Evaluating Carbon and Climate Sensitivities of the NOAA/GFDL Earth System Model ESM2Mb to Forcing Perturbations during the Paleocene-Eocene Thermal Maximum

Abstract

The Paleocene-Eocene Thermal Maximum (PETM, ~55.8 Myr) was a period of rapid warming resulting from major changes in the carbon cycle and has been cited as the closest historical analogue to anthropogenic carbon release. Up to now, modeling studies of the PETM used either a low-resolution coupled model of the ocean and atmosphere with prescribed CO2 or CH4, or coupled climate-carbon models of intermediate complexity (i.e. simplified ocean or atmosphere with an interactive carbon cycle). In this study I create a suit of numerical experiments with the NOAA/GFDL comprehensive atmosphere-ocean coupled model, known as an Earth System Model (ESM2Mb). I analyze the output from ESM2Mb simulations with greenhouse gas forcings from the pre-PETM and PETM. I examine changes in temperature and ocean circulation patterns as well as vegetation distribution, permafrost, and carbon storage in terrestrial ecosystems from pre-PETM to PETM conditions. I found that with the present day land/sea mask and distribution of land glaciers, changes in only greenhouse gas concentrations (CO2 and CH4) from pre-PETM to PETM levels induce global warming of ~2.5-4 °C in ESM2Mb simulations, on the low range of estimates from paleo-proxies. Replacement of glacier types with an ice-free soil type increases the warming by 0.5 °C. Changes in the global temperature and terrestrial carbon storage depend on geological conditions such as glaciation. This study illustrates how models designed for studying recent historical and future climate change can capture past paleo-events, such as the PETM, and how modern day geological conditions may affect climate and carbon cycle sensitivities.