Doubling of atmospheric carbon dioxide is an oft-talked-about threshold, and today's climate models include accepted values for the climate's sensitivity to doubling. Using these accepted values and the PETM carbon data, the researchers found that the models could only explain about half of the warming that Earth experienced 55 million years ago.
Which is to say that something else must have caused the other half of the PETM warming. Co-author Richard Zeeb explains:
But Zeebe said this [the release of CO2] could only explain a 1 to 3.5 degree Celsius rise in temperatures, adding that a commonly accepted scientific range for a doubling of CO2 is between 1.5 and 4.5 degrees Celsius.
This meant other factors must have been at work to drive up temperatures between 5 and 9 degrees Celsius.
And if there are unknown factors that contributed to the PETM warming being greater than current models predict, then it is quite possible that future warming will be greater than what current models predict, as these unknown factors add their weight to the equation:
"If this additional warming which we do not really understand, was caused as a response to the CO2 warming, then there is a chance that also a future warming could be more intense than people anticipate right now," Zeebe said.
In addition, and contra Singer, who argues on CCNET that
Their discussion simply *assumes* that CO2 should be the cause of the sudden warming 55 my ago.
...the paper, more specifically Zeebe's later discussion with Reuters, makes no such assumption, at least if "cause" = "PETM trigger event".
"There may have been an initial trigger," he told Reuters on Wednesday from Hawaii. This could be a deep ocean warming that caused a catastrophic release of methane from hydrate deposits under the seabed.
...which is one of the standard theories in the relevant PETM literature.
