Greenland’s Disappearing Glaciers—A Tale of Fire and Ice

NOVA

May 19, 2014

By Hannah Hoag

...Dark Snow’s goal was to link a specific set of fires to a specific melting event. “We saw an unprecedented melting in Greenland in 2012, beyond what can be explained by climate warming alone. Our hypothesis is that it was the light-absorbing impurities seen in the record-setting forest fires,” says McKenzie Skiles, a snow hydrologist and PhD candidate at the University of California, Los Angeles and manager of the snow optics lab at NASA’s Jet Propulsion Laboratory in California who joined Dark Snow to analyze the snow samples.

One group Dark Snow was up against was the NSF-funded team led by Chris Polashenski '07 Th'07, a geophysicist, and Zoe Courville Th'03, '07, a research mechanical engineer, at the U.S. Army Cold Regions Research and Engineering Laboratory, in Hanover, New Hampshire. The team secured $2.3 million from NSF and NASA for the project, the Sunlight Absorption on the Greenland ice sheet Experiment, known as SAGE.

In late 2012, Box and Polashenski independently pulled a NASA image taken by the CALIPSO satellite in July showing a smoky plume drifting over Greenland’s ice sheet. It added to their arsenal of evidence, but it wasn’t enough proof. “It set the stage that the big melt event happened because the wildfires in Siberia emitted black carbon and caused more sunlight to be absorbed and caused the ice sheet to melt,” Polashenski says. If it turned out to be true, he says, “It would be a good story.” ...

... In May 2013, Polashenski’s three-person crew set out on snow machines from Summit Station, 10,551 feet above sea level, along a traverse route northward and then west towards Thule, covering 50 to 90 miles a day. They towed sleds, heavily laden with equipment and food, including over 80 pounds of chocolate bars. They soon fell into a routine, setting out at midday and arriving at the next station at 2 p.m., where they would dig a snow pit to sample the last two years of snow accumulation and any carbon trapped within.

Each pit was a just over six feet square and just as deep, dug by hand in temperatures far below freezing. “You’re digging and digging and digging and sweating in that hole,” he says. “But then you get concerned about contaminating it. So, you climb out, put on clean gear, get back in and scrape another six inches off the sides.” Only then would Polashenski begin cutting Rubik’s cube-sized snow samples from the wall, one every inch, from top to bottom. They would finish by 2 a.m., eat, and fall into their sleeping bags. By mid-June, they had dug 35 snow pits, taken 1,100 snow samples, and made 1,190 albedo measurements...

Chris Polashenski and Carolyn Stwertka
Polashenski and Carolyn Stwertka, Dartmouth engineering PhD student, digging a snow pit. (Photo courtesy of NOVA/PBS)

...At a lecture at Dartmouth in December, Polashenski’s enthusiasm for the black carbon hypothesis was beginning to fade. His preliminary analysis of the snow samples suggested the levels of the pollutant were too low for black carbon to be the driver of the widespread melting seen in 2012. “It’s hard to conceive that this black carbon layer made a big enough change to cause melt all through the ice sheet—it isn’t enough,” he says. In all his calculations and estimates, black carbon only seemed to account for a small fraction of the melt. By working with samples collected during the summer of 2012, he says, “We’re studying the perfect storm. We know black carbon can get to Greenland and we know that it reduces albedo, but the question is by how much and does it matter?” ...

...Box’s optimism is further buoyed by a study recently published in the Proceedings of the National Academy of Sciences. Using ice cores that date back to 1750 and surface samples from 2012 taken at four study sites in Greenland, including Summit Station, Kaitlin Keegan, a graduate student [in engineering] at Dartmouth, looked for links between black carbon from wildfires and widespread melting of the Greenland ice sheet. The 262-year record shows four black carbon peaks at depths that correspond to 1868, 1889, 1908, and 2012, and that the ice sheet melted extensively in 1889 and again in 2012. The other two years—1868 and 1908—didn’t see significant melting because temperatures at the surface of the ice sheet were too cool in 1868 and the black carbon fell too late in the year in 1908 to trigger widespread melting...

...Keegan and her colleagues predict that with rising temperatures and an increase in the frequency of forest fires, widespread melting should occur on average every five years by 2100. Wildfires in the Western U.S. have become larger and more frequent over the last 30 years, and the length of the fire season expanded by 78 days between 1987 and 2003...

...Both teams are back in Greenland this spring and summer to collect more samples and make more measurements. Polashenski’s group was unable to reach the area between Summit and NEEM this year, and instead focused on getting a better understanding of the distribution of black carbon across Greenland in 2012, along the Thule peninsula. Box is focusing on the biology of the ice sheet and how dark-colored microbes alter its albedo. His team plans to camp on the ice sheet for two and a half months, using a drone to measure the ice sheet’s reflectance. “By bringing the biology and the glaciology together, some innovation can occur and we can learn something new,” Box says.

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