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Figure 2. Time series of 5-day running mean Arctic sea ice extent (10 6 km 2 ) from 1 January 199 to 31 October 200
(top) with and (bottom) without the long-term mean seasonal cycle.
The time series of the dates of maximum and minimum
sea ice extent (Figure 3, right) show that there has been a
slight upward trend (indicative of a progressively later date)
in the date of maximum sea ice extent of 4 days per decade
(or 12 days over the period 1979-2007; significant at the
90% level), while there has been little change in the date
of minimum ice extent (1 day per decade). The length of
time between maximum and minimum extent has decreased
slightly at a rate of 3 days per decade or 9 days over the
period January 199 to June 200, but this does not pass the
90% significance threshold. The mean dates of maximum
and minimum sea ice extent occur on March and 1 Sep-
tember, respectively. It should be noted that the record low
minimum sea ice extent in 200 occurred on 16 September,
close to the mean date of minimum sea ice extent.
The magnitude and sign of the linear trends in Arctic sea
ice extent as a function of time of year are shown in Figure
4 based on 5-day running mean data for the period 1 January
199 to 30 June 200 (the record low values since June 200
are purposefully omitted from the trend calculation). The
trends are expressed in terms of actual magnitude (square
Figure 3 isolates the behavior of the maximum and mini-
mum values of sea ice extent shown in Plate 1 and their dates
of occurrence during 199-200. Minimum and maximum
values were determined by comparing adjacent 5-day run-
ning means. Linear trend lines in the dates and values of
maximum and minimum Arctic sea ice extent, determined
by linear least squares “best fit” regression lines to the
5-day running mean data during January 199 to June 200
(note that data after June 200 were purposefully omitted
from this calculation), are superimposed on the original time
series in Figure 3. Figure 3 (left) shows that there has been a
downward trend in both the maximum and minimum sea ice
extent values (-0.5 × 10 6 km 2 per decade and - 0. × 10 6 km 2
per decade, respectively: significant at the 99% level), with
a corresponding increase in the amplitude of the annual cy-
cle (0.15 × 10 6 km 2 per decade, although this does not pass
the 90% significance threshold). In terms of percentage of
the mean maximum (15.2 × 10 6 km 2 ) and mean minimum
(6.4 × 10 6 km 2 ) sea ice extent, the downward trends in maxi-
mum and minimum extent are -9.0% per decade and -3.4%
per decade, respectively.
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