Far more ice has melted on Greenland's this year than before in the history of melt measurements. Measuring the amount of melting precisely is challenging, but scientists studying Greenland have found that they can make a good first estimate by adding up the amount of area melted each day of the melt season. This year, the season has been longer than ever to date and a larger area has melted per day. With weeks to go in the melt season, all previous melt records have been smashed.
Map of the 2012 anomaly of the number of melting days with respect to the 1980 - 1999 average (e.g., red color indicates areas where melting lasted up to 50 days above the 1980 - 1999 mean). Updated through August 8th.The melting index is computed from passive microwave satellite measurements and it can be seen as a measure of the ‘strength’ of the melting season: the higher the index the more melting occurred. With more melting yet to come during August, 2012 will position itself way above the old records, likely becoming the 'Goliath' of the melting years during the satellite record (1979 - to date). From the map below, we see that the cumulative melting index record is due to extensive increased melting occurring all over Greenland, especially at high elevations where melting lasted up to 50-60 days longer than the average. This means that some of the areas at high elevations in south Greenland are generally subject to a few days of melting (if it happens at all) and this year they underwent melting for more than 2 months (so far).
So, how is this record different from the one that happened in mid July of 2012 and received so much coverage ?
The extreme melting detected at high elevations in mid July (covering ~ 97 % of the Greenland ice sheet, see image on the left) generated liquid water that refroze after a few days, changing the physical properties of the snowpack but very likely not contributing to the meltwater that run offs from the ice and can potentially contribute to sea level rise. The event was exceptional in the sense that it is a rare event (imagine a postcard of Rio de Janeiro under a thin layer of snow !) but it happened in the past, according to the research of colleagues from the Dartmouth College at Summit. The record set by the overall melting has implications on the meltwater that goes into the ocean and it can impact ice dynamics, through basal lubrication or through its impact on the subglacial and englacial drainage systems. Also, the increased melting at higher elevations might remove the seasonal snow and expose more bare ice. The removal of bare ice (which is darker and the absorb more solar radiation and it is therefore more prone to melting than snow), is actually contributing to the net mass loss of Greenland.
This year's mega-melt is not caused by one freak summer of warm weather. Summer temperatures above the ice sheet began rising rapidly after the super El Nino of 1997-1998. Temperatures rose about 1° C from 1998 to 2011. Final numbers aren't in yet for 2012 but it's clear from weather instruments and melt data that this summer is even warmer. The warm temperatures over the ice cap correlate with warm sea surface temperatures in the seas around Greenland. Warm Atlantic ocean water has been expanding northward into the Labrador and Greenland seas since 1998. The heat content of the north Atlantic ocean has risen the fastest of all the ocean basins on earth since 1998. This rapidly rising heat content is changing northern hemisphere weather and rapidly increasing the air temperatures over Greenland.
Greenland ice sheet summer surface air temperatures: 1840-2011The high temperatures this summer, likely the warmest in 1000 years, caused multiple episodes of large-scale melting this year.The episode that triggered melt over 97% of Greenland's surface was one of 4 episodes this summer where melting caused Greenland's reflectivity to drop to very low levels. A dome of warm air, likely related to the warm seas around Greenland, covered Greenland much of the summer.
Developing a new manuscript (Box et al. submitted), I’ve managed to update the Box et al. (2009) near-surface air temperature reconstruction and am struck after incorporating 4 more years, it seems little doubt that recent summer air temperatures for Greenland ice are the highest in at least 172 years. Summer temperatures in the late 2000s are roughly 0.5 C warmer than in the 1930s and even warmer than at any time since at least 1840s. Because the reconstruction captures the end of the Little Ice Age, it is further reasonable to think that Greenland probably hasn’t been as warm in summer than since the time the Norse colonized Greenland beginning in 982. Implications for the recent warmth are of course grabbing headlines. I’ll be adding 2012 data soon.
lowest albedo since year 1150?
The 16 July low was the lowest in the satellite observational record and coincided with 97% of the ice sheet surface area melting. Previous maximum melt extent values since 1978 (when satellite obseravations begin, this is what NASA meant by “unprecedented”) are under 60% of the ice sheet area. Because the 2012 summer temperature was warmer than previous years (as I tweeted 5 August: June 2012, warmest on record for Greenland’s capital Nuuk since at least 1866 when continuous record keeping began, +7.2 C vs +4.3 C average), warmer than 1929 by at least 0.5 deg. C, and if the near surface air temperature records, continuous since 1840, are any indication (Box et al. 2009) this albedo anomaly and accompanying melt extent is probably without precedent since the Medieval Warm Period when the Norse settled Greenland. Greenland temperature variability is high and there is evidence during the late Medieval Warm Period of a warm period in year 1150, that is 862 years before present (Kobashi et al. 2011). Other factors than warming that could have temporarily lowered Greenland ice reflectivity include the effect of major volcanic eruptions or wild fires. The latter I speculated here. The former has a noteworthy cooling effect but could conceivably still blanket the ice sheet with low reflectivity soot.