Whoa this is pretty much the worst news I've read on the subject since I first started reading.

Just published in the online edition of Science

http://www.sciencemag.org/cgi/content/abstract/1178296

The CO2 content of the atmosphere has varied cyclically between ~180 and ~280 ppmv over the last 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, pCO2 is much less certain and the relationship between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the last 20 million years (myr). During the Middle Miocene, when temperatures were ~3 to 6°C warmer and sea level 25 to 40 meters higher than present, pCO2 was similar to modern levels. Decreases in pCO2 were synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago; Ma) and Late Pliocene (~3.3 to -2.4 Ma).
And their press release

http://www.climate.ucla.edu/news/art...?parentid=4676

"The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland," said the paper's lead author, Aradhna Tripati, a UCLA assistant professor in the department of Earth and space sciences and the department of atmospheric and oceanic sciences.
"A slightly shocking finding," Tripati said, "is that the only time in the last 20 million years that we find evidence for carbon dioxide levels similar to the modern level of 387 parts per million was 15 to 20 million years ago, when the planet was dramatically different.
Some projections show carbon dioxide levels rising as high as 600 or even 900 parts per million in the next century if no action is taken to reduce carbon dioxide, Tripati said. Such levels may have been reached on Earth 50 million years ago or earlier


These projections actually make an even worse prediction that the ones I've been making, and mine have been very dire.

It makes complete sense though. Fossil fuels are a very gradual collection of carbon normally sequestered into living beings which were never really atmospheric except maybe when the Earth was very young, and when we burn them, we release all this 'unnatural' material into the atmosphere all at once.

For a while, I've speculated that Fermi's Paradox (basically the question of why we don't see extraterrestrial beings anywhere) is because all species of advanced intelligence did the same thing we're doing, and thus cooked themselves to death, and so there really never has been chance for super advanced societies in the first place. It's actually a rather plausible scenario since planets with advanced species would also very likely have loads of fossil fuels, and they would very likely get discovered, and they would very likely get exploited, etc...

Anyways, this is the first bit of material I've seen that puts a very strong case for AGW taking us further than we ever imagined