Abstract
Abstract. We investigate the late Paleocene/early Eocene (PE) climate using the coupled atmosphere-ocean-sea ice model ECHAM5/MPI-OM. The surface in our PE control simulation is on average 297 K warm and ice-free, despite a moderate atmospheric CO2 concentration of 560 ppm. Compared to a pre-industrial reference simulation (PR), low latitudes are 5 to 8 K warmer, while high latitudes are up to 40 K warmer. This high-latitude amplification is in line with proxy data, yet a comparison to sea surface temperature proxy data suggests that the Arctic surface temperatures are still too low in our PE simulation. To identify the mechanisms that cause the PE-PR surface temperature differences, we fit two simple energy balance models to the ECHAM5/MPI-OM results. We find that about 2/3 of the PE-PR global mean surface temperature difference are caused by a smaller clear sky emissivity due to higher atmospheric CO2 and water vapour concentrations in PE compared to PR; 1/3 is due to a smaller planetary albedo. The reduction of the pole-to-equator temperature gradient in PE compared to PR is due to (1) the large high-latitude effect of the higher CO2 and water vapour concentrations in PE compared to PR, (2) the lower Antarctic orography, (3) the smaller surface albedo at high latitudes, and (4) longwave cloud radiative effects. Our results support the hypothesis that local radiative effects rather than increased meridional heat transports were responsible for the "equable" PE climate.
Highlights
IntroductionEvidence for the warm Paleocene/early Eocene (PE) climate is provided by a wide range of proxies. Sea surface temperatures (SSTs) inferred from oxygen isotopes, Mg/Ca ratios, and biomarkers suggest that the tropics were moderately warmer than at present, while high latitudes and especially Arctic temperatures were much warmer (e.g., Thomas et al, 2002; Tripati and Elderfield, 2004; Zachos et al, 2003, 2006; Sluijs et al, 2006). Estes and Hutchinson (1980) found warm-climate proxies such as salamanders, lizards, snakes, turtles, and an alligator on the Canadian Archipelago (see Markwick, 1994, 1998). Greenwood and Scott (1995) inferred from the existence of high-latitude palm trees that a large part of the Earth surface, including continental interiors, had climates with winter temperatures much higher than today
Simulating warm periods in Earth history is a major challenge in climate research
We find that about 2/3 of the Paleocene/early Eocene (PE)-pre-industrial reference simulation (PR) global mean surface temperature difference are caused by a smaller clear sky emissivity due to higher atmospheric CO2 and water vapour concentrations in PE compared to PR; 1/3 is due to a smaller planetary albedo
Summary
Evidence for the warm PE climate is provided by a wide range of proxies. Sea surface temperatures (SSTs) inferred from oxygen isotopes, Mg/Ca ratios, and biomarkers suggest that the tropics were moderately warmer than at present, while high latitudes and especially Arctic temperatures were much warmer (e.g., Thomas et al, 2002; Tripati and Elderfield, 2004; Zachos et al, 2003, 2006; Sluijs et al, 2006). Estes and Hutchinson (1980) found warm-climate proxies such as salamanders, lizards, snakes, turtles, and an alligator on the Canadian Archipelago (see Markwick, 1994, 1998). Greenwood and Scott (1995) inferred from the existence of high-latitude palm trees that a large part of the Earth surface, including continental interiors, had climates with winter temperatures much higher than today. Huber and Sloan (2001) revisited the hypothesis of increased oceanic heat transport, and simulated the Eocene with a fully coupled atmosphereocean-sea ice GCM, the Climate System Model (CSM) version 1.4 developed at the National Center for Atmospheric Research (NCAR). Their Eocene model solution showed a near-modern meridional temperature gradient, and a nearmodern oceanic heat transport. Kump and Pollard (2008) found that increased cloud droplet radii and precipitation efficiency could cause an additional warming and high-latitude amplification They argued that this change of the cloud properties could have been a response to a reduced global primary production by temperature stress, causing a reduction in cloud condensation nuclei concentration.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
