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

interpreting past sea levels can include much
uncertainty. We highlight two major reasons
for this uncertainty.
First, many islands well within the crustal
tectonic plate that underlies the Pacific Ocean,
for example, are part of hot-spot volcanic
chains. (A major source of internal heat, called
a hot spot, leads to a volcano on the overriding
tectonic plate; as the plate drifts laterally, the
slower moving hot spot becomes positioned
below a different part of the plate, and a new
volcano is formed as the previously active volcano
becomes extinct. Eventually, a chain of
volcanoes is produced, such as the Hawaiian-
Emperor seamount chain.) As a volcano grows
in elevation, its weight isostatically depresses
the land it sits on in the same way that the
weight of an ice sheet does, and the cold upper
elastic layer of the Earth flexes to form a broad
ring-shaped ridge around the low caused by the
volcano. Oahu, in the Hawaiian Island chain, is
a good example of an island that is apparently
experiencing slow uplift, and an associated local
sea-level fall, due to volcanic loading on the
“Big Island” of Hawaii (Muhs and Szabo, 1994).
Second, the existence of a sea-level highstand of
a given age in a stable geologic setting does not
necessarily imply that ice volumes were lower
at that time relative to the present day, even if
the highstand is dated to a previous interglacial.
As discussed above, glacial isostatic adjustment,
because it involves slow viscous flow of rock,
produces global-scale changes in sea-level even
during periods when ice volumes are stable. As
an example, for the last 5,000 years (long after
the end of the last glacial interval), ocean water
has moved away from the equatorial regions and
toward the former Pleistocene ice complexes
to fill the voids left by the subsidence of the
peripheral bulge regions produced by the ice
sheets. As a result, sea level has fallen (and
continues to fall) about 0.5 mm/yr in those farfield
equatorial regions (Mitrovica and Peltier,
1991; Mitrovica and Milne, 2002). This process,
known as equatorial ocean siphoning, has developed
so-called 3-meter beaches and exposed
coral reefs that have been dated to the end of
the last deglaciation and that are endemic to the
equatorial Pacific (e.g., Dickinson, 2001). Thus,
the interpretation of such apparent highstands
requires correction for glacial isostatic adjustments
such that the residual record reflects true
changes in ice volume.
Past Climate Variability and Change in the Arctic and at High Latitudes
5.2.2E gEodEtic indicators
Geodetic data are yielding both local and
regional constraints on recent changes in the
mass of ice-sheets. As an example, land-based
measurements of changes in gravity and crustal
motions, estimated by using the geographic
positioning system (GPS), are being used to
monitor deformation (associated with changes
in the distribution of mass) at the periphery of
the GreenlAnd ice Sheet (e.g., Kahn et al., 2007).
A drawback of these techniques is that few sites
have been monitored because of the difficulty of
establishing high-quality GPS sites. In contrast,
data from the Gravity Recovery and Climate
Experiment (GRACE) satellite mission are
revealing trends in gravity across the polar ice
sheets (at a spatial resolution of about 400 km)
from which estimates of both regional and
integrated mass flux are being obtained (e.g.,
Velicogna and Wahr, 2006). A general problem
in all attempts to infer recent ice sheet balance,
whether from land-based or satellite gravity,
GPS, or even altimeter measurements of ice
height (e.g., Johannessen et al., 2005; Thomas
et al., 2006), is that a measurements must be
corrected for the continuing influence of glacial
isostatic adjustments. As discussed above (section
5.2.2C), this correction involves uncertainty
associated with both the ice sheet history and the
viscoelastic structure of Earth.
Accurate glacial isostatic adjustment corrections
are also central to regional estimates of ice-sheet
mass balance. For the last century global sealevel
change has been inferred principally by
analyzing records from widely distributed tide
gauges (simple sea-level monitoring devices).
Most residual rates (those corrected for glacial
isostatic adjustment) of tide gauges yield an
average 20th century sea-level rise in the range
1.5–2.0 mm/yr (Douglas, 1997) (for additional
information on recent trends in sea level, see
Solomon et al., 2007).
Furthermore, geographic trends in the residual
rates may constrain the sources of the meltwater.
In particular, Mitrovica et al. (2001) and Plag and
Juttner (2001) have demonstrated that the rapid
melting of different ice sheets will have substantially
different signatures, or fingerprints,
in the spatial pattern of sea-level change. These
patterns are linked to the gravitational effects of
the lost ice (sea level is raised near an ice sheet
because of the gravitational attraction of the ice
Rapid melting of different
ice sheets will have
substantially different
signatures, or fingerprints,
in the spatial pattern of
sea-level change.
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