In the winter of 2013, Beijing and other cities in the East China Plains became blanketed in thick, eye-stinging, gray smog. The pollution levels were off the charts: people were told to stay indoors, roads were closed, and hundreds of people were hospitalized with respiratory issues. On some days, the smog was so bad that it could be seen in satellite photos.

News articles at the time attributed the so-called “airpocalypse” to China’s heavy coal emissions. But that’s only part of the story, according to a new study published today in Science Advances. There was another culprit: climate change.

What happened was this: the sea ice in the Arctic has been shrinking and the snowfall in Siberia has been increasing. This has shifted the winter monsoon that wipes out the smog in the East China Plains, allowing pollution to build up in the air. The study shows how global climate change can have real impacts on air quality at a local level, and should prompt countries around the world to work together to reduce greenhouse gas emissions. Climate change is not a faraway problem, this study says — it’s already affecting China’s citizens.

“This is the first study that shows that climate changes in the high Arctic had a significant effect in winter haze in China,” says study co-author Yuhang Wang, a professor in Georgia Tech’s School of Earth and Atmospheric Sciences. “It really is a very connected problem.”

After the 2013 airpocalypse, which sparked outrage in China and abroad, the Chinese government introduced stricter targets to cut down emissions. But even though emissions are down, the winter haze continues in this part of China, Wang says. That was the first suggestion that something larger was at play — it wasn’t just that the smog was bad. Wang had a hunch, though, and that hunch came directly from LA.

Similar geography, and similarly stagnant air

Los Angeles has historically bad pollution problems because it sits in a small basin region with the ocean to the west and the mountains to the east. The East China Plains — where the capital Beijing sits, as well as a bunch of other industrial centers — are a bit like LA: it has mountains to the west and sea to the east. “It’s like a mirror image of Southern California,” Wang says. Similar geography, and similarly stagnant air.

For that reason, this area relies on the East Asian Winter Monsoon — and its winds — to wipe out the smog. But in 2013, the monsoon shifted to the east, toward Japan and Korea, missing the East China Plains and allowing pollution to build up. Why did this happen? Wang and his colleagues pored over years of data on air temperature, wind speeds, pollution, and climate features like sea ice, snowfall, and El Niños to find the answer. They saw that there was a correlation between stagnant air conditions over China and Arctic sea ice, which reached a record low in 2012, as well as snowfall in Siberia, which reached a record high that winter.

Less sea ice allows more ocean water to evaporate, which leads to a more humid atmosphere and greater snowfall in Siberia, says Anthony Wexler, director of the Air Quality Research Center at University of California, Davis. The snow reflects sunlight — and energy — back into space, changing the pattern and strength of the East Asian Winter Monsoon that forms up north. That means bad air quality in the East China Plains. “When you shut down the winter monsoon, there’s not much going on in that region and the haze just accumulates,” Wang says.

The study has limitations — it shows a correlation, not a direct causal link, between ventilation in China and sea ice and snowfall in the polar region. But that’s normal, especially when you’re analyzing meteorological conditions at such a large scale, says Daniel Mendoza, a postdoctoral research associate at the University of Utah’s Department of Atmospheric Sciences. “They tried to be as thorough as possible,” he says.

Now, researchers should try to better understand why exactly that correlation exists, says Geeta Persad, a postdoctoral research scientist at the Carnegie Institution for Science's Department of Global Ecology at Stanford University. Also, we need to understand whether this information can be used to predict when future airpocalypses are going to happen, Persad says, so the health effects can be limited. “That predictability aspect is very important for local officials getting the resources in place they need to prevent the loss of life and human impact of these events,” Persad says.

What’s important is that the study shows how global climate change can have ripple effects at the local level. It also implies that, unless global warming is leveled or stopped, similar airpocalypses will continue to occur. So while China can try to reduce its own greenhouse gas emissions, this is a problem that goes beyond China. “The reality is the whole world is interconnected,” Mendoza says. “It can’t just be one city. It has to be beyond a country or region, it has to be the whole world addressing this. Together as a world community we need to think about all these effects that are happening.”