Since it’s the school holidays and I’m spending some time at home, I thought I might write a short post about a recent paper by Martinez-Boti et al. (2015), that tries to compare the climate sensitivity during the Pliocene and during the Pleistocene. The Pliocene is the period from about 5.3 to 2.6 million years ago and is an interesting period since the conditions are thought to have been very similar to what we have today. The Pleistocene is the period after the Pliocene, ending at the start of the Holocene; about 11,700 years ago. During the Pleistocene, there were extended periods of glaciation.
This is not my area of expertise, so I may get some of this wrong, but from reading the paper, it seems that there has been a suggestion that climate sensitivity is higher in warmer climates than in cooler. This paper suggests that this is not the case. As I understand it, they determine the response to a change in CO2 only, which means that everything else is a feedback and this then gives the Earth System Sensitivity (ESS). They then use estimates for sea-level rise, to determine the contribution due to ice-albedo feedback which – if they remove – then gives the Equilibrium Climate Sensitivity (ECS).The basic result is shown in the figure on the right. The term represents the global mean annual air surface temperature change, and the x-axis is climate sensitivity in K/Wm-2. The red graphs are for the Pleistocene, while the blue re for the Pliocene. What seems clear is that the correction for ice-albedo feedback, brings the ECS for both periods into the same range (bottom two panels).
The paper concludes with
Our findings suggest that, if the Earth system behaves in a similar fashion to how it did during the Pliocene as it continues to warm in the coming years, an ECS of 1.5–4.5 K per CO2 doubling probably provides a reliable description of the Earth’s equilibrium temperature response to climate forcing, at least for global temperature rise up to 3 K above the pre-industrial level.
once all feedbacks have played out for future CO doubling, ESS will very probably (95% confidence) be < 5.2 K and will probably (68% confidence) fall within a range of 3.0–4.4 K.
So, it seems that this paper is suggesting that there isn’t any evidence for a different ECS in warmer and cooler climates, but that the estimates for the ECS during both the Pliocene and Pleistocene are consistent with the recent IPCC range. It also suggests that slower feedbacks could produce an ESS as high as 5K, with a likely range of 3K – 4.4K. So, no great surprises, but interesting nonetheless.