Switching from fossil fuels to renewable energy in the Sahara region make their immediate surroundings both warmer and rainier, and could turn parts of the Sahara green for the first time in at least 4,500 years.
The scientists behind the research published in Science by Yan Li and colleagues. looked at the maximum amount of solar and wind energy that could be generated in the Sahara desert and the transition region to its south, the Sahel.
The two regions were picked as they are relatively plausible sites for such an enormous roll-out of renewable energy, being fairly near to substantial demand from Europe and the Middle East, while having limited other demands on the land. Both have substantial potential resources of wind and solar energy.
As the two regions are so large, the solar and wind farms could be vastly bigger than any existing solar and wind farms, and could provide up to four times as much energy as is currently consumed globally.
This would prompt quite significant changes in the local environment – massive wind farms would raise temperatures by around 2℃ for instance, similar to the amount of global warming we are concerned about. Solar would cause a smaller temperature change, around 1℃.
Wind farms largely cause temperature increases because their turbine blades bring warmer air down to the surface, especially at night. This has been observed in field studies and using remote sensing. They have also been shown to increase moisture in the air.
Solar panels mean more solar radiation is absorbed and less of the sun’s energy is reflected back into space. This causes the land surface to warm up. Several studies have shown this, including one which showed that the effect of warming caused by fossil fuels, via carbon emissions, was 30 times greater than the warming caused by solar photovoltaics absorbing more solar radiation. However, temperature effects may vary within the solar park and with season.
In the Sahara simulation, extra rainfall happens because wind turbines represent an obstacle to free-flowing air, slowing it down and reducing the effect of the Earth spinning on air flow. This lowers the air pressure, and the difference in pressure between the Sahara and surrounding areas causes wind to flow there. When the air meets, or converges, in the Sahara it has nowhere else to go but up. As the air rises, water vapour in it condenses and rain drops form.
For solar, the process is slightly different: warmer air, heated by the panels, simply rises. However, this also promotes low pressure, causing air to flow there, converge and rise.
Large-scale wind and solar would mean more new rain in some areas than others. Eviatar Bach, CC BY-SA
More rainfall also means more vegetation. This increases surface roughness, as with wind turbines, and causes more solar radiation to be absorbed, as with solar panels. This reinforcing cycle is known as a “climate feedback” and incorporating these vegetation feedbacks is a novel aspect of the research by Li and colleagues.