ice NASA researchers found that glacier retreat in the mid-19th century was accelerated by black soot from industrialization in Europe. The soot was deposited on the lower slopes of the glaciers, absorbing sunlight and speeding up melting. Peter Holy/NASA

For a long time scientists have tried to understand why glaciers in the Alps began retreating in the late 19th century — ending the Little Ice Age — even though slightly cooler temperatures than the previous century and ample snowfall means they should have kept growing.

New research suggests that black soot from coal used to heat homes and power factories during Europe's period of rapid industrialization beginning in the 1850s sped up the melting of glaciers, starting in 1865.

The findings were reported in the Sept. 2 issue of the journal Proceedings of the National Academy of Sciences.

The Little Ice Age was a period between the 14th and 19th centuries when glaciers expanded and the temperature dropped. After 1860, glaciers began retreat, but temperatures continued to drop and precipitation stayed the same. Scientists often believed this to be end of the Little Ice Age, even though glacier behavior did not match temperature data. 

Until now, most climatologists believed that the retreat of glaciers in the mid-1800s was "due to a natural climatic shift, distinct from the carbon dioxide-induced warming that came later in the 20th century," study researcher Thomas Painter, a snow and ice scientist at NASA, said in a statement. "This result suggests that human influence on glaciers extends back to well before the industrial temperature increases."

By studying ice cores from European mountain glaciers, researchers found that "black carbon concentrations increased abruptly in the mid-19th century and largely continued to increase into the 20th century, consistent with known increases in black carbon emissions from the industrialization of Western Europe."

When tiny particles landed on the lower slopes of glaciers, the black soot absorbed sunlight and heat (unlike the white ice, which reflects it), speeding up melting.

The scientists write in the paper that black carbon is "now considered to be the second most potent climate warmer behind carbon dioxide."