@Akikako I hesitate to bring real world politics into the discussion, but global warming was observed as a problem for 60 years ago (when oil companies had to start altering their rig designs to accommodate for thinning icesheets), and it's just become more and more obvious for everyone ever since - yet there's still a lot of "old bosses" who prefer to ignore the problem rather than try to invest some effort into it.
(It doesn't even matter if it is man-made or not - we need to invest a lot in adapting if we want to mitigate the effects.)
As for the manga: Once the colony has only five years left, then most of those people will suddenly realize that "certain death" (of staying in the colony) suddenly looks a lot less fun than "probable death" (look for new colony), and by the time there's about six months left, those who are still alive will probably try to do something. But with twenty years left.. well, everyone willing to think that far ahead has already moved. What you have left in the colony are those who would rather look away from the truth, making up whatever excuse they can just to pretend that life goes on as normal and that there is no need to change.