Researchers have long studied atmospheric black carbon, or soot, by drilling and examining ice cores from frigid regions. Black carbon, which is considered a major contributor to global warming, precipitates out of the air quickly, often within a week. If it doesn’t get blown to, say, Greenland, then deposited on snow, it’s lost to science.
But a new study is offering a novel way of measuring historical black carbon: in its accumulation on bird feathers.
Two researchers photographed more than a thousand dead birds from museum collections in Chicago. The birds had been collected from 1880 to 2015 across America’s Rust Belt, a swath of northeastern and midwestern states that once formed a hub of heavy industry. The five species of birds normally have white bellies, but during the peak of the industrial era, not so much.
By carefully photographing the birds in a setting with calibrated lighting, the scientists were able to determine relative concentrations of soot from 1880 to the present. Their results, published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) on 9 October, track fairly well with existing soot measurements and models, with a few key differences.
“We wanted to do this project that really was able to leverage this rich resource [of museum collections]…to use these collections to learn something about past atmospheres,” said Shane DuBay, a coauthor of the new study.
“We wanted to place these museum specimens back into the context they had been stripped from,” said the other coauthor, Carl Fuldner.
The researchers, who are both graduate students at the University of Chicago (DuBay is also a resident graduate student at the Field Museum of Natural History, also in Chicago), are careful to stress that their data show relative rather than absolute concentrations of atmospheric soot.
The birds have dark, sooty bellies from the beginning of the period sampled to about 1910, where there’s a slight dip in the amount of soot. There’s a bigger dip after the beginning of the Great Depression, tied to a decrease in coal consumption during the economic downturn. And from 1955 onward—the year the Air Pollution Control Act, a precursor to the Clean Air Act, was passed—there’s a strong downturn in the birds’ sootiness.
This timeline aligns fairly well with the widely used emissions model called the Speciated Pollutant Emissions Wizard (or SPEW). One area where it differs is in the years 1880–1910, when, according to the bird bellies, there was a higher concentration of airborne soot than estimated by the model.
That means—potentially—that black carbon’s modeled role in the climate of 100 years ago may need tweaking.
“Models that have been used in the past decade,” Fuldner said, “make a very convincing case that black carbon is one of the leading contributors to anthropogenic climate change, and that argument is based on historic emission inventories. We’re helping to clarify and adjust those estimates for black carbon around the turn of the century.”
How, exactly, those estimates will change climate models is a question that Fuldner and DuBay said they are leaving to the climate scientists because neither of them is one.
A Disciplinary Stretch
He and DuBay admit that in some ways they are out of their “disciplinary comfort zone,” as Fuldner puts it. DuBay is a biologist by training, and Fuldner is a historian. “We ended up writing what is kind of most centrally an atmospheric science paper. We’ve been excited to share this research with atmospheric scientists.”
So far, the response has been promising. “I was impressed by the work they did,” said Shuka Schwarz, a research physicist at the National Oceanic and Atmospheric Administration in Boulder, Colo., who specializes in black carbon and was not involved in the bird study. “The thing that excited me was this idea of digging birds out of vaults and trying to look at their feathers in order to get extra information. I think the fact that they matched some trends is quite compelling.”
Schwarz questioned the team’s conclusion, however, that black carbon concentrations were higher in the late 1800s. Because DuBay and Fuldner concentrated on birds from the industrialized parts of the United States and SPEW is a nationwide model, “are they really representing the same things?”
Fuldner said that the majority of soot emissions at the time came from bituminous coal, which comes from the region of the country they studied. “So, not a perfect correlation, but enough overlap [between the bird data and the SPEW model] where we can feel confident about making the comparison.”
Veerabhadran Ramanathan, a professor of atmospheric and climate sciences at the Scripps Institution of Oceanography, University of California, San Diego, approved the paper for publication in PNAS. He said that the study authors’ backgrounds were “unusual, but that’s exactly the type of out of the box thinking we need.” He added that the research “provides additional data points” for researchers studying the historical climate effects of black carbon.
Schwarz said he considered this study “the opening of the door. I think the potential is when they continue doing it, when they have a wider inventory.”
The Beauty of Birds
Birds in museum collections turn out to make great environmental monitors. Birds molt once a year, so each year they grow a set of new, clean feathers. That means that “we can tie each of these birds to a single year,” said Fuldner—the year the bird was collected.
Once the birds go into the drawer at the museum, they don’t change much: The pair ruled out postcollection soiling because the birds were evenly sooted all around their bodies, even the parts that were facing down against the drawer. And they ruled out postcollection cleaning because “naturalists tend to be very diligent about marking down everything they might do,” Fuldner said.
Records indicate that someone attempted to wash a few birds in the collection, he added. They excluded those birds from the study, but it seems that soot is so oily and sticks so well to feathers that conservationists eventually gave up trying to clean the birds in their collections.
Museum collections have been mined for environmental data before. A much cited study showing the link between the pesticide DDT and eggshell thinness used 1,700 eggs in 39 museums and played a major role—along with Rachel Carson’s book Silent Spring—in finally convincing the government to ban DDT. Other researchers have sampled the insides of bird feathers for the presence of heavy metals. But this may be the first time, Fuldner speculated, that collections have been mined for something as deceptively simple—and literally superficial—as the stuff on the surface of a bird’s body.
This research was a natural fit for both DuBay and Fuldner. “We have a real common interest in the history of what field naturalists did…and how those records can be used in unexpected ways that they might never have anticipated.”