How well do Earth System Models reproduce observed aerosol changes during the Spring 2020 COVID-19 lockdowns?

Abstract. One side effect of the Spring 2020 COVID-19 lockdowns was a rapid reduction in aerosol and aerosol precursor emissions. These emission reductions provide a unique opportunity for model evaluation, and to assess the potential efficacy of future policy decisions. We investigate changes in observed regional aerosol burdens during the COVID-19 lockdowns, and compare these observed anomalies to predictions from Earth System Models forced with COVID-19-like reductions in aerosol and greenhouse gas emissions. Despite the dramatic economic and lifestyle changes associated with the pandemic, most anthropogenic source regions do not exhibit detectable changes in satellite retrievals of total or dust-subtracted aerosol optical depth. Only India exhibits an aerosol optical depth anomaly that exceeds internal variability. These conclusions are broadly reproduced by Earth System Models when confounding factors have been accounted for. We present a systematic assessment of the contributions of internal variability, model input uncertainty, and observational sampling to the aerosol signal, and highlight the impacts of observational uncertainty on model evaluation analyses. These results are encouraging, suggesting that current Earth System Models may be able to realistically capture the effects of future emission reductions.