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Assessing the influence of COVID-19 on Earth’s radiative balance

February 19th, 2021

Key Findings

  • The authors investigated the impacts of the worldwide reduction in aerosol emissions resulting from the COVID-19 pandemic, using model simulations with GFDL’s AM4 to separate the effects of meteorology and emissions.
  • Pandemic-related emission reductions account for approximately one-third of the large, precipitous decrease in solar clear-sky reflection (when the sky is not covered by clouds) over the East Asian Marginal Seas in March 2020.
  • The remainder can be attributed to weather variability and long-term emission trends.
  • By contrast, no robust signal is identified in the negative anomaly in solar all-sky reflection. The presence of clouds makes it harder to detect any signal from COVID.

Yi Ming, Norman G. Loeb, Pu Lin, Zhaoyi Shen, Vaishali Naik, Clare E. Singer, Ryan X. Ward, Fabien Paulot, Zhibo Zhang, Nicolas Bellouin, Larry W. Horowitz, Paul A. Ginoux, V. Ramaswamy. Geophysical Research Letters. DOI: 10.1002/essoar.10503579.1

The ongoing COVID-19 pandemic led to a worldwide reduction in aerosol emissions. Anecdotal effects on air quality and visibility were widely reported. Less known are the impacts on the planetary energy balance, and by extension, on weather and climate. By separating the impacts from meteorology and emissions with model simulations, the authors found that about one‐third of the clear‐sky anomalies can be attributed to pandemic‐related emission reductions, and the rest to weather variability and long‐term emission trends.

The authors studied the underlying mechanisms of the large, precipitous decrease in solar clear-sky reflection (3.8 W m-2) over the East Asian Marginal Seas in March 2020, using satellite observations and model simulations. Although these changes are consistent with a sharp cut in aerosol emissions due to the lockdown put in place to curb the spread of COVID‐19, the possible role played by weather conditions, such as winds and humidity, could not be ruled out.

The model used (GFDL’s AM4) is skillful at reproducing the observed interannual variations in solar all‐sky reflection, under both clear and cloudy-sky conditions. This allowed the scientists to distinguish forced signal from weather variability, a prerequisite for interpreting observations. In the all‐sky analyses, no robust COVID‐19 signal was detected, indicating that the observed negative anomaly in Fall for March 2020 was likely caused by weather variability.

The COVID-19 pandemic provides an opportunity for evaluating the model representation of the aerosol-cloud-radiation interactions, a major source of uncertainty in global weather and climate modeling. Although the observational evidence for aerosol direct effects is unequivocal, and their model representation is satisfactory, it is more difficult to draw definitive conclusions about aerosol indirect effects from the observed shortwave all-sky flux. By leveraging the latest observational and modeling capabilities, the framework described in this work is ideal for studying the radiative impacts of the ongoing COVID-19 pandemic, and the resulting perturbations to the energy balance, in other parts of the world (such as Europe and North America).

(a) Times series of the anomaly in aerosol optical depth (AOD) over the East Asian Marginal Seas in March from 2003 to 2020. The black line is from MODIS, and the blue line is from the control simulation. The vertical bar denotes the detection limit (one standard deviation of the differences between the observations and the control simulation from 2003-2019). The orange, green, and red dots denote the perturbation simulations of 20%, 40%, and 60% emissions reductions, respectively. r is the correlation coefficient. (b) Same as (a), but for CERES clear-sky shortwave radiative flux.