Kaitlin Keegan in a snow pit at the Summit camp in Greenland. Sunlight coming through the wall she’s facing shows the layers of snow and firn. (Polar TREC blog)

“I have been studying melt layers (seen in ice cores) for the past year” says Kaitlin Keegan, a PhD degree student in polar science at Dartmouth College. “I have lots of ideas about (ice sheet) melt” she says. “You couldn’t hope for anything better than to be studying melt in Greenland at the time when it’s happening.”

Related: Greenland ice sheet surface melt: massive meltdown or meaningless trickle?

When temperature rose above 32 degrees each day from July 11 through 14 at the National Science Foundation’s Summit, Greenland, Observatory, polar scientists realized they were seeing a kind of warming that had last occurred in 1889.

Summit is 419 miles north of the Arctic Circle and 10,530 feet above sea level, near the highest part of the Ice Sheet where above-freezing temperatures are rare.

Scientists knew about the 1889 warming because that event and similar ones occurring on the average of each 150 years for the last 1,500 years show up in the cylindrical pieces of ice―called ice cores―pulled up from the two miles of ice under the highest parts of Greenland’s ice sheet at Summit and European camps.

Detailed studies of what happens during a rare warm-up will help scientists learn more about the past events.

Since 1997 (the first year Summit stayed open all year) scientists have been making detailed year-round studies of exactly what happens during snow’s transformation into firn, which is snow that’s lasted at least a year but hasn’t yet been squeezed into dense, glacier ice by the weight of the firn above. Firn at Summit is approximately 266 feet deep, dating back to around 1755.

Fortunately for scientists, snow that falls in summer and winter transforms into firn in different ways, which create light and dark bands that scientists use to count back the years in ice cores, much in the way a tree’s rings show its age.

Seeing exactly how snow becomes firn when temperatures have been above freezing, as Keegan and others did in mid-July, makes it easier to understand ice cores showing such transformations maybe hundreds of years ago.

Bubbles of gas and other objects in snow that becomes glacier ice give scientists clues about past climates. Even more important, slight differences in the ice’s water molecules act as thermometers telling them temperatures when the snow that’s now ice formed. Ice cores supply key information about the ups and downs of temperatures in the past.

Cores drilled at Summit and European camps on the ice sheet contain records of Greenland’s and global climates back to approximately 125,000 years ago.

Keegan began studying the 1889 melt last year when she saw a one-centimeter (0.39 inch) part of an ice core that “is mostly transparent like an ice cube.” The core above and below this area is much less dense firn.

“When comparing this layer to the brightness and darkness of other firn layers, it is definitely more bright,” Keegan says. “You can imagine that light can transmit fairly straight through an ice cube but not a snow cone or shaved ice. Also, in the firn you can easily see different grains whereas [in] the ice layer you cannot [just like you cannot see the grains in an ice cube]. It’s definitely the appearance that attracted me to studying the layer. It’s just so very different than the rest of the core.”

After seeing the layer of clear ice, talking about it with other scientists and reviewing the scientific literature, she “realized that what happened in 1889 was a widespread melt event. The study of this melt event, and other smaller, local melt events, will be one chapter of my PhD thesis.”

Until this July, Keegan and other scientists studying the 1889 melt and similar events in the past were confined to what they could tease out from small cross sections of ice cores. “This makes it hard to understand how the melt layer formed and how evenly distributed it is,” Keegan says.

“The most interesting part of being at Summit Station just after the melting is that the melt layer formation could be observed,” she says. “This presents a unique opportunity for us to understand how previous melt events occurred.”

Mary Albert, one of Keegan’s two PhD advisors, who is also executive director of the U.S. Ice Drilling Program Office, says: “One real coincidence with the current melt is the timing relative to a paper Kaitlin and I have been working on.

“We are finishing a paper—telling the interesting story of the 1889 layer―we started on that before we went to Summit,” Albert says. “The melt that then occurred at Summit is pure serendipity with our investigation of the 1889 event. Our investigation of the 1889 ice layer centers on ‘why’―there are lots of warm spells when the snow does not melt at Summit—why did it melt then? Summit is right on the edge and we’re in the process of explaining in this paper what happened.”

News reports of the warming over Greenland described the surface of the ice sheet as melting. But those who were there would describe it as more like the top few inches of the ice sheet turning to slush.

Keegan says that when she arrived at Summit on July 15―the first day with 24 hours of below freezing temperatures after four days of high temperatures of 34 to 36 degrees, “the main difference I noticed it was fairly easy to walk on the snow”

Normally with temperatures always below freezing, she said walking on the four or so inches of fresh snow is difficult, it’s like “walking on a ski area that hadn’t been groomed.” When she arrived at Summit this year the slush had frozen to create a firm layer under the new snow that made walking easier.