Continuous sterane and phytane δC record reveals a substantial pCO decline since the mid-Miocene.

Constraining the relationship between temperature and atmospheric concentrations of carbon dioxide (pCO) is essential to model near-future climate. Here, we reconstruct pCO values over the past 15 million years (Myr), providing a series of analogues for possible near-future temperatures and pCO, from a single continuous site (DSDP Site 467, California coast). We reconstruct pCO values using sterane and phytane, compounds that many phytoplankton produce and then become fossilised in sediment. From 15.0-0.3 Myr ago, our reconstructed pCO values steadily decline from 650 ± 150 to 280 ± 75 ppmv, mirroring global temperature decline. Using our new range of pCO values, we calculate average Earth system sensitivity and equilibrium climate sensitivity, resulting in 13.9 °C and 7.2 °C per doubling of pCO, respectively. These values are significantly higher than IPCC global warming estimations, consistent or higher than some recent state-of-the-art climate models, and consistent with other proxy-based estimates.