A just released study of Greenland’s Arctic ice sheet reveals that without the presence of at least regular small amounts of fresh snow, ice will age, darken, and change shape, resulting in far more solar energy absorption and accelerating melting.
Dartmouth researcher Gabriel Lewis measures reflectivity on Greenland's ice sheet during a 2016 research expedition. According to a new Dartmouth College research paper, a reduction in fresh snowfall has caused parts of Greenland to become darker and may lead to additional surface melt. Photo: Forrest McCarthy, Dartmouth College
That is the conclusion of researchers from Dartmouth College, as disclosed in a recent edition of the peer-reviewed journal Geophysical Research Letters.
The finding came in conjunction with the study of recent weather patterns which are directing fresh snowfall away from parts of Greenland’s ice sheet.
Without the regular fresh coating of light colored snow on top, older and darker snow beneath is exposed at the surface of the existing ice sheets. The aging ice, which has a different physical structure and darker color, has substantially lower levels of albedo, a measure of the reflectivity of the ice sheet when exposed to sunlight.
That lower albedo causes the ice to absorb more heat, which then results in much faster melting of the ice.
"As snow ages, even over hours to a few days, you get this reduction in reflectivity, and that's why the fresh snow is so important," said Erich Osterberg, associate professor of earth sciences at Dartmouth and the principal investigator of the study.
According to the research, the decrease in snowfall is the result of "atmospheric blocking" in which persistent high-pressure systems hover over the ice sheet for up to weeks at a time. The systems, which have increased over Greenland since the mid-1990s, push snowstorms to the north, hold warmer air over Western Greenland, and reduce light-blocking cloud cover.
"It's like a triple whammy effect," Osterberg said. "This all contributes to Greenland melting faster and faster."
According to the research, the result isn't only less snowfall, it's a different type of snow on the surface.
As snowflakes melt or evaporate, they become rounded and less reflective than newer, crystal-shaped snow. This causes the snow surface to become darker. According to the research team, a 1% change in reflectivity across Greenland's ice sheet could cause an additional 25 gigatons of ice to be lost over three years.
"Fresh snow looks like what you would draw in a kindergarten class or cut from a piece of paper - it's got all these really sharp points, and that's because it's extremely cold in the atmosphere when the snow falls," said Gabriel Lewis, the first author of the study, who conducted the research as a PhD candidate at Dartmouth. "Once it falls and sits on the surface of the ice sheet in the sun, it changes shape and the snow grains become larger over time."
The team gathered data for the study during a two-summer 2,700-mile snowmobile trek across a region of Greenland's ice sheet known as the western percolation zone.
The researchers found only about 1 part per billion of impurities in the snow. This helped them determine that the changing shape of snowflakes, forced by the persistent high-pressure systems, was the likely cause of the darkening, rather than soot, dust, or microorganisms.
"It's some of the cleanest snow in the world," said Lewis, "In our research area, the impurities do not appear to be enough to account for the change in albedo other research teams have reported."
According to research cited in the study, the Greenland ice sheet has warmed about 2.7 degrees Celsius (4.85 degrees Fahrenheit) since 1982. The continent is experiencing the greatest melt and runoff rates in the last 450 years, at a minimum, and likely the greatest rates in the last 7,000 years.
Based on this research, the changing nature of weather patterns which used to produce continual replenishment of the fresh snow on ice in the Arctic – and elsewhere in the cryosphere – must be considered yet another feedback loop effect which will contribute to increased global glacial melting and sea level rise.
The increased absorption of heat resulting from this change will also accelerate the overall heating of the planet.
The paper, “Atmospheric Blocking Drives Recent Albedo Change Across the Western Greenland Ice Sheet Percolation Zone,” by Gabriel Lewis, et. al., was just published in Geophysical Research Letters.