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Study: Climate change exacerbates Henan storms in 2021

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From July 17 to 23, 2021, a rare torrential rain swept across Henan. Cities and villages have become swamped countries, hospitals have power outages, subways have been flooded, and river banks have burst. The three-day rainfall in Zhengzhou, the provincial capital with a population of nearly 13 million, is close to that of the whole year in previous years. Continental meteorological observation records the historical extreme value, reaching 201.9 millimeters. Meteorologists called this rainstorm that lasted for several days “21·7” extreme rainstorm in Henan.

According to the survey report released by the Disaster Investigation Team of the State Council, the severe rainstorm disaster in 2021 caused 398 deaths and missing persons in Henan Province due to the disaster, more than 14 million people were affected, and the direct economic loss reached 120.06 billion yuan (about 16.5 billion U.S. dollars). Among them, Zhengzhou is the most severely affected place.

Recently, a research team composed of Chinese scientists used model simulations to confirm the link between climate change and this extreme weather event: climate change increased the precipitation intensity of this torrential rain by 7.5%. The findings were published in the October 31 issue of Science Bulletin. “Science Briefing” is a multidisciplinary academic journal under the supervision of the Chinese Academy of Sciences.

After the disaster, whether climate change caused by human activities contributed to this rare torrential rain has become one of the focal issues of media attention. Many experts interviewed by Caixin, Beijing News and other media at the time said that although climate change is closely related to the occurrence of extreme weather, for extreme weather that occurs in small and medium-scale areas such as Henan and even Zhengzhou, there are many influencing factors, so it is attributable to complex, it is difficult to draw firm conclusions. The above research provides an intuitive and quantitative answer to this question.

Specifically, the researchers set the simulation time for the three days from July 19th to 21st, and conducted four sets of simulation experiments using a meteorological research and forecast model with a horizontal resolution of 4km.

The first set is the standard weather hindcast simulation (A2021), and the other three sets of experiments represent the scenario without human-induced climate change (N2021)—the control group, and the low-emissions scenario (SSP1-2.6) and medium Projections of future conditions under emissions scenarios (SSP2-4.5). The so-called hindcast means that the researcher inputs the relevant data of past events into the model, and compares the matching degree of the output result with the known result to test whether the model is valid. In A2021, the simulation results accurately reproduced the heavy rain in Henan.

According to the setting of various emission scenarios in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), SSP1-2.6 means that global carbon emissions will begin to decline before 2025, and will reach carbon emissions after 2050. Neutralization, by the end of this century, the global temperature will be 1.8°C higher than before industrialization; SSP2-4.5 means that the current level of global carbon emissions will continue until 2050, and then begin to decline, but carbon neutrality will not be achieved by the end of this century, At the same time, the global temperature rise will reach 2.7°C.

All experiments differ only in parameters such as temperature and humidity that determine the thermodynamic state of the weather system, so the differences in simulation results can be attributed to warming and humidification caused by human activities.

According to the difference between the simulation results of A2021 and N2021, the researchers found that the warming and humidification of the climate caused by human activities increased the rainfall intensity of the heavy rain in Henan by about 7.5%, because the increase in ocean surface temperature caused by human activities evaporated more water vapor into the atmosphere, and these water vapors were easily transported by two typhoons in the northwest Pacific Ocean at that time, In-Fa and Cempaka. And in warmer climates, wetter storm interiors release more latent heat, making convection more intense.

“Considering the extreme nature of the “21·7″ rainstorm in Henan, the 7.5% increase in precipitation is not a small amount. In addition, the 7.5% increase in precipitation is the result of the regional average of the entire Henan Province, and some areas with a large increase in precipitation can reach 15- 20%. Therefore, the impact of global warming on the ’21•7′ Henan rainstorm cannot be ignored,” said Wang Jun, one of the authors of the paper and an associate professor at the Institute of Atmospheric Physics, Chinese Academy of Sciences.

The researchers pointed out that although a resolution of 4 km is sufficient for attribution studies at the Henan provincial scale, it is not sufficient for attribution at the local scale because it is difficult to determine the impact of specific topography on convection. This may also be the reason why the observed maximum rainfall center in Henan is in the north-central region (left image), while the simulated Henan maximum rainfall center is in the south (right image).

图片来源: Wang et al., On the role of anthropogenic warming and wetting in the July 2021 Henan record-shattering rainfall, 2022

However, the researchers also said that although higher horizontal resolution may be able to better simulate local convection conditions, doing so is too computationally expensive and time-consuming, and has very limited impact on quantitative attribution at the provincial scale .

In order to assess the impact of climate change on the extreme rainfall that may occur in the future, the researchers also simulated the intensity of extreme rainfall at the end of this century under the two emission scenarios of SSP1-2.6 and SSP2-4.5, and found that the total rainfall of 3 days will be That’s 14.3% and 21.9% higher than the horrific rainfall in 2021, respectively.

In addition, the researchers found that warming and wetting due to historical emissions may have doubled the probability of catastrophic extreme hourly rainfall events (100mm/h) (from 0.0021% to 0.0049%), while at more Under the high emission scenario, by the end of this century, the probability of extreme rainfall events will quadruple from the current level (from 0.0049% to 0.0203%).

“We found a systematic increase in the risk of more severe events as the climate warmed,” the researchers wrote in their paper.

About the author: Xia Zhijian is a freelance writer who focuses on environmental issues and lives in Chengdu.

Note: This article was originally published on the environmental website“chinadialogue”. FTChinese.com reprinted this article with the authorization of chinadialogue. chinadialogue is a Chinese-English bilingual website dedicated to environmental issues, headquartered in London, is an independent non-profit organization.

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