In China, the COVID-19 social and economic shutdown coincided with record rainfall in eastern provinces in 2020 — and it’s no coincidence. This record rainfall quickly turned into flooding, leaving behind hundreds of deaths and millions evacuated. In a new study, researchers found that the extreme rainfall could have been caused by the drop in emissions registered in the country.

After rising swiftly for decades, global greenhouse gas emissions (GEI) dropped 6.4% in 2020, as the pandemic limited economic and social activities worldwide. The US contributed to most of the decline, while China saw a minor decrease (1.4%) because its economy recovered quickly after the outbreaks in early 2020, according to a 2021 study.

The emissions drop was linked with persistent extreme precipitation in the early and middle summer of 2020 in eastern China, a densely populated and urbanized region. The accumulated rainfall was so big that it broke its 60-year-record since 1961. Rain in the Yangtze River Delta, for example, exceeded the 41-year average by 79%.

Studies have looked at what could have caused this significant change in rainfall, with some suggesting it had something to do with the extreme weather conditions in the Indian Ocean. But a group of researchers wasn’t entirely convinced with this, suggesting that the abrupt drop in emissions was a key factor of the steep rain.

“Aerosols can affect clouds, precipitation, hydrological cycle and atmospheric circulation through microphysical as well as dynamical processes,” the researchers wrote. “In the last four decades, summer precipitation over eastern-central China has decreased significantly, which has been reported to be closely related to the increase in aerosols.”

Rain and greenhouse gas emissions

For the study, the researchers used the Energy Exascale Earth System Model, an ongoing, state-of-the-science Earth system modeling, simulation, and prediction project. It simulates most of the anthropogenic aerosol species, including sulfate, black carbon, and primary organic matter along with sea salt aerosols and natural dust.

Aerosols, usually linked with the burning of fossil fuels, can reduce the frequency of large-scale storms, leading to fewer rains. This new study suggests that the absence of these particles and lower greenhouse gas emissions caused the opposite effect, a big increase in rain. But the events that link the pandemic with the flood are more complex, the researchers said.

Lead author Yang Yang from Nanjing University told BBC that the aerosol reduction caused heating over land, while the decline in emissions caused cooling over the ocean. This intensified the temperature difference between land and sea while increasing sea level pressure and intensifying the winds bringing moist air to eastern China, which saw the intense rain.

Looking ahead, the researchers said the sudden change of the climate system driven by the Covid-19 crisis in 2020 would be very different from the changes triggered by the continuous but gradual emissions reduction to tackle global warming. This doesn’t mean we shouldn’t worry. If the world can’t reduce emissions faster, this could trigger more extreme weather events.

The study was published in the journal Nature.