Quantifying Equilibrium Climate Sensitivity to Atmospheric Chemistry and Composition Representations in GFDL‐CM4.0 and GFDL‐ESM4.1

Abstract Equilibrium climate sensitivity (ECS) quantifies surface warming in response to doubled pre‐industrial CO 2 (2xCO 2 ). Uncertainty in estimates arises from diverse model representations and climate‐chemistry feedbacks. We quantify ECS with five atmospheric chemistry‐composition model representations (sea salt, dust, organics, ozone, sea ice masking) with slab ocean configurations of the GFDL‐CM4.0 physical climate (4.1 ± 0.1 K) and GFDL‐ESM4.1 coupled climate‐carbon‐chemistry Earth system (3.4 ± 0.1 K) models. Estimated ECS is 0.7 K lower with GFDL‐ESM4.1, attributed to additive 0.1–0.3 K decreases as each GFDL‐ESM4.1 driver is included and a 0.4–0.6 K decrease with the addition of all five GFDL‐ESM4.1 model drivers in combination, relative to estimated ECS with GFDL‐CM4.0 neglecting these negative climate‐chemistry feedbacks. Interactive stratospheric ozone included in GFDL‐ESM4.1 contributes to the largest ECS decrease in response to 2xCO 2 of all five drivers, characterized by reduced polar amplification. We demonstrate strong climate sensitivity to complex but often ignored climate‐chemistry feedbacks.