Sensitivity of Historical Climate Simulations to Uncertain Aerosol Forcing

Abstract

The relative importance of anthropogenic aerosol in decadal variations of historical climate is uncertain, largely due to uncertainty in aerosol radiative forcing. We analyze a novel large ensemble of simulations with HadGEM3‐GC3.1 for 1850–2014, where anthropogenic aerosol and precursor emissions are scaled to sample a wide range of historical aerosol radiative forcing with present‐day values ranging from –0.38 to –1.50 Wm–2. Five ensemble members are run for each of five aerosol scaling factors. Decadal variations in surface temperatures are strongly sensitive to aerosol forcing, particularly between 1950 and 1980. Post‐1980, trends are dominated by greenhouse gas forcing, with much lower sensitivity to aerosol emission differences. Most realizations with aerosol forcing more negative than about –1 Wm–2 simulate stronger cooling trends in the mid‐20th century compared with observations, while the simulated warming post‐1980 always exceeds observed warming, likelydue to a warm bias in the transient climate response in HadGEM3‐GC3.1.

Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models

“…Such relationships between amplitude of aerosol forcing and ECS have been noted in previous generations of models (46,47). However, because the aerosol forcing varies with time, it is difficult to tune the ECS based on responses to aerosols over different periods of the 20th century (48). On the basis of the information in Table 3 provided by a subset of CMIP6 modeling groups, Fig.…”

Uncertainty in Aerosol Radiative Forcing Impacts the Simulated Global Monsoon in the 20th Century

“…The scaling factors were chosen to sample a broad range of the uncertainty in presentday aerosol radiative forcing according to Myhre et al (2013), and correspond to forcings of −0.38 W m −2 , −0.60 W m −2 , −0.93 W m −2 , −1.17 W m −2 and −1.50 W m −2 respectively. More detail on the SMURPHS ensemble is presented by Dittus et al (2020). The performance of HadGEM3 in representing various monsoon properties is compared against observations and reanalysis in the Appendix (Figure A1), in which we demonstrate the fidelity with which the model reproduces the monsoon properties evaluated in this study.…”

“…The SMURPHS dataset consists of historical climate simulations run over the period 1850-2014 using a fully coupled version 5 of HadGEM3-GC3.1 at resolutions of N96 and 1° in the atmosphere and ocean respectively (Kuhlbrodt et al, 2018;Williams et al, 2018). The model version used here is a development version towards the UK submission to CMIP6 (Andrews et al, 2020;Dittus et al, 2020;Hardiman et al, 2019). HadGEM3 uses the GLOMAP two-moment aerosol scheme that includes representations of the cloud albedo and cloud lifetime effects (Mulcahy et al, 2018 and references therein).…”

“…In this study, we use all five members from each of the five experiments, but include years from 1900 onwards, to allow 50 years for the model to adjust to the scalings (after Dittus et al, 2020). When considering climatological quantities, we consider the ensemble mean for each experiment to be the model estimate of the climate system under those scaling conditions and indicate uncertainty across ensemble members in terms of inter-member standard deviation.…”

“…In this study, we quantify the impact of the uncertainty in present-day aerosol radiative forcing on the global monsoon system using a set of historical climate simulations produced as part of the SMURPHS ("Securing Multidisciplinary Understanding and Prediction of Hiatus and Surge Events") project (Dittus et al, 2020). The SMURPHS ensemble consists of a set of historical climate simulations with AA emissions scaled by various factors, allowing us for the first time to investigate the sensitivity of the climate system to the strength of the forcing.…”