Adding clean energy to the Sahara could make it rain (and not just figuratively)... To green or not to green?

Pedro_P

A view of part of the Sahara near Chad.
NASA
▼ Think of the Sahara, with its windswept dunes shining in the sunlight. Some people might see barren land, with minimal water or life and scorching temperatures. Others see a potential solution to a looming energy crisis, and one that could potentially make it rain in one of the largest deserts in the world.
In a paperpublished this week inScienceresearchers found that by building out huge wind and solar farms across the desert, they could not only provide a stunning amount of power to Europe, Africa, and the Middle East, but they could simultaneously change the climate—increasing heat, but also increasing precipitation and vegetation in areas that could sorely use the added greenery. They estimate that such a venture could double the rainfall in the region, and increase vegetation cover by about 20 percent.
How much green are we talking? The Sahara covers 3.55 million square miles (9.2 million square kilometers). In the study, the researchers ran computer models that placed wind turbines across the desert close to a mile apart, and covered 20 percent of the desert with solar panels in different configurations (sometimes the panels were spread across the desert in a checkerboard pattern, and in other cases were concentrated in quadrants). Smaller coverage produced smaller climate impacts—in this case, less precipitation—but much of it depended on the location of the turbines and panels as well. For example, installing panels in the northwest corner had a larger impact than the other three desert options.
Covering parts of the desert with darker solar panels meant that less sun bounced off the Saharan sand, which is unusually light in color, and therefore has a higher albedothan other non-polar deserts. That means that typically the Sahara reflects more light and heat back into the air. Reducing the albedo by installing darker solar panels could actually increase precipitation in the region even as it increases the temperature around the solar panels. Warmer air rises to areas in the atmosphere where it’s cooler, and moisture there condenses and falls as rain.
"In 1975 Jule Charney, my PhD advisor at MIT, proposed a feedback mechanism to help explain the drought in the Sahel, the semi-arid transition region south of the Sahara: Overgrazing increased surface albedo (reflectivity), reduced precipitation, and in turn further reduced vegetation. About a decade ago, I had the idea that this feedback would work in the opposite direction in the presence of large solar panel farms, since they would reduce the surface albedo," said Eugenia Kalnay, a lead author of the paper in a press release. "Similarly, wind farms would increase land surface friction and convergence of air, thus producing upward motion and precipitation.”
With more rain, grass and trees could slowly grow back into the once-lush landscape, sprouting between turbines and solar panels as they do on existing farms.
The solar farm would produce an estimated average of 79 terawatts of power, and the wind farm would produce about 3 terawatts—without producing greenhouse gas emissions. To put that in perspective, the authors say that the entire world used about 18 terawatts of power last year.
But building a massive solar farm doesn’t happen overnight, so the researchers programmed the computer model to look at what would happen during a 100 year buildup period, and during the 100 years after the plants were built.
“If we can finish building all the wind and solar farms immediately, some of the effects on atmosphere would be observed almost immediately. But the effects due to the vegetation–albedo–precipitation feedback mechanism would take some time before it could be observed, because vegetation needs time to grow (within a few years). In reality, the effect would grow as the size of wind and solar farms that have been installed grows,” two of the lead authors, Yan Li and Safa Motesharrei, said in an email. (▪ ▪ ▪)

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