In the mid-1800s, a peculiar thing happened to the glaciers of the European Alps. Rather than growing as would have been expected given the temperature and precipitation at the time, they began to shrink. Scientists now think that soot from the Industrial Revolution might be to blame for the glaciers’ abrupt retreat.

Between the end of the 13th century and the middle of the 19th century, the glaciers in the European Alps were considerably larger than they are today. Around 1865, they began to retreat rapidly, generally becoming shorter than they had been in the previous 500 years—a retreat that continues to this day. Their retreat marked the end of the Little Ice Age. But according to the temperature and precipitation records, the glaciers should have continued to grow until around 1910. The discrepancy led scientists to consider other factors that weren’t being captured by the climate data.

At about the same time that the Alps’ glaciers were melting, human civilization was going through a massive change: the Industrial Revolution. In Western Europe, this transition began in Great Britain in the mid-18th century, spread to France in the early 19th century, and moved to Germany and the rest of Western Europe by the mid-19th century. Along with the machinery and factories that defined the Industrial Revolution came an increase in emissions of black carbon, which came from coal combustion for industry and the burning of coal and biomass for heating.

In the midst of the intensive industrialization and black carbon emissions stood the Alps. Rail systems were built to parallel existing roads and became more extensive as transport and tourism grew. This greatly expanded the human footprint in the region and brought new emissions into close proximity to the regions’ glaciers. Estimates of historical emissions for Europe show that black carbon emissions increased dramatically after 1850.

Ice cores from high elevations in the Alps reflect the increased black carbon emissions, with an indication of its presence starting between 1850 and 1870 and continuing until well into the 20th century. Scientists from NASA’s Jet Propulsion Laboratory now think these increases in black carbon may be responsible for the glaciers’ retreat that began around 1865.

Glaciers exist in a delicate balance of accumulation of new ice and melting. Each summer, the glacier melts at lower elevations when the snow cover disappears completely, exposing the darker glacier ice to increased energy fluxes, and each winter it accumulates more snow. The difference between accumulation and melt controls whether the glacier grows or shrinks. Snow and ice covered in black carbon, more commonly known as soot, absorb more sunlight, leading to surface warming and increased melting.

Black carbon is now considered the second leading cause of global warming after carbon emissions. Soot’s impact on glacier mass in the mid-19th century pushes back scientists’ understanding of when humans began altering Earth’s climate.

PNAS, 2013. DOI: 10.1073/pnas.1302570110 (About DOIs).