Arctic Wildfires Speed Melting of Greenland Ice: Study

By Andrew Freedman

SAN FRANCISCO — Smoke from Arctic wildfires may have contributed to the melting of the Greenland ice sheet during the 2012 melt season, according to new research to be presented at a scientific conference in San Francisco on Friday. The research, led by Jason Box of Ohio State University, provides the first satellite-based evidence that smoke from Arctic wildfires is reaching the Greenland ice sheet, where it acts as an amplifier of the warming already occurring from manmade emissions of greenhouse gases, such as carbon dioxide.

Aerial photo of the Greenland ice sheet surface from mid-August, 2005. The dark areas are concentrations of light absorbing impurities. This example the impurities are some combination of dust, soot, and microbial activity.
Click to enlarge the image.
Credit: Jason Box.

Wildfire smoke contains dark-colored soot and other particles which, when deposited onto the Greenland ice sheet, cause the ice and snow surfaces to darken and absorb more incoming solar radiation, and therefore melt more readily. Box said that wildfire smoke may be responsible for a dramatic expansion in the portion of Greenland that experiences melting during the summer season.

During July of this year, melting was observed even at the highest elevations in Greenland, with the melt extent reaching a record 97 percent of the ice sheet on a single day. This is nearly four times greater than the typical 25 percent average melt extent observed during the 1981 to 2010 period, according to a separate Arctic report released on Wednesday.

As was first reported by Climate Central in June, Box observed a record low in the reflectivity of the Greenland ice sheet during the 2012 melt season, which he attributed partially to the influence of dark-colored particles on the bright, highly reflective snow and ice cover. He and his colleagues have found that the reflectivity of the Greenland ice sheet has declined by 6 percent in the last decade, which is helping to cause melting even in the coldest areas of the island. However, exactly how much of that decline is attributable to soot from wildfires is not yet clear.

Profile of the atmosphere above Greenland's ice sheet, showing areas of what researchers say is wildfire smoke. 
Click to enlarge the image.
Credit: Ohio State/NASA CALIPSO satellite.

“Soot is an extremely powerful light absorber,” Box said in a press release. “It settles over the ice and captures the sun's heat. That's why increasing tundra wildfires have the potential to accelerate the melting in Greenland.”

A comprehensive annual report on the state of the Arctic, which was released on Wednesday, found that global warming is causing an increase in Arctic wildfires, as vegetation cover increases and thunderstorms, which are a key ignition source for wildfires, become more numerous during the Arctic summer. The report found that conditions have been particularly favorable for Alaskan wildfires during the past several years.

The high reflectivity of snow is one factor that has kept Greenland so cold by bouncing incoming heat from the sun back out toward space. But with several factors combining to increase temperatures in Greenland and reduce the reflectivity of the snow and ice cover, the ice sheet is becoming less efficient at reflecting that heat energy, and as a consequence melt seasons are becoming more severe.

Freshly fallen snow reflects up to 84 percent of incoming sunlight, but during the warm season the reflectivity declines as the ice grains within the snowpack change shape and size. In addition, once snow cover melts completely it often reveals underlying ice that has been darkened by dust and other particles, whose surface absorbs more solar energy, promoting heating.

Greenland ice melt extent trends, showing the extensive melt in 2012.
Credit: 2012 Arctic Report Card.

Greenland is the world's largest island, and it holds 680,000 cubic miles of ice. If all of this ice were to melt — which, luckily won't happen anytime soon — the oceans would rise by more than 20 feet.

Drew Shindell, a NASA climate researcher who was not involved in this research, said there are still many open questions about how wildfires are contributing to Greenland melt. “The link between fires, black carbon [soot] and melting seems quite reasonable, but I too would want to see some more measurements to know if the contribution from this was important or not,” Shindell said in an email message. “It'd be good to see how variations in fires from year-to-year correspond with variations in melting to reinforce their claim.”

To find evidence of soot deposition from these fires, Box and his colleagues, including Thomas Painter of NASA's Jet Propulsion Laboratory and graduate student McKenzie Skiles of the University of California, Los Angeles, first used thermal images from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) to spot large Arctic wildfires. They then calculated possible smoke particle trajectories, which showed that smoke could be reaching Greenland, particularly from fires in Alaska and parts of Canada. They then used those trajectories to scrutinize thermal imagery from another NASA satellite to pinpoint sooty aerosols over Greenland.

In order to get an accurate measure of the extent of the soot particle contribution to the Greenland melt, it's also neccesary to obtain ground-based evidence. To accomplish this task, Box is planning an expedition to the ice sheet in 2013, called the Dark Snow Project. He said it would be a funded through crowdsourcing, making it the first such scientific ice expedition of its kind.

Box has been an outspoken advocate of reducing greenhouse gas emissions, going as far as taking part in protests against the Keystone XL oil pipeline at the White House in 2011. 

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