COVID-19 Emissions Reductions Contributed to Record Summer Rainfall in China
E3SM’s Contribution to Advancing Understanding of the Impact of COVID-Induced Aerosol Emissions Reduction on Weather and Climate
Figure 1: The COVID-19 pandemic massively disrupted human activities, causing abrupt emissions reductions that made a critical contribution to 2020’s record rainfall and flooding over eastern China. (Image by Whisper of the heart | Wikipedia)
Background
The COVID-19 pandemic has caused massive disruptions in public life worldwide since February 2020. China, followed by many other nations, adopted a series of restrictions, such as limiting nonessential activities and gatherings, temporarily shutting down industries, and restricting transportation. These dramatic restrictions substantially reduced anthropogenic emissions of greenhouse gases (GHGs), aerosols, and their precursor gases, which have been inferred from mobility data and atmospheric observations. The reduction in emissions could have detectable impacts on weather and climate but quantifying such impacts is challenging. Yang et al. (2020) reported an anomalous surface warming over the Northern Hemisphere continents caused by a fast climate response to aerosol reductions, based on atmospheric model simulations with prescribed sea surface temperatures. A multi-Earth system model intercomparison project (MIP), called CovidMIP, was initiated in mid-2020, which received very rapid support from the international modeling community with 12 participating Earth system models, including E3SMv1. The CovidMIP protocol uses the SSP2-4.5 scenario as the baseline, against which COVID-induced GHGs and/or aerosol emissions reductions are applied during 2020-2022 to study their short-term impact on climate (Jones et al., 2021). The initial analysis of 12 models published by Jones et al. (2021) showed that the impact of COVID-induced changes on societal activities is detectable in atmospheric composition and solar radiation reaching the surface, especially over southern and eastern Asia. No robust signal or consistent patterns of changes in surface air temperature and rainfall were found on the annual timescale or across all models. Further analysis of changes in regional extremes and atmospheric circulations at shorter timescales was recommended.
In the summer of 2020, persistent extreme precipitation events occurred in eastern China (Fig. 1). The accumulated rainfall broke its record since 1961. From the perspective of climate variability, the strong Indian Ocean Dipole event in 2019, the notably delayed withdrawal of the Meiyu-Baiu front caused by enhanced Arabian Sea warming, and the sub-seasonal phase transition of the North Atlantic Oscillation were postulated as contributors to the unprecedented 2020 summer rainfall in China (Liu et al., 2020; Wang et al., 2021; Zhou et al., 2021). In addition to natural variability, human influence may also have a dramatic impact on extreme precipitations. Aerosols have long been recognized for their important effects on clouds, atmospheric circulations, and rainfall. Because of the complex processes involved, the role of aerosols in climate change and extreme weather remains very uncertain. Yang et al. (2022) conducted and analyzed E3SM simulations, in addition to the CovidMIP experiments, to reveal that the COVID-induced abrupt emissions reductions made a critical contribution to the summer 2020 record rainfall and flooding over eastern China. In the climate model experiments, the dramatic aerosol reductions affected atmospheric heating and regional meteorological conditions, resulting in a strong moisture convergence, which increases the regional rainfall.
Figure 2: (a) Spatial distribution of observed anomalies of June–July (JJ) mean precipitation rate (mm/day) over China in 2020 relative to the historical period of 1979–2019. (b) Time series of observed JJ mean precipitation rate (mm/day) in eastern China, marked by the pink box in (a). The black dashed line marks the JJ mean precipitation rate during 1979–2019, and the red bar is for 2020. Spatial distribution of changes in JJ precipitation rate (mm/day) due to COVID-induced reductions in (c) aerosol emissions and GHGs and (d) aerosol emission alone. The stippled areas in (c) and (d) indicate statistically significant differences at the 90% confidence level based on a two-tailed Student’s t-test.
Summary of Results
A 60-year record high summer precipitation occurred over eastern China in 2020 when worldwide human activities and aerosol emissions were dramatically reduced. Yang et al. (2022) investigated the impact of abrupt, COVID-induced emissions reductions on the unprecedented summer precipitation using E3SM and observational datasets, supplemented by other CovidMIP climate model experiments. Model experiments were analyzed to contrast meteorological conditions, including regional total precipitation, atmospheric heating, and circulation changes between business-as-usual emissions (baseline SSP2-4.5) and pandemic-induced reduced emissions (SSP2-4.5 with COVID aerosol emissions, and SSP2-4.5 with COVID aerosol & GHGs emissions).
Results show that pandemic-induced aerosol reductions are a crucial factor in enhancing the prolonged summer precipitation over eastern China. The precipitation in June–July (JJ) of 2020 averaged over the Yangtze River Delta was 11.3 mm per day, exceeding the average of 1979–2019 of 6.3 mm per day by 79% (Figure 2a, b). Considering the reductions in aerosol emissions and GHGs concentrations, E3SM remarkably reproduces the spatial pattern of observed precipitation anomalies over China. Statistically significant rainfall increase attributed to COVID emissions reductions appears primarily over the Yangtze River Delta, where the JJ mean increase of 1.3 mm per day is about one-fourth (26%) of the observed increase (Figure 2c). The simulated rainfall increase is mainly contributed by the decreases in aerosols and precursor emissions (Figure 2d), which alone account for about one-third (32%) of the observed precipitation increase. Other CovidMIP models show similar results, although the magnitude of the precipitation response varies among the different climate models.
