Greenland Ice Sheet
M. Tedesco1,2, J.E. Box3, J. Cappelen4, X. Fettweis5, T. Jensen3, T. Mote6,
A.K. Rennermalm7, L.C. Smith8, R.S.W. van de Wal9, J. Wahr10
1The City 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
5Department of Geography, University of Liege, Liege, Belgium
6Department of Geography, University of Georgia, Athens, GA, USA
7Department of Geography, Rutgers, The State University of New Jersey, New Brunswick, NJ, USA
8Department of Geography, University of California Los Angeles, CA, USA
9Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands
10Department of Physics and CIRES, University of Colorado, Boulder, CO, USA
December 5, 2013
- Surface air temperatures in summer 2013 in Greenland were near the long-term average of 1980-2010.
- Melting occurred over as much as 44% of the surface of the ice sheet in summer 2013, 14th in the 33-year record (1981-2013) and much lower than the record 97% in 2012.
- Melting occurred on >100 days, consistent with the long-term (1981-2010) average at some locations along the southwestern margin of the ice sheet, where the surface mass balance along a transect (the K-transect) was close to the 1990-2010 average.
- The average albedo of the ice sheet surface during summer 2013 was the highest since 2008, curtailing a period of increasingly lower albedo values since 2007, but close to the average for the period of record (2000-2013).
Melt estimates across the Greenland ice sheet obtained from passive microwave data (Mote and Anderson 1995, Mote 2007) indicate that the June-July-August (JJA) 2013 melt period was near the long-term average for the period 1981-2010. Melt extent during summer 2013 did not deviate significantly from the 1981-2010 long-term average, and sporadic melt spikes were much smaller in magnitude than those of the extreme 2012 melt season (Fig. 54a). The maximum area of the Greenland ice sheet subjected to melting during summer 2013 was 44% on July 26th, a much smaller area than the record 97% of 2012 (Nghiem et al. 2013, Tedesco et al. 2013a), ranking 14th in the 33-year period of record (1981-2013). The average melt extent for summer 2013 was ~17%, ranking 16th in the period of record, was the lowest annual value since 2000. For comparison, the average melt extent during extreme summer of 2012 was ~34%.
The cumulative spatial extent of melt across the Greenland ice sheet during 2013 closely followed the long-term average of 1981-2010 (Fig. 54b, updated through 23 September 2013, when melting can be considered to have ceased). Consistent with the long-term average, melting during 2013 occurred in excess of 100 days in some locations along the southwestern margin of the ice sheet. The frequency of melting was slightly higher than the 1981-2010 average along the western and northwestern coasts along but less frequent than average along the southern and southeastern coasts (Fig. 54c).
Surface Mass Balance and Runoff
Since measurements began in 1990, there has been a trend of decreasing surface mass balance (mass loss increasing) at the 'K-transect' (lowest elevation located ~20 km east from Kangerlussuaq between 340 m and 1500 m above sea level, a.s.l.; van de Wal et al. 2005, 2012) in west Greenland (Fig. 55a). In 2013 (red box, Fig. 55a), measurements made at individual points along the transect (Fig. 55b) indicate that there was slightly less melting in the lower ablation zone compared to the 1990-2010 average, particularly near the ice margin. The estimated equilibrium line altitude (the highest altitude at which winter snow survives) in 2013 on the K-transect was close to the long-term average position of 1500 m a.s.l. (in contrast to its upslope migration to ~2700 m in 2012, when record mass losses occurred at high elevations during an exceptionally warm summer, e.g., Tedesco et al. 2013b).
Rivers draining from the ice sheet transport meltwater runoff to the ocean, but are currently monitored at only a few sites around Greenland (Rennermalm et al. 2013a). Consistent with surface mass balance estimates at the K-transect, river discharge observations of a small basin (with an estimated ice catchment area of ~31-60 km2 and altitude of 400-850 m a.s.l.; Rennermalm et al. 2012, Rennermalm et al. 2013b) of the Kangerlussuaq catchment (largely including the K-transect) reveal a later melt season onset in 2013 and lower flow conditions compared to previous years. Cumulative river discharge (e.g., ice sheet meltwater export, Fig. 55c) in 2013 was the lowest recorded during the instrumental record for this site (2008-2013). Also, consistent with the lower air temperatures reported below, meltwater temperatures were considerably lower than the 2008-2013 average (Fig. 55d). While runoff from this single catchment cannot be extrapolated to other parts of the ice sheet, this short record suggests that meltwater runoff from the ice margin for this area of southwestern Greenland was lower than previous years.
Total Ice Mass
GRACE satellite gravity data (Velicogna and Wahr 2006) are used to estimate monthly changes in the total mass of the Greenland ice sheet (Fig. 56). Unfortunately, GRACE mass loss estimates are not available for August and September 2013, when the K-band ranging system was switched off to preserve battery life. The next estimate, for October 2013, will be available in early 2014. Nevertheless, there are sufficient GRACE data to provide some information about the total mass of the ice sheet in 2013 relative to previous years.
