What would happen if we built canals and let ocean and sea water into the sahara desert?

The general rule of thumb is that plants that start to grow in a high Sodium-Chloridic environment die quickly because their roots get blocked by said NaCl molecules, as they are larger than H2O molecules. H2O can then not flow into the root canals and for that reason fotosynthesis can't take place. For that reason ATP can't be produced and the plant can't grow and will eventually die. You're absolutely right that there are exceptions, as there always are. There are locations where plants have learned to survive or even thrive in a usually harsh environment but those are specific plants that have developed over thousands of years, and often not of value to humans. That could happen in some locations over time throughout the map OP provided, but in general, no, it would just be an expensive project that doesn't really serve purpose.

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