I already said this in response to one person on the thread but... #4 is the only one on this list that, in my opinion, we must realize as a species. The end of the Earth is unavoidable. Even if we avoid every anthropogenic cause of it, there are still countless other ways we could face "the end" - imagine an asteroid like what killed the dinosaurs. Perhaps it wouldn't cause us to go extinct but we'd lose a lot of people, a lot of civilization, and a lot of our advancement. And even if we avoided that, the dying sun will inevitably consume the planet (and yes, I am being implausibly optimistic with my assumptions that mankind will make it long enough to see that) so the only solution is to abandon our ancestral home and make a new one, somewhere. If mankind has any hope of assured, permanent survival, the Earth will have to become the childhood home of our species. Otherwise it will be our grave.