Efficacy of Climate Forcings in PDRMIP Models.

T Richardson ,R W Portmann,B H Samset,J Mülmenstädt,Gunnar Myhre ,Christopher J Smith ,Apostolos Voulgarakis ,Oliviér Boucher ,Dilshad Shawki ,Timothy Andrews ,Dirk Olivié ,Tom Wood ,Philip Stier ,Alf Kirkevåg ,D T Shindell ,Øivind Hodnebrog ,Amanda C Maycock ,Piers M Forster ,Gregory Faluvegi ,Dagmar Fläschner ,J Lamarque ,Matthew Kasoar ,Toshihiko Takemura ,Duncan Watson-Parris

doi:10.1029/2019jd030581

Abstract

Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multimodel analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy‐induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling when estimated using the historical GSAT trend.

Highlights

Quantifying the effectiveness of different physical drivers in altering surface temperature is important for understanding historical and future temperature trends (Kummer & Dessler, 2014; Marvel et al, 2015)
Quantifying the effectiveness of different forcing agents in driving surface temperature change is important for understanding the real‐world climate sensitivity and for projecting future levels of surface warming for different emission pathways
Consistent with previous studies, we find that efficacies calculated using instantaneous radiative forcing (IRF) vary significantly across drivers

Summary

Introduction

Quantifying the effectiveness of different physical drivers in altering surface temperature is important for understanding historical and future temperature trends (Kummer & Dessler, 2014; Marvel et al, 2015). Global mean near‐surface air temperature (GSAT) change is typically understood in terms of the well‐ established radiative forcing and response framework (equation (1); Hansen et al, 1997; Boer & Yu, 2003; Gregory, 2004; Myhre et al, 2013). Radiative forcing represents the change in the Earth's energy balance due to an imposed perturbation, such as increasing greenhouse gases. Journal of Geophysical Research: Atmospheres which is held constant, GSAT will increase or decrease until the climate system reaches a new equilibrium. The temperature response depends on both the magnitude of the initial forcing and the temperature‐driven radiative feedbacks, which are represented by the climate feedback parameter (α) in equation (1)

Methods

Results

Conclusion