In the decades following the 1850s, industrialization of Western Europe saw the use of coal to heat homes and power transportation and manufacturing, spewing huge quantities of black carbon and other dark particles into the atmosphere, the researchers said.
It came at the end of the Little Ice Age, loosely defined as a cooler period between the 14th and 19th centuries, when temperatures dropped and glaciers expanded.
But glacier records show that between 1860 and 1930, as temperatures continued to drop and snowfall remained adequate, large valley glaciers in the Alps abruptly retreated by an average of nearly 0.6 mile.
Glaciologists and climatologists have struggled to reconcile this apparent conflict between climate and glacier records.
"Something was missing from the equation," study leader Thomas Painter, a snow and ice scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said. "Before now, most glaciologists believed the end of the Little Ice Age came in the mid-1800s when these glaciers retreated, and that the retreat was due to a natural climatic shift, distinct from the carbon dioxide-induced warming that came later in the 20th century.
Analyzing the levels of carbon particles trapped in ice core layers, the researchers were able to estimate how much black carbon -- soot -- was deposited on glacial surfaces.
Black carbon is the strongest sunlight-absorbing atmospheric particle that, when it settles on the snow blanketing glaciers, darkens the snow surface, speeding its melting and exposing the underlying glacier ice to sunlight and warmer spring and summer air earlier in the year.
The industrial soot was sufficient to cause glacier retreat despite cooling temperatures, the researchers said.
"This [study] result suggests that human influence on glaciers extends back to well before the industrial temperature increases," Painter said.
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