Terraform the Deserts?

What would it take to terraform the deserts of the world?

If my science is right, then introducing massive amounts of water to the area would prevent drastic temperate change ..but it would probably take a long time to change normally.

Would be more beneficial to wait decades for natural changes to take place after taking measures like stated above, or would forcing it be better for humans?
 
Deserts are already as terraformed as it gets.
Basically, you have a ring of Hadley cells in the atmosphere (a convection mechanism) and where the air flows downwards, you have permanent (or very long lived) high pressure areas. Meaning air flows out of them rather than in, and being a cold air from the top of the troposphere being warmed up by descent, it is pretty dry. Where this zone lies over land, you have sunshine all the time. The water you bring in - unless a massive amount of it flows in - will simply evaporate and be carried by the Hadley convection to the north or to the south to rain off there.
Of course you can pump water and use it for irrigation, thereby moisturizing a local area, but it does not stop the desert from being desert. It just remains a desert into which someone brought some water.

If you build an artificial, self-filling lake - filling the Qattara depression for example, or the Dead Valley, by a channel to the sea - you will get a belt of moist area around the lake, and still a lot more desert further away from it.

If you want to change the Hadley cells - good luck. They are pretty much a constant given by a planetary diameter. they can at most expand/shrink relative to each other, thus moving the desert belt north/south. You will still always get a high pressure belt around each hemisphere.
 
changing deserts would be a generations long task... and you're never going to change the heart of them, they are deserts for lots of perfectly good geological reasons. What you can do is shrink them a little by tinkering on the edges... judicial watering, keep grazing down the first few decades, etc. You have to be very careful when watering dry areas like these; too much, and you start getting salts and alkalies in the soil rising to the surface and ruining the land. Light watering on a regular basis could improve the growth of the native grasses/shrubs/brush that already lives there, and the land will slowly improve. You have to be careful with grazing such places, generally do it very fast and move on, giving the plants time to recover.
 
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