New paper finds climate sensitivity to CO2 is about 63% less than IPCC claims

A paper under review for Climate of the Past finds on the basis of reconstructed temperatures and CO2 during the Oligocene epoch that the climate response to increased CO2 is only about 1/3 of that estimated by the IPCC. According to the author,

"Climate sensitivity estimated from the [fossil evidence] is 1.1 ± 0.4 °C ...compared with the IPCC central value of 3 °C."

In addition, the author finds that the low climate sensitivity of 1.1°C per doubling of CO2 levels is confirmed by being "of similar order to sensitivity estimates published from satellite observations of tropospheric and sea-surface temperature variations."

Clim. Past Discuss., 8, 4923-4939, 2012
www.clim-past-discuss.net/8/4923/2012/
doi:10.5194/cpd-8-4923-2012
Estimate of climate sensitivity from carbonate microfossils dated near the Eocene-Oligocene global cooling

M. W. Asten
School of Geosciences, Monash University, Melbourne, VIC 3800, Australia
 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.

 Discussion Paper (PDF, 1101 KB)   Interactive Discussion (Final Response, 13 Comments)   Manuscript under review for CP