A new discussion paper at Climate of the Past presents a new estimate of climate sensitivity from proxy records (H/T Bob Carter). The figure it comes up with is 1.1±0.4°C. This is in line with the Forster and Gregory estimate, and far below the IPCC's figure.
Climate sensitivity is a crucial parameter in global temperature modelling. An estimate is made at the time 33.4Ma using published high-resolution deep-sea temperature proxy obtained from foraminiferal δ18O records from DSDP site 744, combined with published 5 data for atmospheric partial pressure of CO2 (pCO2) from carbonate microfossils, where 11B provides a proxy for pCO2. The pCO2 data shows a pCO2 decrease accompanying the major cooling event of about 4 C from greenhouse conditions to icecap conditions following the Eocene-Oligocene boundary (33.7 My). During the cooling pCO2 fell from 1150 to 770 ppmv. The cooling event was followed by a rapid and huge 10 increase in pCO2 back to 1130 ppmv in the space of 50 000 yr. The large pCO2 increase was accompanied by a small deep-ocean temperature increase estimated as 0.59±0.063 C. Climate sensitivity estimated from the latter is 1.1±0.4°C (66% confidence) compared with the IPCC central value of 3 C. The post Eocene-Oligocene transition (33.4 Ma) value of 1.1 C obtained here is lower than those published from 15 Holocene and Pleistocene glaciation-related temperature data (800 Kya to present) but is of similar order to sensitivity estimates published from satellite observations of tropospheric and sea-surface temperature variations. The value of 1.1 C is grossly different from estimates up to 9 C published from paleo-temperature studies of Pliocene (3 to 4 Mya) age sediments. The range of apparent climate sensitivity values available 20 from paleo-temperature data suggests that either feedback mechanisms vary widely for the different measurement conditions, or additional factors beyond currently used feedbacks are affecting global temperature-CO2 relationships.
Climate of the Past has an open review process. I imagine this will be, ahem, colourful.