Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 266

each otherbyrelatively immobile ice. Theultimate fate ofmuchoftheice oftheGreenland
ice sheet is to be lost through solid mass discharge as icebergs into marine fjords.
At the time of the SWIPA report, total mass balance estimates indicated that the
Greenland ice sheet was losing ice at an increasing rate: from 50 Gt/year 13 between 1990
and 2000 to 205 Gt/year between 2005 and 2006. The latter seemed to be made up of
roughly equal amounts of surface mass balance loss and solid mass discharge.
A more recent study used a method to detect changes in ice mass by looking at how
the loss (or gain) in ice is reflected in changes in the local gravitational field. This is
made possible by the NASA gravity field satellite mission GRACE ( Gravity Recovery and
Climate Experiment ). A recent analysis of GRACE satellite gravity data is roughly consist-
ent with the SWIPA results and estimated that the Greenland ice sheet lost a total mass at
a rate of 222 (+/−9) Gt/year for the period 2002-2010. This is equivalent to a global sea
level rise of 0.62 (+/−0.03) mm/year. At the end of 2012, a “reconciled” estimate of ice
sheet mass balance was published by Andrew Shepherd and colleagues. The word recon-
ciled means that the study utilized a variety of quite different methodologies. It estimated
the following changes in mass between 1992 and 2011 in Gt/per year for the world's ma-
jor ice sheets: Greenland: −152 (+/−49), West Antarctica: −65 (+/−26) and the Antarctic
Peninsula: −20 (+/−14). It is generally believed that East Antarctica is not losing ice at the
present time. The combined impact of mass loss from the ice sheets was a contribution of
11.1 mm (+/−3.8) to the total global mean sea level rise since 1992. Greenland maintained
a rate of ice loss that is five times that of 1992.
Undoubtedly, 2012 was an extraordinary year to be on the Greenland ice sheet. Melt-
ing started about two weeks earlier than average at low elevations and occurred over a
longer period at any given elevation than in 2010 (the previous record year) for most of
June until mid-August. Once melting began, the surface albedo dropped even further (an
example of positive feedback due to melting ice having a reduced albedo). Some areas
in north-west Greenland between 1,400 and 2,000 metres above sea level had nearly two
months more melt than during the 1981-2010 reference period. At low elevations in some
parts of south-west Greenland, melting lasted for 20-40 days longer than the mean. On one