TIGHT COUPLING BETWEEN ATMOSPHERIC ρCO2 AND TEMPERATURE CHANGE DURING THE LATE TRIASSIC: OBSERVATIONAL EVIDENCE FOR ENHANCED CLIMATE SENSITIVITY IN A HOTHOUSE STATE

Earth system sensitivity (ESS) is the change in global equilibrium surface temperature per doubling of atmospheric ρCO₂. Climate models and paleodata estimate modern climate sensitivity to a value of 3 °C with a range of 2-4.5 °C per doubling of CO₂ based on fast-feedback processes (Charney sensitivity). However, fast-feedback processes may not be representative of ice-free hothouse states that have dominated most of Earth history where sensitivity may be higher. Few opportunities exist to empirically determine climate sensitivity during a hothouse state based on contemporaneous observations of ρCO₂ and temperature. Here, we present evidence for tight coupling between ρCO₂ and temperature during the Late Triassic (Norian through Rhaetian) from the Newark basin and Lagonegro/Sicani basins, respectively. Detailed assessment of magnetostratigraphy and biostratigraphy allows for correlation between the Lagonegro and Sicani basins (Italy), which are magnetostratigraphically correlated to the Newark basin. Temperature is calculated from δ¹⁸O values of conodont apatite published from the Lagonegro and Sicani basins, while ρCO₂ estimates are from pedogenic carbonates in the Newark basin. We find a distinct rise and subsequent fall in atmospheric ρCO₂ that is precisely mirrored by a contemporaneous rise and fall in temperature. Between 212-209 Ma, we observe a concomitant increase in ρCO₂ (1900 to 4800 ppm) and temperature (20 to 27°C), followed by a more protracted concomitant decrease in atmospheric ρCO₂ (4800 to 2200 ppm) and temperature (27 to 21°C) from 209-202 Ma. We use simple numerical methods to calculate ESS for the Late Triassic from these complementary data sets and find that sensitivity through both a doubling and subsequent halving of pCO₂ are on the order of 4°C, in close agreement with empirically based model assessments from younger sections. We note sensitivities as high as 5°C/doubling are observed when using the lowest pCO₂ estimate allowed by the formal error window assigned to the paleosol data. These are the oldest contemporaneous and tightly coupled ρCO₂ and temperature estimates, and among the only empirical indications of enhanced Earth System Sensitivity in a demonstrably ice-free world, confirming previous findings that ESS is enhanced during a hothouse state when compared to the modern.