The study, led by Marco Tedesco, director of the Cryospheric Processes laboratory at the City College of New York, showed that the melting index had broken the previous record, set in 2007.
Melting in 2010 started exceptionally early at the end of April and ended quite late in mid- September, Tedesco said in a statement. This past melt season was exceptional, with melting in some areas stretching up to 50 days longer than average.
A melting Greenland ice sheet contributes to sea level rise, which has occurred at a mean rate of about 1.8 millimeters per year over the past century. If the Greenland ice sheet were to melt completely it would raise sea levels by 7 meters. But that is unlikely to happen for several centuries at least.
One reason for the record-breaking melt was that summer temperatures in the Arctic were 2-3 degrees C (5.4 degrees F) warmer than normal. Greenland's capital, Nuuk, experienced temperatures higher than any since 1873, when weather records started being been kept there. NASA data showed that 2010 was tied with 2005 as, globally, the warmest year on record.
Combined with reduced snowfall, the bare ice was more exposed to the sun, causing more of it to melt and faster. Other factors that influence ice melt are soot left on the surface, which absorbs heat, and the lakes that form on the surface, which also warm the ice because they are darker.
The data was gathered from satellites operated by the National Oceanic and Atmospheric Administration. By measuring the amount of microwave radiation emitted by the ice, snow and water of the ice sheets in Greenland, and comparing that with data gathered on the ground, the researchers were able to measure how much of the ice sheet is subject to melting.
Adding up the areas subject to melting each day gives an index, which can then be used to see how much more melting (or less) is happening in any given year. For the baseline year the researchers took an average of the melting between 1979 and 2009.
The index is presented as a multiple of the standard deviation from the mean. The deviation is the spread around the average. For example, a group of 10 people might have a mean height of 67 inches tall, with the shortest people at 60 inches and the tallest at 74 inches. The standard deviation is seven inches, and a basketball player who is 81 inches tall is two standard deviations off of the average.
In the melt index measurements, 2010 got a value of just over 2, similar to 2007. Previous record-setting years were 2005, 2002, and 1998.
Tedesco said if the variability were random, then over a 30-year period one would expect the record years to be evenly distributed - they would be just as likely to be in 1970 as they are in 2001. But the data shows a gradual upward curve, and even the fact that the melt index was just about average in 2008 and 2009 doesn't alter the trend.
He added that the real concern about the ice sheets is that the melt process is not linear. That is, twice as much meltwater on the ice surface doesn't mean only twice as much melting of the underlying ice sheet. That's because many processes are feedbacks.
For example, more meltwater on the surface means more heat, which melts more ice and makes more meltwater. A river of water running over ice transfers heat to the ice because it is above freezing, melting yet more ice and increasing the volume of water. If the feedbacks accelerate past a certain point, the ice sheet will melt even faster, Tedesco said.
Tedesco said he is planning future trips to Greenland to find out more about the contribution to melting of the water that seeps under the ice sheet from the surface.