Another thing that I wanted to write about in the solar-and-wind-will-green-the desert story is the region that was modeled by the researchers. I understood from the BBC article that the researchers distinguished two regions: the Sahara and the Sahel. Which puzzled me at the time. These are two distinct regions. The Sahara is a desert, while the Sahel is a transitional zone between the Sahara Desert to the north and the more humid savanna belt to the south.
It is not very clear from the BBC article what the exact relationship is between the two in that paper. For example this statement (my emphasis):
“Our model results show that large-scale solar and wind farms in the Sahara would more than double the precipitation, especially in the Sahel, where the magnitude of rainfall increase is between 20mm and 500mm per year,” said Dr Yan Li, the lead author of the paper from the University of Illinois, US.
That is rather confusing. The title is “Large-scale wind and solar power ‘could green the Sahara'”, but this quote seems to suggest that the large-scale installation in the Sahara would have the most effect in the Sahel, which seems to contradict the title since the biggest effect is not in the Sahara, but in a nearby region of the Sahara.
Also what was found is rather confusing. I can understand the bottom value of 20 mm (probably the northern part of the Sahel/southern part of the Sahara). If a region is a desert, then a precipitation increase of 20 mm/year will not change much. Unless the precipitation is close to 250 mm/year (which is widely taken as the upper limit of being a desert). On the other part of the spectrum there is the 500 mm/year increase (probably the southern part of the Sahel). That much precipitation increase would make a desert green, but the Sahel is not really a desert.
There was however a graph in the BBC article that contained some numbers that I could use to shed some light on the issue. Let’s go back to that graph: