Climate Response at the Paleocene–Eocene Thermal Maximum to Greenhouse Gas Forcing—A Model Study with CCSM3

Abstract

The Paleocene-Eocene Thermal Maximum (PETM; 55 Ma) is of particular interest since it is regarded as a suitable analog to future climate change. In this study, the PETM climate is investigated using the Community Climate System Model (CCSM3) with atmospheric CO 2 concentrations of 43, 83, and 163 the preindustrial value.Simulated climate change from 43 to 83 atmospheric CO 2 concentration, possibly corresponding to an environmental precursor of the PETM event, leads to a warming of the North Atlantic Ocean Intermediate-Water masses, thus lowering the critical depth for methane hydrate destabilization by ;500 m. A further increase from 83 to 163CO 2 , analogous to a possible massive methane hydrate release, results in global oceanic warming and stratification. The increase in the radiative surface warming, especially at high latitudes, is partially offset by a decrease in the ocean heat transport due to a reduced overturning circulation. Surface temperature values simulated in the 163CO 2 PETM run represent the closest match to surface temperature reconstructions from proxies for this period.Simulated PETM precipitation, characterized by a slight northward shift of the intertropical convergence zone, increases at higher CO 2 concentrations, especially for the northern midlatitudes as well as the high latitudes in both hemispheres. Data-inferred precipitation values and gradients for North America and Spain, for instance, are in good agreement with the 163CO 2 simulation. Increasing atmospheric CO 2 concentrations might also have favored the release of terrestrial methane through warmer and wetter conditions over land, thus reinforcing the greenhouse gas concentration increase.

Investigating vegetation–climate feedbacks during the early Eocene

“…that cause this shallow temperature gradient are unknown or not fully understood so cannot be accounted for (Winguth et al, 2010;Sloan and Morrill, 1998).…”

“…Until recently, models have had great difficulty in replicating this feature of the climate, instead producing temperatures that are cooler than indicated by the data in the high latitudes or temperatures that are higher than indicated by the proxies in the tropics (e.g. Heinemann et al, 2009;Winguth et al, 2010;Shellito et al, 2009;Roberts et al, 2009;…”

Ocean biogeochemistry in the warm climate of the late Paleocene

“…Hitherto, the focus of modeling the late Paleocene with complex earth system models (ESM) was set to the physical ocean and the atmospheric system (Huber and Sloan, 2001;Heinemann et al, 2009;Winguth et al, 2010). Studies of the PETM background climate show a wide range of intermodel variability, using prescribed atmospheric CO 2 concentrations ranging from 2× to 16× pre-industrial CO 2 (Lunt et al, 2012).…”

Low clouds suppress Arctic air formation and amplify high-latitude continental winter warming

“…Climate models can simulate warm conditions over the ocean, but they have difficulty simulating continental warmth away from the moderating effects of the ocean, especially if tropical warming is constrained to be K10°C. Although recent work suggests a relaxation of such tropical constraints (2), model−data agreement has been found only for model CO 2 concentrations that seem unrealistically high, and the mechanisms that maintain high-latitude warmth over land remain poorly understood (2,3). Previous proposed mechanisms to explain the overall reduction of the equator−pole temperature contrast in equable climates include polar stratospheric clouds (4,5), dramatic expansion of the Hadley circulation (6), increased poleward ocean heat transport due to ocean mixing by stronger tropical cyclones (7,8), and a convective cloud feedback (9).…”