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Figure 13.23 Arctic September sea ice extent from observations (thick red line, 1953-2006) and from 13 models
participating in the IPCC Fourth Assessment Report, together with the multi-model mean (solid black line) and
standard deviation (dotted black line). The inset shows nine-year running means.
Source: Stroeve et al.(2007, fig. 1). Courtesy of the American Geophysical Union.
to the effects of greenhouse gas loading. The IPCC
models indicate that ice-free conditions in
September might be realized any time from
the year 2050 to well beyond 2100. Given the
discrepancy between modeled and observed
trends over the period of overlap, ice-free condi-
tions may well occur much sooner.
Antarctic sea ice extent, which has been
accurately monitored since 1979, actually shows
small upward trends in most months (based on
data through 2007). While perhaps counter-
intuitive, this is largely in accord with projections
from even very early generation GCMs of a much
slower and delayed response of the Antarctic to
greenhouse gas loading in comparison to the
Arctic. Recall from earlier discussion (Section
13E.2) the very different nature of the ocean
circulation in southern high latitudes, in which
heat input to the ocean surface tends to be rapidly
mixed to deeper ocean levels. The small upward
trends that have been observed appear to reflect
the especially persistent zonal circulation of
the atmosphere that for several decades has
characterized the region surrounding the ice sheet
(a persistently positive Southern Annular Mode).
This has helped to keep the region cool. The
notable exception is the Antarctic peninsula, which
has warmed by about 2.5 o C in the past 50 years,
(see also Figure 13.9 ). An interesting aspect of the
Antarctic peninsula which seems broadly related
to this warming is the major calving events that
have occurred during the past 10-15 years along
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