Arcadia

Overexploitation of planet resources causes natural counterattack - seawater lever rises, climate deteriorates, and majority of alive species die with humanity being next in queue. Mankind hurriedly develops space colonization plans and sends a mission to planet Arcadia. Humanity rebuilds there to the best of its ability.
Main character is an ordinary high school teacher. But he meets a boy who bears a mysterious secret able to change the fate of the world.

[From vndbreview]