Early Miocene CO₂ estimates from a Neotropical fossil leaf assemblage exceed 400 ppm

The global climate during the early Miocene was warmer than the present and preceded the even warmer middle Miocene climatic optimum. The paleo-CO₂ records for this interval suggest paradoxically low concentrations (<450 ppm) that are difficult to reconcile with a warmer-than-present global climate.

In this study, we use a leaf gas-exchange model to estimate CO₂ concentrations using stomatal characteristics of fossil leaves from a late early Miocene Neotropical assemblage from Panama that we date to 18.01 ± 0.17 Ma via ²³⁸ U/²⁰⁶ Pb zircon geochronology. We first validated the model for Neotropical environments by estimating CO₂ from canopy leaves of 21 extant species in a natural Panamanian forest and from leaves of seven Neotropical species in greenhouse experiments at 400 and 700 ppm.

The results showed that the most probable combined CO₂ estimate from the natural forests and 400 ppm experiments is 475 ppm, and for the 700 ppm experiments is 665 ppm. CO₂ estimates from the five fossil species exhibit bimodality, with two species most consistent with a low mode (528 ppm) and three with a high mode (912 ppm).

Despite uncertainties, it is very likely (at >95% confidence) that CO₂ during the late early Miocene exceeded 400 ppm. These results revise upwards the likely CO₂ concentration at this time, more in keeping with a CO₂ -forced greenhouse climate.