CO2 up, temperature up

A study of what happened with increasing CO2 and higher temperatures 40 million years ago indicates that the CO2 came first, not vice versa:

“We found a close correspondence between carbon dioxide levels and sea surface temperature over the whole period, suggesting that increased amounts of carbon dioxide in the atmosphere played a major role in global warming during the MECO,” said Bohaty.

The researchers consider it likely that elevated atmospheric carbon dioxide levels during the MECO resulted in increased global temperatures, rather than vice versa, arguing that the increase in carbon dioxide played the lead role.

“The change in carbon dioxide 40 million years ago was too large to have been the result of temperature change and associated feedbacks,” said co-lead author Peter Bijl of Utrecht University. “Such a large change in carbon dioxide certainly provides a plausible explanation for the changes in Earth’s temperature.”

And what conclusion do they reach about climate sensitivity:

The authors conclude that the climate sensitivity during the MECO led to a 2- to 5-degree C increase per doubling of atmospheric CO2.

Which is pretty much the range the IPCC expects.