I don't see what's so bad about this? Temperatures will rise, sea levels will rise, what's the big deal?

Certain parts of the world that are low-lying or subject to extremely high temperatures will become uninhabitable. Too bad for those who thought it would be a good idea to settle there, they'll have to migrate.

Once all the ice has melted, sea levels will plateau. By the 22nd Century (notwithstanding a global nuclear war between now and then) we will have mastered nuclear fusion and will no longer be dependent on fossil fuels.

So what's the big deal?