> But if a billion people have to move within 20 or 30 years because large poor cities can no longer be defended against the sea or because technology to deal with desertification

That seems perfectly reasonable. Nowhere near doom and gloom. There's no doom there, that I can see. Moving a billion people is not that big of a deal, as long as there is incentive. The idea that "the sea" will push people around has been going on forever and doesn't cause mass migrations in terms of the earth. It's the food and distribution a population has to worry about that will cause trouble. Everything else is well within human abilities to mitigate (even when average lifespans decrease by a few decades).