Diagnosing Stalled Warming in CMIP6 Models

Abstract

Global coupled models provide essential constraints on future Earth warming; however, significant spread across climate simulations creates uncertainty in the rate of warming. Nonlinearities in the rate of warming at different levels of CO2 contribute to spread even within a single model. We consider eight CMIP6 models for which both abrupt quadrupling (abrupt-4xCO2) and abrupt doubling (abrupt-2xCO2) of pre-Industrial CO2 experiments are available. Within these eight GCMs, two models have the largest difference in warming rate between abrupt-4xCO2 and abrupt-2xCO2: CESM2 and MRI-ESM2. Using estimated equilibrium statistics, a linearized energy balance model, and an analysis of ocean dynamics, we evaluate whether the net radiative feedback, the effective radiative forcing, or the effective heat capacity contribute to differences in the rate of warming across forcings and by what mechanism. We find that on average, radiative feedbacks are the dominant cause of forcing-dependent differences. However, for CESM2 and MRI-ESM2-0, the effective heat capacity is a significant, or even dominant, mechanism of slowed warming under abrupt doubling CO2. We conclude that changes in the low cloud cover and depth of heat storage due to declines in the Atlantic Meridional Overturning Circulation (AMOC) play an important role in modulating the transient climate.