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

Although linked to site temperature, δ 18O can
be affected by many factors (Jouzel et al., 1997;
Alley and Cuffey, 2001), such as change in the
ratio of summertime to wintertime precipitation.
Hence, additional means of determining
past temperatures are required. One of the most
reliable is based on the physical temperature of
the ice. Just as a frozen turkey takes a long time
in a hot oven to warm in the middle, intermediate
depths of the central GreenlAnd ice Sheet
are colder than ice above or below. Surface ice
temperatures equilibrate with air temperature,
and basal ice receives some warmth from Earth’s
heat flow, but the center of the ice sheet has not
finished warming from the ice-age cold. If ice
flow is understood well at a site, the modern
profile of the physical temperature of the ice with
increasing depth provides a low-time-resolution
history of the surface temperature with increasing
time. Joint interpretation of the isotopic
ratios and temperatures measured in boreholes
(Cuffey et al., 1995; Cuffey and Clow, 1997),
or independent interpretation of the borehole
temperatures and then comparison with the
isotopic ratios (Dahl-Jensen et al., 1998), helps
to outline the history of surface air temperature.
Furthermore, the relation between isotopic ratio
and temperature (α ‰ per °C) becomes a useful
paleoclimatic indicator, and changes in this ratio
α with time can be used to test hypotheses about
the overall changes in seasonality of snowfall
and other factors.
The isotopic composition of gases trapped in
bubbles in the ice sheet provides an additional indicator
of temperature. New-fallen snow contains
many interconnected air spaces. Snow turns to
ice without melting in central regions of cold
ice sheets through solid-state mechanisms that
operate more rapidly under higher temperature
or higher pressure. Snow in an ice sheet usually
transforms to ice within the top few tens of
meters. The intermediate material is called firn,
and the transformation is complete when bubbles
are isolated so that the air spaces are no longer
interconnected to the surface. Wind moving
over the ice sheet typically mixes gases in the
pore spaces of the firn only in the uppermost
few meters or less. Diffusion mixes the gases
deeper than this. Gases are slightly separated by
gravity (Sowers et al., 1992), with the air trapped
in bubbles slightly isotopically heavier than in the
free atmosphere, proportional to the thickness
of the air column in which diffusion dominates.
Past Climate Variability and Change in the Arctic and at High Latitudes
If a sudden temperature change occurs at the
surface, the resulting temperature change of the
firn beneath requires typically about 100 years
to penetrate to the depth of bubble trapping.
However, when a temperature gradient is applied
across gases in diffusive equilibrium, the
gases are separated by thermal fractionation as
well as by gravity, with the heavier gases moving
thermally to the colder end (Severinghaus et al.,
1998). Equilibrium of gases is obtained in a few
years, far faster than the time for heat flow to
remove the temperature gradient across the firn.
Within a few years after an abrupt temperature
change at the surface, newly forming bubbles
will begin to trap air with very slight (but easily
measured) anomalies in gas-isotope compositions,
and this trapping of slightly anomalous
air will continue for a century or so. Because
different gases have different sensitivities to
temperature gradients and to gravity, measuring
isotopic ratios of several gases (such as argon
and nitrogen) allows researchers to determine
the temperature difference that existed vertically
in the firn at the time of bubble trapping
and to determine the thickness of firn in which
wind was not mixing the gas (Severinghaus et
al., 1998). If the surface temperature changed
very quickly, the magnitude of the temperature
difference across the firn will peak at the
magnitude of the surface-temperature change;
for a slower change, the temperature difference
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