Equilibrium Climate Sensitivity dependence on changing climate, geography, and ocean heat transport

Equilibrium climate sensitivity (ECS) – the global temperature response to doubling CO ₂ – can be better understood by examining past climate states and the feedback mechanisms regulating CO ₂ -induced warming. While studies agree that ECS increases with higher CO ₂ , the range of Earth’s historical ECS and its underlying drivers remain incompletely understood. Here, we use slab ocean Community Earth System Model simulations to analyze four distinct periods in Earth's climate history with substantially different continental configurations: the late Cretaceous, early Eocene, late Oligocene, and preindustrial. Our results show that ECS varies by over 2.5°C, ranging from 4.04°C to 6.66°C. We analyze the contributions of CO ₂ background, geography, and ocean heat transport to ECS variability and decompose the total climate feedback parameter into the water vapor, cloud, surface albedo, and temperature feedbacks. Using a consistent model framework across geological epochs, we provide new constraints on ECS sensitivity to boundary conditions and offer insights into its variability throughout Earth's history.