Tight coupling between atmospheric ρCO2 and temperature change during the Late Triassic: observational evidence for enhanced climate sensitivity in a hothouse state

Abstract:

Climate sensitivity is the change in global equilibrium surface temperature per doubling of atmospheric ρCO₂. Modern climate sensitivity, based on paleoclimate data and fast-feedback processes (Charney sensitivity), is observed to be ~3°C per doubling of CO₂. However, Charney sensitivity 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 (end-Norian through Rhaetian) from the Newark basin and Lagonegro/Sicani basins, respectively. Detailed 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 climate sensitivity 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 5°C, in close agreement with empirically based model assessments from younger sections. We note sensitivities as high as 7°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 climate sensitivity is enhanced during a hothouse state when compared to the modern.