Greenland Ice Sheet

M. Tedesco^1,2, J. E. Box³, J. Cappelen⁴, R. S. Fausto³, X. Fettweis⁵, K. Hansen⁴, T. Mote⁶, C. J. P. P. Smeets⁷, D. van As³, R. S. W. van de Wal⁷, J. Wahr⁸

¹The City College of New York, New York, NY, USA
²Lamont Doherty Earth Observatory of Columbia University, Palisades, NY, 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 & Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

In memoriam. This essay is dedicated to the memory of John Wahr⁸ (June 1951 – November 2015) for his exceptional contribution to studying and promoting the understanding of the interactions between the solid layers of the Earth and the overlying atmosphere, oceans and ice sheets, and for his unique and outstanding human qualities.

Highlights

• Melt area in 2015 exceeded more than half of the ice sheet on July 4th for the first time since the exceptional melt events of July 2012, and was above the 1981-2010 average on 54.3% of days (50 of 92 days).
• The length of the melt season was as much as 30-40 days longer than average in the western, northwestern and northeastern regions, but close to and below average elsewhere on the ice sheet.
• Average summer albedo in 2015 was below the 2000-2009 average over the northwest and above the average over the southwest portion of the Greenland ice sheet. In July, albedo averaged over the entire ice sheet was lower than in 2013 and 2014, but higher than the lowest value on record observed in 2012.
• Ice mass loss of 186 Gt over the entire ice sheet between April 2014 and April 2015 was 22% below the average mass loss of 238 Gt for the 2002- 2015 period, but was 6.4 times higher than the 29 Gt loss of the preceding 2013-2014 season.
• The net area loss from marine-terminating glaciers during 2014-2015 was 16.5 km². This was the lowest annual net area loss of the period of observations (1999-2015) and 7.7 times lower than the annual average area change trend of -127 km².

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Surface Melting

Estimates of the spatial extent of melting across the Greenland ice sheet in 2015, derived from spaceborne brightness temperatures recorded by the Special Sensor Microwave Imager/Sounder (SSMIS) passive microwave radiometer (e.g., Mote 2007; Tedesco 2007; Tedesco et al. 2013), show that melting occurred over more than half of the ice sheet for the first time since the exceptional melt events of July 2012 (Nghiem et al. 2012). The 2015 melt extent exceeded two standard deviations above the 1981-2010 average, reaching a maximum of 52% of the ice sheet area on 4 July (Fig. 3.1a). For comparison, melt extent in 2014 reached a maximum of 39% of the ice sheet area and ~90% in 2012.

The number of melting days along the southwestern and southeastern margins of the ice sheet was close to or below the long-term average (Fig. 3.1b), with maximum negative anomalies (increased relative to the 1981-2010 average) being of the order of 5-10 days. In contrast, the number of melt days in the northeastern, western and northwestern regions, was up to 30-40 days above the 1981-2010 average and setting new records for meltwater production and runoff in the northwestern region (Tedesco et al. manuscript in preparation).

Surface Mass Balance

The surface mass balance for September 2014 through September 2015 measured along the southwestern portion of the ice sheet at the K-transect (Fig. 3.1b; van de Wal et al. 2005, 2012), was the third least negative since the beginning of the record in 1990; not since the 1991-1992 (when melting was low due to the Mount Pinatubo eruption) and 1995-1996 balance years has so little ice been lost (Fig. 3.2a). This is consistent with the negative anomalies detected by the SSMI/S along the southwestern portion of the ice sheet. Data from location S5 (~540 m a.s.l., 67.10°N, 50.09°W), in the ablation area, show that the melt season started approximately 15-20 days later than the 2008-2014 average, then decreased substantially near the end of July and remained low during August (Fig. 3.2b). At station S9 (~1500 m a.s.l. near the equilibrium line [the lowest altitude at which winter snow survives], 67.05°N, 48.25°W), melting ceased due to snowfall at the end of July and there was no melting during August (Fig. 3.2c).

Anomalously low melt in 2015 with respect to the 2008-2014 average at PROMICE network stations (van As et al. 2011; Fausto et al. 2012) is consistent with the K-transect. At all PROMICE stations (Fig. 3.1b), ablation in summer 2015 was relatively low with respect to the 2008-2014 observational period, except at the most northerly latitudes (Kronprins Christian Land, KPC, 80°N, 25°W; Thule, THU, 76°N, 68°W), where melt totals were slightly above average. The highest recorded melt in 2015, 5.1 m on the Qassimiut lobe (QAS_L station, 61°N, 47°W), was little more than half the record-setting 9.3 m at that site in 2010 (Fausto et al. 2012).

Albedo

Albedo, a measure of surface reflectivity, is the ratio of reflected solar radiation to total incoming solar radiation. A relatively low albedo promotes increased melting, with net solar radiation being the major driver of summer surface melt over Greenland. Average albedo in summer 2015, derived from data collected by the Moderate-resolution Imaging Spectroradiometer (MODIS, after Box et al. 2012), was below the 2000-2009 average in the northwestern region and above the average in the southwestern region (Fig. 3.3a). This is consistent with the negative surface mass balance and melting day anomalies measured over the same region. The 2000-2009 reference period is used here because MODIS observations began in 2000. The trend of mean summer albedo over the entire ice sheet for the period 2000-2015 is -5.5±0.4% (Fig. 3.3b).

