Geoscience Reference
In-Depth Information
Mote,
2007
; Tedesco et al.,
2010
). The summer of 2007 saw extensive melt, 60 per-
cent greater than the previous high recorded in 1998, which was then surpassed by
another remarkable melt event in 2012. Satellite data show that during July 2012,
more than 98 percent of the ice sheet experienced at least a brief period of surface
melt. Evaluation of ice core data indicate that the last time such extensive melt
occurred was in 1889 and the next previous event was approximately seven centu-
ries earlier (Nghiem et al.,
2012
), Extensive melt events can be related to anomalous
atmospheric circulation patterns; the 2012 event was linked with a very strong ridge
in the mid-tropospheric flow. Where melt is observed in higher regions, it may only
be occurring in a near surface layer. At lower elevations, meltwater that is formed
will percolate to lower depths and re-freeze. It is only near the coast that actual run-
off is observed, either directly from the surface or from the base of glaciers draining
the ice sheet. The surface melt in 2012 caused extensive local flooding, notably
in the town of Kangerlussuaq, which hosts one of the island's busiest commercial
airports.
8.1.7
Mass Balance
The changing mass balance of the Greenland and Antarctic Ice Sheet has been has
been a subject of increasing interest as well as concern. As of the writing of this text-
book, since 1998, there have been at least twenty-nine individual mass balance esti-
mates (Shepherd et al.,
2012
). Satellite remote sensing resources to estimate mass
balance include gravimetric measurements from NASA's GRACE and ICESat mis-
sions and Interferometric Synthetic Aperture Radar. Satellite information has been
complemented by data from NASA's Operation IceBridge series of aircraft over-
flights, regional models, such as RACMO2 (Regional Atmospheric Model Version
2) and surface observations. Although estimates have ranged widely depending on
the time period examined and the techniques employed, it is clear that the mass bal-
ance of both ice sheets is negative, contributing to the current estimated sea level
rise of 3.1 mm per year, that the mass losses for Greenland are larger than for the
Antarctic Ice Sheet, and that the mass loss from Greenland in particular has accel-
erated over the past decade. Authors of many past studies recently collaborated on a
major effort to provide a reconciled estimate of mass balance.
Figure 8.6
shows the
cumulative annual mass balance (Gigatonnes and equivalent sea level rise) for the
Greenland and Antarctic ice sheets and for the ice sheets combined. The accelerat-
ing mass loss from Greenland stands out.
Ice sheet mass balance represents the difference between accumulation (precipi-
tation) and mass losses, the latter primarily representing runoff, evapo-sublimation
and iceberg discharge (calving). As assessed for the 2000-2008 period, M. van den
Broeke et al. (
2009
) estimate that the mass loss from Greenland is roughly split
between surface processes (precipitation and runoff, the latter associated with sea-
sonal melt just discussed) and ice sheet dynamics. Attention has focused on the
observation that some of the major glaciers that drain the Greenland ice sheet,
including the Jakobshavn, Kangerdlugssuaq, and Petermann glaciers, are exhibiting
