Changes in equilibrium climate sensitivity and associated feedbacks through past greenhouse and icehouse climate simulations

Changes in climate feedback processes drive fluctuations in equilibrium climate sensitivity (ECS), the measure of global warming associated with a doubling of atmospheric CO₂. Warming in different past climates resulted in various responses in ECS that can be explored to further understand how distinct feedbacks and forcings control CO₂-induced warming. Previous studies generally agree that ECS increases with increases in the CO₂ background state. We investigate this further through simulations of different time slices from ~100 million years ago to present, all run with the same model version. We will use Community Earth System Model (CESM1.2) simulations to study Earth’s response to an increase in CO₂ radiative forcing under past greenhouse and icehouse climates. We compare time slices that have differences in geography, vegetation, and ice, which affect feedbacks that drive ECS. We will use slab ocean model and fully-coupled CESM1.2 simulations of the late Cretaceous, early Eocene, late Oligocene, mid Miocene, and preindustrial (PI) all at modern orbital parameters, with greenhouse climate simulations at 840 ppm and 1680 ppm CO₂ and icehouse climate simulations at 280 ppm and 560 ppm CO₂, in order to compare changes in temperature resulting from changes in albedo, ocean heat flux, and nonlinearities in atmospheric water vapor and cloud feedbacks. We will compare simulations of greenhouse and icehouse climates, past climates to the PI climate, and our CESM1.2 simulations to previously published simulations run on other earth system models, like the Hadley Centre Coupled Model, to study the degree of model-dependence in ECS. Analyzing ECS in past climates, with a control on model version, and comparing differences in climate feedbacks will help constrain the sensitivity of ECS to boundary conditions and the range of ECS through Earth’s history.