Measurements of soot trapped on the feathers of bird specimens in natural history museums suggest that scientists have underestimated past levels of a type of air pollution that contributes to climate change, according to a study published yesterday in the Proceedings of the National Academy of Sciences.
Soot – or more precisely, the fine particles known as black carbon – from the burning of fossil and biomass fuels may be the second largest contributor to warming aside from carbon dioxide. In order to model future climate change more accurately, we need to know how much of the stuff humans emitted in the past. But data on this form of pollution prior to the 1950s are very spotty.
Scientists have tracked black carbon deposition in Greenland ice cores, and they have developed computer models to estimate how emissions of this pollution have changed over time. But both of these methods involve assumptions, and their calculations do not always agree.
For another approach to the question, two graduate students at the University of Chicago, evolutionary biologist Shane DuBay and photography historian Carl Fuldner, turned to natural history museums. “Natural history collections are powerful resources for tracking environmental pollutants through time because specimens provide durable snapshots of the past environments from which they were drawn,” they write.
The researchers took photographs of 1,097 bird specimens collected between 1880 and 2015 in the U.S. states of Pennsylvania, Ohio, Indiana, Michigan, Illinois, and Wisconsin. In the late 19th century these states had a lot of heavy industry and relied on bituminous coal, which produces large amounts of soot.
They chose birds belonging to five species that all have pale breast and belly plumage: Field Sparrow (Spizella pusilla pusilla), Grasshopper Sparrow (Ammodromus savannarum pratensis), Eastern Towhee (Pipilo erythrophthalamus erythrophthalamus), Horned Lark (Eremophila alpestris pratensis), and Red-headed Woodpecker (Melanerpes erythrocephalus).
The researchers examined the feathers under a high-powered microscope to confirm that feathers were darkened because of soot particles clinging to them rather than because of altered pigmentation of the feathers themselves. They calculated the reflectance – the proportion of light reflected from an object –of each bird’s underside in the images. Black carbon absorbs light and decreases reflectance, so these measurements enabled them to track how the relative sootiness of birds changed over time.
For example, the researchers found that black carbon on bird specimens was at its highest during the first decade of the 20th century. Soot deposition decreased sharply during the Great Depression, rebounded during World War II, and then showed a gradual and sustained drop after about 1950. This lasting decline reflects that people were no longer using coal in residential furnaces, and factories in the region switched to less-sooty anthracite coal and adopted technologies to burn it more efficiently.
The analysis suggests that levels of black carbon in the air during the early Industrial era were higher than computer models have estimated. In turn, that means that past warming due to black carbon may have been underestimated as well. That matters because getting a handle on temperature increases during the early Industrial era is key to calculating our current carbon budget – the amount of greenhouse gases left to emit before we hit climate tipping points.
Of course, the new analysis comes with its own limitations. Chief among these is that it tracks the relative sootiness of birds over time, not the actual amount of black carbon produced by human activities. But museum specimens could help with this, too. Researchers could look at specimens recently collected in cities like Beijing and Delhi that currently have high black carbon levels. This would enable them to calibrate the reflectance of a bird’s feathers with the amount of black carbon in the air. More detailed analysis of the soot on older bird specimens could also provide more information about the distribution of different sizes and shapes of black carbon particles, which affects warming but which we don’t know much about prior to the 1950s, the researchers say.
Source: DuBay SG and CC Fuldner “Bird specimens track 135 years of black carbon and environmental policy.” Proceedings of the National Academy of Sciences. 2017
Image: Field Sparrows from 1906 (Top) and 1996 (Bottom). The top bird is coated in soot. Credit: Proceedings of the National Academy of Sciences.