I don't see any reason to imagine that lies within our power to do. If we exerted all the singular, concerted effort of which our entire species is capable, we might at most annihilate ourselves and make a mess that the biosphere will need as much as a million years to evolve beyond. But a million years is nothing on a planetary scale.

And if you are worried about that - all the more reason to favor expansion into space, I should think! The thermonuclear genie will never go back into the bottle. Nor will any other. Better, don't you think, to ensure we have at least some part of our species beyond the reach of at least some of those genies, should they turn on us?

A million years? I think you over-estimate how insignificant we are to this planet.

I'm not advocating we destroy the environment, but we really can't do a whole darn lot to it. Can we set off every nuclear weapon at once? Sure, but even that will likely just ruin the surface for 50,000 - 100,000 years. Life will continue, just not humans and most likely nothing of any size.

If we manage to throw the planet into a run-away greenhouse cycle like Venus, yeah, life's done here but that seems like a tall order (though I am not well read or well versed on this subject matter so I could be wrong on this count.

Ok, off the soap-box, you were making the same point as me :)