FAHRENHEIT 481 MYR: A RECONSTRUCTION OF GLOBAL TEMPERATURES FROM THE ORDOVICIAN THROUGH THE HOLOCENE (Invited Presentation)

A consistent and reliable record of Earth’s global mean surface temperature (GMST) across the Phanerozoic Eon is essential to understanding the complex relationships and feedbacks between climate and tectonics, the chemistry of the atmosphere and oceans, the evolution of life, and intervals of mass extinction. However, reconstructing a Phanerozoic temperature history is challenging. With increasing geologic age, the density and geographic distribution of proxy data decreases and climate model boundary conditions become progressively less constrained. Here, we present PhanDA, a reconstruction of GMST spanning the last 480 million years based on the integration of paleotemperature proxy data with climate model simulations within the statistical framework of paleoclimate data assimilation. PhanDA shows good agreement with previously published time-slice reconstructions during the Cenozoic and independent evidence for glaciation, which lends confidence to the robustness of the reconstruction. The reconstruction reveals that, at the geologic stage level, GMST has a larger dynamic range than previously thought, fluctuating by more than 20^oC over the Phanerozoic. PhanDA reproduces known climatic events, but also revises our view of GMST evolution during key periods, such as the Late Paleozoic Ice Age. Using quantiles of GMST, we separate Phanerozoic climate into five states, which we use to explore time-independent patterns in the latitudinal temperature gradient, estimate CO₂ thresholds under different climate regimes, and demonstrate that the Earth has spent more time under warmer (ice sheet-free) conditions than in cold states. We find a remarkably strong correlation between reconstructed atmospheric CO₂ concentrations and PhanDA GMST, which firmly identifies CO₂ as the dominant control on variations in global climate for the past 480 million years. The strength of the relationship decreases when other climate forcings, such as solar irradiance, are considered, suggesting a near-cancelling of non-CO₂radiative forcings and/or a set of compensating feedbacks operating during both warm and cold intervals.