Changing atmospheric CO₂ concentration was the primary driver of early Cenozoic climate

The Early Eocene Climate Optimum (EECO; ~51-53 million years ago (Ma)), was the warmest interval of the last 65 million years with mean annual surface air temperature over 10 ^oC warmer than pre-industrial. Subsequent global cooling in the middle and late Eocene, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene (~ 33.6 Ma). However, existing estimates place atmospheric carbon dioxide (CO₂) levels during the Eocene at 500-3000 ppm, and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we utilise recent analytical and methodological developments to generate a new high fidelity record of CO₂ concentrations using the boron isotope (δ¹¹Β) composition of well-preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates6. Although species-level uncertainties make absolute values difficult to firmly constrain, CO₂ concentrations during the EECO were ~1400 ppm. The relative decline in CO₂ through the Eocene is more robustly constrained at ~50%, with further decline into the Oligocene. Provided the latitudinal dependency of sea-surface temperature change for a given climate forcing was similar in the Eocene to the late Quaternary, this CO₂ decline was sufficient to drive the well documented high and low latitude cooling that occurred through the Eocene. Once the change in global temperature between the pre-industrial and Eocene due to the action of all known slow feedbacks (apart from those associated with the carbon cycle) are removed, both the EECO and late Eocene exhibit Equilibrium Climate Sensitivity relative to pre-industrial of 2.1-4.6 ^oC per CO₂ doubling (66% confidence), which is similar to the canonical range (1.5-4.5 ^oC), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by elevated CO₂ concentrations, and that climate sensitivity was relatively constant through this period.