2014 Ice Sheet Assessment

Aaron Letterly

Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA

05 June 2015

With a combined area of nearly 16 million square kilometers and volume of nearly 30 million cubic kilometers, the Antarctic and Greenland ice sheets affect many aspects of the cryosphere. The high albedo (reflectivity) of these ice sheet surfaces controls regional surface energy balances and, on a larger scale, moderates global temperatures. Furthermore, runoff, melting, and iceberg calving into the ocean from these ice sheets have implications for the thermohaline circulation and world climate. The combined volume of water stored in these two regions alone is enough to raise sea levels by approximately 65 meters, if melted. These ice sheets are in constant mass flux, accumulating area and mass due to piling of winter snow that does not melt in summer and losing mass due to melting and movement into the ocean. Recent trends of increased temperatures have accelerated melting in most of the Greenland ice sheet, but Antarctica has experienced only regional increases in melting and ice loss (Tedesco et al., 2014).

Greenland Surface Melting

In 2014, the Greenland ice sheet had a greater number of surface melt days as well as a larger melting extent than the 1981-2010 average (Figure 1). A greater number of melting days occurred on the northern and western Coasts of Greenland throughout June, July, and August. The lack of anomalous melting days at higher elevations in the Greenland interior is different from the surface melt pattern in 2012 when Greenland saw a much greater melt during this time period (Figure 2). Regions with fewer melting day anomalies correspond well to 2014 seasonal temperature anomalies, showing negative or no temperature anomalies in central or Southeast Greenland, especially when compared to the north or west coasts. Using Figure 1 (bottom) to compare the seasonal variances in melting fraction, the significant melting events in 2014 (fractional values above the 1981-2010) were observed earlier than melting events in the previous two years (Tedesco et al., 2014).

This increase in surface melting from the 1981-2010 mean is associated with a decreased surface albedo. Greenland saw its second-lowest albedo since 2000, possibly due to less precipitation over the ice sheet in 2014 resulting in a somewhat darker surface. The Geological Survey of Denmark and Greenland recorded a low albedo in May, which may have prompted the early melting events seen in Figure 1.

Greenland Ice Sheet Mass Balance

In 2014, the Greenland Ice Sheet surface mass balance - the difference between annual snow accumulation and annual melting - was below average though not as low as in 2012. This can be seen in Figure 2, where the first part of 2014 is represented by the 2013-2014 curve and the latter part of 2014 is shown by the 2014-2015 curve. Positive temperature anomalies in southwest Greenland led to extensive melting there and set a record there for the warmest summer. Surprisingly, between June 2013 and June 2014, Greenland experienced minimal ice loss compared to average losses of 250 gigatons (annually). This may be an effect of decreased surface melting in the Greenland interior or negative temperature anomalies in the southeast leading to greater accumulation of mass. Though Greenland was more than one standard deviation below the 1990-2011 mean (shaded) for surface mass balance in the first half of 2014, it was far from the 2012 record low.

The warmest summer on record in western Greenland was partially due to multiple, persistent anticyclonic circulations over the region in the summer of 2014. Clockwise motion advected warm air from the south along Greenland’s western coast, and the sinking air reduced precipitation that would have normally occurred over the summer. Increased melting in these regions as well as Greenland-wide decreased mass accumulation may be seen in Figures 1 and 2, respectively.

Regional Ice Sheet Trends: A Tale of Two Glaciers

Regional differences in mass accumulation over Greenland are observed when individual glacial mass balances are compared. The 2014 spike in mass balance of the Freya Glacier sharply contrasts the trend of generally decreasing ice mass for glaciers in other regions of Greenland (Figure 3). The Freya Glacier in eastern Greenland is in the area that experienced a reduction in the number of melting days in 2014 compared to the 1981-2010 average. The fewer melting days allowed falling snow to accumulate more readily than along the southern and western coasts (where Mittivakkat and other glaciers are located), capping off the 2014 Freya Glacier mass balance with a nearly 400 mm w.e. increase. To the south, the downward-trending ice mass of Mittivakkat glacier has experienced runoff more than .5 meters w.e. since its first year of observation in 1996. Despite Freya Glacier’s increase in the surface mass balance in 2014, Greenland-wide increases in absorbed sunlight and temperature led to an overall 2014 mass loss equivalent to 1.2 mm of sea level rise, only 0.1 mm from the record melt in 2012.

Antarctic Surface Melting

In 2014, the Western Antarctic Ice Sheet (WAIS) received considerable coverage in both scientific journal and media outlets, partly due to the results of a study by NASA and University of California-Irvine researchers. The research detailed the rapid and “irreversible” melting of several western Antarctic glaciers into the sea. This increase in glacial outflow and movement may be an indicator of basin-wide change in the Amundsen Sea region and Antarctic Peninsula (Rignot et al., 2014). The negative trend in continental mass balance should not be confused with the record-high 2014 Antarctic sea ice concentrations and extent (http://www.nasa.gov/content/goddard/antarctic-sea-ice-reaches-new-record-maximum).

Other literature described an increase ice loss on an almost continental scale (Mcmillan et al., 2014). Elevation changes (observed by a spaceborne altimeter) in Antarctica’s largest glaciers. Increased melting at the base of the glacier, a lack of snowfall, or both, can affect changes in glacier elevation. Figure 4 shows changes in elevation observed by the CryoSat-2 satellite. Dark red coloration in Western Antarctica corresponds to warnings in Rignot et al. (2014), where extreme thinning of ice sheets leads to increased glacial outflow and motion towards the sea (Figure 4).

There is a linkage between temperatures of the north and tropical Atlantic Ocean and Antarctic Peninsula ice (Li et al., 2014). As water in the Atlantic warms, the “conveyor belt” of the thermohaline circulation brings it southward, and into contact with the ice shelf of Western Antarctica. As the bottom of this shelf melts, the ice mass pushes faster and further into the ocean from on land. Sea surface temperature anomalies in the southern Atlantic are higher than usual for 2014 and 2015, possibly foretelling sustained or increased melt in the next few years (Figure 5).

The mass of the Antarctic ice sheet continues on its downward trend. Negative mass balances over the last 20 years have led to reductions in total ice mass of multiple glaciers (Figure 6). The rapid increase in Pine Island melting, represented by red circles, is confirmed by Rignot et al. (2014), and mimics the combined trend of ice mass decreases in the Amundsen Sea sector.

References

Li, X., D. M. Holland, E. P. Gerber, and C. Yoo, 2014. Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice. Nature, 505, 538-542, doi:10.1038/nature12945.

McMillan, M., A. Shepherd, A. Sundal, K. Briggs, A. Muir, A. Ridout, A. Hogg, and D. Wingham, 2014. Increased ice losses from Antarctic detected by CryoSat-2. Geophys. Res. Lett., doi: 10.1002/2014GL060111.

Rignot, E., J. Mouginot, M. Morlighem, H. Seroussi, and B. Scheuchl, 2014. Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011. Geophys. Res. Lett., doi: 10.1002/2014GL060140.

Tedesco, M., J. E. Box, J. Cappelen, X. Fettweis, T. Mote, R. S. W. van de Wal, C. J. P. P. Smeets, J. Wahr, 2014, Greenland Ice Sheet, in Arctic Report Card: Update for 2014, http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html.