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Abstract PremiseThe Aptian–Albian (121.4–100.5 Ma) was a greenhouse period with global temperatures estimated as 10–15°C warmer than pre‐industrial conditions, so it is surprising that the most reliable CO₂estimates from this time are <1400 ppm. This low CO₂during a warm period implies a very high Earth‐system sensitivity in the range of 6 to 9°C per CO₂doubling between the Aptian‐Albian and today. MethodsWe applied a well‐vetted paleo‐CO₂proxy based on leaf gas‐exchange principles (Franks model) to twoPseudotorelliaspecies from three stratigraphically similar samples at the Tevshiin Govi lignite mine in central Mongolia (~119.7–100.5 Ma). ResultsOur median estimated CO₂concentration from the three respective samples was 2132, 2405, and 2770 ppm. The primary reason for the high estimated CO₂but with relatively large uncertainties is the very low stomatal density in both species, where small variations propagate to large changes in estimated CO₂. Indeed, we found that at least 15 leaves are required before the aggregate estimated CO₂approaches that of the full data set. ConclusionsOur three CO₂estimates all exceeded 2000 ppm, translating to an Earth‐system sensitivity (~3–5°C/CO₂doubling) that is more in keeping with the current understanding of the long‐term climate system. Because of our large sample size, the directly measured inputs did not contribute much to the overall uncertainty in estimated CO₂; instead, the inferred inputs were responsible for most of the overall uncertainty and thus should be scrutinized for their value choices. 

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.