The early Eocene equable climate problem revisited

Abstract

The early Eocene "equable climate problem", i.e. warm extratropical annual mean and above-freezing winter temperatures evidenced by proxy records, has remained as one of the great unsolved problems in paleoclimate. Recent progress in modeling and in paleoclimate proxy development provides an opportunity to revisit this problem to ascertain if the current generation of models can reproduce the past climate features without extensive modification. Here we have compiled early Eocene terrestrial temperature data and compared with climate model results with a consistent and rigorous methodology. We test the hypothesis that equable climates can be explained simply as a response to increased greenhouse gas forcing within the framework of the atmospheric component of the Community Climate System Model (version 3), a climate model in common use for predicting future climate change. We find that, with suitably large radiative forcing, the model and data are in general agreement for annual mean and cold month mean temperatures, and that the pattern of high latitude amplification recorded by proxies can be reproduced

Warm ocean processes and carbon cycling in the Eocene

“…Indeed, the shape of the profiles implies that the majority of remineralization of sinking organic matter was occurring at a much shallower depth than is typical in the modern. Reconstructed sea surface temperatures offshore of Tanzania and in the Gulf of Mexico in the Eocene were only 2-4 • C greater than today, which is not sufficient to affect remineralization rates greatly, particularly at temperatures so much higher than 20 • C. However, at a depth of around 150 m, Eocene water temperatures were approximately 10 • C higher than today (figure 4) [82,83]. This means that if, for example, heterotrophic community respiration had a Q 10 value of 2 (a conservative estimate [6]), then respiration rates could quite reasonably have been twice as high at these depths as in the modern.…”

“…Modern temperature-depth measurements for this area were obtained from the World Ocean Atlas database [84]; the curve in figure 4 A logarithmic regression of the data generated by the NCAR model from the surface to a depth of approximately 300 m (consistent with the neritic zone palaeoenvironment of the Guayabal Formation) and absolute water depths determined as for Tanzania. The significance of the model results is discussed elsewhere [82,83]. The carbon and oxygen isotope cross-plots and the step-by-step reconstruction of the δ 18 C DICdepth profiles are illustrated in figure 5.…”

“…The δ 18 O-derived temperatures for each data point (derived in step 2) were input into the equation generated by this regression to give an absolute water depth. For the single dataset from the Guayabal Formation, Mexico, a temperature-depth profile was also generated using the NCAR model [83] and a climate forcing of 2240 ppmv pCO 2 for an area in the southeast part of the Gulf of Mexico from the coast down to 1500 m water depth (figure 4). Modern temperature-depth measurements for this area were obtained from the World Ocean Atlas database [84]; the curve in figure 4 A logarithmic regression of the data generated by the NCAR model from the surface to a depth of approximately 300 m (consistent with the neritic zone palaeoenvironment of the Guayabal Formation) and absolute water depths determined as for Tanzania.…”

“…In the case of Tanzania, these were generated by the National Center for Atmospheric Research (NCAR) model and also the Hadley Centre Model version 3 (HadCM3) for comparison. The NCAR model profile [83] was generated for a continental margin adjacent to the East African coast centred around a latitude of approximately 18.2 • S and for a slope extending from the water surface down to a water depth of 1500 m (figure 4). Experiments using different climate forcings of 1120, 2240 and 4480 ppmv atmospheric pCO 2 changed the absolute temperatures but not the shape of the temperature-depth profiles.…”

Was the Arctic Eocene ‘rainforest’ monsoonal? Estimates of seasonal precipitation from early Eocene megafloras from Ellesmere Island, Nunavut

“…However, other proxy and modeling data for Arctic regions implies Eocene polar rain forests consistent with no or low precipitation seasonality (Eldrett et al, 2009;Greenwood et al, 2010;Eberle and Greenwood, 2012;Huber and Goldner, 2012). The hydrological cycle of a post-PETM high latitude environment, as evidenced by climate models and paleo-precipitation reconstructions from paleobotanical proxy data, was likely a significant component in maintaining high-latitude warm and equable climates (Abbot et al, 2009;Heinemann et al, 2009;Greenwood et al, 2010;Speelman et al, 2010: Tindall et al, 2010Huber and Caballero, 2011;Huber and Goldner, 2012;Pross et al, 2012;Schubert et al, 2012;Kiehl and Shields, 2013).…”