Proxy-Informed Model Estimates of Early Paleogene Climate

As global temperatures approach levels unprecedented in human history, past warm periods offer insights into climate variations associated with increasing greenhouse gas concentrations. The Early Paleogene (65-49 million years ago) is a prime example of a warmer climate state, with average temperatures 10 to 19 K higher than today. However, our understanding of this period is limited by sparse proxy data and uncertainties in key model parameters, preventing a comprehensive analysis of its climate. In this study we use a new compilation of temperature proxies to select the least biased simulations from the Deep-Time model intercomparison project for each of six Early Paleogene time periods. These ensembles align well with proxy temperature and precipitation data across all six periods of the Early Paleogene. This method also provides an independent estimation of CO2 concentrations, suggesting that Early Paleogene CO2 levels were around 600-1200 ppm, except for the warmest periods, the Paleocene Eocene Thermal Maximum (PETM) and Early Eocene Climatic Optimum where CO2 concentrations were 1100-1800 and 900-1500 ppm, respectively. The ensembles suggest temperatures in high-latitude regions increased by up to 13 K during the PETM hyperthermal, while tropical temperatures rose by approximately 1 K, indicating substantial polar amplification during the nearly ice-free Early Paleogene. Precipitation increased almost everywhere with warming, at a global mean rate of 2% per Kelvin of global warming, matching theorized and modeled estimates from the modern climate. Additionally, precipitation changes suggest a poleward shift in global atmospheric circulation with warming, aligning with theories of modern climate change.