Abstract
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.