Trends in sea ice minimum ice extents, with a rate of decrease of 2.5 per
cent per decade for March and 8.9 per cent per decade
Passive microwave sensors on satellites have monitored for September
5–7
(Figure 5.4).
the extent of the sea-ice cover since 1978
2
. This technique
is widely used to investigate fluctuations in ice extent There are major regional differences (Figure 5.5), with
over the seasons, variability between years, and long- the strongest decline in ice extent observed for the
term trends. The seasonal variation of ice extent is much Greenland Sea (10.6 per cent per decade). The small-
greater in the Antarctic where there is about six times as est decreases of annual mean sea-ice extent were found
much ice in winter as in summer. Currently, in the Arctic, in the Arctic Ocean, the Canadian Archipelago and the
ice approximately doubles from summer to winter. Figure Gulf of St. Lawrence. In the marginal Arctic seas off Si-
5.3 shows the average minimum and maximum extents beria (the Kara, Laptev, East Siberian and Chukchi Seas)
of Arctic and Antarctic sea ice in recent decades. a slight negative, but not significant, trend in ice extent
was observed between 1900 and 2000
8
.
Northern Hemisphere trends
Figure 5.6 compares the Arctic sea-ice extent in Septem-
Despite considerable year-to-year variability, significant ber for the years 1982 (the record maximum since 1979)
negative trends are apparent in both maximum and and 2005 (the record minimum). The ice extent was 7.5
Ice extent anomaly
(million km
2
)
Northern Hemisphere
1.5 Whole N Hemisphere -3.2%
March
Greenland Sea -10.6
1
September Baffin Bay -8.6
Sea of Okhotsk -7.4
0.5
Kara-Barents Sea -6.0
0
Hudson Bay -5.0
Bering Sea -1.8
-0.5
Arctic Ocean -1.3
Gulf of St Lawrence -0.6
-1
Canadian Archipelago -0.4
-1.5 Change in annual mean sea ice
1980 1985 1990 1995 2000 2005 extent (% per decade)
Figure 5.4: Time series of the difference in Arctic sea-ice extent Figure 5.5: Regional changes in Arctic annual mean sea-ice ex-
in March (maximum) and September (minimum) from the mean tent (% per decade) for the period 1979–2004.
values for the time period 1979–2006. Based on a linear least
Source: Data courtesy of NASA 2007a
9
squares regression, the rate of decrease in March and Septem-
ber was 2.5% per decade and 8.9% per decade, respectively.
Source: Data courtesy of National Snow and Ice Data Center (NSIDC)
68 GLOBAL OUTLOOK FOR ICE AND SNOW
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