The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth system models

Abstract

Abstract. The Earth system models (ESMs) that participated in the sixth 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 sulfate aerosols. Structural uncertainties between ESMs have a larger impact on the anomalous cooling than internal variability. Historical simulations with and without anthropogenic aerosol emissions indicate that the anomalous cooling in the ESMs is attributed to the higher aerosol burden in these models. The aerosol forcing sensitivity, estimated as the outgoing shortwave radiation (OSR) response to aerosol concentration changes, cannot well explain the diversity of pothole cooling (PHC) biases in the ESMs. The relative contributions to aerosol forcing sensitivity from aerosol–radiation interactions (ARIs) and aerosol–cloud interactions (ACIs) can be estimated from CMIP6 simulations. We show that 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 between 64 % and 87 % of the aerosol forcing sensitivity in the models and is the main source of the aerosol forcing sensitivity 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), with the cloud-amount term accounting for most of the inter-model differences.

The Role of Anthropogenic Aerosol Forcing in the 1850–1985 Strengthening of the AMOC in CMIP6 Historical Simulations

“…Such a conclusion is consistent with a number of studies that document a too large AA driven cooling in CMIP6 historical simulations (Flynn and Mauritsen 2020;Dittus et al 2020;Wang et al 2021;Zhang et al 2021). As there is a strong relationship between the strength of the AA forcing/response and the changes of the AMOC in CMIP6-class models (i.e.…”

UKESM1.1: Development and evaluation of an updated configuration of the UK Earth System Model

“…The UKESM1.1 model configuration demonstrates how a seemingly small number of model changes can have a significant impact on the simulated global surface temperature and highlights the large sensitivity of historical climate to aerosols and in particular, to simulated burdens of SO 2 , a key anthropogenic aerosol precursor. This is consistent with the work of Zhang et al (2021) who demonstrate a strong correlation between sulphate aerosol loading and the global mean surface temperature anomalies across a number of the CMIP6 models. Competing interests.…”

“…More generally, Earth System models (ESMs) with more complex chemistry and aerosol schemes have a larger cold bias in CMIP6 than their physical model counterparts (Zhang et al, 2021). A strong correlation is found between the anthropogenic sulphate burden and surface temperature change over the historical period, further strengthening the argument for the role of an overly strong sulphate aerosol forcing in these models (Zhang et al, 2021).…”

Anthropogenic aerosol impacts on Pacific Coast precipitation in CMIP6 models

“…Models also have systematic biases in simulating ENSO processes, particularly in the topical Pacific (Timmermann et al 2018, Bayr et al 2019, which may impact Pacific Coast precipitation (Allen and Luptowitz 2017). We also note that CMIP6 models may overestimate AA-driven cooling of the NH mid-latitudes during the mid-20th century (Zhang et al 2021), which in turn may lead to overestimation of the associated hydro-dynamical responses along the Pacific Coast. The causes of this, however, are likely due to several factors.…”