In July 2015, when extensive melting occurred (Fig. 3.1a), albedo averaged over the entire ice sheet was 68.1%, i.e., lower than the 2013 and 2014 values and higher than the lowest average July albedo of 65.8% recorded in 2012 (Fig. 3.3c). Albedo in July 2015 was anomalously low (as much as 15-20% below average) along the northwestern ice sheet and along the west coast, where large positive melting days anomalies were observed (Fig. 3.1b). But, over the entire summer, the albedo anomaly along the west coast was positive. The summer mean albedo in 2015 was higher than the albedo recorded in 2014 and close to the value recorded in 2013 (Fig. 3.3b), mostly because of the relatively short melt season and early snowfall in August.

Total Ice Mass

GRACE satellite data (Velicogna and Wahr 2013) are used to estimate monthly changes in the total mass of the Greenland ice sheet (Fig. 3.4). Between mid-April 2014 and mid-April 2015, roughly corresponding to the period between the beginning of the two consecutive melt seasons, the 186 Gt of ice loss was 22% lower than the average April-to-April mass loss (238 Gt) during 2002-2015. For comparison, since GRACE measurements began in 2002, the smallest April-to-April mass loss was 29 Gt during 2013-2014 and the largest was 562 Gt during 2012-2013.

Marine-terminating Glaciers

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 flow of the ice (see the essay on Greenland Ice Sheet Surface Velocity: New Data Sets for further information on the velocity of these glaciers). Analysis of LANDSAT and ASTER imagery since 1999 of 45 of the widest and fastest-flowing marine-terminating glaciers reveals that they continue to retreat, although the rate of retreat has slowed since 2012 (Fig. 3.5). Between the end of the 2014 melt season and the end of the 2015 melt season, 22 of the 45 glaciers had retreated, but the advance of 9 relatively wide glaciers resulted in a low 1-year net area loss of 16.5 km². This is the lowest annual net area loss in the 16-year period of observations (1999-2015) and 7.7 times lower than the annual average area change trend of -127 km² (Fig. 3.5). The advance of Petermann Glacier (0.684 km advance across a width of 17.35 km gives an area increase of 11.87 km²) and Kangerdlugssuaq Glacier (1.683 km advance across a width of 6.01 km gives an area increase of 10.12 km²) contributed to the low net area loss of the 45 glaciers in 2014-2015.

Weather

Measurements at weather stations of the Danish Meteorological Institute (DMI, Cappelen 2015, Table 3.1) during spring 2015 indicate that temperatures at Nuuk (64.2°N,51.8°W), Paamiut (62.0°N, 49.7°W), Narsarsuaq (61.2°N,45.4°W), Qaqortoq (60.7°N, 46.0°W) and Prins Christian Sund (60.0°N,43.2°W) in south and southwest Greenland were 1.5 standard deviations below the 1981-2010 average, with temperature anomalies (relative to the 1981-2010 average) as much as -2.6°C at Narsarsuaq. The average May temperature at Danmarkshavn (76.8°N, 18.8°W) set a new record low, with a -4.6°C anomaly relative to the 1981-2010 average. These widespread low temperatures are consistent with the strong negative spring temperature anomaly centered over Greenland (see Fig. 1.2c in the essay on Air Temperature). Danmarkshavn also experienced the warmest January on record, with a +7.7 °C anomaly relative to the 1981-2010 average. Summer average temperature anomalies were positive at most stations around the Greenland coastline, with anomalies exceeding one standard deviation at Pituffik (76.5°N, 68.8°W, +1.2ºC), Upernavik (72.8°N, 56.2°W, +1.2°C), Nuuk (64.2°N, 51.8°W, +1.1°C) and Danmarkshavn (+0.9 °C). A new record August low temperature of -39.6 °C occurred on August 28 at Summit (3216 m a.s.l., 72.5796°N, 38.4592°W).

Summer 2015 was characterized by negative North Atlantic Oscillation (NAO) conditions, with a mean summer value of -1.3, similar to those of the summers (JJA) of 2007-2012 when enhanced surface melting occurred (Tedesco et al. 2013, 2014). NAO is defined as the difference in atmospheric pressure at sea level between the Icelandic Low and the Azores High and it has been shown to be connected to extreme melting events over Greenland (McLeod and Mote, 2015). However, a distinct difference in the atmospheric circulation associated with the NAO in summer 2015 and the summers of 2007-2012 affected melting on the ice sheet. During the summers of 2007-2012, the 500 mb geopotential height anomaly (typically used to describe NAO atmospheric circulation) with respect to the 1981-2012 average was persistently centered over the ice sheet and southerly air flow advected warm air across the ice sheet. In the summer of 2015 the anomaly was centered over the north-central ice sheet in July, which promoted warm, southerly airflow and enhanced melting. On the other hand, in June and August 2015 the anomaly was centered over the Labrador Sea southwest of Greenland, which promoted the advection of cold air from the Arctic Ocean and reduced melting (Tedesco et al., unpublished manuscript). The spatial distribution of albedo and melting observed by remote sensing (Fig. 3.1) and with the in-situ surface mass balance measurements (e.g., Fig. 3.2) and summer air temperatures (Table 3.1) are consistent with these atmospheric circulation patterns.

References

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December 7, 2015