From the end of April 2012 through the end of April 2013, which corresponds reasonably well to the period between the beginning of the 2012 and 2013 melt seasons, the cumulative ice sheet loss was 570 Gt, over twice the average annual loss rate of 260 Gt y-1 during 2003-2012. The 2012-2013 mass loss is the largest annual loss rate for Greenland in the GRACE record, mostly reflecting the large mass loss during the summer of 2012 (Tedesco et al. 2013b). The mass loss during the 2013 summer melt season is likely to be considerably smaller than during 2012, based on other evidence such as the reduced surface melt extent, surface mass balance and runoff described above. A lower mass loss during summer 2013 can also be inferred from the much smaller difference between the April (blue asterisks) and July 2013 mass values (orange asterisks), particularly relative to each of the three previous years (Fig. 56).
The average ice sheet-wide albedo derived from the Moderate-resolution Imaging Spectroradiometer (MODIS, e.g., Box et al. 2012) during summer 2013 was the highest since 2008 (Fig. 57a), interrupting a period of increasingly negative and record albedo values (Box et al. 2012, Tedesco et al. 2011, 2013a). Overall, albedo for the period JJA 2013 was well above the 2000-2011 average along the southwest, northwest and northeast regions and coasts of the ice sheet, but it was below the average for the east and southeast regions (Fig. 57b).
Near surface air temperature (NSAT) data recorded by automatic weather stations (Cappelen 2013, http://www.dmi.dk/fileadmin/Rapporter/TR/tr13-04.pdf) indicate that the outstanding surface temperature feature for calendar year 2013 was a consistent warm anomaly along the west Greenland coast during March (see Fig. 3b in the essay on Air Temperature). A record warm March was recorded at Pituffik/Thule AFB, where the NSAT anomaly relative to 1981-2010 baseline was +7.7°C, the warmest on record since 1948. Similarly, the Upernavik and Kangerlussuaq March NSAT anomalies were +7.7°C and +8.6°C, respectively.
In contrast to the previous six summers, summer 2013 was characterized by a positive North Atlantic Oscillation (NAO) and persistently lower-than-normal 500 hPa geopotential heights. Consequently, warm, southerly air masses were diverted eastward away from Greenland and cool northerly airflow in west Greenland (see Fig. 4 in the essay on Air Temperature) promoted cooler, wetter and cloudier weather than normal, and less melting than in recent years, as reported above. This is reflected in the NSAT data (Table 7), which show that during the summer months (June, July, August) NSAT values were generally near or below one standard deviation of anomalies relative to the 1981-2010 baseline period, indicating that summer 2013 NSATs were "normal" with respect to that period. Wide-area air temperature anomalies (Fig. 3d in the essay on Air Temperature) are broadly consistent with the data for individual stations (Table 7).
(lat., °N; lon., °W)
|-1.0 (-0.6)||-0.8 (-0.5)||-0.4 (-0.4)||-0.8 (-0.6)|
|0.7 (1.1)||0.7 (1.10)||-0.2 (0.2)||0.4 (1)|
|1.4 (1)||0.5 (0.4)||-0.6 (-0.8)||0.4 (0.4)|
|0.8 (1.1)||-0.4 (0.1)||-1.1 (-0.5)||-0.3 (0.4)|
|1.2 (1.0)||0.4 (0.3)||0.5 (0.4)||0.7 (0.7)|
|0.4 (0.3)||0.9 (0.9)||0.9 (0.7)||0.7 (0.7)|
|-1.1 (-0.7)||0.0 (0.0)||-0.2 (-0.1)||-0.4 (-0.4)|
|-0.8 (-0.3)||-0.7 (-0.5)||-0.1 (-0.1)||-0.5 (-0.4)|
|-1.3 (-1.1)||-1.1 (-0.9)||-0.6 (-0.4)||-1 (-1)|
|0.7 (0.7)||0.8 (1.10)||0.8 (0.9)||0.8 (1.2)|
|1.3 (1.6)||1.1 (1.4)||1.2 (1.5)||1.2 (1.6)|
|0.7 (0.3)||0.5 (0.2)||-0.3 (-0.2)||0.3 (0.2)|
|Prince Christian Sund
|0.5 (0.3)||0.1 (0.6)||0.5 (1.2)||0.8 (0.8)|
|0.3 (0.1)||0.1 (-0.1)||-2 (-0.9)||-0.5 (-0.4)|
Marine-terminating glaciers are the outlets via which the inland ice sheet discharges to the ocean. When in balance, the rate of iceberg calving (by area) is balanced by the seaward motion of the ice. LANDSAT and ASTER images of 17 of the widest marine-terminating glaciers in summer 2013 indicate a net area change of -11.3 km2 since summer 2012. This retreat is the 4th lowest in the 13-year period of observations (2000-2013) and equivalent to 13% of the trend of -84 km2 yr-1 for the period (Fig. 58). The largest increases in area between 2012 and 2013 occurred at Petermann (+15.9 km2) and Nioghalvfjerdsbrae/79 (+3 km2) glaciers. The largest area loss occurred at the Zachariae glacier (-15.6 km2).
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