A question often posed by naturalists, climatologists.
We already know that it is possible to transform an arid land into a forest by creating stone carpets at least 1 meter deep to retain water, minerals and the compost generated by the degradation of organic matter. This ancestral technique called zaï has been used successfully several times. It was unveiled to the world by Yacouba Sawadogo in Burkina Faso.
In Egypt, the use of semi-treated wastewater allowed to grow trees in a desert without using the drinking water needed by the populations. This water is rich in nitrogen and phosphates. Water is redirected to the desired area from a drainage basin. Trees grow there even faster than normal, as it takes only 15 years for the wood to reach a cutting height, compared to 60 years in Europe.
For the Sahara to be entirely wooded, it is necessary to be able to retain the clouds and make them rain, which engenders rivers and streams. And therefore be able to create artificial mountains, since the Sahara is globally rather flat. The maximum altitude of the Sahara is 463 meters. Eventually, the forest will increase condensation and make it rain.
Therefore, the question that should really be asked is: how to create artificial mountains?
You can use blocks of concrete or rocks that you accumulate as well as to create dykes. The problem, besides the fact that it will be expensive to import these blocks, is that it will be dangerous to walk on such mountains; They can sag on themselves at any time. It will be necessary to let nature follow its course for hundreds of years before hoping to discover it and exploit it.
China is currently changing its main desert, the Gobi Desert, into the forest.