Abstract
Abstract. The equilibrium climate sensitivity (ECS) of climate models is calculated as the equilibrium global mean surface air warming resulting from a simulated doubling of the atmospheric CO2 concentration. In these simulations, long-term processes in the climate system, such as land ice changes, are not incorporated. Hence, climate sensitivity derived from paleodata has to be compensated for these processes, when comparing it to the ECS of climate models. Several recent studies found that the impact these long-term processes have on global temperature cannot be quantified directly through the global radiative forcing they induce. This renders the prevailing approach of deconvoluting paleotemperatures through a partitioning based on radiative forcings inaccurate. Here, we therefore implement an efficacy factor ε[LI] that relates the impact of land ice changes on global temperature to that of CO2 changes in our calculation of climate sensitivity from paleodata. We apply our refined approach to a proxy-inferred paleoclimate dataset, using ε[LI]=0.45-0.20+0.34 based on a multi-model assemblage of simulated relative influences of land ice changes on the Last Glacial Maximum temperature anomaly. The implemented ε[LI] is smaller than unity, meaning that per unit of radiative, forcing the impact on global temperature is less strong for land ice changes than for CO2 changes. Consequently, our obtained ECS estimate of 5.8±1.3 K, where the uncertainty reflects the implemented range in ε[LI], is ∼50 % higher than when differences in efficacy are not considered.
Highlights
Equilibrium climate sensitivity (ECS) expresses the simulated equilibrated surface air temperature response to an instantaneous doubling of the atmospheric CO2 concentration
We have incorporated the concept of a constant efficacy factor (Hansen et al, 2005), which interrelates the global temperature responses to radiative forcing caused by land ice changes and CO2 changes, into our framework of calculating specific paleoclimate sensitivity S[εCO2,LI]
The aim of this effort has been to overcome the problem that land ice and CO2 changes can lead to significantly different global temperature responses, even when they induce the same globalaverage radiative forcing
Summary
Equilibrium climate sensitivity (ECS) expresses the simulated equilibrated surface air temperature response to an instantaneous doubling of the atmospheric CO2 concentration. We first illustrate our refined approach by applying it to transient simulations over the past 5 Myr using CLIMBER-2 (Stap et al, 2018a), obtaining a quantification of the effect on global temperature of CO2 changes and the accompanying short-term feedbacks from a simulation forced by both land ice and CO2 changes. We compare this result to a simulation where CO2 changes are the only operating long-term process. The climate sensitivity resulting from applying this range provides a quantification of the consequence of the uncertain efficacy of land ice changes
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