Tuesday, February 7

How COVID caused record floods in China: the curious (and unexpected) impact of lockdowns on the weather

After two years of a long, raw and stubborn pandemic, COVID has little room left to surprise us. Or not. Researchers at the Nanjing University of Information Science and Technology in China have just come to a striking conclusion: in its own way, the coronavirus is behind the downpour that hit the east of the country in the summer of 2020, when the Giant Asiatic suffered rains like not remembered in 60 years and floods that left serious damage and victims. The most curious thing is that, technically, the cause was not COVID, but how we deal with it.

In an article published in Nature Communications, Professor Yang Yang and his colleagues point out that approximately a third of the rain that was recorded then can be explained by a sum of factors that originate from the confinements with which the local authorities —as happened in Spain and other many countries – tried to stop the escalation of infections. His conclusion is that these measures favored a decrease in emissions and air pollution, which, in turn, brought heavier than usual rainfall.

Abrupt and unexpected reductions

“Abrupt emission reductions during the pandemic strengthened summer atmospheric convection over eastern China, resulting in a positive sea-level pressure anomaly over the northwestern Pacific Ocean. The latter enhanced moisture convergence to eastern China and further intensified rainfall in that region,” the study details.

For their research, Yang and the rest of his colleagues, among whom are researchers from other centers at an international level, developed a model that reflects the impact that emissions greenhouse gases had in the rainfall recorded in the country over the last four decades. After this analysis they concluded that the increase in pollution and aerosol particles have reduced the volume of rainfall.

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By decreeing the quarantine to stop the spread of COVID-19 —in Wuhan it was already applied at the beginning of 2020— traffic and industrial activity stopped and with them a large part of the polluting emissions. Result: the trend was reversed, intensifying the convention and favoring the moist air from the ocean to reach the continent. The authors of the study specify, in fact, that the decrease in aerosol emissions can explain approximately a third of the increase in extreme rainfall recorded in eastern China during the summer.

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“Modeling experiments show that the reduction in aerosols had a stronger impact on precipitation than the decrease in greenhouse gases. We conclude that through abrupt reductions in emissions, the pandemic contributed significantly to the extreme rains of summer 2020 in eastern China”, conclude the experts, who point to the decrease in local emissions as the main source, not from other parts of Asia.

Does that mean that if we want to avoid floods like the ones that hit China two years ago, we must continue to pollute? No. Far from it. The problem is not that the emission of gases has been reduced, but that this cut was made in a “sudden” and “abrupt” way, a word that the authors even carry in the title of their research. In fact, scientists emphasize that a programmed and longer response can be totally different.

“Emissions have been reduced in recent years in China to improve air quality. Why was the response of precipitation to COVID-induced emission reductions in eastern China during 2020 so different from the response of precipitation reduced emissions in previous years?In particular, emissions dropped sharply in early 2020 when the pandemic suddenly emerged, causing an immediate and abrupt change in various components of the climate system,” Yang and his colleagues write.

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“Sudden change in the climate system could be very different from changes in response to continuous but gradual policy-driven emission reductions,” continues the team of researchers from China, stressing that much of what happened is explained by the behavior of the aerosols themselves and their effects. “Even additional small reductions during the pandemic could lead to a more dramatic change in the Earth system.”

“Because the emissions were drastically reduced in early 2020 when the pandemic emerged, it caused an immediate and abrupt change in various components of the climate,” Yang says in statements to the BBC; “There was warming over land due to reduced aerosols, but also cooling over the ocean due to a decrease in greenhouse gases, which intensified the temperature difference between land and sea in the summer. This, in turn, increased sea level pressure over the South China Sea/Philippines and intensified winds that brought moist air to eastern China, which then experienced heavy precipitation.”

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Throughout the study, its authors also point out the effect of pollution on the climate, which has caused a significant decrease in summer rainfall in the east and center of the Asian Giant, precisely due to the increase in aerosol particles in the atmosphere.

With COVID, lockdowns through, that scenario left heavy rains and floods about a year and a half ago. As the study recalls, a record amount of water was accumulated throughout the summer, the largest since 1961. The Yangtze River, for example, recorded the most intense rainfall in six decades, with an increase of 79% in June and July compared to the average of the last four decades. Extreme weather left damage, evacuations and victims. By mid-July, there were already 140 missing and deceased in the center and east of the country.

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Cover image | Jean Beller (Unsplash) and Nuno Alberto (Unsplash)

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