Also I don't think you understand the concept of "third world". First world countries are never going to be third world countries. Third world countries exist because they have fundamental, insurmountable (for now) flaws and problems in their governments, socio-economics and culture. These problems preclude the given third world country from being able to form stable governments, economies and social order. Consequently these countries with vanishingly small exceptions have no money for anything. Their people live in grinding poverty in lives that are "nasty, brutish and short". A first world nation like China, Japan or the USA can engineer preventive measures to ameliorate the effects of a slowly rising sea level. Remember, we are talking in terms of 50 to 100 years of gradual change. That is plenty of time to generationally react to such changes.

I recommend that you study such futurist topics as "GRAIN". That is an acronym for "Genetics, Robotics, Artificial Intelligence and Nanotechnology. I would add practical fusion and materials science to that.

I hope this helps to make my position clearer and supplies the facts that you require.