Efforts to curb air pollution in China, a country already facing dire health impacts from high levels of soot and smog, will likely become increasingly difficult as the planet warms, a new study shows.

Increased heat waves and more periods of stagnant air resulting from global warming will worsen existing air pollution across much of China, the scientists concluded. This presents a heightened challenge for a country already choking on airborne pollutants that cause more than 1 million premature deaths there each year.

“For Chinese policy makers working to improve current air quality and protect public health, our finding is a daunting conclusion, and one that underscores the need to tackle the challenges of both climate change mitigation and air quality at the same time,” the authors wrote in the study, published Monday in the Proceedings of the National Academy of Sciences.

China’s government has been working to reduce emissions since a public outcry began several years ago over air pollution that has left cities blanketed in haze for days on end. During periods of severe pollution, officials in Beijing and other major cities have shut down factories, suspended schools and placed restrictions on driving.

A study published last fall found that the country’s concentrations of particulate matter and sulphur dioxide declined from 2015 to 2017 as the government took steps to reduce air pollution, but that concentrations of ozone, fueled by emissions from cars, power plants and chemical factories reacting with sunlight, still increased. And since then, coal use in the country has edged upward again.

The new study looked at how worsening global warming would affect China’s air pollution and human health by mid century. It used a combination of climate, air quality and epidemiological models and the assumption that countries will take some steps to reduce global greenhouse gas emissions so the temperature rise is kept to about 1.4 degrees Celsius at that point.

Under those warmer conditions, climate change would adversely affect future air quality for more than 85 percent of China’s population by mid century, the study found.

It would lead to increased concentrations of fine particulate matter and ozone — both of which can cause heart and lung damage — that would result in an additional 21,000 deaths per year in China. (A similar study published in July that assumed much higher global greenhouse gas emissions by mid century projected three times that many additional premature deaths. Both used current population and pollution levels to focus on the impact of warming.)

The biggest factor, the new study found, is stagnation of the atmosphere related to climate change.

A Bit Like 19th Century London’s Smog

“Stagnant air masses are in a way the worst conditions for air pollution to really bite,” said Hans Joachim Schellnhuber, director emeritus of the Potsdam Institute for Climate Impact Research and a co-author of the study.

“It is a bit like the 19th Century when the London smog happened, because it was a combination of the pollution from the chimneys and the factories and the other thing was the atmospheric conditions,” he said.

In 2013, when Beijing and other cities in China faced a pollution-filled haze dubbed the “airpocalypse,” scientists pointed to stagnant air conditions as contributing to the air pollution accumulation. In parts of heavily populated eastern China, the mountains help create conditions that make it easier for haze to sit in cities when high pressure systems and high humidity affect the region. The new study shows how big of a role those stagnant air events can play.

“Our results indicate that future climate change is likely to increase the risk of severe pollution events in China,” Qiang Zhang, a chemical engineering professor at Tsinghua University in Beijing and a co-author of the study, said via email. “Managing air quality in China in a changing climate will thus become more challenging.”

The study points to research that has connected haze-prone conditions in China with weather patterns caused by Arctic sea ice decline, the rapid warming of the Arctic compared to the lower latitudes, and the weakening of the winter monsoon.

Global warming is also expected to affect air quality elsewhere, including in the United States, the study says, “but compared with the United States, we project roughly an order of magnitude more climate-induced air pollution-related deaths in China.”

‘The Two Crises Are Hand in Hand’

The findings underscore the urgency with which China and other countries need to reduce emissions of greenhouse gases as well as other pollutants, said Ranping Song, the developing country climate action manager for the World Resources Institute.

“We always knew of the synergies and co-benefits of tackling climate change and air quality together, but now we know, if you want to take on air quality alone, without tackling climate change, it will be very challenging or those efforts will be in vein because the two crises are hand in hand,” Song said.

China’s commitment to the Paris climate agreement calls for its carbon emissions to peak by 2030. Given climate’s role in health, Schellnhuber said, policymakers may want to increase their greenhouse gas reduction goals.

“Because you have these exacerbating effects, China should try to peak much earlier regarding its emissions rather than 2030,” he said. “Of course you can reduce air pollution, but given the exacerbating factor of global warming, China should also do everything it can to bring down greenhouse gas emissions.”