Records of historical air pollution are hiding in sooty birds’ feathers

A hundred years ago, the skies over the northern USA did not look as blue as they do today. Burning of fossil fuels over the Rust Belt, a group of states near the Great Lakes, caused air pollution to soar. 

Now, a group of researchers has used an unusual piece of history to measure exactly how much air pollution has changed in the industrial region in the last century.

Instead of having yellow chins and white bellies like they do now, the Horned Larks that lived in the Rust Belt 100 or so years ago were grey. The soot that filled the air stained their feathers.                                                                                               

Now, using museum exhibits, researchers have studied the history of the colour of these birds to reveal a hidden history of air pollution. The results, published in PNAS, show black carbon levels peaked at the start of the 1900s, and the air at this time was more polluted than previously thought.

“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 US cities like Chicago and Pittsburgh were once like,” said Shane DuBay, from the University of Chicago and one of the authors of the study. “Using museum collections, we were able to reconstruct that history.”

The researchers studied 1,300 birds collected over the last 135 years, to determine the effects of soot in the air over cities in the Rust Belt. 

They measured how much black carbon was in the air based on the soot on the feathers. Black carbon gets into the atmosphere when fossil fuels are burnt with a poor oxygen supply, and it is a major constituent of soot.

“The soot on these birds’ feathers allowed us to trace the amount of black carbon in the air over time, and we found that the air at the turn of the century was even more polluted than scientists previously thought,” said DuBay. 

The results of the study are the most complete description of air pollution in the area, and it is hoped this timeline will be used to improve our estimates of atmospheric levels of black carbon.

“The big finding and implication of our study is that we are recovering relative concentrations of atmospheric black carbon that are higher than previously estimated from other methods,” said DuBay.

“It helps constrain and inform how we understand the relative role of black carbon in past climate and by understanding that we can more accurately model future climate scenarios.”

The team hopes the same methods could be used to analyse the history of the UK.

“We are very excited about the prospects of expanding the project into the UK which has a longer history of industry and also a longer history of natural history collecting,” co-author Carl Fuldner from the University of Chicago told the BBC.

“Some of the resources in the UK go back much farther in time, so the results we could find in a comparative study would be very exciting.”