Estimate of climate sensitivity from carbonate microfossils dated near the Eocene-Oligocene global cooling

Abstract. Climate sensitivity is a crucial parameter in global temperature modelling. An estimate is made at the time 33.4 Ma using published high-resolution deep-sea temperature proxy obtained from foraminiferal δ¹⁸O records from DSDP site 744, combined with published data for atmospheric partial pressure of CO₂ (pCO₂) from carbonate microfossils, where δ¹¹B 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 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 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 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.