The melting of Greenland
We’ve already seen that 2010 was the warmest year on record for Greenland. That was part of the evidence which lead the National Oceanic and Atmospheric Administration to conclude that the Arctic has entered a “new and different climate state” — one that is not reversible. It should therefore not be a surprise to learn of the extraordinary melting that took place last year.
As the above map from NASA’s Earth Observatory page shows, substantial sections of Greenland, particularly in the south and west, experienced a much longer melting period (in some places up to 50 days longer) than the average for the previous 30 years. The data on which the map is based come from microwave readings taken by satellite. Wet and dry snow emit different microwaves. The above chart shows how long (relative to a 30 year average) areas emitted the microwaves of wet (or melting) snow.
The anomalies follow a trend (but this year much more pronounced) that was noted by NASA observers three years ago. Below is a similar map, posted on NASA’s Earth Observatory page on November 30, 2007. The data were compiled in the same way. Note that the longest anomalies then were 30 days as opposed to the 60 days experienced in 2010.
Greenland is not the only place where substantial melting took place in 2010; in fact just a week ago we saw that in January 2011 the Arctic overall had a record low sea ice cover. In a recent paper in Environmental Research Letters, however, Marco Tedesco, an assistant professor at the graduate center of City College of New York, describes the melting in Greenland in detail. And the explanation is somewhat different than that attributed to the low sea ice cover. See M. Tedesco, X. Fettweis, M.R. van den Broeke, R.S.W. van de Wal, C.J.P.P. Smeets, W.J. van de Berg, M.C. Serreze, & J.E. Box, “The role of albedo and accumulation in the 2010 melting record in Greenland,” 6 Environmental Research Letters (January 21 2011) (abstract and access to full paper).
First, the summer of 2010 saw record surface temperatures along the coast of Greenland. The capital Nuuk, for instance, recorded its warmest year in at least 138 years, when record-keeping began in 1873. Thus, the melting began earlier than usual. When snow melts its albedo (roughly, its reflectivity) decreases. This is because snow grains size increases during a melt. The larger grains reflect light more poorly than the finer grains. With lower albedo, the snow absorbs more energy, which causes greater melting. In the southwest there was significantly less snowfall during the summer than average. In fact, snowfall in that area was two standard deviations below the mean of 1979 to 2009. The reduced snowfall together with the high surface temperatures led to a prematurely exposure of bare ice, which in turn led to reduced albedo. This can be seen from comparing the three phenomena in figure 2 from the paper.
The map on the right (c) shows the anomalies of summer snowfall anomalies in millimeter of water equivalent relative to the mean of the period 1979–2009. The middle map (b) shows the “standardized anomaly” of the number of days when bare ice was exposed. (This “standardization” is calculated by dividing the departure from the mean by the standard deviation of the distribution of the number of days.) The map on the left (a) shows the shortwave white-sky albedo anomaly for the months of May–August relative to the 2004–2009 mean. The strongly negative anomalies along the southern and western coasts correlate with the positive melting anomalies from the satellite data.
Bare ice is also darker than snow, which means that exposed ice absorb more energy than snow and continue the melting. The melting lasted longer than usual in 2010 (as well as starting earlier).
All of these factors, according to Tedesco, “paints a portrait of strongly negative surface mass balance during 2010 promoted by a strong warmth and enhanced by large negative anomalies of albedo and accumulation and large positive anomalies of days when bare ice was exposed.” This means that the mass of the large ice sheet that covers almost all of Greenland was reduced last year. When that happens fresh-water is poured into the ocean and the ocean level rises. It’s estimated that the water locked in the Greenland ice-sheet, if melted, could raise sea levels by seven meters. It’s currently thought that at current warming Greenland will contribute about a half meter rise in ocean level by 2100. But given the feedback mechanisms described by Tedesco, it is possible that melting will occur at a faster rate with a faster rise in ocean levels.