If you spend enough time on the “educational” side of Youtube, you will come across short documentaries that talk about projects aimed at stopping desertification and climate change. One example is the green wall of trees, that multiple sub-saharan countries want to plant in order to stop the sahara from spreading. Another example are the plans to turn parts of the australian outback in to humid land by planting trees and building dams. My question is: Is there even enough fresh water to sustain all of those projects? Could you theoretically turn all deserts into humid forests?
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The short answer is no, you can’t turn all deserts into green lands, simply because there’s a reason why deserts are where they are. They mostly occur along the subtropics (30°N and S), where high pressure is persistent, meaning rain cannot form, because to get rain you need air moisture and low pressure (rising air) whereas high pressure (sinking air) fights cloud formation and thus rain. The purpose of the Green Belt in the Sahel is to feed moisture into a weak low pressure system hoping it’s enough to increase rain. This is achieved by planting vegetation, which retains moisture and makes air humidity increase locally. That moisture is then brought to high altitudes by low pressure, where it condenses into rain. You can’t do that in the Sahara, because there is little to no low pressure to begin with, so all the humidity you could possibly add to the system will never be able to rise and condensate into decent amounts of rain. What you can do is using underground freshwater reservoirs, which are already there, and create an oasis. You could build an entire city with that, and it has been done, but it will not last forever and it cannot be applied to entire countries because there is a limit to how much water a system of canals can transport.
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