These problems are definitely going to be serious. But how serious they will be and whether or not all the doom and gloom is justified depends on how our societies react to the challenges.

It depends on how quickly we can develop technology and make it available to everyone. It depends on how quickly people can adapt to migration situations. It depends on countless non linear interactions that are unpredictable.

I think what we can say is that the pace of change is absolutely crucial. If land is lost to deserts or to the sea at a pace noticable only by asking your great grand parents, then I think we're good.

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 is only available to rich countries then I fear the gloom and doom is more than justified.