000 years ago when the monsoon’s intensity and its northward or southerly extent changed (SN: 1/18/17). It was during the 1930s that Hungarian explorer László Almásy, who served as the inspiration for actor Hugh Grant’s character in the 1996 film The English Patient, uncovered Neolithic cave and rock art in the Libyan Desert depicting humans swimming.
In the past, there has been a correlation between variations in Earth’s orbit and the amount of incoming solar energy that heated up the West African monsoon. Francesco Pausata, a climate dynamicist at the Université du Québec à Montréal who ran the new simulations, thinks that orbital cycles do not reveal the complete story. Researchers now realize that changes in plant cover and dustiness can greatly accelerate the monsoon movements, according to him.
Deepak Chandan, a paleoclimatologist at the University of Toronto who was not involved in the research, argues that greater vegetation “helps create a local pool of moisture,” with more water cycling from soil to atmosphere, increasing humidity and so rainfall. Chandan adds that plants create a darker land surface compared to the white sands of the desert. This allows the ground to absorb more heat. The amount of dust in the atmosphere is reduced even further when vegetation is present. Solar radiation is reflected back to space by dust particles, so less dust means more solar energy may reach the land When all of these things are taken into account, the land is hotter and more humid than the water, resulting in a greater disparity between atmospheric pressures. Stronger monsoon winds are a logical consequence of this.
It was in the 1970s and ’80s, when the once-fertile Sahel region began to dry up and become barren, that the idea for Africa’s Great Green Wall was born. The idea of creating a vegetative barrier to slow the spread of the desert has been around for a long time. To prevent the spread of the Dust Bowl, President Franklin D. Roosevelt enlisted the help of the U.S. Forest Service and the Works Progress Administration in the 1930s. To stem the southerly advance of Gobi Desert sand dunes since the 1970s, China has undertaken its own large desert vegetation project, also known as the Great Green Wall.
There has been a 15 percent completion rate for African Union’s Great Green Wall project, which began in 2007. Supporters anticipate the completed tree line would not only halt the expansion of the desert, but will also enhance food security and provide millions of jobs in Senegal and Djibouti.
Few studies have examined the potential effects of the completed greening on the local, regional, and global climate, but Pausata believes that more studies should be conducted. It is important to consider the potential consequences of every geoengineering endeavor, he argues, because that is what it is at its core.
Computer simulations of future global warming, both with and without a virtual wall of plants along the Sahel, were built by Pausata to study these probable implications. Global warming would reduce midsummer temperatures in the Sahel by up to 1.5 degrees Celsius if the Great Green Wall were built.
The warmest parts of the Sahel, though, would see temperatures rise by up to 1.5 degrees Celsius. Researchers believe that the increased vegetation will also enhance rainfall over the region, possibly even more than doubling it in some spots.
Preliminary findings, according to Pausata, and only for a high-emissions future warming scenario called RCP8.5 were given at the symposium. Ongoing simulations are being conducted for scenarios with moderate and lower levels of emissions.
The calculations imply that the consequences of greening the Sahara may spread well beyond the region. Larger air circulation patterns could shift westward as a result of a stronger West African monsoon, which could affect other climate patterns like the El Nio Southern Oscillation and modify the path of tropical storms.
He agrees with Chandan on the importance of knowing how large-scale planting might affect the Sahara and emphasizes that gains in understanding what led to past changes are essential for simulating its future. “The climate system is full of interactions,” he adds, “so the Great Green Wall’s impact could be far-ranging.”
