Ancient siderites reveal hot and humid super-greenhouse climate

Joep van Dijk, Alvaro Bremer Fernandez, Stefano M Bernasconi, Jeremy Caves-Rugenstein, Simon R Passey, Tim White

Earth’s climate is warming as the rise in atmospheric CO2 (pCO2) contributes to increased radiative forcing. State-of-the-art models calculate a wide range in Earth’s climate sensitivities due to increasing pCO2, and, in particular, the mechanisms responsible for amplification of high latitude temperatures remain highly debated. The geological record provides a means to evaluate the consequences of high radiative forcing on Earth’s climate. Here we present clumped (Δ47) and oxygen (δ18O) isotope data from latest Paleocene/earliest Eocene (LPEE; 57-55 million years ago) pedogenic siderites, a time when pCO2 peaked between 1400 and 4000 ppm. Continental mean annual temperature reached 41 °C in the equatorial tropics, and summer temperatures reached 23 °C in the Arctic. Reconstruction of the oxygen isotopic composition of precipitation reveal that the hot LPEE climate was characterized by a globally averaged increase in specific humidity with a corresponding increase in the average residence time of atmospheric moisture and a decrease in the subtropical-to-polar specific humidity gradient compared to the present-day. Pedogenic siderite data from other ancient super-greenhouse periods support the evidence that with higher global mean temperatures and a decreased meridional temperature gradient the increase in specific humidity is subject to polar amplification.