WHEN THE WORLD fully transitions from cars that run on dinosaur juice to cars that run on electricity, humanity will have eliminated a major source of planet-warming carbon dioxide and a major threat to human health—air pollution kills nearly 550,000 children under age 5 each year. But a hidden environmental threat from cars will persist, and perhaps get worse as more of the world enters the middle class, putting more vehicles on the road: the microplastics that shear off cars’ tires and brakes. Tires are made of rubber but also contain synthetic elastomers and fibers to improve stability; brakes are a mixture of metal and plastic. Little fragments of these materials erode with friction whenever rubber meets the road or you hit the brakes, and these pieces end up in the gutter. Later, they wash out to sea in rainwater, or get caught up in the wind.
Today in the journal Nature Communications, researchers model how microplastics from our cars are traveling from densely populated regions into the environment. These little automotive bits pour from the cities of Europe, Asia, and the Americas, and settle out in the Arctic, Greenland, and the world’s oceans. The researchers find that the mean lifetime for the smallest particles, which more easily get caught up in winds, is nearly a month. Their modeling calculates that 52,000 tons of the smallest particles end up in the sea each year, and 20,000 tons end up in remote snowy and icy regions.
By combining data on tire and brake wear with existing methods of calculating the transportation of pollutants in the atmosphere, the scientists build on a growing body of evidence that the wind is dispersing an astonishing amount of microplastics, both near and far. “Small particles are lofting higher, of course. But they also weigh less than larger ones and can easily reach remote regions under favorable meteorological conditions,” says Nikolaos Evangeliou, senior researcher at the Norwegian Institute for Air Research and lead author of the new paper. “Larger particles are usually deposited near the sources.”
This jibes with fieldwork that over the last few years has found microplastics far away from human activity, such as on the tops of the French Pyrenees, in formerly-pristine regions like the Arctic, and falling from the sky onto protected areas in the western US national parks. “Generally speaking, it is an important study because it highlights just how important the atmosphere is in terms of microplastic transport, especially to the ocean and remote areas such as the Arctic,” says marine ecologist Melanie Bergmann of the Alfred Wegener Institute for Polar and Marine Research, who studies microplastics but wasn’t involved in this new work.
Bergmann’s own field research last year found that microplastics are indeed blowing from Europe into the Arctic. A whole lot of them, too: In a single liter of snow, she found 14,000 plastic particles. This new research, she says, “helps to explain why we found such high amounts of microplastic in Arctic snow, but also in our Arctic Ocean samples, all the way down to the deep Arctic seafloor—13,000 microplastic particles per kilogram of sediment.”
But because this new research is based on atmospheric models rather than fieldwork, she continues, “we need more empirical data and experiments to validate the results and understand the underlying processes, especially to find out if colored microplastic in ice and snow does decrease the reflectance of sun light and thereby enhances global heating.”
This reflectance is known as albedo, and it’s a serious concern in the Arctic. Because ice is white, it reflects a good deal of the sun’s energy back into space compared to the land, which is darker and absorbs more energy. One of the reasons the Arctic is warming twice as fast as the rest of the planet is that as ice disappears in a warming world, it exposes darker waters or land, further heating a region in a nasty feedback loop. Now that scientists know the Arctic is laced with microplastics blown in from Europe, and now that this new work has modeled that route of transport in fine detail, they are concerned that synthetic particles might be darkening snow and ice, accelerating melting.
“We believe it might be the case,” says Evangeliou. “We are currently making simulations to calculate the climatic parameter of the microplastic dispersion, but it is rather speculated in the paper as a possible impact on the climate.”
The easy transportation of these particles comes down to the dual charms—and evils—of plastics: These materials are both extremely lightweight and extremely tough. Car tires and brakes are meant to last, and their chemical composition means that after the particles slough off, they just break into smaller pieces as they tumble through the environment. That plastic never actually goes away—it just disintegrates.
Given what scientists already know about the transportation of objects in the atmosphere, perhaps it’s not surprising that microplastics from cars are so readily blowing around the world. In June, the Sahara desert did its annual burping of dust, much of it quite coarse, which blew clear across the Atlantic and settled in the southern US. “That dust is largely mineral material, which is way more dense than plastics are,” says Janice Brahney, an environmental scientist at Utah State University. (Her own research has found that microplastics are falling out of the sky, but she wasn’t involved in this new research.) Spiders ride the wind, too: Some species let out a bit of silk that lofts them high into the air, known as ballooning, flying hundreds of miles.
But the flight of microplastic particles gets tricky, because they come in such a wide array of shapes. Wispy microfibers might take to the air much like spider silk, but tiny chunks of tires and brakes presumably behave differently in the atmosphere. “It is very difficult to understand the physics of what might keep a particle afloat that isn’t a perfect sphere,” says Brahney.
Different kinds of plastics might behave differently as well, given their varying densities. This almost certainly complicates the patterns that the authors of the new paper noticed: Smaller particles travel farther. Say two microfibers are the same shape, but different weights. Or say that a microfiber and a fragment of microplastic are different shapes, but the same weight. How does this affect how they travel through the atmosphere?
And even if they stay at ground level, microplastic particles from tires and brakes end up escaping the freeways, in this case by flowing out to sea. Last year, scientists calculated that the densely-populated cities surrounding the San Francisco Bay are washing 7 trillion—yes, with a “t”—microplastic particles into that body of water each year. Tiny bits of plastic slough off tires and brakes and accumulate on roads before rainwater washes all the gunk into the sea.
The big question that scientists are just beginning to answer: What implications might this insidious new kind of pollution have for ecosystems and human health? Microplastic particles are certainly small enough to penetrate deep into our lungs, and could be leaching their component chemicals while in there. In the sea at least, microplastics have also been shown to accumulate bacteria and viruses—whether they’re doing the same while floating in the air is to be determined.
“It seems to be one of the first questions people ask me,” says Brahney. “And I’m not a doctor, so I have a hard time answering it. But like: How are we ingesting it?” Might the littlest bits of microplastic actually pass through the gut lining and work their way into other organs? How long do the particles persist in the lungs? These are all questions that scientists are starting to chip away at.
And while we may not have those answers right away, in the meantime, we will probably have more cars. One recent Goldman Sachs report estimated that by the year 2025, new car sales in emerging markets would be more than double those in other nations, a consequence of per capita income gains. The pandemic may only accelerate the trend as people avoid public transportation for fear of contracting Covid-19, or as transit agencies trim schedules or try carshare-style alternatives. More wheels and more brakes mean more microplastics. Bumpy roads are ahead, that’s for sure.
wired.com, 14 July 2020