Passive Desalination with photovoltaic pumping

[Lissa]

I'm no engineer, but I've always wondered why large-scale desalination isn't the obvious solution to fresh water shortages, especially on the coasts.

Yes, me as well. I wonder why North Africa couldn't use it's solar power to reclaim parts of the Sahara from west to east, increasing the relative humidity of the entire region.

Well, the issue with the Sahara is due to the change in tilt of the Earth that occurred 25k years ago. Prior to that the Sahara was very lush, but after the change in tilt, the desert grew.

Another aspect is that ancient man unknowingly caused some deserts to grow due to irrigation with sea water. A prime example of this is the growth of the Arabian desert. At one time, probably close to 6k AD, people started irrigating parts of Arabia using waters from the Persian Gulf, Red Sea, and Arabian Sea and this salted the earth there and destroyed the arable lands there thus causing the desert to grow to its present state.