It's a striking image—two rows of birds lined up side-by-side, one with white bellies, and the other nearly entirely black.
The birds are the same species of horned lark, a naturally white bird with a yellow chin, and have become part of an unconventional record of air pollution in the U.S. Rust Belt over the past 135 years.
Looking at more than 1,300 birds from five species that flew over the Rust Belt and were contained in natural history collections, two graduate students—Shane DuBay and Carl Fuldner from the University of Chicago—were able to tell how much black carbon, also known as soot, accumulated in bird feathers during the year it was collected. While the bird species have noticeable differences in color, the exact amount of black carbon contained within their feathers was measured by photographing the amount of light that bounced off of them.
Black carbon is a type particulate matter emitted largely by gas and diesel engines and coal-fired power plants. The particles effectively absorb sunlight and prevent it from being reflected into the atmosphere. Birds, which only molt a new set of feathers once a year, became unwitting feather dusters, collecting the particulate matter. Using these photographs, DuBay and Fuldner were able to effectively collect a visual record of how much black carbon the birds collected.
"If you look at Chicago today, the skies are blue. But when you look at pictures of Beijing and Delhi, you get a sense for what U.S. cities like Chicago and Pittsburgh were once like," DuBay said in a press release announcing the research, which was published in the Proceedings of the National Academy of Sciences this week. (See what it's like to live in the world's most polluted city.)
Both researchers described working with the birds as similar to handling a newspaper—the soot wasn't easily removed from the specimens, but the gloves they used to handle the carcasses collected inky black stains.
Using data from the birds, DuBay and Fuldner were able to see how black carbon levels corresponded to historical data on U.S. environmental regulations or fuel consumption. For example, during the Great Depression, coal consumption dropped, and they saw less soot on bird feathers. But during the WWII production boom, bird feathers darkened once again.
While the birds present an unconventional historic record, they're hardly a relic of the past. Particulate matter, while less prevalent in the U.S., is still emitted and can still exacerbate health conditions like asthma and lung disease, according to the American Lung Association.
"We know black carbon is a powerful agent of climate change, and at the turn of the century, black carbon levels were worse than previously thought," DuBay said. "I hope that these results will help climate and atmospheric scientists better understand the effects of black carbon on climate." (Learn how soot can melt snow.)
The new research comes at a time when the burning of fossil fuels remains hotly debated. On Monday, the Trump administration announced plans to roll back the Clean Power Plan, a sweeping Obama program that aimed to, among other regulations, reduce the amount of coal burned on a state-by-state basis. (See a running list of how Trump is changing the environment.)
So what does that have to do with black carbon?
While much of the world's black carbon comes from developing nations, 0.51 million metric tons of black carbon were emitted in the U.S. in 2011, according to a report by the Arctic Council. The lion's share came from transportation, but a few percentage points were contributed by coal-fired power plants.
"This study shows a tipping point when we moved away from burning dirty coal, and today, we're at a similar pivotal moment with fossil fuels," said DuBay.