A reconstruction of the Earth’s climatic history during a key hot period 55 million years ago has highlighted a yawning gap in our understanding: this period’s rise in carbon dioxide accounts for just half of its warming. Some as-yet-unidentified climate feedbacks could be at work, the scientists behind the research conclude.
The era under scrutiny is the Palaeocene-Eocene Thermal Maximum (PETM). Paleoclimatologists believe that the PETM could mimic our own future climate, because it’s thought to have kicked off with a pulse of carbon dioxide roughly equivalent to what humans are currently pumping out by burning fossil fuels…
According to the IPCC’s best guess at climate sensitivity, that 70% rise should have pushed up global temperatures 3.5 degrees Celsius at most. Other proxy records indicate, though, that temperatures soared by 5 to 9 degrees. In other words, the consensus climate sensitivity – the value, devilishly hard to pin down, for how much warming will result from a given greenhouse gas increase – doesn’t seem to be holding.
This is yet another example of the conservative nature of the IPCC reports. But that is to be expected from a consensus document.
And this isn’t the only research indicating the the IPCC’s estimate of climate sensitivity might be low.
UPDATE: It looks like deniers are trumpeting this study, because it highlights that the current crop of models are uncertain. Unfortunately for them the uncertainty highlighted by this study is in the wrong direction, it indicates that the models may be underestimating the expected warming, and as I pointed out in my post this isn’t even a surprising revelation.
The IPCC estimate of climate sensitivity is commonly called the Charney sensitivity; it excludes a bunch of long term feedbacks. This new study (along with other studies, such as Hansen’s 350 ppm paper) provide evidence that the Charney estimate of sensitivity is low. How low? I don’t think we can answer that yet with much confidence, but as was pointed out on RealClimate when the Hansen paper was making the rounds:
However, even with the (substantial) uncertainties in the calculations and underlying assumptions, the conclusion that the Earth System sensitivity [which includes these slow feedbacks] is greater than the Charney sensitivity [which doesn’t] is probably robust. And that is a concern for any policy based on a stabilization scenario significantly above where we are now.
So basically this study says that the models may be underestimating the expected warming. And yet deniers are excited by it? Obviously they didn’t read/understand the study. And they actually have the audacity to complain when they are not taken seriously!