A Classic Whodunit: Industrial Soot, Volcanoes, and Europe’s Shrinking Glaciers

In the second half of the 1800s, glaciers in the Alps rapidly shrunk in length, some by hundreds of meters. Their alarming retreat, documented in photographs, has often been a symbol of the human influence on global climate, as the accelerated melting aligned with increased production of industrial soot. But were there other factors that drove the rapid glacier recession in the Alps at the end of the Little Ice Age?

Surface darkening from mineral dust and soot deposited on the Aletsch glacier. The Colle Gnifetti drilling site lies in the background (Source: Michael Sigl).

A new study in The Cryosphere led by Michael Sigl, a chemist and climatologist at the Paul Scherrer Institute (PSI) in Switzerland, challenges the notion that human-made industrial soot, or more formally black carbon, from European industrialization was primarily responsible for the observed deglaciation during the 15-year period between 1860 and 1875.

Based on their comparison of high-resolution black carbon deposition records from ice cores from the Colle Gnifetti glacier in the Swiss Alps and historical data of the changing lengths of major Alpine glaciers, the researchers discovered that “when black carbon concentrations started to significantly rise (around 1875), Alpine glaciers had already experienced 80 percent of their 19th century retreat, meaning that black carbon was not the first responsible for this retreat, contrary to what was suggested in a previous study,” team member Dimitri Osmont, a doctoral student at the PSI, told GlacierHub, referring to earlier research published in the Proceedings of the National Academy of Sciences of the United States of America.

“Of course, this doesn’t mean that black carbon didn’t contribute at all (especially during the 20th century when concentrations are significantly higher, and also today in the case of Himalayan glaciers), but it was not the first driver,” Osmont told GlacierHub.

Sigl further elaborated on the discrepancies between his team’s findings and that of previous research in discussion with GlacierHub. “If the glaciers had actually been forced to retreat by more abundant soot impurities in the snow, one would expect the glaciers’ retreat to have been synchronous with or slightly lagging increases in black carbon deposition. But we observe the exact opposite and conclude that other factors, predominantly volcanism, account for most of past glacier variability,” he said.

The TUNU ice-core in Greenland containing a continuous archive of global volcanism (Source: Michael Sigl).

Volcanoes? Indeed, a series of massive volcanic eruptions in the early 1800s, like the catastrophic Mount Tambora in 1815 behind Europe’s Year Without a Summer, resulted in a few decades of cooler and wetter conditions conducive for the Alpine glaciers to surge and grow. Not to belittle the sheer devastation experienced locally and the socioeconomic effects of altered agricultural patterns across the globe, other positive takeaways of the eruptions included artistic inspiration for vibrant sunsets in J. M. W. Turner paintings, the backdrop of Mary Shelley’s Frankenstein, and the peak of larger glaciers in the Alps to phenomenal lengths in the middle of the 1850s.

The team argues that this more favorable atmosphere for the glaciers allowed them to grow to their peak size in the 1850s and that the rapid retreat from 1860 to 1875 was the glaciers simply returning to their “normal” size. They conclude that whatever role anthropogenic black carbon had in Alpine glacier retreat before 1875 was negligible in comparison to the natural decadal factors.

But other scientists disagree with their findings, including Thomas Painter, the author of the study whose hypothesis was tested and a principal scientist at NASA’s Jet Propulsion Laboratory in California. “Sigl et al. performed admirable work with their ice core analysis, and it is alone an important contribution to understanding deposition dynamics of atmospheric constituents,” Painter told GlacierHub. However, he found that the study “attacked a strawman argument that the glacier retreat in the 19th century predated the emergence of black carbon deposition and its additional absorption of sunlight in the snowpack.” He challenges this new study’s claims that they disprove his hypothesis. “The glaciers did start retreating from a cold period, but they then kept on strongly retreating to lengths not seen in the previous centuries, while air temperature and precipitation didn’t change sufficiently to cause this,” he said.

Image of the Colle Gnifetti glacier in 2015. It’s the ice-core drill site hosting a continuous archive of air pollution since 1741 A.D. (Source: Michael Sigl).

Regardless of the differing conclusions, none of the scientists from the recent study contacted by GlacierHub discounted the role of human activity on glacier retreat. “Just to be very clear, the study in no way neglects the generally significant contribution of anthropogenic emissions to the ongoing observed worldwide glacier retreat, but black carbon, at least for the alpine region, was not a major factor for the 19th century retreat,” stated Theo Jenk, another co-author of the study from PSI. Painter and Jenk’s colleagues are sure to butt heads further, but all in the name of sound scientific endeavor.