Bird specimens track 135 years of atmospheric black carbon and environmental policy

Abstract

Atmospheric black carbon has long been recognized as a public health and environmental concern. More recently, black carbon has been identified as a major, ongoing contributor to anthropogenic climate change, thus making historical emission inventories of black carbon an essential tool for assessing past climate sensitivity and modeling future climate scenarios. Current estimates of black carbon emissions for the early industrial era have high uncertainty, however, because direct environmental sampling is sparse before the mid-1950s. Using photometric reflectance data of >1,300 bird specimens drawn from natural history collections, we track relative ambient concentrations of atmospheric black carbon between 1880 and 2015 within the US Manufacturing Belt, a region historically reliant on coal and dense with industry. Our data show that black carbon levels within the region peaked during the first decade of the 20th century. Following this peak, black carbon levels were positively correlated with coal consumption through midcentury, after which they decoupled, with black carbon concentrations declining as consumption continued to rise. The precipitous drop in atmospheric black carbon at midcentury reflects policies promoting burning efficiency and fuel transitions rather than regulating emissions alone. Our findings suggest that current emission inventories based on predictive modeling underestimate levels of atmospheric black carbon for the early industrial era, suggesting that the contribution of black carbon to past climate forcing may also be underestimated. These findings build toward a spatially dynamic emission inventory of black carbon based on direct environmental sampling.

Keywords: aerosols; air pollution; climate change; natural history; soot.

Fig. 1.

Comparison of two Field Sparrows (S. pusilla pusilla), one from 1906 and one from 1996. (Lower) SEM micrographs of belly feathers plucked from the specimens in Upper. SEM micrographs of the Field Sparrow from 1906 show black carbon aggregates composed of small sphericals [a detailed description of black carbon morphology with microscopy images can be found in Bond et al., 2013 (1)]. The feather from the 1996 specimen lacks black carbon deposition. Both specimens were collected during spring months in the vicinity of Chicago. SEM images were made with a Tescan LYRA3 field emission microscope with secondary electron (SE) detection and an acceleration voltage (HV) of 3.0 kV. Feather samples were carbon-coated before imaging.

Fig. S1.

Additional SEM micrographs, taken at different magnifications, from the Field Sparrows (S. pusilla pusilla) in Fig. 1. A–D are from the soiled 1906 specimen. E–H are from the clean 1996 specimen. The micrographs for each specimen are progressively higher in magnification. The white boxes in D and H outline the areas shown in Fig. 1.

Fig. S2.

Map showing the collection localities for 1,345 of 1,347 specimens used in this study. The remaining two specimens lack county locality data. Counties are shaded based on the density of sampling within the county. The number of specimens from each county is printed within each county.

Fig. S3.

Comparisons of old and young specimens for the four species pairs not shown in Fig. 2. (A) Grasshopper Sparrows (A. savannarum pratensis) from 1907 (Upper) and 1996 (Lower). (B) Horned Larks (E. alpestris pratensis) from 1904 (Upper) and 1966 (Lower). (C) Eastern Towhees (P. erythrophthalmus erythrophthalmus) from 1906 (Upper) and 2012 (Lower). (D) Red-headed Woodpeckers (M. erythrocephalus) from 1901 (Upper) and 1982 (Lower).

Fig. S4.

Monthly trends in black carbon deposition for each species before 1950. Inverse reflectance is reported rather than reflectance to express drops in black carbon emissions, which register as increased reflectance values. The shaded areas are the months excluded from final analyses for each species, which are applied to all years. Sampling is sparse for Grasshopper Sparrows and Field Sparrows in the US Manufacturing Belt during fall and winter months because these species predominately migrate out of the region.

Fig. S5.

Black carbon deposition for all 1,347 individuals sampled for this study, showing that specimens from molting months (red points) are substantially cleaner than specimens from winter–summer (black points). Black points are individuals included in the final dataset (n = 1,097), and red points are individuals from molting months that were excluded in final analyses (n = 250) (Fig. S4). Inverse reflectance is reported rather than reflectance to express drops in black carbon emissions, which register as increased reflectance values. Before 1950, individuals from molting months are noticeably cleaner than individuals from the rest of the year, warranting the exclusion of specimens from these months for all years.

Fig. 2.

Black carbon deposition on specimens of five bird species from the US Manufacturing Belt, collected between 1880 and 2015. Each point represents the z score for an individual specimen (n = 1,097) based on the inverse raw reflectance value taken from its breast and belly feathers. The black line is a GAM (k = 20) with 95% confidence limits (indicated by the shaded area), determined from the individual specimens (details on how k was determined can be found in SI Materials and Methods and Fig. S10. Fig. S13 shows species-specific trends). The orange line is consumption for coal in the United States expressed in British thermal units (BTUs) (US Energy Information Administration). Before 1950, coal consumption data are available in 5-y intervals. After 1950, coal consumption data are yearly. The purple line shows estimates of total US black carbon (BC) emissions from Bond et al., 2007 (11), which uses fuel consumption data and emission factor data to generate a historical emission inventory. The dashed line at 1910 denotes the progressive shift in cities within the US Manufacturing Belt from prosecuting to educating emissions violators. The dashed line at 1960 denotes the approximate moment after which black carbon emissions become decoupled from coal consumption.

