Estimates of late Early Cretaceous atmospheric CO 2 from Mongolia based on stomatal and isotopic analysis of Pseudotorellia.

Premise: The 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 CO ₂ estimates from this time are <1400 ppm. This low CO ₂ during a warm period implies a very high Earth-system sensitivity in the range of 6 to 9°C per CO ₂ doubling between the Aptian-Albian and today.
Methods: We applied a well-vetted paleo-CO ₂ proxy based on leaf gas-exchange principles (Franks model) to two Pseudotorellia species from three stratigraphically similar samples at the Tevshiin Govi lignite mine in central Mongolia (~119.7-100.5 Ma).
Results: Our median estimated CO ₂ concentration from the three respective samples was 2132, 2405, and 2770 ppm. The primary reason for the high estimated CO ₂ but with relatively large uncertainties is the very low stomatal density in both species, where small variations propagate to large changes in estimated CO ₂ . Indeed, we found that at least 15 leaves are required before the aggregate estimated CO ₂ approaches that of the full data set.
Conclusions: Our three CO ₂ estimates all exceeded 2000 ppm, translating to an Earth-system sensitivity (~3-5°C/CO ₂ doubling) 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 CO ₂ ; instead, the inferred inputs were responsible for most of the overall uncertainty and thus should be scrutinized for their value choices.
(© 2024 Botanical Society of America.)