Much to-do has been made this week about surface melt in Greenland. For anyone who hasn't been following the story, NASA released findings that shows surface melt has been observed on more than 97 percent of the Greenland ice sheet this summer a rate unprecedented in the era of satellite observation.
The subsequent media uproar has centered mainly on the revelation that this year's melt is not an isolated occurrence. The deep interior of Greenland is high enough in elevation to normally stay frozen year-round, but ice cores show us that extreme melt events do happen from time to time. This year's extreme melt episode is noteworthy for being the first such in nearly 125 years, as well as the first to be observed by satellites.
Climate naysayers, as one might expect, have been critical of NASA's use of the word "unprecedented." The fact that this has happened many times in the past only confirms their belief that there's nothing to be concerned about.
Climate activists, meanwhile, have expressed frustration with NASA for calculating a historical trend over a time span that poorly frames recent observations. It has been perceived by some to be a deliberate and overly conservative hedge to avoid calling a spade a spade.
Lost in the fray is the simple salient fact that Greenland is losing more and more of its ice to the sea every year. These latest findings, unprecedented or not, don't change the fact that this melt season is one more in a longstanding trend of increasingly higher melt seasons.
It is also worth noting that surface melt constitutes approximately half of Greenland's total contribution to sea-level rise. The other half of the equation comes via discharge from outlet glaciers. In other words, last week's findings are in fact only part of the story of ice mass loss and global sea level rise.
Through the last century, the ice sheets were considered fairly stable -- losses and gains were fairly close enough to offset each other. It was thought that a lot of time -- on the order of centuries -- was needed to for an ice sheet to undergo significant growth or loss. Since the late '90s, many Greenlandic outlet glaciers have been accelerating, some dramatically so. Antarctic glaciers have been accelerating as well, pushing the southern continent from what used to be a net gain in ice to a net loss. The rapidity of change -- on the order of decades -- has garnered considerable attention.
Currently, Greenland and Antarctica contribute approximately 1.3 millimeters to sea level rise each year, but this rate is increasing. Under the current rates of acceleration for ice sheet loss, we could expect 56 centimeters of sea level rise by 2100, from the ice sheets alone. Whether this month's extreme melt event was truly unprecedented, or part of a larger cycle, is not really the point. There exists many years of data, from multiple sources of sea level rise, to justify concern. We need not glom onto (nor dismiss) one extraordinary number to come to that conclusion.