Further analysis revealed the mechanisms of enhanced rainfall by the emission reductions. The pandemic-induced decreases in aerosols resulted in anomalous atmospheric heating over eastern China and downwind areas, which increased atmospheric instability and produced an anomalous ascending flow over eastern China (Figure 3). The anomalous ascent over land resembles the observed pattern but with a weaker strength (Figure 3a). The enhanced ascending flow and convection over eastern China strengthened the dynamical condition for prolonged heavy precipitation. Aerosol reduction is identified as the main reason for the ascent since it appears in both experiments (with and without GHGs emissions reduction). Due to a compensating cooling effect associated with the decrease in GHGs concentrations, the anomalous heating and ascent due to reduction in both aerosols and GHGs (Figure 3b) are not as strong as those due to aerosol reduction alone (Figure 3c). This explains the stronger impact of aerosol-only reductions on the record rainfall in eastern China than the combined effect of aerosols and GHGs (Figure 2c, d).
Figure 3: (a) Changes in observed JJ mean meridional-vertical wind (m/s, vectors) with the vertical component (pressure velocity, Pa/s) multiplied by –100, averaged over 105–125°E in 2020, relative to the historical average over 1979–2019 from ERA5 reanalysis data. (b) and (c) show the changes in JJ mean meridional-vertical wind (m/s, vectors) and atmospheric heating rate (K/day, shaded colors) due to COVID-induced reductions in aerosol emissions and GHGs and aerosol emission alone, respectively.
The Impact and Outlook
E3SM has a comprehensive, advanced representation of aerosols in the climate system. The findings of Yang et al. (2022) reveal an important role of aerosols in modifying regional weather conditions and extreme events. The profound impact of the abrupt emissions reductions is different from previous reports of the climate system’s response to continuous but gradual emissions reductions, which also has important implications for improving the prediction of climate responses to dramatic changes in aerosols. They motivate the need to improve the modeling of aerosol-cloud-precipitation processes and their impacts on the water cycle and extreme weather and hydrologic events. Ongoing relevant E3SM work focuses on understanding how the effects of human-induced regional and global aerosol emissions changes depend on the historical climate, natural climate variability, and future warming climate.
Model and Data
The source code of E3SM is available at GitHub – E3SM-Project/E3SM: Energy Esascale Earth System Model Source Code, and the E3SM CovidMIP simulations can be found at COVID-19 Simulation Data Now Available.
Publications
Jones, C. D., Hickman, J. E., Rumbold, S. T., Walton, J., Lamboll, R. D., Skeie, R. B., et al., “The climate response to emissions reductions due to COVID-19: Initial results from CovidMIP”. Geophysical Research Letters, 48, e2020GL091883. (2021). [DOI: 10.1029/2020GL091883]
Yang, Y., Ren, L., Li, H., Wang, H., et al. (2020). “Fast climate responses to aerosol emission reductions during the COVID-19 pandemic”. Geophysical Research Letters, 47, e2020GL089788. (2020). [DOI: 10.1029/2020GL089788]
Yang, Y., Ren, L., Wu, M., Wang, H. et al., “Abrupt emissions reductions during COVID-19 contributed to record summer rainfall in China.” Nature Communications 13, 959 (2022). [DOI: 10.1038/s41467-022-28537-9]
Other References
Liu, B., Yan, Y., Zhu, C., Ma, S. & Li, J. Record-breaking Meiyu rainfall around Yangtze River in 2020 regulated by the subseasonal phase transition of North Atlantic Oscillation. Geophys. Res. Lett. 47, e2020GL090342 (2020).
Wang, J., Liu, Y., Ding, Y. & Wu, Z. Towards influence of Arabian Sea SST anomalies on the withdrawal date of Meiyu over the Yangtze-Huaihe River asin. Atmos. Res. 249, 105340 (2021).
Zhou, Z., Xie, S. & Zhang, R. Historic Yangtze flooding of 2020 tied to extreme Indian Ocean conditions. Proc. Natl Acad. Sci. USA 118, e2022255118 (2021).
Related Links
- E3SM Effort on COVID-19
- COVID-19 Simulation Data Now Available
- Covid shutdown linked to record rainfall in China – BBC News
- Deadly link between sharp emissions fall and China’s record 2020 rains, South China Morning Post
Funding
- This research is partly supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research (BER), Earth System Model Development program area as part of the E3SM project. This research used National Energy Research Scientific Computing Center, a DOE Office of Science User Facility, and BER’s Compy computing resources located at Pacific Northwest National Laboratory (PNNL).
Contact
- Hailong Wang, Pacific Northwest National Laboratory
- Ruby Leung, Pacific Northwest National Laboratory
