Last July, something unprecedented in the 34-year satellite record happened: 98 percent of the Greenland Ice Sheet鈥檚 surface melted, compared to roughly 50 percent during an average summer. Snow that usually stays frozen and dry turned wet with melt water. Research led by the Cooperative Institute for Research in Environmental Sciences now shows last summer鈥檚 extreme melt could soon be the new normal.
鈥淕reenland is warming rapidly, and such ice-sheet-wide, surface-melt events will occur more frequently over the next couple of decades,鈥� said Dan McGrath, a University of Colorado Boulder doctoral student who works at CIRES. McGrath is lead author of a paper published online May 20 in Geophysical Research Letters and which reports a significant warming trend on the Greenland Ice Sheet.
McGrath and his coauthors calculate that by 2025 ice-sheet-wide melt events will have a 50 percent chance of occurring each year. That would signal the loss of the last major dry-snow zone -- regions where the snow stays almost perpetually frozen -- in the Northern Hemisphere, McGrath said.
In the study, the researchers used air and snow temperature data from meteorological stations and boreholes to generate a 60-year record of air temperatures at the Summit research station, the highest and coldest station on the ice sheet.
From 1982 to 2011, near-surface temperatures increased by an average of 0.16 degrees Fahrenheit every year. 鈥淭his is six times faster than the global average,鈥� McGrath said.
The warming at Summit is also accelerating. From 1950 to 2011, the average rate of warming was 0.04 degrees F per year. But from 1992 to 2011, that number jumped to 0.22 degrees F per year.
The warming has had a dramatic effect on the ice sheet鈥檚 structure, the scientists report. The ice sheet鈥檚 ablation zone -- the lower parts that lose more snow and ice each year than they accumulate -- is expanding up the ice sheet by about 145 feet per year.
鈥淭his increases the area over which the ice sheet sheds mass while shrinking the zone that gains mass,鈥� McGrath said. 鈥淭hat will have an obvious impact on the ice sheet鈥檚 mass balance.鈥澨�
Additionally, the dry-snow line -- above which the snow doesn鈥檛 melt -- is migrating up the ice sheet by about 115 feet per year. 鈥淭hese zones are indicators of the health of the ice sheet,鈥� McGrath said. 鈥淎nd the changes we are observing are an early but important sign that the ice sheet is in transition.鈥�
The changes could increase the amount of solar radiation the ice sheet absorbs -- since wet snow reflects less sunlight than dry snow -- increasing the melt rate as well. It also could potentially speed up the ice sheet鈥檚 flow, though more work needs to be done to untangle these impacts.
These findings are supported by results from other researchers who have found that the ice sheet is losing more than 275 billion tons of ice per year -- equivalent to the weight of 750,000 Empire State Buildings. 鈥淭his imbalance is making a significant contribution to sea-level rise,鈥� McGrath said.
The summit of the Greenland Ice Sheet has experienced surface melt in the past, McGrath says. But the melt events in the past were rare, happening once every century or two听 -- in fact, only eight times in the last 1,500 years -- the exception rather than the norm. Now the norm is shifting toward a new, slushy set point.
鈥淧rogressive increases in surface melt have occurred throughout the satellite record, but the last decade has been exceptional,鈥� McGrath said. 鈥淚f each of these events keeps being so far above the average, the average will change to reflect that.鈥�
The scientists鈥� findings come at a time when Arctic sea ice extent is also at record lows. 鈥淚ce-sheet-wide melting coupled with the loss of Arctic sea ice points to profound changes occurring to the Arctic climate system,鈥� McGrath said. 鈥淭hese are not small, insignificant events we鈥檙e witnessing.鈥�
NASA Cryospheric Sciences funded the research, with additional field logistical support provided by the National Science Foundation鈥檚 Office of Polar Programs. Co-authors include former CIRES Director Konrad Steffen, CIRES adjunct research associate William Colgan, former CIRES doctoral student Atsuhiro Muto, and current CIRES doctoral student Nicolas Bayou.
CIRES is a joint institute of the National Oceanic and Atmospheric Administration and CU-Boulder.
颁辞苍迟补肠迟:听
Dan McGrath, 303-492-6881
Daniel.mcgrath@colorado.edu
Katy Human, CIRES communications director, 303-735-0196
Kathleen.Human@colorado.edu
A new study shows summer melt events on the Greenland Ice Sheet are increasing in frequency due to rising temperatures. 听Photo courtesy CIRES
