Past Climate Variability and Change in the Arctic and at High Latitudes

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The U.S. Climate Science Program Chapter 5
The Greenland Ice
Sheet contains by far the
largest volume of any
present-day Northern
Hemisphere ice mass.
More-recent trends
are toward warming
temperatures, increasing
snowfall, and more
rapidly increasing
meltwater runoff.
5.1 THE GREENLAND ICE SHEET
5.1.1 Overview
The GreenlAnd ice Sheet* (Figure 5.1) contains
by far the largest volume of any present-day
Northern Hemisphere ice mass. The ice sheet
is approximately 1.7 million square kilometers
(km 2) in area, extending as much as 2,200 km
north to south. The maximum ice thickness
is 3,367 m, its average thickness is 1,600 m
(Thomas et al., 2001), and its volume is 2.9 million
km 3 (Bamber et al., 2001). Some of the
bedrock beneath this ice has been depressed
below sea level by the weight of the ice, and a
little of this bedrock would remain below sea
level following removal of the ice and rebound
of the bedrock (Bamber et al., 2001). However,
most of the ice that rests on bedrock is above sea
level and so would contribute to sea-level rise
if it were melted: if the entire ice sheet melted,
it is estimated that sea-level would rise about
7.3 m (Lemke et al., 2007).
The ice sheet consists primarily of old snow
that has been squeezed to ice under the weight
of new snow that accumulates every year.
Snow accumulation on the upper surface tends
to increase ice-sheet size. Ice sheets lose mass
primarily by melting in low-elevation regions,
and by forming icebergs that break off the
ice margins (calving) and drift away to melt
elsewhere. Sublimation, snowdrift (Box et
al., 2006), and melting or freezing at the bed
beneath the ice are minor terms in the budget,
although melting beneath floating extensions
called ice shelves before icebergs break off may
be important (see section 5.1.2, below).
Estimates of net snow accumulation on the
GreenlAnd ice Sheet have been presented
by Hanna et al. (2005) and Box et al. (2006),
among others. Hanna et al. (2005) found for
1961–1990 (an interval of moderately stable
conditions before more-recent warming) that
surface snow accumulation (precipitation minus
*For bold terms, refer to Glossary; for italic terms, refer
to Plate 1; for geologic ages, refer to Plate 2.
evaporation) was about 573 gigatons per year
(Gt/yr) and that 280 Gt/yr of meltwater left
the ice sheet. The difference of 293 Gt/yr is
similar to the estimated iceberg calving flux
within broad uncertainties (Reeh, 1985; Bigg,
1999; Reeh et al., 1999). (For reference, return
of 360 Gt of ice to the ocean would raise global
sea level by 1 millimeter (mm); Lemke et al.,
2007.) More-recent trends are toward warming
temperatures, increasing snowfall, and more
rapidly increasing meltwater runoff (Hanna et
al., 2005; Box et al., 2006). Large interannual
variability causes the statistical significance of
many of these trends to be relatively low, but the
independent trends exhibit internal consistency
(e.g., warming is expected to increase both
melting and snowfall, on the basis of modeling
experiments and simple physical arguments,
and both trends are observed in independent
studies (Hanna et al., 2005; Box et al., 2006)).
Increased iceberg calving has also been observed
in response to faster flow of many
outlet glaciers and shrinkage or loss of ice
shelves (see section 5.1.2, below, for discussion
of the parts of an ice sheet) (e.g., Rignot and
Kanagaratnam, 2006; Alley et al., 2005). The
Intergovernmental Panel on Climate Change
(IPCC; Lemke et al., 2007) found that “Assessment
of the data and techniques suggests a mass
balance of the GreenlAnd ice Sheet of between
+25 and –60 Gt (–0.07 to 0.17 mm) SLE (sea
level equivalent) per year from 1961–2003
and –50 to –100 Gt (0.14 to 0.28 mm SLE) per
year from 1993–2003, with even larger losses
in 2005.” Updates are provided by Alley et al.
(2007b) (Figure 5.2) and by Cazenave (2006).
Rapid changes have been occurring in the ice
sheet, and in the ability to observe the ice sheet,
so additional updates are virtually certain to
be produced.
The long-term importance of these trends is
uncertain—short-lived oscillation or harbinger
of further shrinkage? This uncertainty
motivates some of the interest in the history of
the ice sheet.