May 20, 2014
Warming And Ash From Fires Drove 2012 Greenland Ice Sheet Melt
April Flowers for redOrbit.com - Your Universe Online
In 1889 and 2012, there was large-scale melting of the Greenland ice sheet. Scientists have assumed these events were driven by warming alone, but a new study from the Thayer School of Engineering at Dartmouth College and the Desert Research Institute reveals that rising temperatures and ash from Northern Hemisphere forest fires combined to cause the ice melting events.
The findings, published in the Proceedings of the National Academy of Sciences (PNAS), indicate that by 2100, continuing climate change will cause a nearly annual widespread melting of the ice sheet's surface. This may set into motion a positive feedback mechanism. Instead of affecting sea level rise, meltwater from the dry snow region percolates into the snowpack and refreezes. This lowers the albedo, the surface's ability to reflect sunlight of the ice sheet, making it more susceptible to future melting.
"The widespread melting of the Greenland ice sheet required the combination of both of these effects -- lowered snow albedo from ash and unusually warm temperatures -- to push the ice sheet over the threshold," says Kaitlin Keegan, Dartmouth doctoral student. "With both the frequency of forest fires and warmer temperatures predicted to increase with climate change, widespread melt events are likely to happen much more frequently in the future."
Each year, the massive Greenland ice sheet melts at low elevations near the coastline. In the dry snow region near the center of the ice sheet, however, surface melt is rare. The first widespread melt during the era of satellite observation occurred in July of 2013 when more than 97 percent of the ice sheet experienced surface melt. Keegan, who studies "firn," the newly deposited layers of snow atop the two-mile-thick ice sheet, contributed critical information to NASA's announcement of the 2013 melt.
The research team analyzed samples from six Greenland shallow ice cores retrieved from the dry snow region. These samples confirmed that the most recent prior widespread melt before 2012 occurred in 1889. Analysis of an ice core from the center of the ice sheet revealed that black carbon sediment deposited from Northern Hemisphere forest fires combined with exceptionally warm temperatures to reduce the ice sheet's albedo below a critical threshold, causing large scale melting events in both 1889 and 2012.
The source of the ash was not a particular focus of the study; however the presence of a high concentration of ammonium concurrent with the black carbon suggests large boreal summer forest fires in Siberia and North America in June and July 2012 were the culprits. Just before the widespread melt event in the dry snow region, air masses from these two regions reached the ice sheet. Historical records of testimony to the US Congress point to large-scale forest fires in the Pacific Northwest in the summer of 1889. It would be difficult, however, to pinpoint exactly which fires were responsible for the ash deposits that summer.
Data from the Intergovernmental Panel on Climate Change (IPCC) was also incorporated into the model to predict the widespread surface melting into the year 2100. Climate change is expected to continue to cause rising Arctic temperatures and rising forest fire frequency. The findings indicate that large-scale melt events may begin to occur almost annually on the Greenland ice sheet by the end of the current century. Such events would likely alter the surface mass balance of the ice sheet, which would, in turn, leave the surface susceptible to further melting.
"Our Earth is a system of systems," says Thayer Professor Mary Albert, director of the U.S. Ice Drilling Program Office. "Improved understanding of the complexity of the linkages and feedbacks, as in this paper, is one challenge facing the next generation of engineers and scientists — people like Kaitlin."
Earlier this week, redOrbit reported on a separate, but related, study from UC Irvine and NASA suggesting that the Greenland is far more vulnerable to warm ocean waters from climate change than previously thought. The Thayer study focused on the dry snow region at the center of the ice sheet, but the UC Irvine study focused instead on previously uncharted deep valleys that stretch for dozens of miles beneath the ice sheet. The results of this study were published in Nature Geoscience.
Subtropical Atlantic waters push against hundreds of glaciers, and these bedrock canyons allow the warm waters to erode the edges much farther than previously assumed. As the Greenland ice sheet is the second largest body of ice in the world, this erosion will release far greater amounts of water than predicted.
It is known that ice melt from the subcontinent has already begun to speed up as the warmer marine currents have migrated north. Prior models, however, predicted that as higher ground was reached, the warm water induced melting would slow or halt, mitigating the effect such melt would have on sea level.
“That turns out to be incorrect. The glaciers of Greenland are likely to retreat faster and farther inland than anticipated – and for much longer – according to this very different topography we’ve discovered beneath the ice,” said Mathieu Morlighem, a UCI associate project scientist. “This has major implications, because the glacier melt will contribute much more to rising seas around the globe.”
The new research was supported by the National Science Foundation and NASA.