Run out of drinkable water? What kind of trash is that? We can purify the damned water, expensive as it might well be, and the combustion engine is a relic of one-hundred years ago, I have no worries about our ability to replace it, the bigots at the oil industry and the president will string us along, but the worst it will get in the next few years is them jacking up the price to roll in more green.
Most people don't believe in global warming, and the hole in the Ozone layer is hardly an apocalyptic event, if anything, we need more skin cancer, humanity be damned, those of us too idiotic to wear sunscreen to the beach should be cut out of the gene pool.
While I'm sure that a few million years is a blink of the eye in terms of Earth, (that's what opinionated scientists keep telling us, right?) I'm sure we'll have found and advanced to another planet by then, (According to Mr. President, it might even be in our lifetimes! 
)
We don't live to die, we die to live. More generations faster= more genetic progress.