|July 03, 2012|
Greenland Ice Sheet Melt Nearing Critical 'Tipping Point'
|Last year, scientists explained that the Greenland Ice Sheet "could undergo a self-amplifying cycle of melting and warming" that is "difficult to halt." A new study finds we may be close to a "tipping point." |
The Greenland ice sheet is poised for another record melt this year, and is approaching a "tipping point" into a new and more dangerous melt regime in which the summer melt area covers the entire land mass, according to new findings from polar researchers.
The ice sheet is the focus of scientific research because its fate has huge implications for global sea levels, which are already rising as ice sheets melt and the ocean warms, exposing coastal locations to greater damage from storm surge-related flooding.
Greenland's ice has been melting faster than many scientists expected just a decade ago, spurred by warming sea and land temperatures, changing weather patterns, and other factors. Until now, though, most of the focus has been on ice sheet dynamics --- how quickly Greenland's glaciers are flowing into the sea. But the new research raises a different basis for concern.
The new findings show that the reflectivity of the Greenland ice sheet, particularly the high-elevation areas where snow typically accumulates year-round, have reached a record low since records began in 2000. This indicates that the ice sheet is absorbing more energy than normal, potentially leading to another record melt year --- just two years after the 2010 record melt season.
"In this condition, the ice sheet will continue to absorb more solar energy in a self-reinforcing feedback loop that amplifies the effect of warming," wrote Ohio State polar researcher Jason Box on the meltfactor.org blog. Greenland is the world's largest island, and it holds 680,000 cubic miles of ice. If all of this ice were to melt --- which, luckily won't happen anytime soon --- the oceans would rise by more than 20 feet.
In a new study, Box and a team of researchers describe the decline in ice sheet reflectivity and the reasons behind it, noting that if current trends continue, the area of ice that melts during the summer season is likely to expand to cover all of Greenland for the first time in the observational record, rather than just the lower elevations at the edges of the continent, as is the case today.
The high reflectivity of snow is what has kept Greenland so cold by redirecting incoming heat from the sun back out toward space. But with several factors combining to increase temperatures in Greenland and reduce the reflectivity of the snow and ice cover, the ice sheet is becoming less efficient at reflecting that heat energy, and as a consequence melt seasons are becoming more severe.
Freshly fallen snow reflects up to 84 percent of incoming sunlight, but during the warm season the reflectivity declines as the ice grains within the snowpack change shape and size. In addition, once snow cover melts completely it often reveals underlying ice that has been darkened by dust and other particles, whose surface absorbs more solar energy, promoting heating.
Box's research has shown that the change in the reflectivity of the Greenland ice sheet during the 12 summers between 2000 and 2011 allowed the ice sheet to absorb an extra 172 "quintillion joules" of energy, nearly twice the amount of energy consumed in the U.S. in 2009. This extra energy has gone into raising the temperature of the snow and ice cover during summer.
"If the area of the Greenland Ice Sheet experiencing net melt expands to eclipse the accumulation zone of the ice sheet, the ice sheet will, by definition, be tipped into a state of inevitable decline," said William Colgan, a research associate at the Cooperative Institute for Research in Environmental Sciences (CIRES) who did not participate in the new research.
In an interview shortly before leaving for a Greenland research expedition, Box said the new findings reveal that the normally snowy, windswept higher-elevation areas of Greenland are rapidly transitioning to melt during the summer in a similar manner to the lower reaches of the ice sheet.
"It appears that we're about to cross a threshold in summer . . . you might even call it a tipping point as we go into a net energy absorption" of the higher elevations, Box said. "Then we'll see the melt area expanding abruptly and potentially covering the entire ice sheet in summer for the first time in observations."
Box said the shift may take another decade or so, provided that current trends continue. "We're right on the threshold."
Colgan said it's difficult to make predictions based on the reflectivity record, but it would be significant if 2012 becomes another record melt year.
"The frequency with which Greenland record melt years are being established is exceptional, and certainly supports the notion that the Arctic climate is warming, and moving away from an equilibrium climate state in which extreme or record events are relatively rare," Colgan said in an email conversation.