Greenland Ice Sheet High Elevation: Not Melting Due to Soot or Carbon

While scientists have been aware in recent years that the ice sheet in Greenland appeared to be getting darker and seemed no longer to be reflecting sunlight back into space, a recent study from Dartmouth College found that the "dirty ice" or "dark snow" is not the result of soot or dust from forest fires or oil/gas pollution, but because of satellite sensors that have degraded and are showing inaccurate information regarding the ice. However, they noted that the findings of this study are only the case for higher elevations on the ice sheet. 

The study also says that the ice sheet most likely is more reflective than we have thought. The research team believes that concentrations from dust and black carbon have not increased as much as perceived, either. Those findings were recently reported in the journal Geophysical Research Letters.

This contradicts anecdotal and observational findings before this, which noted that the albedo -- or ice's ability to turn the sun's energy back toward the atmosphere -- has broken down since 2001 because of carbon and soot. The thinning has seemed most apparent at the ice edges, according to a release.

It's important to note that a couple of properties are related to dry-snow reflectivity. One of these is snow-grain size; they become more absorbent and larger as they melt and spread out. The second is dark impurities that take in the sun's energy. These last are usually mineral dust and black carbon, and they make snow melt faster. While black-carbon concentrations in the Greenland Ice Sheet's higher levels is usually low, wildfires in Canada and Siberia in 2012 and winds may have triggered ice-sheet surface melting that year, said the release.

In response to all this, study lead-author Chris Polashenski of Dartmouth and colleagues looked at snow-pit samples from 2012-2014 snowfalls in northern Greenland, comparing them with earlier-year samples. There was very little discernible difference in the amount of black carbon deposit over the past 60 years. Also, compared with the last 12,000 years, there was no significant change in the amount and types of minerals present. The team also ruled out algae growth as a culprit in darkening the ice, in this case, noted the release.

The research instead concluded that sensors in NASA's MODIS satellites, which are aging, are degrading. The scientists think that the ice decline indications will end when new measurements are processed again. In other words, the sensors are getting old and picking up incorrect information. MODIS is an instrument on NASA's Terra and Aqua satellites; they shoot images of the Earth's surface and clouds every two days. The Terra mission was launched in 1999 and the Aqua mission went up in 2002. The plan was for them to gather information for 15 years, the release noted.

The researchers noted that their findings do not apply to ice at lower elevations in Greenland, because in those areas the melting, soot and dust are causing reflectivity declines, and warmer temperatures may also increase algae growth. All of those may further erode reflectivity. 

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