What was done
The authors analyzed boron isotope composition in planktonic foraminifera to reconstruct the pH-depth profile of ancient seawater.  Using the pH data they were then able to estimate atmospheric CO2 concentrations during the middle Eocene some 43 million years ago.

What was learned
Atmospheric carbon dioxide concentrations during the middle Eocene were determined to lie between 180 and 550 ppm, with a best estimate of 385 ppm.

What it means
The middle Eocene climate has been estimated to have been as much as 5°C warmer than today.  Conventional wisdom has it that such temperatures were the product of an enhanced greenhouse effect, where atmospheric CO2 concentrations were five times higher than they are presently.  If the authors' calculation of middle Eocene atmospheric CO2 concentration is correct, then, in their own words, "it implies either that earth's climate is very sensitive to small changes in pCO2, or that the global cooling since the Eocene was not driven primarily by changes in pCO2, but rather reflects reorganization of ocean circulation resulting from tectonic opening and closing of oceanic gateways."

Given the recent flood of articles we have reviewed that indicate a decoupling between atmospheric CO2 concentration and global temperature (see our reviews on Nearly Half a Million Years of Climate and CO2, Miocene Climate and CO2, CO2 and Temperature: Ice Core Correlations, and The Holocene Climatic Optimum: Paradise Lost?), the second of these conclusions seems most likely to be correct.  And, we would add, this finding further weakens any claim of future CO2-induced global warming that might result from the burning of fossil fuels.  Indeed, in the words of paleoclimatologist Thomas Crowley, as quoted in Science (Vol. 284, p.1745), "it could be the whole carbon dioxide paradigm is crumbling," at least, as news writer Richard Kerr adds, "when it comes to explaining very long-term climate change."  We predict that the fall of the paradigm will someday extend to shorter time scales as well.