Sooty air from coal burning triggered the initial melting of the mountain glaciers in the European Alps in the second half of the 19th Century when it caused the snow to turn grey and so reflect less sunlight back into space, scientists said.
The soot, known as black carbon, caused the glaciers to absorb more heat than usual, causing the ice to recede year by year even though the regional temperatures were colder than today, the researchers found.
Alpine glaciers have receded significantly over the past century but much of this melting is believed to have resulted from rising global temperatures caused by climate change. Scientists could not explain why glaciers started to melt as long ago as the 1860s, when Alpine temperatures were still low.
However, the initial melting appears to have been caused by deposits of black carbon building up on the pristine snow covering the glaciers as a result of industrial expansion around Europe in the late 19th Century fuelled by coal burning, said Thomas Painter of the University of Colorado at Boulder.
“Snow without soot is the brightest surface on the planet. When soot is deposited on snow, it absorbs sunlight and then conducts that energy to the surrounding snow. This is effectively an additional way for sunlight to warm and melt the snow,” Dr Painter said.
“The soot on the glacier ice has a relatively small impact because the glacier ice is already quite dark. Soot darkens snow, warms it and melts it earlier, exposes glaciers to sunlight earlier and so leads to faster glacier melting and retreat,” he said.
Records of glacial retreat in the central European Alps go back as far as the 1500s. This data shows that between 1860 and 1930, the large valley glaciers of the Alps retreated on average by nearly 0.6 miles, yet the local temperatures had continued to cool by nearly 1C during the same period of time.
This mismatch between the regional temperatures of the Alps and what was happening to the glaciers posed a problem for scientists, Dr Painter said. “Something was missing from the equation,” he said.
The study, published in the journal Proceedings of the National Academy of Sciences, looked at ice core samples from a series of glaciers and posited that the heat absorption caused by industrial soot landing on the snow.
Computer models showed that the effect of the black carbon could explain why glaciers started to melt at a time when temperatures were continuing to be low at the end of a cooler-than-average period in Europe known as the ‘little ice age’, which had lasted about 300 years.
“Where soot was deposited on snow cover, the computer modelling data suggest that the impact reached at times to the equivalent of a 4C increase in air temperature – markedly greater than current warming by the modern industrialisation increases in carbon dioxide,” Dr Painter said.
“However, where the soot fell on vegetation or rock, the impact was negligible. They were already very absorptive of sunlight in the visible wavelengths, so the soot did little to change their energy fluxes. In effect, the soot targeted the glaciers,” he said.
In recent decades, sooty deposits are less of a problem in western European countries because of clean-air legislation and the use of filtering technology that has cleaned up industrial emissions of black carbon.
“The region of focus of soot into snow is now Asia. Soot is increasing in India and China due to industrialisation – sloshing back and forth across the snows of the Himalaya. The measurements are so sparse there, but initial results suggest profound impacts from dust and soot,” Dr Painter said.