A recent paper by Carolyn Snyder published in Nature this week presented a record of global average surface temperature (GAST) reconstructed from a compilation of sea surface temperature records. This is a great approach and the GAST record, and the techniques used, will be very valuable in efforts to use the geological past to understand how the climate system works.
However, towards the end of the abstract, and the focus of the press release, was a statement about Earth System Sensitivity (the temperature change in response to CO2 change – subtly different to equilibrium climate sensitivity – see here for detail). Earth system sensitivity, unlike equilibrium climate sensitivity, includes the action of all climate feedbacks (except those relating to the carbon cycle) and is very dependent on the background climate state. Synder calculated that ESS was around 9 oC per CO2 doubling over the last 800 thousand years. As discussed here, this is not a relevant measure of how our future climate system will behave because the high ESS during the cold Pleistocene was driven in part by the changes in surface albedo caused by the waxing and waning of the continental ice sheets of North America and Europe – ice sheets that are no longer present on the Earth so are not able to act as a feedback in the future. ESS for the future is therefore much lower than what Synder calculates. In a paper last year we @thefosterlab used new Pliocene CO2 data from the boron isotope proxy to show that a future relevant ESS is more likely in the range 2.2 to 5.2 oC per CO2 doubling (which is in good agreement with modelling studies; Lunt et al. 2010). This suggests that estimates of equilibrium climate sensitivity from the IPCC (1.5 to 4.5 oC per CO2 doubling) do adequately describe the long-term (1000 year) response of the future climate system to CO2 change because the northern hemisphere ice sheets are currently at close to their minimum extent.
The figure above is from Martinez-Boti et al. (2015) showing the Sensitivity Parameter for the Pleistocene (red) and Pliocene (blue) for two different temperature records - mean surface air temperature (left) and sea surface temperature (right). The Sensitivity Parameter describes the temperature change for a given change in climate forcing. Panels (a) and (c) show the sensitivity parameter due to CO2 change only. ESS = this sensitivty parameter multiplied by 3.7 (the radiative forcing from a doubling of CO2). ECS is approximately the senstivity parameter in panels e and g multipled by 3.7 (because here we also take into account the radiative effect of the ice sheets). As in the Synder paper we also found a high ESS for the Pleistocene because of the large ice sheets present (ca. 2 x 3.7 = 7.4 oC per CO2 doubling). The Pliocene however lacked large ice sheets and ESS is about equal to ECS, which both fall in the IPCC range (ca. 1 x 3.7 = 3.7 oC per CO2 doubling).