Thanks, Shadron, for the explanation.
Is it possible that the methane in the permafrost will be released slowly enough to be absorbed/converted to its final byproducts as it is released? I'm not doubting that even a slow release could have a warming effect, but it seems as though we won't turn into Venus over the course of a summer season...
Unfortunately, the methane that is held in the permafrost is merely held there mechanically - it is not dissolved in the ice or bound chemically to it, so the only thing holding it back from joining the atmosphere is the physical ice structure of the permafrost. Once that heats up to 32 degrees in the summer, and then the considerable further latent heat of melting is added, the ice turns to water and the methane formerly held by that ice is permanently released. Now, obviously not all the permafrost, from 3" down through 20 or so feet will melt in a single year, but each years melt will add methane which will add more greenhouse effect to make the next summer even warmer, releasing more methane, etc, etc, until it is all released. A classic positive feedback loop.
How long might that take? A hundred years, a thousand? People thought the melting of the Artic ice might take a long time too, but the last twenty years have proven those predictions to be disastrously underestimated. In term of geological time, of course, a thousand years is nothing; and anything less is hugely quick - not that the Earth cares, but you and I (or more likely, our grandchildren) will.
Nothing in the very beautiful movie convinces me that the effects we're seeing aren't the ending of the last ice age. It's just that we're convinced that we're the cause of it. We must be, because we're here, and we're recording it, and we're humans after all: we are the source, the cause, and the result of all nature's efforts.
Well, the evidence from the IPCC and the hundreds of papers show that there is something different about this period of warming. The biggest difference is the speed with which it is happening - the amount of change that we've seen since, say, 1970, might have taken on the average 5 ot 6 thousand years in previous episodes of warming. The other side of the coin, as one person noted to me, that all the coal and oil we are burning must be having some effects, and these are the most obvious effects it could be having. These are not proof, of course, but I'm not a climatologist, so I have to trust them on that.
At any rate, that is what the narrator meant when she said that melting the permafrost poses a serious threat to life. I disagree to the extent that I believe that Earth and life in general will continue on, but it will, I think, provide our civilization and our species with a challenge the magnitude of which it hasn't faced for over 70,000 years. I'm sorry to say.
I have two series of videos that explain the backgrounds and the controversies about global warming. The first is from an Australian journalist who was trained as a geologist and has spent most of his career following the global warming debate. You'll find the first of four videos (more to come, he says) here:
http://www.youtube.com/view_play_list?p=A4F0994AFB057BB8&playnext=1&playnext_from=PL
The second is a video blog about specific controversies about global warming, called the Climate Denial Crock of the Week:
http://www.youtube.com/profile?user=greenman3610&view=videos