An early warning system for joint heat and ozone extremes in China

High temperatures exacerbate ground-level ozone production, resulting in a deadly combination of extreme heat and poor air quality that is particularly hazardous for vulnerable populations such as children, seniors, and individuals with preexisting respiratory issues.

China, like many other countries, is facing rising temperatures and more frequent and longer heatwaves. However, the rapid and energy-intensive development in China has led to increased production of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x), which are the main precursors of ozone. This combination poses a significant health risk, especially in densely populated urban areas such as Beijing.

A collaborative team of researchers from the Harvard-China Project at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Hong Kong Baptist University has now identified large-scale climate patterns that can be used to predict the co-occurrence of extreme heat and ozone days in China months in advance. Similar to predictions for hurricane and wildfire seasons, these forecasts could help the government prepare resources and implement policies to mitigate the severity of the season.

The findings of the research were recently published in the Proceedings of the National Academy of Sciences.

“We have already witnessed record-breaking heatwaves worldwide, including in China, where local emissions have resulted in significant ozone pollution,” explained Fan Wang, a visiting fellow at SEAS and the Harvard-China Project, Ph.D. candidate at Hong Kong Baptist University, and co-lead author of the study. “Our research could have important implications in the future, allowing agencies such as the Ministry of Ecology and Environment in China to prepare for high summer heat and ozone in springtime.”

The research team, led by Michael McElroy, the Gilbert Butler Professor of Environmental Studies at SEAS and the faculty chair of the Harvard-China Project, and Meng Gao, professor at Hong Kong Baptist University and former postdoctoral researcher at SEAS, analyzed past meteorological data and daily ozone levels to identify patterns that could be used for predictions.

Due to the limited availability of long-term daily observations of ground-level ozone concentrations, the researchers utilized a sophisticated machine learning model to reconstruct levels dating back to 2005. Using this dataset, the team identified patterns in sea surface warming in the western Pacific Ocean, the western Indian Ocean, and the Ross Sea, off the coast of Antarctica, that preceded summers with high heat and ozone in northeast China, including Beijing.

Warm sea surface temperatures in these regions lead to a decrease in precipitation, cloud cover, and circulation in the North China Plain, which is home to approximately 300 million people.

“These sea surface temperature anomalies influence precipitation, radiation, and more, which affect the co-occurrence of heatwaves and ozone pollution,” said Gao, co-first author of the paper and Associate of the Harvard-China Project.

The team’s model successfully correlated these anomalies with increases in heatwaves and ozone approximately 80 percent of the time.

Governmental agencies could utilize these predictions to issue warnings for human health and agriculture and to reduce ozone and its precursors in the atmosphere before the onset of extreme heatwaves.

“The ability to forecast the likelihood of unusually hot summers and high levels of summertime ozone in China based solely on temperature patterns observed months earlier in remote ocean regions is truly exciting,” said McElroy.

This study was co-authored by Yihui Ding, Zhiwei Wu, Yangyang Xu, Xiao Lu, Zifa Wang, and Gregory R. Carmichael.

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