Soot pouring out of European factory chimneys caused a sudden melting of Alpine glaciers that has puzzled experts, according to new research.

Records show that between 1860 and 1930, glaciers filling large valleys in the European Alps retreated by an average of nearly one kilometre, or 0.6 miles.

Yet during this time, which pre-dated the current era of global warming, temperatures in Europe cooled by almost one degree Celsius.

Scientists have struggled to explain the mismatch between what was happening to the climate and the glaciers.

Now a new study has come up with the likely answer - black carbon spilled into the air by rapidly industrialising European cities.

The sooty particles coated the snow-covered ice, increasing heat absorption and reducing reflectivity, researchers believe.

As the snow melted, the underlying glacier ice became exposed and vulnerable to the actions of sunlight and relatively warm air. The net result was a rapid increase in glacier melting.

Researchers came to the conclusion after comparing ice core data with computer simulations.

Professor Waleed Abdalati, one of the scientists from the University of Colorado in Boulder, US, said: "This study uncovers some likely human fingerprints on our changing environment. It's a reminder that the actions we take have far-reaching impacts on the environment in which we live."

The mid-19th century is loosely defined as the end of the Little Ice Age in Europe. During this time, Europe underwent dramatic changes driven by industrialisation which involved the large-scale burning of coal.

To determine how much black carbon was in the atmosphere and the snow when the Alpine ice began to retreat, the scientists studied cores drilled from several mountain glaciers.

Measurements of the levels of sooty particles trapped in the ice layers allowed them to estimate the extent of black carbon deposition.

Adding the carbon particle data to the computer simulations yielded an impact on glaciers consistent with the historical record.

The findings, reported in the journal Proceedings of the National Academy of Sciences, have implications for the fate of mountain glaciers today.

Co-author Dr Georg Kaser, from the University of Innsbruck in Austria, said: "We must now look closer at other regions on Earth, such as the Himalaya, to study the present-day impacts of black carbon on glaciers."

Mountain glaciers are a vital source of fresh water for many millions of people around the world.

Earlier this year scientists reported that glaciers in the tropical Andes had shrunk by up to 50% since the 1970s.

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