CESM-simulated responses of HWOP frequency. Image: Gao, McElroy et al, PNAS

Joint heat and ozone extremes in China

10 August 2023

Scientists from Hong Kong Baptist University and Harvard have developed an early warning system to predict the joint occurrence of heat and ozone extremes.

The combination of extreme heat and poor air quality, exacerbated by high temperatures, poses a significant threat to vulnerable groups like children, seniors, and people with preexisting respiratory illnesses. The situation is particularly alarming in the light of increasing temperatures and heat waves, coupled with energy-intensive development with associated ozone precursors, volatile organic compounds (VOCs), and oxides of nitrogen (NOx).

The collaborative research team, Dr Meng Gao from Hong Kong Baptist University and Professor Michael McElroy from the School of Engineering and Applied Sciences, Harvard University, used past meteorological data and daily ozone levels to identify patterns that would help predict these extreme events. The research was published in the Proceedings of the National Academy of Sciences.

Due to the lack of long-term daily observations of ground-level ozone concentrations, a sophisticated machine learning model was employed to reconstruct levels dating back to 2005. The team recognized 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.

Fan Wang, a member of the research team, said “Our research could have important implications in the future that would allow agencies such as the Ministry of Ecology and Environment in China to prepare for high summer heat and ozone in springtime.”

Warm sea surface temperatures in these regions lead to a decrease in precipitation, cloud cover and circulation across this region of China, known as the North China Plain, which is home to roughly 300 million people.

“These sea surface temperature anomalies influence precipitation, radiation and more, which modulate the co-occurrence of heat waves and ozone pollution,” said Gao.

The team’s model correlated these anomalies with increases in heat waves and ozone about 80 percent of the time.

Governmental agencies could use these predictions to not only issue warnings for human health and agriculture but to reduce the components of ozone and its precursors in the atmosphere before the extreme heat waves hit.

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