Abstract
AbstractThe methods to quantify equilibrium climate sensitivity are still debated. We collect millennial‐length simulations of coupled climate models and show that the global mean equilibrium warming is higher than those obtained using extrapolation methods from shorter simulations. Specifically, 27 simulations with 15 climate models forced with a range of CO2 concentrations show a median 17% larger equilibrium warming than estimated from the first 150 years of the simulations. The spatial patterns of radiative feedbacks change continuously, in most regions reducing their tendency to stabilizing the climate. In the equatorial Pacific, however, feedbacks become more stabilizing with time. The global feedback evolution is initially dominated by the tropics, with eventual substantial contributions from the mid‐latitudes. Time‐dependent feedbacks underscore the need of a measure of climate sensitivity that accounts for the degree of equilibration, so that models, observations, and paleo proxies can be adequately compared and aggregated to estimate future warming.
Highlights
The equilibrium climate sensitivity (ECS) is defined as the global- and time-mean, surface air warming once radiative equilibrium is reached in response to doubling the atmospheric CO2 concentration above pre-industrial levels
We collect millennial-length simulations of coupled climate models and show that the global mean equilibrium warming is higher than those obtained using extrapolation methods from shorter simulations
27 simulations with 15 climate models forced with a range of CO2 concentrations show a median 17% larger equilibrium warming than estimated from the first 150 years of the simulations
Summary
The equilibrium climate sensitivity (ECS) is defined as the global- and time-mean, surface air warming once radiative equilibrium is reached in response to doubling the atmospheric CO2 concentration above pre-industrial levels. Light gray dots indicate annual means of the first 150 years of a step forcing simulation, requested by the Coupled Model Intercomparion Project Phase 5 and 6 protocols (CMIP5 and CMIP6; Eyring et al, 2016; Taylor et al, 2011) and widely used to infer ECS (Andrews et al, 2012; Geoffroy et al, 2013) We refer to this time scale as “decadal to centennial”. The two-layer model including ocean heat uptake efficacy (ΔTEBM−ε; e.g., Geoffroy et al, 2013; Winton et al, 2010) results in a multimodel median equilibrium warming estimate which is 9% lower than ΔTbest est, again both for all simulations and the subset of CO2 doubling and quadrupling Both methods are described and illustrated in the supporting information.
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