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The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO₂record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO₂thresholds in biological and cryosphere evolution.

Throughout the Phanerozoic, estimated CO₂levels from CO₂proxies generally correlate well with independent estimates of temperature. However, some proxy estimates of atmospheric CO₂during the Late Cretaceous and early Paleocene are low (<400 ppm), seemingly at odds with elevated sea surface temperature. Here we evaluate early Paleocene CO₂by applying a leaf gas‐exchange model toPlatanitesleaves of four early Paleocene localities from the San Juan Basin, New Mexico (65.66–64.59 Ma). We first calibrate the model on two modernPlatanusspecies,Platanus occidentalisandP. × acerifolia, where we find the leaf gas‐exchange model accurately predicts present‐day CO₂, with a mean error rate between 5% and 14%. Applying the model to the early Paleocene, we find CO₂varies between ∼660 and 1,140 ppm. These estimates are consistent with more recent CO₂estimates from boron, leaf gas‐exchange, liverwort, and paleosol proxies that all suggest moderate to elevated levels of CO₂during the Late Cretaceous and early Paleocene. These levels of atmospheric CO₂are more in keeping with the elevated temperature during this period.