How Climate Change Covered China in Smog

When smog gets bad, the air becomes more than a coolness on your skin or a haze on the horizon. When smog gets bad, you can taste it.“Today, Shanghai air really has a layered taste. At first, it tastes slightly astringent with some smokiness. Upon full contact with your palate, the aftertaste has some earthy bitterness, and upon careful distinguishing you can even feel some dust-like particulate matter,” Alan Yu, a gourmet chef who lives in Shanghai, told The Telegraph in 2013.Yu got to know those tastes pretty well. In January of that year, a Greenland-sized cloud of toxic air pollution smothered the plains of eastern China, shrouding the skylines of China’s largest cities in a sickly gray-yellow fog. The U.S. Embassy in Beijing reported that levels of PM 2.5—a type of particulate air pollution that seeps into lung, vein, and heart tissue—surged above 850 micrograms per cubic meter; the UN says that 20 micrograms per cubic meter is the highest safe level.
Schools, highways, and airports closed. More than 7,000 children were sent to Beijing Children’s Hospital on a single day. At least 50 of the country’s 74 major cities exceeded national air-quality standards. The episode remains the worst haze pollution ever recorded in the month of January.Something was strange about the smog. Usually smog will dissipate when sources of air pollution—like cars or factories—shut down for a time. But this cloud was stubborn. As part of solar new year celebrations in February 2013, millions of families drove their cars out of Beijing to go on vacation, and the government ordered all factories to cease operations. The smog didn’t subside much, and less than a week later the full-on “airpocalypse” returned as bad as before.What made the winter smog so bad that year—and in the winters since, which have also been stubbornly smoggy? Two new studies revisit the episode. Both of them argue that climate change will make this kind of smog event much more common. And, remarkably, one of them asserts that the Chinese smog of January 2013 was worsened by two weather phenomena thousands of miles away. Because the Arctic Ocean froze less than it usually does, and because higher-than-usual snowdrifts piled up across the boreal forests of Russia, millions of Chinese people were subjected to some of the worst air pollution ever measured.That’s because the smog of January 2013 wasn’t the result of emissions alone: Weather played the accomplice. For most of that winter, air over eastern China barely circulated. Trade winds went dormant, so smog could not ventilate to the east; and vertical circulation slowed, meaning particulate matter could not float up into the higher atmosphere. And as is typical for Chinese winters, rain never arrived to wash air pollution out.

The first paper, published Monday in Nature Climate Change, uses an ensemble of 15 climate models to suggest that those wind conditions—which lead to a stagnation of air across China’s most populated regions—are more likely to occur in the century to come. The haze-inducing conditions are 50 percent more likely to form between 2050 and 2099 than they were from 1950 to 1999. And once those patterns fall into place in any one episode, they are 80 percent more likely to persist.
These calculations assumed a worst-case emissions scenario, so any reduction in projected carbon-dioxide emissions (to meet the goals of the Paris Agreement, for instance) would likely reduce the frequency and persistence of haze episodes.A second paper goes further. A team at Georgia Tech asked: What in the world, specifically, shaped those wind patterns? They applied a rudimentary form of artificial intelligence to historical climate and pollution data. Ultimately, they found two big influences: the extent that the Arctic Ocean has frozen over, and the amount of Siberia and the Russian far east covered in snow. These shape the climate of China mostly through their differing temperatures. And when the Arctic is especially warm, and Siberia is covered with snow and especially cold, they combine to reduce the atmospheric-pressure gradient across Asia. This weakens the pull of the East Asian monsoon wind and leaves air stagnating across eastern China.“In our analysis, these two effects have a similar impact. And if you put them together, you have a bigger impact,” said Yuhang Wang, a professor of atmospheric chemistry at Georgia Tech.These two weather events seem to be getting more common, and more intense, as the climate keeps warming. This means that even as China succeeds in reducing some of its emissions, the winter haze may worsen for meteorological reasons. Really, this is what’s already happening: Even in January 2013, there was no reported massive surge in factory emissions. The sudden build-up of smog was all meteorology. “I see this as an offsetting effect,” Wang told me. “Polar, high-Arctic changes are offsetting the effort that China has been putting into emissions reductions.”
Noelle Selin, a professor of Earth and atmospheric sciences at the Massachusetts Institute of Technology, said the paper was important because it showed how local weather could shape global air pollution. “It’s a real different approach than has been traditionally taken,” she told me. Most smog research uses an atmospheric-chemistry model and not a model of the planet’s entire climate system. “And I think we definitely need both approaches.”She added that she had pondered whether sea ice or Eurasian snow were shaping air-pollution patterns in the United States or Europe. “Being able to quantitatively test [that effect] is a really unique contribution of this paper,” she said.Though there’s been research into how climate affects air pollution, it mostly focuses on the more developed parts of the world. Wang’s team at Georgia Tech, for instance, has found that dry, warm, autumnal air in the American southeast is leading to record-breaking ozone pollution in the region, even though industrial emissions levels are down.Fewer studies have focused on China because its contemporary air-pollution problem is so dire. But studies like this help reveal why addressing air pollution and climate change are twin political priorities for Beijing. If air pollution is a political problem today, imagine how bad it could be when climate change intensifies it. And President Xi also has a near-term reason to figure out winter pollution: Beijing is hosting the Winter Olympics in 2022.Speaking of low Arctic ice and smog over Beijing, Selin said she could think of few other atmospheric phenomena where such distant events were so closely correlated. The best examples, she said, were the periodic temperature swings in the oceans, like El Niño and La Niña. Yet the fact of this world is that there is always a vast and planetary dance, where silent icebergs in a cold, dark sea shape the pollution that skips across an urbane chef’s tongue.