Air pollution isn't just bad for your health. It can have dramatic effects on weather and climate. In fact, a team of scientists believes that air pollution from industries and traffic could have caused the extreme floods that devastated southwest China in 2013.
In July of that year, China's Sichuan province was racked with floods from the worst storms it had seen in 50 years. The greatest damage occurred in a mountainous region northwest of the Sichuan Basin, where nearly 30 inches of rain fell over several days. A combination of flooding and a landslide triggered by the rain left over 50 people dead, more than 100 missing and thousands without homes.
Watching news coverage of the flooding, atmospheric scientist Jiwen Fan noticed a pattern: The mountains with the worst flooding were downwind of the Sichuan Basin, a valley with air that's highly polluted from factories and cars. Surrounding mountains rise steeply from the valley floor, trapping polluted air over the basin. She suspected that the lingering cap of soot had something to do with the heavy rainfall that fell in the mountains, but not the valley.
Where did this soot come from? Soot is a type of pollutant called black carbon, which is produced by burning biomass or fossil fuels. When these fuels are burned, the process can release byproducts like carbon monoxide and very fine particles of black carbon. The black carbon particles are suspended in the atmosphere as an aerosol, which we call soot. Black carbon as soot affects climate and temperature by absorbing the sun's radiation before it gets to the earth. According to the Environmental Protection Agency, this can mean increasing temperatures, faster melting of snow and ice, and changes in clouds.
And, as Fan knew, soot can affect weather systems.
Fan, who researches at the Pacific Northwest National Laboratory, gathered a team of scientists to figure out the cause of the extreme weather. Their results were published in June in Geophysical Research Letters. The scientists built computer models of the weather in the Sichuan Basin, both with the current amount of air pollution, caused by decades of industrialization, and with clean air as it had been 40 years previously. In both models, they ran simulations of the weather on the day before and the days of the storm.
When they compared the two models, they saw a dramatic difference.
Without pollution, Fan says, the basin would have experienced mild rain during the day before the storm hit. Moist air would have moved in from the Indian Ocean over the land, risen and formed a cloud. Moisture condenses inside clouds to form droplets of water, which, when they get heavy enough, fall to the ground as rain. In this scenario, by the time that air rose over the mountains, it would have already lost a lot of its moisture, and extremely heavy rains wouldn't form.
Instead, she says, the soot prevented rain from forming over the basin. The pollution particles absorbed heat from the sun, warming the air higher up while blocking that warmth from getting to the surface. The moist air at ground level couldn't rise and form storms. It was stuck beneath the soot during the day, where it gathered moisture and energy, traveling downwind and reaching the mountains at night. As it rose, following the contours of the land below, it finally formed into clouds and released all that pent-up power. The result: massive storms that flooded the region.
"So basically," Fan says, "the pollution transformed this mild rain over a very large area of the basin during the daytime into a heavy rain, focused on a narrow area over the mountains." According to the team's calculations, the presence of air pollution made it rain up to 60 percent harder over the mountains than it would have without the pollution effect. So more rain fell in less time.
"We were surprised at the scale of the pollution effect," Fan says. The scientists suggest that in areas with similar geography, pollution might exacerbate weather in similar ways, leading to more extreme weather events.
Geeta Persad, a Ph.D. student in Princeton University's Atmospheric and Oceanic Sciences program, thinks this study is compelling. "It does a neat job of giving a concrete example of how air pollution in these systems can have detectable effects," she says. And in Persad's opinion, studies that link mechanisms with real-world events — such as air pollution causing flooding — are more useful to people making policy decisions than more general studies.
But, she says, it's important not to generalize too much. "The conditions they're pointing out may not be universal," Persad says. The soot layer in other regions, for instance, might be at a different height — either above or below the rain cloud. Also, other locations might not get the same kind of moist air inflow, or might have different topography or be polluted with a different aerosol.
Fan is aware of these limitations. Right now, she says, her team is working to understand the weather effects of different types of aerosol pollutants in several locations. In the meantime, she thinks weather reports should take into consideration the aerosols in the atmosphere. Since air pollution particles have a short life span, cutting down on pollution could have immediate effects in preventing severe weather events.