“…the equilibrium climate change associated with an increase in CO2 is likely to be significantly larger than has traditionally been estimated.” Lunt et al.
We last visited the Pliocene several months ago . The Pliocene is of special importance to us because the warm climates during the middle Pliocene between 3 and 3.3 million years ago (Myr) and the early Pliocene between 4 and 4.2 Myr are similar to what we expect to occur this century due to human emissions of greenhouse gases. Peak atmospheric carbon dioxide was about 400 parts per million by Volume (ppmV). The current level is 393 ppmV but climbing. As we described earlier, the continents had nearly the same positions as today and the sun’s luminosity was nearly the same as well. By studying the Pliocene, palaeoclimatologists hope to better assess the Earth’s climate sensitivity to the radiative forcing caused by increases in greenhouse gases.
While remarkably similar to current conditions, the mid-Pliocene was between 2.4 and 2.9oC warmer than pre-industrial climate and the early Pliocene was about 4oC warmer on average than the pre-industrial climate. Sea levels were 25 meters higher than today due to smaller ice-sheets in Greenland and Antarctica. These temperatures are much hotter than those estimated by the IPCC for the same atmospheric carbon dioxide levels.
The Charney Climate Sensitivity (CS) is the expected warming caused by a doubling of atmospheric carbon dioxide after the climate equilibrates. The value of CS is assumed to be between 1.5 and 4.5 degrees Centigrade with the most likely value considered to be 3oC. However, CS only takes into account fast short-term feedbacks acting on timescales of years to decades such as water vapor, snow albedo, sea-ice albedo and clouds. Slow long-term feedbacks are not included in the estimates of Charney Sensitivity. Thus the CS will underestimate the eventual warming of Earth to a doubling of atmospheric carbon dioxide. Long-term feedbacks include changes in dust and other aerosols, vegetation, ice sheets and ocean circulation. These effects will all modify the eventual equilibrium surface temperature. The long-term temperature response to a doubling of atmospheric carbon dioxide is called the Earth system sensitivity or ESS. By studying the sudden warming concluding the Last Glacial Maximum (LGM) Hansen has estimated the ESS to be closer to 6oC or 11oF . Hansen was kind enough to acknowledge me for reviewing this paper prior to its publication.
Two recent papers attempt to better estimate ESS by studying the Pliocene climate. Lunt et al.  expand work originally begun by Harry Dowsett and others at the USGS  called Pliocene Research, Interpretation and Synoptic Mapping (PRISM) Project. Dowsett is a co-author of the Lund paper. The second paper is by Mark Pagani et al. . The Lunt paper examines only the mid-Pliocene warming. It uses the USGS extensive Pliocene data to reconstruct the boundary conditions such as surface topography or orography  as well as sea surface temperature, sea level, ice sheet extent, vegetation, etc. In all, PRISM comprises 28 global data sets all available for free downloading from the USGS. These boundary conditions are then used in simulations using the HadCM3, the UK Met Office fully coupled atmosphere-ocean general circulation model (GCM). The advantages of using a GCM are that the effects of different forcings can be isolated. Two forcings were known to play a role in mid-Pliocene climate: an increase in atmospheric carbon dioxide due to an imbalance between tectonic related emissions and weathering and orographic changes. Orbital forcings are assumed to be minor during this time. The Lunt paper concludes that Earth system sensitivity is between 30 and 50% higher than the value assumed by the IPCC of 3oC.
The Pagani paper studies both the mid and early Pliocene climate. Their results are shown in Figure 1. They show that if changes in carbon dioxide and associated feedbacks were the primary agents forcing climate sensitivity for the middle and early Pliocene and if estimates of global temperature are correct then the ESS for a doubling of atmospheric CO2 would be 7.1 +/- 1.0oC and 9.6 +/-1.3oC. Unlike Lunt et al., they do not take into account the impacts of orographic changes on the climate.
Figure 1, Estimated CO2 trends considering probable oceanographic changes at each site. Each line represents a modified CO2. Vertical grey lines intersect CO2 consentrations at 3.0-3.3 Myr and 4.0-4.2 Myr time intervals representing the Earth-system climate sensitivity estimates.
While differing in their results, both papers agree that equilibrium climate sensitivity may be higher than the consensus view and we may see an unexpected increase once the oceans warm up or equilibrate to the new higher level of carbon dioxide. It is also sobering to appreciate that sea levels were 25 meters higher than they are today for the same level of atmospheric carbon dioxide. Hansen reminds us that forming glaciers and ice sheets is a dry process and takes millennia but melting glaciers is a wet process which we don’t yet fully understand but can proceed rapidly. The Pagani paper observes that the east-west sea surface temperature gradient of about 1.5oC resembles a possible permanent el Nino-like state. The record warming during 1998 and earlier this year occurred during el-Nino conditions. These estimates of ESS ranging from 30% higher to more than double the CS value are worrisome especially considering the rate of increase in atmospheric CO2 today is far more dramatic than at any time in Earth’s history and we don’t appear to be willing to slow down greenhouse gas emissions even by a little. Furthermore, we now have an entire political party, Republicans, completely disassociating themselves from any attempt to comprehend reality.
To view maps of the locations of continents in the Earth’s past see Chirstopher R. Scotese’s fascinating web site .
 Hansen, J., Sato, M., Kharechal, P., Beerling, D., Berner, R., Masson-Delmotte, V., Pagani, M., Raymo, M., Royer, D. L., and Zachos, J. C., Target Atmospheric CO2: Where Should Humanity Aim?, The Open Atmospheric Science Journal, 2008, 2, 217-231.
 Daniel J. Lunt, Alan M. Haywood, Gavin A. Schmidt, Ulrich Salzmann, Paul J. Valdes, and Harry J. Dowsett, Earth system sensitivity inferred from Pliocene modelling and data, published online: 6 Decembder 2009 | DOI: 10.1038/NGEO706
 Harry J. Dowsett, John A. Barron, Richard Z. Poore, Robert S. Thompson, Thomas M. Cronin, Scott E. Ishman and Debra A. Willard, Middle Pliocene Paleoenvironmental Reconstruction: PRISM2, U.S. GEOLOGICAL SURVEY OPEN FILE REPORT 99-535, 1999.
 Mark Pagani, Zhonghui Liu, Jonathan LaRiviere and Ana Christina Ravelo, High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations, published online: 20 December 2009 | DOI: 10.1038/NGEO724