Humanity's overall predicament is far worse than anything Gore has imagined, of course. Global Warming is only one of a number of extinction threats, and as such it is not the most likely to actually lead to extinction.
It does now seem likely to most of the relevant experts, as far as I can tell, that human activities have probably contributed to global warming, and that the effects on the biosphere could be quite serious, and possibly grave. However, few are predicting Global Warming will cause the rapid extermination of the human race. Sure: massive crop failures; mass migrations by large chunks of the world population; deaths in the millions... but these are all things the very determined and hairless apes have bounced back from before. The climate change we are seeing is rapid -- but not 'The Day After Tomorrow' rapid, not even nuclear winter rapid. Chances are extremely good that plenty o' humans will survive the disaster Gore says we have but ten years to avert.
Not to rain on his parade, then, but I must point out that a number of countries still have plenty of atomic weapons sleeping in their silos, and any war where a bunch of those get chucked around is likely to create climate change on a different order altogether. If you take the historical record as a good indicator of the general frequency of major wars, well, things don't look so good.
It's possible that humanity would survive even a serious nuclear winter. Aside from the microscopic stuff and the cockroaches and so on, I personally vote this species of ours as Most Likely To Survive (Even If Survival Sucks). But could we survive a supervolcano? A comet impact? Given a long enough time frame, these events are as sure as the rising of the sun is apparent.
And so I have long believed in the overwhelming importance of creating stable and self-sufficient colonies in space. Indeed, one broad misgiving I continue to harbor about environmentalism in public policy is that badly-thought-out government environmental policies could compromise the development of the kind of scientific, industrial and economic power we will need to build giant ships capable of towing populations of people between stars. (And not JUST to build ships. That's just the first item on a long list of prerequisites for successful permanent space colonization...)
Anyway, I suspect most people think I am crazy when I rant about how humanity must escape the Earth. But today Stephen Hawking said the same thing, with a lot more reporters listening: http://apnews.myway.com/article/2006061
I would like to thank him, on behalf of my great-great-great-grandchildren, the bioengineered-yeast-sucking dome-dwellers of Alpha Centauri B's second planet, 'Gurdjieff's Gulch'.