Carbon Dioxide at Miocene levels

I made an observation at Kunstler's blog that bears repeating.

[I]f there is an endless supply of oil, then we'll be content to burn us [it--my Freudian slip was showing] until we reach Jurassic level[s] of carbon dioxide and global warm temperatures. As I also tell my students, that was a great world for dinosaurs, but there weren't any people in it.

It turned out my comment was even timelier than I thought, as Andrew Freedman at Climate Central reported The Last Time CO2 Was This High, Humans Didn’t Exist.

The last time there was this much carbon dioxide (CO2) in the Earth's atmosphere, modern humans didn't exist. Megatoothed sharks prowled the oceans, the world's seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now.

As we near the record for the highest CO2 concentration in human history — 400 parts per million — climate scientists worry about where we were then, and where we're rapidly headed now.

According to data gathered at the Mauna Loa Observatory in Hawaii, the 400 ppm mark may briefly be exceeded this month, when CO2 typically hits a seasonal peak in the Northern Hemisphere, although it is more likely to take a couple more years until it stays above that threshold, according to Ralph Keeling, a researcher at the Scripps Institute of Oceanography.
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 The news that CO2 is near 400 ppm for the first time highlights a question that scientists have been investigating using a variety of methods: when was the last time that CO2 levels were this high, and what was the climate like back then?

There is no single, agreed-upon answer to those questions as studies show a wide date range from between 800,000 to 15 million years ago. The most direct evidence comes from tiny bubbles of ancient air trapped in the vast ice sheets of Antarctica. By drilling for ice cores and analyzing the air bubbles, scientists have found that, at no point during at least the past 800,000 years have atmospheric CO2 levels been as high as they are now.

That means that in the entire history of human civilization, CO2 levels have never been this high.

Other research, though, shows that you have to go back much farther in time, well beyond 800,000 years ago, to find an instance where CO2 was sustained at 400 ppm or greater.

For a 2009 study, published in the journal Science, scientists analyzed shells in deep sea sediments to estimate past CO2 levels, and found that CO2 levels have not been as high as they are now for at least the past 10 to 15 million years, during the Miocene epoch.

As I wrote in the comments to the repost of this report over at Desdemona Despair, "If the 10-15 million year figure is right, the last time CO2 was this high, Antarctica still had forests."  Not only did humans not exist at this time, current evidence indicates that great apes had barely diverged from gibbons.  It may not be the Jurassic, but the Miocene is certainly far enough in the past to worry me!