The above figure summarizes what seems to me the key conclusions from Tripati et al, Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years from Science last December (which I somehow missed at the time, alas, but Climate Progress links to it tonight).
The paper in question describes a new method of estimating the carbon dioxide content of the atmosphere from fossilized skeletons of sea creatures buried in sedimentary rocks, validates that it is in fair agreement with the ice core record from Antarctica for the last 800,000 years, and then shows their estimates for what CO2 was like from 5 to 20 million years ago.
I made the above mashup from one of their figures, which shows their CO2 estimates. I've also shown the immediate preindustrial value (green line) and the 2010 value (red line). The only time their CO2 estimate was higher than present values was during the Middle Miocene Climate Optimum (vertical blue line), about 15 million years ago. At that time, they say, sea level was 80-130 feet higher than at present, and there were no or very small ice caps on Greenland and Antarctica. There were crocodiles in Germany.
I've also shown (orange line) the level of CO2 that the US Environmental Protection Agency estimates the atmosphere would stabilize at if Waxman-Markey was adopted and the rest of the world adopted similar goals and policies. It is just above the level that Tripati et al estimate for the mid-Miocene.
One thing to remember though is that North and South America joined up only 5 million years ago, so the ocean currents were significantly different then as well.
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