Around the middle of the 19th century, the Arctic took a sooty turn for the worse, according to researchers studying how humans have affected the climate.

Soot can darken the snow, causing it to absorb sunlight, warm up and melt. That, in turn, can add to local warming by exposing darker ground, which absorbs more energy from the sun that the white snow would have reflected.

Ice cores from before about 1850 show most soot came from forest fires. But since then, soot in Arctic snow has increased several times over and most now comes from industrial activities, according to a paper in Thursday's online edition of the journal Science.

The soot study was done by a team led by Joseph R. McConnell of the Desert Research Institute in Reno, Nev.

The researchers analyzed black carbon levels in ice from Greenland, covering the last 215 years.

They found that the older soot samples contained vanillic acid, an indicator of burning conifer trees.

In the more recent years the soot was seven times more common and contained a larger concentration of non-ocean sulfur, an indicator of industrial emissions.

Soot concentrations peaked in 1906-1910 and remained high for decades. Sulfur emissions declined following the U.S. Clean Air Act in 1970, they noted.

In the early 20th century, the Arctic warmed more than anywhere else on Earth, Richard B. Alley of the California Institute of Technology observes in a commentary on the report, noting a "broad correspondence between the soot peak and the observed warming."