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Declining atmospheric CO2 during the late Middle Eocene climate transition

The transition from the extreme greenhouse of the early Paleogene (∼52 Ma) to the present-day icehouse is the most prominent change in Earth’s Cenozoic climate history. During the late Middle Eocene climate transition (42-38 Ma), which preceded the onset of long-lived, continental-scale ice sheets, there is concordant evidence for brief pulses (<1 m.y. in length) of global warmth and ice sheet growth but few constraints on atmospheric CO2. Here we estimate the concentration of atmospheric CO2 during this critical interval using stomatal indices of fossil Metasequoia needles from ten levels in an exceptionally well-preserved core from the Giraffe kimberlite locality in northwestern Canada. Reconstructed CO2 concentrations are mainly between 700 to 1000 ppm, but include a secular decline to 450 ppm towards the top of the investigated section. Because the CO2 threshold for nucleating continental ice sheets at this time was ∼500 to 750 ppm, the CO2 decline is compatible with a rapid (<104 yrs) transition from warm, largely ice-free conditions to cooler climates with ice sheets. These fossils provide direct evidence that high-latitude deciduous forests thrived in the geological past under CO2 concentrations that will likely be reached within the 21st century (500-1000 ppm).