Desalinating seawater now 3X more efficient:

Originally posted by sonhouse
Damming up the Ms river could help but it is also very silty,

Its silty because of poor farming practices that cause erosion. The silt itself is good for irrigation. The most fertile land in the world is silt on river banks and deltas - partly because it is renewed regularly.
Of course stopping the silt altogether would have dire consequences for the New Orleans region which is sinking and thus relies on the silt to build up the coast.

BTW, a recent study of the last major heating cycle, besides losing all the polar ice, caused deserts in half the world and lasted 200,000 years before some kind of reset occurred that brought things back to what we would call normal. That was 55 million years ago, 10 million years AFTER the dinosaurs got whacked.
But presumably large parts of the Arctic (where much of the worlds land is and is currently frozen desert) would become habitable. Tropical rain forests would also expand I believe.
The real costs are not total areas of desert etc but change.
Change results in most species suffering and struggling to move /adapt to new environments.
Change also is very expensive for people.