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

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

The event was dramatic, causing the death of hundreds of people and forcing millions more from their homes. Historic rainfall in eastern China and subsequent deadly flooding in the summer of 2020 has left climatologists and meteorologists around the world baffled ever since. Entire and multidisciplinary publications have even been devoted 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 North 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 strict containment measures and factory closures would have “played a crucial role in the extreme increase in precipitation during the summer”, they assure in an article published in the magazine Nature Friday February 18.

“It’s an approximation that had to be thought about because it is counter-intuitive given that the multiplication of extreme weather events is attributed to the increase in pollution and its effect on global warming, and not the reverse”, explains to France 24 Cathy Clerbaux, research director at the Atmospheres, Environments, Space Observations Laboratory (Latmos), who has studied the impact of confinements on atmospheric pollution.

The record rainfall in China would be a kind of unprecedented exception to the rule. The team of scientists first checked that these rains had no historical equivalents in the databases in order to ensure that it was indeed a conjunction of elements unique to 2020 that had to be sought to understand this deadly summer.

Once this work was done, we had to face the facts: 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 in order to fight against Covid-19.

The atmospheric consequence of these confinements is known, especially in China where the horizon usually obstructed by pollution has quickly cleared up. “It is in China and India that we have observed the most brutal and rapid fall 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 the formation of clouds and can therefore have an impact on the level of precipitation,” explains Wang Hailong, one of the co-authors of the article and researcher at the Pacific Northwest National Laboratory in Washington, interviewed by the South China Morning Post. The increase in these pollutants in the air has even “already been linked in the past to the observed decrease in rainfall in eastern China”, specify the authors of the study.

In fact, the lockdown in China in the spring of 2020 had a double impact on the climate: “The drop in particulate emissions led to a warming of the atmosphere above the earth, while the drop in gas emissions greenhouse effect (such as CO2) resulted in a cooling above the sea”, notes Cathy Clerbaux.

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

But the two effects did not cancel each other out. In total, the atmosphere was warmer than usual, note the authors of the study. The greater difference between the temperatures on land and those at sea “increased atmospheric instability, contributing to the creation of winds over the sea that carried humidity to eastern China”, summarizes Wang Hailong to the BBC.

This is how the period of heavy rain would have turned into an episode of unprecedented precipitation. The authors of the study do not claim that everything is the fault of the containment and the shutdown of factories throughout 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, like warmer winds over the Indian Oceanmay also have played a role.

But it would be the “crucial” third, adds this team of scientists. In other words, the confinement and the drop in the number of fine particles acted as the trigger that transformed the already traditionally heavy rainfall at that time in eastern China into a natural disaster.

“It is a very seriously conducted study”, recognizes Cathy Clerbaux, who adds that it is however not excluded that the only responsible is a capricious weather.

But if improved air quality really played a role in the deadly rainfall, what does that mean for China’s emissions policy, the BBC wonders. Should the country expect ever more violent rainy seasons? “It’s impossible to say,” 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 event of a controlled decline linked to a policy of progressive reduction of greenhouse gas emissions,” Wang Hailong told the BBC. And then anyway, notes Cathy Clerbaux, we are sure that the alternative – not to fight global warming – would result in a multiplication of extreme weather events.