Comparing the feathers of songbird specimens collected in the Rust Belt over 135 years, researchers at the University of Chicago report there was more soot in the air during the early 20th century than originally estimated.

Why it matters: Soot or black carbon from burning coal contributes to climate change but is difficult to study because it falls out of the atmosphere after a few days or weeks. Researchers want to be able to accurately assess soot's role in past warming in the past in order to improve predictions about future contributions.

How they did it: Carl Fuldner and Shane DuBay measured how much light was reflected in the feathers of 1347 birds representing five species collected between 1880 and 2015. The birds molt each year so the soot in their feathers is an accurate proxy of that in the air during the year they are collected.

What they saw: The birds' feathers were covered in the most soot between 1880 and 1929. During the Great Depression when less coal was burned, the feathers were cleaner but then became dirty again during World War II. After the war ended in 1945, the amount of soot on the feathers declined, a trend that continued as air pollution legislation was enacted in the middle of the century.

What it means: The AP's Seth Bornstein writes some scientists think black carbon emissions should be a focus of fighting climate change while others "say the impacts of the study on projections for future warming would likely be modest — at best — in part because black carbon stays in the atmosphere for such a short time."