Is there enough fresh water on the planet to turn all deserts green?

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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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I don’t think that’s a useful way to think about it. Fresh and salt water aren’t two different things that remain separate all of the time. If you look at the water cycle a lot of fresh water will end up in the ocean. And a lot of salt water from the ocean will turn into fresh water when it evaporates and rains down over land. So in the end it’s about implementing a micro climate where there’s enough water to sustain the vegetation. As for the green wall that means stopping soil degradation, as degraded soil tends to lose a lot more water to evaporation than plant covered soil.

Nope and even if it was it probably wouldn’t be a good idea. China’s great green wall may actually be making the situation worse. https://youtu.be/VTUTJ1vuhcQ

The Amazon rainforest relies upon the Bodele depression which is near lake Chad at the southern end of the Sahara https://youtu.be/Ggeu_M7HRR4

One useful thing is the use of “magic stones” or stone lines to prevent the loss of topsoil https://youtu.be/gW8GE4ilJhk

It’s not just about water really. Take the classic explanation of the water cycle. Ocean water evaporates. That evaporated water sits in the atmosphere where it’s transported by wind. This wind is in turn caused by the dynamics between warm and cold air as well as warm and cold oceanic currents.

Atmospheric water moves on air currents until it coalesces into clouds and eventually falls down to Earth in the form of rain, snow, hail and so on or freezes into glacial ice. All that water eventually flows into rivers, either on the surface or as groundwater and the rivers transport it back to the ocean for the next cycle.

The Sahara is so dry exactly because the local climate conditions cause moisture to evaporate from the Sahara and get transported out to sea. There currently is no moisture cycle that brings water into the Sahara, only a cycle that takes it out. Most deserts have environmental causes like that which cause the arid nature of the region.

So if you just start dropping buckets of water into the Sahara, you won’t be achieving much. You drop in a bucket of water and the climate conditions will suck it right out and drop that water back into the ocean.

And with all the greenhouse gasses we’re pumping into the atmosphere, the climate catastrophe is only speeding up and intensifying that process.

So the question do we have enough fresh water on the planet isn’t quite the right question. Trying to fix the Sahara by throwing water on it is a bit like trying to fill a bucket without a bottom.

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.

There is enough water to turn all the deserts green. But you need plenty of desalination plants to handle the conversion of all of the salt water you would need to use. I don’t believe you could gather enough fresh water to make this happen.