Fig. S7.

Black carbon deposition on specimens (five bird species) from the US Manufacturing Belt, collected between 1880 and 2015. Each point represents the z score for an individual specimen (n = 1,097), based on the inverse raw reflectance value taken from its breast and belly feathers. The black line in Upper is a GAM (k = 20) with 95% confidence limits (indicated by the shaded area), determined from the individual specimens (details on how k was determined can be found in SI Materials and Methods and Fig. S10. Fig. S13 shows species-specific trends). The colored lines are consumption trends for biofuels and fossil fuels expressed in British thermal units (BTUs) (US Energy Information Administration). Before 1950, fuel consumption data are available in 5-y intervals. After 1950, fuel consumption data are yearly. Lower shows estimates of total US black carbon (BC) emissions from Bond et al., 2007 (11), which uses fuel consumption data and emission factor data to generate a historical emission inventory. The dashed line at 1910 denotes the progressive shift in cities within the US Manufacturing Belt from prosecuting to educating emissions violators. The dashed line at 1960 denotes the approximate moment after which black carbon emissions becomes decoupled from coal consumption.

Fig. S10.

GAMs with various smoothing functions applied to the normalized 1,097-specimen dataset. k = 10–12 applies an overly powerful smoothing operation in the GAM; k = 13–35 recovers trends that are effectively identical, which appear to recover important signals in the data absent from the k = 10–12 models; and k = 36 (and greater) generates a toothy trend that overrepresents random variations within the sample set.

Fig. S13.

Species-specific trends in black carbon deposition. Each point represents an individual specimen. The colored lines are GAMs (k = 20) with 95% confidence limits (shaded area) for each species [fall-month birds are excluded (Fig. S4)]. Inverse reflectance is reported, rather than reflectance, to visualize drops in atmospheric black carbon.

Fig. S6.

Black carbon deposition on specimens plotted against US coal consumption for the three time periods defined by the dashed lines in Fig. 2. (A) Between 1880 and 1910, black carbon deposition is not correlated with coal consumption. Black carbon deposition is high and remains relatively constant, trending upward only slightly as consumption increases sharply. (B) Between 1911 and 1960, black carbon deposition and coal consumption are positively correlated. (C) After 1960, black carbon deposition is decoupled from coal consumption. As consumption increases, black carbon deposition remains low. Before 1950, fuel consumption data are only available in 5-y intervals. We thus interpolated consumption values between points to estimate consumption for the year in which each specimen was collected before 1950. After 1950, yearly fuel consumption data are available.

Fig. S8.

Black carbon deposition on specimens plotted against black carbon (BC) emissions estimates from Bond et al., 2007 (11) for the three time bins defined in Fig. 2. The second two time bins (1911 to 1960 and 1961 to 2014) are combined to illustrate the strong correlation across both intervals. (A) Before 1910, we recovered relatively constant, high levels of black carbon deposition on specimens, while Bond et al., 2007 (11) estimated a sharp rise in black carbon emissions. (B) After 1910, black carbon deposition is positively correlated with black carbon emissions estimates from Bond et al., 2007 (11). Our results independently recovered similar trends in atmospheric black carbon. Bond et al., 2007 (11) report BC emissions in 5-y intervals. We thus interpolated emissions values between points to estimate values for the year in which each specimen was collected.

Fig. S11.

Ten Horned Larks (E. alpestris pratensis) at The Field Museum, showing that specimens collected in nonindustrial regions do not exhibit comparable levels of soiling to birds collected within the US Manufacturing Belt. The five specimens in Left were collected in Illinois, inside the US Manufacturing Belt. The five specimens in Right were collected along the western coast of North America, outside of the US Manufacturing Belt. All 10 specimens were collected during nonmolting months (January–April) between 1903 and 1922.

Fig. S12.

Images of the dorsal side of specimens from Fig. 1 and Fig. S3. These images, paired with Fig. 1 and Fig. S3, show that even soiling appears over the entire bird, indicating that the soiled birds in our sample acquired black carbon from the environment while alive. (A) Field Sparrows (S. pusilla pusilla) from 1906 (Upper) and 1996 (Lower). (B) Grasshopper Sparrows (A. savannarum pratensis) from 1907 (Upper) and 1996 (Lower). (C) Horned Larks (E. alpestris pratensis) from 1904 (Upper) and 1966 (Lower). (D) Eastern Towhees (P. erythrophthalmus erythrophthalmus) from 1906 (Upper) and 2012 (Lower). (E) Red-headed Woodpeckers (M. erythrocephalus) from 1901 (Upper) and 1982 (Lower).

Fig. S9.

Raw R, G, and B channel-specific regressions based on the Munsell Neutral Value Scale reflectance standards for each shooting location. The regression equations for each channel were used to calculate channel-specific reflectance from raw CMOS sensor data recovered in RawDigger for each specimen.