Greenland's massive ice sheet experienced record surface melting and runoff last year, according to research released today.
Unusually warm conditions in much of the country helped extend the annual melting season by up to 50 days longer in 2010 than the average observed between 1979 and 2009, researchers found.
Last year also set records for the amount of water runoff from the ice surface, loss of surface ice and the number of days when ice was bare rather than blanketed by snow. Summer snowfall was below average.
"In 2010, generally speaking, surface temperatures were higher than average," said lead author Marco Tedesco of the City College of New York's Cryosphere Processes Laboratory. "It was not just the summer temperatures, but also the spring and later winter temperatures. The melting season started early and lasted much longer than normal."
Tedesco's co-authors include researchers from the University of Liege in Belgium, Utrecht University in the Netherlands, the National Snow and Ice Data Center in Boulder, Colo., and Ohio State University. Their work was published today in the journal Environmental Research Letters.
The researchers based their analysis on satellite observations of surface temperatures, satellite estimates of surface melting, ground observations from automated weather stations on the ice sheet's surface and computer models.
"The overall idea of the project is to improve the capability of the models by ingesting this information from satellites and ground observations," Tedesco said. That will help scientists project how future warming will affect the behavior of Greenland's ice, which could help improve estimates of sea level rise.
Recent studies suggest that sea level will rise between 3 and 6 feet by 2100, with a significant contribution coming from the more rapid melting of huge ice sheets in Antarctica and Greenland.
Mechanisms that accelerated the melting
Last year was the warmest in Greenland's capital, Nuuk, since record keeping began there in 1873. Nuuk, on the country's southwest coast, also set records in 2010 for warmest winter, spring and summer seasons.
Meanwhile, the southern Greenland community of Narsarssuaq recorded its warmest-ever winter and spring, warmest May, and warmest year since observations began in 1951. The town of Aasiat in western Greenland saw its warmest year, winter, spring, and months of May and June since record keeping started in 1951.
But that unusual warmth is just the beginning of the story of what happened last year, Tedesco said.
Greenland's annual melt began earlier than normal last spring. As that melt reduced the amount of snow on the ice surface, it also "aged" the snow that remained so it became less reflective and absorbed more heat from the sun -- accelerating the melting further.
Eventually, the combination of unusual warmth and below-average summer snowfall exposed large portions of bare ice, which is less reflective than snow and absorbs even more heat. That helped extend the length of the melt season well into the fall.
"Everything adds up to accelerate things," Tedesco said. "They are all guilty."
While 2010 was a record-setting year in Greenland, the scientist said his study is important because it used a variety of data, estimates and models to construct a detailed picture of how processes affecting the ice sheet influence each other.
A process that could become 'exponential'
"The bottom line of the paper is that we know that it is important to look at the surface temperature," Tedesco said. "We know the warmer it is, the more melting you have. But this is not just a linear relationship. It can be exponential, and increase much faster. ... It's important to look at all of these processes as a whole."
That can help improve scientists' understanding of how the Greenland ice sheet will respond to future climate change, and how that will affect sea level rise, he said.
The new study didn't examine one of the major processes by which Greenland loses ice and contributes to sea level rise -- the calving of icebergs. But it did examine melting at the surface of the ice sheet, which scientists believe influences the speed at which the ice sheet slides to the sea.
Earlier research has shown that some of the meltwater drains into cracks in the ice down to the bedrock below, where it helps lubricate the ice sheet's movements.