Scientists recently have come up with a bold idea to green the Sahara with green electricity. Using climate models, they found that building large wind and solar farms in the Sahara would increase rainfall and green vegetation in the region, and provide much of the electricity for Europe, Africa, and the Middle East. The theory behind is that power plants that reduce albedo and increase rainfall can increase vegetation cover by about 20 percent, and the resulting vegetation can further increase precipitation. The paper is published on the journal Science.
The first author, University of Illinois at Urbana-Champaign postdoctoral Yan Li, said: "former studies showed that large-scale wind and solar farms in mainlands have a significant impact on climate change, but the lack of vegetation feedback could make the outcome of the simulation very different from the actual result." Given this factor, even in the Sahara desert, Yan Li and his colleagues added vegetation feedback to the model.
The model results show that large-scale construction of wind farms would significantly increase the temperature near the ground, with warm air rising to cooler areas of the atmosphere, allowing moisture to condense and form rain. The data showed that rainfall could increase by an average of 0.25 mm/day, double that of the control trial (0.24 mm/day). This in turn leads to increased vegetation cover, forming a positive feedback loop. However, wind speeds in the region have fallen by about 36 percent due to the use of wind energy.
Solar farms, like wind farms, also increase rainfall and vegetation in the Sahara. That's because the panels directly reduce albedo, raising the temperature near the generator and increasing rainfall by an average of 50 percent. Combining wind and solar farms together would have been even more effective, increasing rainfall by an average of 0.35mm per day.
The Sahara desert is the largest sandy desert in the world, covering an area of 9.06 million square kilometers, accounting for about 32% of the total area of Africa. About 11,000 to 5,000 years ago, the Sahara was a diverse ecosystem, far greener and wetter than it is today, dotted with many lakes where fishermen live. About 8,000 to 4,000 years ago, herders began to emerge, cattle, camels and donkeys were domesticated and livestock ate vegetation, accelerating the process of desertification in the humid Sahara.
In the study, scientists covering almost the entire desert with wind and solar farms found they could generate about 3 TW (TW, terawatt is a unit of power, 1 TW equals to 10 to the 12th watts) and 79 terawatts respectively, comparing to about 18 terawatts of global energy demand in 2017. The researchers also looked specifically at the Sahel region, a belt of more than 3,800 kilometers between the southern Sahara desert and the steppe region of central Sudan that stretches from the western Pacific to the eastern horn of Africa. It is a semi-arid grassland area, with typical transitional characteristics, highly sensitive to land changes. In March 2018, a study by the university of Maryland found that the Sahara desert has grown by more than 10 percent since 1920, propelling it into the fragile Sahel region, which is driven by climate change.
Large-scale installations of wind and solar power in the Sahara could curb this trend. The model shows that the installation of wind power plants can increase rainfall in the region by an average of 1.12mm/day, the installation of solar power plants by an average of 0.57mm/day, and the annual rainfall of hybrid installations by an average of 200mm to 500mm, which is enough to produce significant ecological, environmental and social impacts.
However, the effect won't happen overnight, according to the paper's lead authors Yan Li and Safa Motesharrei. The effects of the vegetation-albedo-precipitation feedback mechanism will take time to be observed, as vegetation takes at least several years to grow.