Soot from coal-burning factories in the northeastern United States might have been the most important factor in the warming of the Arctic region at the turn of the last century, American researchers reported last week.

At its peak, the contribution of soot to warming the region was almost twice as great as that of current carbon dioxide levels, the team reported in the online journal Science Express.

Particulate matter in the atmosphere generally is thought to have a cooling effect because it reflects sunlight back into space. Soot in the atmosphere, however, absorbs sunlight and has a warming effect.

When soot, also known as black carbon, is deposited on top of ice and snow, it has a stronger effect because it blocks the snow’s natural reflectance.

Soot in the air and on the ground has not been measured until recently and it has been “a big unknown in climate studies”, said lead author Joseph McConnell, a snow hydrologist at the Desert Research Institute in Reno, Nevada.

To fill this gap, McConnell and colleagues drilled out an ice column in Greenland, melted it sequentially and analysed the liquid for carbon, vanillic acid — an indicator of pine forest fires — and a type of sulfur that is an indicator of human activities.

Before 1850, levels of black carbon and vanillic acid were closely matched, indicating that forest fires were responsible for most of the soot.

Between 1850 and 1951, black carbon levels rose sharply, peaking in 1908. The levels were matched with rising sulfur levels, particularly in winter. The warming effect from the industrial pollutants was eight times that of forest fires, according to the study.

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