Abstract
The Earth System Models (ESMs) that participated in the 6th Coupled Model Intercomparison Project (CMIP6) tend to simulate excessive cooling in surface air temperature (TAS) between 1960 and 1990. The anomalous cooling is pronounced over the Northern Hemisphere (NH) midlatitudes, coinciding with the rapid growth of anthropogenic sulfur dioxide (SO2) emissions, the primary precursor of atmospheric sulphate aerosols. Historical simulations with and without anthropogenic aerosol emissions indicate that the anomalous cooling within the ESMs is potentially due to in part from overestimated anthropogenic aerosols and the enhanced aerosol-forcing-sensitivity. Structural uncertainties between ESMs that contribute to these two factors have a larger impact on the anomalous cooling than internal variability. CMIP6 simulations can also help us to quantify the relative contributions of aerosol-forcing-sensitivity by aerosol-radiation interactions (ARI) and by aerosol-cloud interactions (ACI). However, even when the aerosol-forcing-sensitivity is similar between ESMs, the relative contributions of ARI and ACI may be substantially different. The ACI accounts for 64 to 87 % of the aerosol-forcing-sensitivity and is the main source of differences between the ESMs. The ACI can be further decomposed into a cloud-amount term (which depends linearly on cloud fraction) and a cloud-albedo term (which is independent of cloud fraction, to the first order). The large uncertainties of cloud-amount term are responsible for the aerosol-forcing-sensitivity differences and further the anomalous cooling differences among ESMs. The metrics used here therefore provide a simple way of assessing the physical mechanisms contributing to anomalous twentieth century cooling in any given ESM, which may benefit future model developments.
Highlights
Surface air temperature (TAS) variation is an essential indicator of climate change, and reproducing the evolution of historical TAS is a crucial criterion for model evaluation
The historical TAS anomaly simulated by the models in the sixth Coupled Model Intercomparison Project (CMIP6) is on average colder than that observed in the mid-20th century, whereas the CMIP5 models tracked the instrumental TAS variation quite well (Flynn and Mauritsen, 2020)
Whilst the overall impact of aerosol forcing will depend on other aerosol species, we adopt this approach because the sulfates dominate estimates of aerosol forcing during this period and other aerosols species can be assumed to have covaried with the SO2 emissions during this period as presented by the Community Emissions Data System (CEDS) inventory adopted by CMIP6 models (Hoesly et al, 2018)
Summary
Surface air temperature (TAS) variation is an essential indicator of climate change, and reproducing the evolution of historical TAS is a crucial criterion for model evaluation. The historical TAS anomaly simulated by the models in the sixth Coupled Model Intercomparison Project (CMIP6) is on average colder than that observed in the mid-20th century, whereas the CMIP5 models tracked the instrumental TAS variation quite well (Flynn and Mauritsen, 2020). This is surprising because the transient climate response in CMIP6 models is generally higher than in CMIP5 models (e.g., Flynn and Mauritsen, 2020; Meehl et al, 2020). As a result of anthropogenic emissions, atmospheric aerosol concentrations increased along with rising greenhouse gases, but with greater decadal variability. The rate of change of global aerosol emissions slowed down in the late 20th century (Hoesly et al, 2018), and the trend
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