The worldwide COVID-19 lockdown impacts on anthropogenic aerosol reductions and climate
―Realistic assessments on changes in aerosol precursor gas emissions based on satellite observations―

1. Key Points

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During the worldwide lockdown early in the COVID-19 pandemic, the amount of anthropogenic aerosols (*1) in the atmosphere decreased by 8–21% in the major emission regions (East Asia, North America, and Europe). This was supported by changes in aerosol precursor gas emissions that were accurately estimated from satellite observations.

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The umbrella effect (*2) faded due to this reduction in anthropogenic aerosols, and the net thermal energy entering the earth increased by 0.14 W/m² compared to normal periods.

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This study was the first global-scale assessment of climate impacts of anthropogenic aerosol reductions based on realistic estimates in emission reductions during the lockdown. The warming effect due to the decrease in aerosols was found to exceed the cooling effect (-0.025 W/m²) due to the decrease in CO2 emissions during the same period.

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This result shows the quantitative relationship regarding the impacts from the reduction of CO2 emissions and the simultaneous reduction in aerosol amounts on the climate. It has important implications for the optimization of climate mitigation measures and climate change projection.

2. Overview

Dr. Takashi Sekiya of the Earth Surface System Research Center, Research Institute for Global Change, and Japan Agency for Marine-Earth Science and Technology, Dr. Kazuyuki Miyazaki of the NASA Jet Propulsion Laboratory (JPL), Professor Kengo Sudo of Nagoya University, and others shows how the reduction in emissions associated with the worldwide lockdown during the COVID-19 pandemic changed the amount of anthropogenic aerosol particles formed in the atmosphere, and how this affected the Earth’s thermal energy balance.

Previous research methods based on observation data have assessed changes in the total amount of aerosol particles, from both anthropogenic and natural sources; however, extracting only the impact of changes in human activity due to lockdowns was difficult. This research team addressed this challenge by using a data assimilation (*3) analysis system developed by JAMSTEC and NASA JPL that can integrate observational information on multiple atmospheric constituents, as well as satellite observation data from the European Space Agency (ESA), in order to estimate the changes in the aerosol precursor gas emissions and the amount of anthropogenic aerosols. Additionally, the research team assessed the impact of reduced anthropogenic aerosols on the global thermal energy balance and found that this reduction weakened the umbrella effect of aerosols and increased the net thermal energy entering the Earth by 0.14 W/m².

This study is the first to use realistic changes in emission amounts back-calculated from satellite observations to assess its ripple effect on global-scale anthropogenic aerosols and the climate during the lockdown. The warming effect of aerosols was found to exceed the potential cooling effect (-0.025 W/m², Forster et al., 2020) from a slight reduction in CO2 emissions during the same period. Aerosol emission reductions, which are thought to simultaneously occur with CO2 emissions reductions due to climate mitigation measures, are known to offset each other. The present study is important for quantifying this relationship. These findings provide valuable information regarding future climate mitigation measures and are expected to contribute to improved climate change projections.

This study was supported by the Ministry of the Environment (MoE) Environment Research and Technology Development Fund (2-2201) and Japan Society for the Promotion of Science KAKENHI (JP22K12353).

These results are scheduled to be published in “Science Advances” on July 29 (Japan time).