Glaciers can be blindingly bright because pure ice and snow are reflective. But add some black carbon—little particles of soot produced by fossil fuel burning—and suddenly glaciers absorb more sunlight, warm up, and can begin to retreat.
Previously, researchers have suggested that the glacial retreat observed in the Alps in the 1860s was linked to the contemporaneous uptick in black carbon during Europe’s industrialization. But now new ice core data and high-resolution glacial length histories suggest that black carbon levels didn’t primarily drive glacial extent in nineteenth century Europe.
Instead, scientists are now proposing that cooling due to airborne particles lofted by volcanic eruptions was likely responsible.
History in an Ice Core
Michael Sigl, an analytical chemist who conducted this work while at the Paul Scherrer Institut in Villigen, Switzerland, wanted to test the long-standing theory that black carbon was responsible for the significant glacial retreat observed in the Alps in the nineteenth century. For this investigation, Sigl and his colleagues needed both accurate records of black carbon levels stretching back well over a century and a handle on the historical extent of glaciers in Europe.
To get at the first data set, the researchers turned to one of the world’s best methods for preserving historical conditions: ice cores. The research team collected ice cores in the Alps near the border between Switzerland and Italy.
Back in the lab, the scientists applied a battery of tests like soot photometry, which involves shining a laser at aerosols made from the ice meltwater, to calculate black carbon concentrations at different points along the cores. The second data set—historical records of the extents of glaciers—came from detailed records of four glaciers in the Western Alps: Mer de Glace (France), Oberer Grindelwald (Switzerland), Unterer Grindelwald (Switzerland), and Bossons (France).
To determine whether the concentrations of black carbon in the ice cores were temporally linked with glacial retreat, the researchers had to age date the ice absolutely. “You need dating markers,” Sigl explained. Without them, the data are not tied to known events in time.
He and his collaborators measured trace elements and minerals in the ice deriving from well-studied and dated volcanic eruptions, nuclear weapons tests, and dust intrusions from the Sahara Desert. The researchers estimated that their ice cores, all together, traced more than 270 years of history, beginning with the year 1741.
A Puzzling Surprise
When Sigl and his colleagues compared the age-dated black carbon concentrations and the historical records of the glaciers’ retreats, they were surprised. By the time black carbon concentrations first rose above their historical, pre-industrialization background levels—around 1875—the glaciers the team examined had already experienced the majority of their nineteenth century length reduction, the team reported last month in The Cryosphere.
“You would expect that the increase in black carbon would start before or at the time of glacial retreat,” said Sigl. “That wasn’t the case.”
Furthermore, in the 1910s and 1920s, when black carbon concentrations were roughly 5 times higher than their pre-industrial values, Alpine glaciers were tending to advance, the team showed. Black carbon apparently wasn’t the primary driver of glacial extent, the scientists concluded. So what was?
Enter the Volcanoes
Sigl and his collaborators have proposed a nonanthropogenic explanation: volcanic eruptions. Eruptions can loft large quantities of sunlight-reflecting particles high into the atmosphere, which can result in pronounced weather changes, including cooling. For context, that’s what scientists believed happened in 1816, which they dub the “Year Without a Summer.”
Sigl and his colleagues noted that five large volcanic eruptions occurred in the tropics in 1809, 1815, 1823, 1831, and 1835; the exact volcanoes that generated these eruptions haven’t yet been pinpointed. Particles lofted from these eruptions cumulatively might have caused cooling that allowed the glaciers in the Western Alps to advance significantly until the 1850s. When those large cooling episodes ceased as volcanism became less prevalent, temperatures would have warmed, and the glaciers would have started to retreat, the team suggested.
In other words, if volcanoes are not providing any additional cooling, glaciers cannot maintain their large tongues, Sigl explained. “They retreat back to their original positions,” he said.
Sigl and his colleagues are quick to point out, however, that their conclusions about volcanic forcing apply only to one part of the Western Alps in the nineteenth century. The modern picture is quite different. “The currently observed mass loss [of glaciers] is global in scale and attributed to anthropogenic greenhouse gas emissions,” the researchers write in their paper.
These findings are important because they help researchers pinpoint the causes of large-scale climate changes, said Kaitlin Keegan, a glaciologist at Dartmouth College in Hanover, N.H., who was not involved in the research. She added that “our ability to accurately model future climate scenarios, including melting events, is only as good as our understanding of how these events have transpired in the past.”