LAUSR

Proxy estimates of atmospheric CO2 are necessary to reconstruct Earth’s climate history. Confidence in paleo-CO2 estimates can be increased by comparing results from multiple proxies at a single site, but so far this strategy has been implemented only for marine-based techniques. Here we present CO2 estimates for the well-studied early Paleocene Castle Rock site in Colorado using four paleobotanical proxies. Median estimates range from 470 to 813 ppm, demonstrating fair correspondence. The synthesis yields a median of 616 ppm (352–1110 ppm at 95% confidence), considerably higher than previous early Paleocene CO2 estimates (~300 ppm). Ash bed geochronology by the high-precision U-Pbmethod places the Castle Rock assemblage at 63.844 ± 0.097 Ma (fully propagated 2σ error). When these results are placed into the broader context of other Cenozoic CO2 estimates from plant-gas-exchange approaches and coeval estimates of global mean surface temperature, a pattern emerges of an Earth system sensitivity around 3 °C per CO2 doubling during the Paleocene and Eocene, a time with little land ice, then steepening to>7 °C after the Eocene once land ice was present on Antarctica. Plain Language Summary As atmospheric CO2 continues to increase, we enter a climate state whose analog in terms of CO2 concentration is found millions of years ago. Information about climate from such distant times is only available to us via proxy methods (i.e., indicators of climate recorded in ancient rocks and fossils); increasing confidence in proxy results is therefore a high priority. Here we compare four different CO2 proxy methods using plant fossils from an exceptionally diverse rainforest that existed near present-day Denver, Colorado, 63.8 million years ago. Estimates are largely congruent and higher than previously thought (~600 ppm). The higher CO2 levels during this warm period are in better agreement with the current understanding of long-term Earth system climate sensitivity, and results from the newer gas-exchange proxy methods paint a coherent picture of Earth system sensitivity evolution over the Cenozoic.