Guest essay by Eric Worrall

Climate mitigation causes more rain? According to scientists analysing the 2020 Chinese floods, the drop in greenhouse gas emissions and coal smoke aerosols during the 2020 Covid shutdown led to increased rainfall.

Climate change: Covid shutdown linked to record rainfall in China

By Matt McGrath
Environment correspondent

Scientists say that a rapid drop in emissions because of Covid played a key role in record rainfall in China in 2020. 

The decline in greenhouse gases and small particles called aerosols caused atmospheric changes that intensified the downpours. 

Hundreds of people died and millions more were evacuated during a summer of record rainfall. 

But long-term cuts in emissions are unlikely to trigger similar events.

Many parts of eastern China experienced severe flooding in June and July in 2020. The researchers say the reductions in emissions contributed about one third of the extreme summer rain.

…

A number of scientific studies have looked at what caused the flooding events, some pointing to the extreme conditions in the Indian Ocean. 

Now an international team has put forward a new theory. They argue that the abrupt reduction in emissions of greenhouse gases and aerosols, caused by shutdowns during the Covid-19 pandemic, was a key cause of the intense downpours.

…

“There was heating over land due to aerosol reductions but also cooling over the ocean due to a decrease in greenhouse gases, which intensified the land/sea temperature difference in the summer,” explained lead author Prof Yang Yang from Nanjing University of Information Science and Technology, in China.

“This in turn, increased sea level pressure over the South China/Philippines sea and intensified the winds bringing moist air to eastern China which then saw intense precipitation.”

…

Read more: https://www.bbc.com/news/science-environment-60423329

The abstract of the study;

Published: 18 February 2022

Abrupt emissions reductions during COVID-19 contributed to record summer rainfall in China

Yang Yang, Lili Ren, Mingxuan Wu, Hailong Wang, Fengfei Song, L. Ruby Leung, Xin Hao, Jiandong Li, Lei Chen, Huimin Li, Liangying Zeng, Yang Zhou, Pinya Wang, Hong Liao, Jing Wang & Zhen-Qiang Zhou 

Abstract

Record rainfall and severe flooding struck eastern China in the summer of 2020. The extreme summer rainfall occurred during the COVID-19 pandemic, which started in China in early 2020 and spread rapidly across the globe. By disrupting human activities, substantial reductions in anthropogenic emissions of greenhouse gases and aerosols might have affected regional precipitation in many ways. Here, we investigate such connections and show that the abrupt emissions reductions during the pandemic strengthened the summer atmospheric convection over eastern China, resulting in a positive sea level pressure anomaly over northwestern Pacific Ocean. The latter enhanced moisture convergence to eastern China and further intensified rainfall in that region. Modeling experiments show that the reduction in aerosols had a stronger impact on precipitation than the decrease of greenhouse gases did. We conclude that through abrupt emissions reductions, the COVID-19 pandemic contributed importantly to the 2020 extreme summer rainfall in eastern China.

Read more: https://www.nature.com/articles/s41467-022-28537-9

Reading the study, what they are saying is the Covid shutdown reduced coal smoke aerosols, which in turn caused the land to heat more than usual, triggering a massive updraft which drew in wet monsoon air from the sea. The enhanced rainfall washed even more aerosols out of the air, causing a positive feedback loop which led to widespread flooding.

They also claim reduced greenhouse gas emissions helped strengthen the weather pattern, by reducing ocean heating, leading to a greater temperature difference between land and sea.

Would a long term emissions and aerosol reduction lead to a long term increase in flood risk?

… “It’s a good question,” said Prof Yang. 

“Because emissions were reduced dramatically in early 2020 when the Covid-19 pandemic emerged, it caused an immediate and abrupt change in various components of the climate system.”

“Such sudden change of the climate system would be very different from changes in response to continuous but gradual policy-driven emissions reductions.” …

Read more: https://www.bbc.com/news/science-environment-60423329

It is an interesting theory. Living in the subtropics by the sea I regularly see onshore winds, presumably triggered by land heating relative to the sea, which seem to lead to towering thunderheads forming near the sea shore.

The authors themselves advise caution, the study is model heavy.

But the study is interesting in that it goes against the flow of most claims, that climate change causes more rain. In this case, scientists are suggesting that climate mitigation, both through Xi Jinping’s efforts to reduce air pollution, and the 2020 Covid shutdown, triggered the deadly floods – although they are more cautious about predicting the outcome of long term, more gradual mitigation.