I have seen before the scientific analysis about Earth's cyclical thermal rise and fall over the very long term (say a billion years), and there seems to be a consensus. But as you yourself point out, civilization has barely been here a blink by that scale, much shorter than humans.... perhaps we can say 5000 years. It isn't the survival of humans per se that is the issue -- chances are there will be at least a few survivors of all but the worst climate change scenario -- but rather, human civilization as we know it today.
As you point out, the prospect of human population being culled to say a tenth of current numbers and a complete shakeup of the way we organize might not be a bad thing, there are so many basic flaws in our social/political/economic systems. But the cost sure looks painful.
I think we can agree that the planetary environment over the last few thousand years has been great for humans -- hey we became the predominant species on Earth during this time! Modern civilization is built on high energy consumption, and I for one, would hate to go back to a limited energy existence. There's no question that burning fossil fuel is what brought us here -- it's hard to imagine how humans could have reshaped the world so dramatically without it. Yes, the great wall of China & the pyramids were built by human labor, but would you want to be doing that? Or building the Suez Canal, the Bay bridge in San Francisco, the 3 Gorges dam w/o heavy machinery or industrial steel mills?
Neil & others have pointed out that non-fossil energy technologies are already here, still being developed very rapidly, and we still have the potential to wean ourselves off oil & gas & coal and move to a clean high energy future. But we need more time and concerted effort to pull it off.
There may not be enough time if the greenhouse gas emissions continued unabated & the earth keeps getting hotter. The biggest risk is that as environmental changes cause food, arable (and livable) land, water, and energy to become increasingly limited, the global cooperation needed to eliminate greenhouse gas emissions simply become impossible -- nations, armies, corporations, people will be too busy scrambling for those dwindling resource -- wars, not cooperation, will be far more likely.
-------------------An aside --
One of the most shameful things about "being a Canadian" is the way the Alberta & Fed. govts have cast CO2 emission and environmental issues aside and rape the tar sands for the $$ from oil. The disaster covers 140,000 square kilometres (54,000 sq mi)—an area larger than England. It's so shortsighted, and the price they get is lower than the much cheaper-to-extract lighter/cleaner Mideast oils. Since Kyoto, Canada's CO2 emissions have gone up by something like 27%, and you can attribute most of that increase to the tar sands operations -- just the heavy machinery operating at the tar sands sites is enough to make Alberta the #1 CO2 province in Canada. They don't have refineries capable of handling the bitumen sludge, which is why it has to go to special refineries in the US, mostly Texas (afaik). By 2011, annual greenhouse gas emissions from the tar sands plants alone will be over 80 million tones of CO2 equivalent - more than that produced by all of Canada's passenger cars in 2007. Then there's the staggering number of ponds and lakes in the area designated for utterly toxic tailings -- the volume is staggering, it covers 130 sq km, & grows at an incredible rate. Poisoning of water is a huge issue. (Too many sites about all this to list.)
The eco-risky 1700-mile-long Keystone pipeline to funnel tar sands oil from Canada into refineries along the Gulf Coast is dead for now, at least... but also proposed is the Enbridge Northern Gateway Pipeline, cutting across Alberta BC to the coast at Kittimat in northern BC for tankers to China & California. It will cross a thousand streams, many of them salmon-bearing streams, and the headwaters for major river systems, like the Fraser River. Any spill would be utterly devastating. Then there's the spill/accident risk to the waters of the entire west coast, which include some of the last remaining wilderness in N. Am. You remember the Valdez...
...speaking of which, how many of you know that Exxon made a $600 million deal in 2009 with California-based Synthetic Genomics Inc., a company founded by J. Craig Venter (who in 2000 mapped the collection of human genes) to develop synthetic fuel from algae that can be used in cars or aeroplanes without the need for any engine modifications? http://www.guardian.co.uk/environment/2 ... xxon-mobil
Talk about hedging your bets! What a turnaround, too, as Exxon was a major contributor to the "what global warming?" disinformation campaign.