Back in 1874, it was hard to see the sun in Chicago. The city was rebuilding after the Great Fire of 1871 by adding row after row of new factories, which belched so much smoke that city-dwellers came home caked in a thin layer of black dust after a day outdoors. And it wasn’t just Chicago: Other cities in the U.S. Rust Belt such as Detroit and Pittsburgh were enveloped by the same dark smog.
The soot even marked small birds, which collected it on their feathers while flying. Now, nearly a century and a half later, modern scientists are using those darker feathers to infer information about the history of pollution. Using digital photography and 1,347 bird specimens from three museums, biologist Shane DuBay and art historian Carl Fuldner were able to track the sootiness of the air in the Rust Belt from 1880 to 2015. Their study was published yesterday in the Proceedings of the National Academy of Sciences.
Scientifically called black carbon, soot is the result of burnt organic matter like wood, oil, and coal. It’s been linked to respiratory illnesses and cancer, and more recently has been recognized as a major contributor to climate change, Gregory Carmichael, a biochemical engineer at the University of Iowa, says. “Unlike other particles in the atmosphere, black carbon absorbs solar radiation because of its color and acts as a greenhouse gas.” And according to the Center for Climate and Energy Solutions, black carbon may be second only to carbon dioxide in driving global warming.
Though traces of black carbon can be found all around, there aren’t many ways to measure its prevalence in the atmosphere. Early naturalists observed that birds living near natural wildfires were darker in color due to soot: John James Audubon wrote in the Downy Woodpecker entry of Birds of America that its breast and belly were “soiled by the carbonaceous matter," noting that the grayed woodpeckers could be mistaken as a different species altogether. To see if darker feather color could be used as a pollution indicator, DuBay and Fuldner, both PhD students at the University of Chicago, scoped out the Field Museum’s vast historic collection, which includes 500,000 data-rich bird specimens. There, they noticed a spike in feather discoloration starting in 1882. “A lot of the earlier specimens are actually much more pristine than the specimen around the turn of the century,” Fuldner says. “This was one of the initial clues.”