Already, according to Randall and Schwartz, the planet is carrying a higher population than it can sustain. By 2020 'catastrophic' shortages of water and energy supply will become increasingly harder to overcome, plunging the planet into war. They warn that 8,200 years ago climatic conditions brought widespread crop failure, famine, disease and mass migration of populations that could soon be repeated.
The skeptic in me, though, notes that they have been saying similar things since the time of Malthus, and that (as Steven Pinker pointed out) the imminent extinction of humanity appears to be a constant perceptual illusion resulting from a tendency to underestimate technological progress. Perhaps that's the case with this report as well, and we'll all survive in shantytowns in the Alps, living off vat-grown genetically-modified algae and/or Soylent Green. Or maybe this time, it's for real, and we're all, not to put too fine a point on it, fucked.
It seems everything has been distorted. The Pentagon's report is about a 'what if?' scenario. I've found it odd since it forecasts opposite conditions - i.e. global warming does mean floods in Holland, but a Syberian climate in Britain would be the result of a mini-ice age, meaning a lower sea level. To me, it seems inspired in the forthcoming blockbuster from the guys who did ID4.
It's obvious climate change is a problem, but such extreme changes are unlikely to happen in a short time. Let's deal with peak oil first - if dealt properly it might even help solve global warming.
The only reason why Britain is warmer than Siberia is the gulf stream, by which warm water from the equatorial Atlantic reaches Britain and northern Europe. The theory is that the rising sea levels from melting ice caps (or possibly the decreasing salinity of the ocean as such) would reach a tipping point, beyond which the gulf stream would very rapidly stop, and Britain would freeze.
Here's what Australia's premier warblogger has to say: http://timblair.spleenville.com/archives/006051.php