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Cyclists in Lianyungang, China, ride through winter smog that could be exacerbated by Arctic weather patterns.

Si wei – Imaginechina/AP

Why is China’s smog so bad? Researchers point far away to a melting Arctic

In China, the winter of 2013 was an “airpocolypse.” A thick soup of harmful smog cloaked its biggest cities, contributing to at least 90,000 deaths and sickening hundreds of thousands more. Things haven’t gotten much better since then, even though the country has enacted tough new emissions controls. A new study may explain why. Melting sea ice and increasing snow across Siberia have altered large-scale weather patterns, replacing currents that used to ventilate China with stagnant air that lets pollution accumulate.

“The ventilation is getting worse,” says study author Yuhang Wang, an atmospheric scientist at Georgia Institute of Technology in Atlanta. “We think climate change, as it is driving rapid warming of the Arctic, is having a large effect on pollution in China.”

Air pollution, a perennial challenge in China’s rapidly growing cities, became a full-blown national crisis during the winter of 2012–13, when 70% of the nation’s 74 largest cities logged pollution levels above government air quality standards. Of special concern were high levels of “PM2.5,” particles smaller than 2.5 micrometers that can lead to heart and lung problems. Citizens flooded the social network Sina Weibo with photos of themselves wearing colored air masks and giant buildings vanishing beneath blankets of smog. Stock prices of air purifier companies surged as the term PM2.5 became a catchphrase.

Since then, the country has deepened its efforts to reduce pollution, with a new focus on monitoring and reducing emissions of PM2.5. But Wang and his co-authors realized that pollution is only part of the problem. Using historic wind and air temperature data across eastern China, they created an index of air stagnancy, dubbed the Pollution Potential Index, or PPI. It accounts for the intensity of westerly winds blowing pollution out to sea and vertical mixing that can dilute it skyward. PPI values showed that “the ventilation conditions in January 2013 were the worst in 3 decades of data,” Wang says.

The researchers also wanted to understand what was driving the stagnant conditions. They examined a host of global phenomena, including El Niño, large-scale circulation patterns in the Pacific Ocean, and changes in snow and ice in the Arctic. Of these, they found that low levels of sea ice and increased snowfall on land were the most correlated with changes in PPI. Both factors are thought to be effects of a warming, wetter Arctic, because some scientists think open seas can lead to the increased evaporation that drives snowfall. And indeed, in the fall of 2012, record lows of September sea ice preceded high snowfalls across northern Eurasia.

Wang’s team then conducted modeling experiments that removed ice and added snow to the Arctic, comparing those scenarios to control runs with the Arctic in its previous state. Both factors appeared to strengthen an observed high-pressure system of cold air above Siberia in December 2013. That mountain of cold air appears to have altered other pressure systems in Asia, nudging the monsoon winds that usually ventilate eastern China east into Japan, the team reports today in Science Advances.

Siberian highs tend to suppress the ventilation in China, notes Jennifer Francis, a meteorologist at Rutgers University in New Brunswick, New Jersey, who has studied possible links between the changing Arctic and global weather patterns. “When it’s strong the atmosphere [in the region] tends to be dry and stable” she says. “The findings are important for the health of billions of people.”

They also mean China’s task in cleaning up its cities will be tougher. Reducing PM2.5 and other pollutants that affect air quality is important, Wang says, but so is cutting the greenhouse gas emissions that are warming the planet and melting the Arctic—a far thornier problem. He says recent trends suggest that low ventilation conditions will endure in China for years to come as the Arctic continues to warm.

The study underscores the interconnectedness of the global climate system, says Judah Cohen, a climatologist at Atmospheric and Environmental Research in Lexington, Massachusetts, who was not involved in the work. “A rapidly changing Arctic has likely influenced and will continue to influence our weather and society in ways we just never considered previously.”