Greenland Ice Sheet
M. Tedesco1,2, J. E. Box3, J. Cappelen4, X. Fettweis5, T. Mote6,
R. S. W. van de Wal7, C. J. P. P. Smeets7, J. Wahr8
**1City College of New York, New York, NY, USA
2National Science Foundation, Arlington, VA, USA
3Geological Survey of Denmark and Greenland, Copenhagen, Denmark
4Danish Meteorological Institute, Copenhagen, Denmark
5University of Liege, Liege, Belgium
6Department of Geography, University of Georgia, Athens, Georgia, USA
7Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands
8Department of Physics and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA **
January 12, 2015
· Melt extent, above the 1981-2010 average for 90% of summer 2014, reached a maximum of 39.3% of the ice sheet area on 17 June 2014. The number of days of melting in June and July 2014 exceeded the 1981-2010 average over most of the ice sheet.
· Average surface mass balance (the difference between annual snow accumulation and annual melting) measured along the K-transect in west Greenland for the period 2013-2014 was slightly below the 1990-2010 average, while the equilibrium line altitude (~1,730 m a.s.l., the lowest altitude at which winter snow survived) was at a higher elevation than the 1990-2010 average of 1,545 m.
· Average albedo during summer 2014 was the second lowest in the period of record that began in 2000; a new record low albedo occurred in August 2014.
· Summer 2014 in Greenland was the warmest on record at Kangerlussuaq, west Greenland, where the average June temperature was 2.3°C above the 1981-2010 average. In January 2014, the average temperature at Illoqqortoormiut, east Greenland and Upernavik, west Greenland were 7.5°C and 8.7°C above the 1981-2010 means, respectively.
· The ice mass anomaly (relative to the average for 2002-2014) of -6 Gt between June 2013 and June 2014 was negligible compared to all previous years since observations began in 2002, and particularly with respect to 2012-2013 when the largest mass loss (-474 Gt) in the GRACE record occurred (Note](http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html)).
With an area of 1.71 million km2 and volume of 2.85 km3, the Greenland ice sheet is the second largest glacial ice mass on Earth. Only the Antarctic ice sheet is larger. The freshwater stored in the Greenland ice sheet has a sea level equivalent of +7.4 m. The discharge of the ice to the ocean my melting and runoff, and iceberg calving would not only increase sea level, but also likely alter the ocean thermohaline circulation and global climate. The high albedo (reflectivity) of the ice sheet surface (together with that of snow-covered and bare sea ice, and snow on land) plays an important role in the regional surface energy balance and the regulation of global air temperatures.
Estimates of the spatial extent of melting across the Greenland ice sheet (Fig. 3.1), derived from brightness temperatures measured by the Special Sensor Microwave Imager/Sounder (SSMI/S) passive microwave radiometer (e.g., Mote 2007, Tedesco et al. 2013a, 2013b), show that melt extent for the period June through August (JJA, hereafter referred to as the summer) 2014 was above the 1981-2010 average 90% of the time (83 of 92 days, Fig. 3.1d). Melting occurred over 4.3% more of the ice sheet, on average, than in summer 2013, but 12.8% less than the exceptional summer of 2012 (Fig. 3.1d). Melt extent exceeded two standard deviations above average, reaching a maximum of 39.3% of the total ice sheet area on 17 June (Fig. 3.1b). Similar values occurred on 9 July and 26 July (Fig. 3.1c). Melt extent exceeded the 1981-2010 average on 28 days in June, 25 days in July, and 20 days in August 2014. For a brief period in early August there was below average melt extent, but by 21 August melting areas covered 29.3% of the ice sheet; this exceeded the 1981-2010 average by two standard deviations.
Fig. 3.1. Melting on the Greenland Ice Sheet in 2014 as described by (a, top left) total number of days when melting was detected at the surface between 1 January and 1 October, 2014; (b, top center) June melt anomaly expressed as the number of days melting that month compared to the 1981-2010 average; (c, top right) July melt anomaly expressed as the number of days melting that month compared to the 1981-2010 average; and (d, bottom) the annual cycle of melt extent expressed as a fraction of the total ice sheet area where melting was detected. In (d), melt extent in 2014 is represented by the blue line and the long-term average is the black line. Black star in (a, top left) indicates the position of the K-transect (discussed in the surface mass balance section).
The number of days of surface melting in June and July 2014 exceeded the 1981-2010 average across most of the ice sheet (Figs. 3.1b and 3.1c), particularly on the western margin, consistent with the above normal temperatures recorded at coastal stations in western Greenland in June and July. Locations with below average days of melting were evident in southeast Greenland (Figs. 3.1b and 3.1c), consistent with below normal temperatures in that region (see Fig. 1.3d in the essay on Air Temperature](http://www.arctic.noaa.gov/reportcard/air_temperature.html), which shows lower temperatures in southeast Greenland than along the western margin of the ice sheet).
Surface Mass Balance
Average surface mass balance (the difference between annual snow accumulation and annual melting) measured along the K-transect in West Greenland (Van de Wal et al. 2005, 2012) for the period 2013-2014 was slightly below the mean for 1990-2010 (measurements began in 1990; thus it is not possible to use the standard 1981-2010 reference period) (Fig. 3.2a). The equilibrium line altitude (the lowest altitude at which winter snow survives), estimated to be 1,730 m above sea level [a.s.l.] in 2014, was at a higher elevation than the 1990-2010 mean (1,545 m). During summer 2014, melt rates below the equilibrium line were not as high as they were in some recent years, e.g., 2010 and 2012.
