In China, containment against Covid-19 responsible for torrential rains?

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An international team of scientists says in a paper published Friday that China’s record rainfall in 2020 and subsequent deadly floods are partly due to containment measures to combat covid-19.

The event was dramatic. killing hundreds of people and forcing millions more from their homes. Historic rains in eastern China and subsequent deadly floods in the summer of 2020 have baffled climatologists and meteorologists around the world ever since. Complete and multidisciplinary publications have even been dedicated end of 2021 to the possible causes of the event.

A new stone has just been added to this building thanks to an international team of scientists, mainly Chinese and American researchers, who believe that the solution to this mystery is… the Covid-19 pandemic.

Fewer fine particles, more rain?

The sudden drop of certain pollutants in the atmosphere due to the strict confinement measures and the closure of factories would have “played a crucial role in the extreme increase in precipitation during the summer”, they assure in an article published in Nature magazine Friday February 18.

“It is an approximation that had to be thought about because it is counterintuitive given that the multiplication of extreme weather events is attributed to the increase in pollution and its effect on global warming, and not the other way around,” Cathy Clerbaux explains to France 24. , research director of the Laboratory of Atmospheres, Environments, Space Observations (Latmos), who studied The impact of lockdowns on air pollution.

China’s record rainfall would be something of an unprecedented exception to the rule. The team of scientists first checked that these showers had no historical equivalents in databases to make sure that it was indeed a conjunction of elements unique to 2020 that needed to be sought to understand this deadly summer.

Once this work was done, the facts had to be faced: the only new parameter in 2020 that could provide the key to the enigma – as unlikely as it may seem – is Beijing’s decision to close the economy and confine the country to fight covid -19.

The atmospheric consequence of these lockdowns is well known, especially in China where the normally smog-clogged horizon has rapidly cleared. “It is in China and India that we have observed the most brutal and rapid drop in the number of particles in the atmosphere and in greenhouse gas emissions,” recalls Cathy Clerbaux.

But what does this have to do with the torrential rains in the east of the country? Everything would come from the concentration of fine particles in the atmosphere. “We have known for a long time that these aerosols are an important factor in cloud formation and therefore can have an impact on the level of precipitation,” explains Wang Hailong, one of the co-authors of the paper and a researcher at the Center. Pacific Northwest National Laboratory in Washington, interviewed by the China South Morning Post. The increase of these pollutants in the air has even “been related in the past to the observed decrease in precipitation in eastern China,” the study authors specify.

In fact, the lockdown in China in the spring of 2020 had a double impact on the climate: “The drop in particle emissions caused a warming of the atmosphere on earth, while the drop in greenhouse gas emissions (such as COtwo) resulted in cooling over the sea,” notes Cathy Clerbaux.

The fight against Covid-19 contributed to “a third” of the rainfall record

But the two effects did not cancel each other out. Overall, the environment was warmer than usual, the study authors note. The greater difference between temperatures on land and at sea “increased atmospheric instability, contributing to the creation of winds over the sea that brought moisture to eastern China”, sums up Wang Hailong to the BBC.

This is how the period of heavy rains would have become an episode of unprecedented rainfall. The study’s authors are not claiming that everything is to blame for the lockdowns and factory closures across China. In fact, “we estimate that this context has contributed to a third of the level of precipitation,” they say. Other factors, already mentioned by other researchers, as warmer winds over the Indian Oceanmay also have played a role.

But it would be the third “crucial”, adds this team of scientists. That is to say, the confinement and the decrease in the number of fine particles acted as the trigger that transformed the already traditionally heavy rains of that time in eastern China into a natural disaster.

“It is a study carried out very seriously,” acknowledges Cathy Clerbaux, who adds that, however, it is not ruled out that the only responsible is a capricious climate.

But if improved air quality really did play a role in the deadly rains, what does that mean for China’s emissions policy? the BBC asks. Should the country expect increasingly violent rainy seasons? “It’s impossible to know,” says Cathy Clerbaux. Everything seems to indicate that it is the sudden drop in emissions that has caused this particular atmospheric alchemy. “It would be very different in the case of a controlled decrease linked to a policy of progressive reduction of greenhouse gas emissions,” Wang Hailong told the BBC. And in any case, Cathy Clerbaux points out, we are sure that the alternative, not fighting global warming, would lead to a multiplication of extreme weather events.

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