Geoscience Reference
In-Depth Information
200]. These data are available from July 199 through
December 2005 on a 100-km
2
grid north of 55ºN.
mum sea ice extent is defined as the 30-day average centered
on the mean date of maximum extent, March, and the min-
imum extent is defined as the 30-day average centered on the
mean date of minimum extent, 1 September. At maximum
extent, all of the Arctic and sub-Arctic seas are ice covered
(sea ice concentrations >15%), while at minimum extent,
only the Greenland and Beaufort seas and the Arctic basin
(central Arctic Ocean) are ice covered.
Arctic sea ice extent serves as a useful starting point for
describing the temporal character of sea ice over the Arctic
as a whole. Following convention, we have defined Arctic
sea ice extent as the area of the ocean covered by at least
15% sea ice concentration based on 5-day running mean
data; note that Hudson Bay and the Baltic Sea have been
excluded from this calculation. Plate 1 shows the 5-day run-
ning mean Arctic sea ice extent during the period 1 January
199 to 31 October 200, with each year overlaid in a dif-
ferent color (see color scale to the right of Plate 1). Plate 1
conveys the regularity of the seasonal cycle throughout the
period of record, with maximum values (14-16 × 10
6
km
2
)
occurring in early March and minimum values (5-.5 ×
10
6
km
2
excluding 200) in the middle of September. In ad-
dition to the regularity of the seasonal cycle, Plate 1 conveys
the systematic reduction of Arctic sea ice extent over time,
with the 1980s exhibiting the highest values (red hues) and
the 2000s showing the lowest values (blue hues). This sys-
tematic retreat of Arctic sea ice extent has occurred in all
months of the year. Arctic sea ice extent reached unprec-
edented minimum values in August-October of 200 (pur-
ple curve). The change in September sea ice extent between
2006 and 200 alone (~1.5 × 10
6
km
2
) is approximately
equivalent to the entire change that occurred between Sep-
tember 199 and September 2006 (~1.3 × 10
6
km
2
).
The 5-day running mean Arctic sea ice extent data shown
in Plate 1 are replotted in Figure 2 (top) as a single con-
tinuous time series starting on 1 January 199 and ending
on 31 October 200. The same record after removing the
long-term 5-day running mean seasonal cycle is shown in
Figure 2 (bottom). The former depiction serves to empha-
size that the seasonal cycle of Arctic sea ice extent is still
the most prominent feature of the record, while the latter
underscores the accelerating downward trend of Arctic sea
ice extent over time. Arctic sea ice extent has declined at a
rate of −0.52 × 10
6
km
2
per decade, or −1.76 × 10
6
km
2
over
the period 1 January 199 to 1 January 200; this trend is
significant at the 99% level. The magnitude of the downward
linear trend has increased from −0.35 × 10
6
km
2
per decade
to −0.9 × 10
6
km
2
per decade from the first half (January
199 to December 1993) to the second half (January 1993 to
October 200) of the record (similar results were found by
Comiso
et al.
[2008]).
3. RESULTS
3.1. Arctic Sea Ice Extent
A map of the locations of the Arctic and sub-Arctic seas
referred to in this study are shown in Figure 1. These place
names are superimposed upon the long-term mean distribu-
tions of maximum and minimum sea ice extent (defined as
marine areas within which sea ice concentrations equal or
exceed 15%), based on the period 199-2006. The maxi-
Figure 1.
Locations of the Arctic and sub-Arctic seas referred to
in this study, superimposed upon the long-term (199-200) mean
sea ice extent at month of maximum (thick black contour) and
month of minimum (thin black contour and light shaded areas).
The maximum (minimum) sea ice extent is defined as the 30-day
average centered on the mean date of maximum (minimum) extent,
March (1 September).
Search WWH ::
Custom Search