Soot, also known as black carbon, could have been the reason behind the abrupt retreat of the mountain glaciers in the Alps starting in the 1860s, according to a new study published in the journal Proceedings of the National Academy of Sciences.
If correct, the theory could solve a longstanding scientific debate as to why the glaciers began melting even decades before global temperatures started rising.
Records dating back to the 1500s show that large valley glaciers in the Alps suddenly melted by an average of 1 kilometer between 1860 and 1930, even as the weather in Europe cooled by nearly 1 degree Celsius during the same time period.
"Something was missing from the equation," Thomas Painter, the lead author of the study and a snow and ice scientist at NASA's Jet Propulsion Laboratory, said in statement.
To figure out what could have triggered this abrupt melt despite the cooler weather, Painter and his colleagues turned to history.
"It dawned on me that industrialization was kicking off then," Painter told NPR. "We have these visions from Charles Dickens and others of that time -- the mid 1800s -- of a huge amount of soot being pumped out into the atmosphere, not just in England but in France and Germany and Italy."
That soot, Painter hypothesizes, settled on the snow, causing it to become darker and thus melt quicker since dark snow absorbs a greater amount of sunlight. As the snow melted, the ice underneath was exposed to a greater amount of sunlight, causing it to melt.
To test this theory, the team of researchers examined ice cores drilled from high up several European mountain glaciers in order to determine the levels of carbon particles trapped in the ice core layers, all the while taking into consideration modern observations of the distribution of pollutants in the Alps.
In doing so, they were able to estimate how much black carbon was deposited on glacial surfaces at lower elevations.
All of this information was run through computer models based on glacier behavior, the results of which were consistent with the historic record of glacial retreat.
"This study uncovers some likely human fingerprints on our changing environment," co-author Waleed Abdalati, the director of the Cooperative Institute for Research and Environmental Studies, said. "It's a reminder that the actions we take have far-reaching impacts on the environment in which we live."
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