If we can't even save our own home world, there is every reason to believe that we will do damage elsewhere. That being the case, best to stay home until we grow up a bit.
That proverbial man who plants trees knowing he will never sit in their shade is proverbial for a reason.. it's an amazing and heartwarming little saying because it's a contrast to how we generally behave, as individuals as well as organizations and masses. We learn from repeated catastrophe.. sometimes, maybe. But this problem is not one we can have a lot of attempts at solving. Right now we are still moving into the wrong direction, and probably even accelerating.. sure, hitting a wall can make people change their direction, but not if it breaks their neck, so to speak.
I'm not enormously optimistic. I'm just saying that compared to interstellar travel, avoiding fossil fuels seems like a fairly small problem.