Fig. 3.2. (a, top) Surface mass balance as a function of elevation along the K-transect for 2013-2014 (large blue squares), the previous four years, and the 20-year (1990-2010) average. (b, bottom) Average surface mass balance for sites located between 400 m and 1500 m a.s.l. A linear regression (red line) of the data gives a correlation coefficient ® of 0.46 (significant at a 97.5% confidence level).
Figure 3.2a shows the mass balance profiles for the last five years and the long-term mean obtained from stations at different elevations. Figure 3.2b shows the average surface mass balance for sites between 400 m and 1500 m a.s.l altitude, and the corresponding linear trend. There was slightly more melt in 2013-2014 than the 1990-2010 average; 2013-2014 had the 7th most negative mass balance of the 24 consecutive mass balance years in the observational record. The trend in the mean mass balance over the ablation area is -3.3 cm per year.
Total Ice Mass
GRACE (Gravity Recovery and Climate Experiment) satellite gravity solutions are used to estimate monthly changes in the total mass of the Greenland ice sheet (Velicogna and Wahr 2006; Fig. 3.3). At the time of writing, data were available only through June 2014. Between the beginning of June 2013 and the beginning of June 2014, which corresponds closely to the period between the onsets of the 2013 and 2014 melt seasons, there was virtually no net change in cumulative ice sheet mass (Fig. 3.3). The very small 6 Gt (Gigatonne) loss during that 12 month period contrasts with the previous eleven consecutive years of large losses, and particularly with the 474 Gt mass loss between June 2012 and June 2013, the highest annual loss observed in the GRACE record (Note](http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html)).
Fig. 3.3. Monthly mass anomalies (in Gigatonnes, Gt) for the Greenland ice sheet since April 2002 estimated from GRACE measurements. The anomalies are expressed as departures from the 2002-2014 mean value for each month. For reference, orange asterisks denote June values (or May for those years when June is missing).
Albedo, also referred to as reflectivity, is the ratio of reflected solar radiation to total incoming solar radiation. Here it is derived from the Moderate-resolution Imaging Spectroradiometer (MODIS, after Box et al. 2012). In summer 2014, albedo was below average over most of the ice sheet (Fig. 3.4a) and the area-averaged albedo for the entire ice sheet was the second lowest in the period of record that began in 2000 (Fig. 3.4b). The area-averaged albedo in August was the lowest on record for that month (Fig. 3.4c). August 2014 albedo values were particularly low at high elevations; such low values have not previously been observed so late in the summer. The observed albedo in summer 2014 continues a period of increasingly negative and record low albedo anomaly values (Box et al. 2012, Tedesco et al. 2011, 2013a, Dumont et al. 2014).
Fig. 3.4. (a, top) Greenland ice sheet surface albedo anomaly for June, July and August (JJA, summer) 2014 relative to the average for those months between 2000 and 2011. (b, lower left) Average surface albedo of the ice sheet each summer between 2000 and 2014. (c, lower right) Average surface albedo of the ice sheet each August between 2000 and 2014. All data are derived from the Moderate-resolution Imaging Spectroradiometer (MODIS).
Slightly negative (-0.7) North Atlantic Oscillation (NAO) conditions in summer 2014 promoted abnormal anticyclonic conditions over southwest and northwest Greenland; these favored northward advection of warm air along its western margin as far as the northern regions of the ice sheet (see Fig. 1.3d in the essay on Air Temperature](http://www.arctic.noaa.gov/reportcard/air_temperature.html)). Further, the anticyclonic conditions reduced summer precipitation (snowfall) over south Greenland. The combination of southerly air flow and lower precipitation contributed to the melting, mass balance and albedo observations reported above.
The advection of warm air towards Greenland is reflected in summer air temperatures. Near surface air temperature data recorded by automatic weather stations (Table 3.1) indicate that summer 2014 in Greenland was the warmest on record at Kangerlussuaq, west Greenland, with June temperatures +2.3°C above the 1981-2010 average. Other west Greenland locations also had anomalously warm summer temperatures. For example, the coastal site of Nuuk had its second warmest summer since 1784, with July temperatures 2.9°C above the 1981-2010 mean.
Warming in winter is greater than in summer (Table 3.1). At Ittoqqortoormiut, east Greenland, where observations began in 1924, the average air temperature during December 2013 to February 2014 equalled the record high set in the same period in 1947, and January temperatures were 7.5°C above the 1981-2010 average. Upernavik, west Greenland, had its 7th warmest January, 8.7°C above the 1981-2010 average, since observations began in 1873.
Table 3.1. Near-surface temperature anomalies relative to the 1981-2010 average at thirteen stations distributed around Greenland. Standard deviation (SD) values, and the years when record maximum and minimum values occurred are also given. Data are from Cappelen (2014) and from the Danish Meteorological Institute (DMI) for the period January-August 2014.
Note: The more positive or more negative the standard deviation (SD) value, the more extreme the positive or negative temperature anomaly. For example, at Ittoqqortoormiut, where winter 2014 was as warm as the previous warmest winter on record, in 1947, the SD value (2.4) of the winter 2014 temperature anomaly is among the most positive in the table.
Abbreviations: SON: September, October, November; DJF: December, January, February; MAM: March, April, May; JJA: June, July, August.
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