Geoscience Reference
In-Depth Information
content of 1 per cent, 89 per cent of floating ice is
submerged. If the air content rises to 10 per cent, as it
can do with some icebergs and shelf-ice, then the
submerged part is reduced to 81 per cent by volume.
Sea-ice can be divided into four categories. The first
consists of the polar shelf- and sea-ice forming a per-
manent cover, 3-4 m thick, over 70 per cent of the
Arctic Ocean and surrounding the Antarctic continent
(Figure 8.6). This ice is very hummocky but may crack
open in summer to form leads. In the following winter,
leads will be squeezed shut by the shifting ice forming
pressure ridges tens of meters high. Sea-ice may also
melt patchily, especially in the Antarctic, forming open
water areas called polynyas. Since the mid-1970s, the
melting of Arctic sea-ice has increased. The area
covered by sea ice has diminished 21 per cent between
1955 and 2000. This varies from a high of 6 per cent
per decade since 1970 in the Atlantic, to 1.5 per
cent per decade in the Canadian Arctic and Bering Sea
(see Figure 8.6 for placenames mentioned in this
section). While the trend has been attributed to global
warming, melting has been a longer term process.
Figure 8.7 illustrates this trend in the Arctic over the
past two centuries using data from Davis Strait
between Canada and Greenland, the Greenland Sea
east of Greenland, and the Nordic Sea in the north
Atlantic. In the latter region, sea-ice extent in April has
decreased by 33 per cent (0.79
Deep-water swell waves of 4 m breaking over the breakwall
to Wollongong harbor, Australia (photograph courtesy
John Telford, Fairymeadow, New South Wales).
Fig. 8.5
The crucial parameters in the generation of freak
waves are high seas and variable wave conditions.
These conditions may be associated with periods of
stormier or windier weather. If such conditions have
not occurred for some period, then people's awareness
of wave behavior - especially along rocky coasts under
high seas - may be poor. This ignorance is the main
reason for a significant loss of life due to people being
swept off rocks by waves in Australia.
SEA-ICE AS A HAZARD
Ice i
n the ocean
10
6
km
2
) with a sub-
stantial proportion of this reduction occurring before
industrial activity accelerated in the middle of the
twentieth century. The dotted lines in Figure 8.7
indicate that changes in sea-ice extent are synchronous
across the Arctic. The limit of sea-ice in the north
Atlantic retreats when the North Atlantic Oscillation
(NAO) is positive, due to an increased northward flux
of warm air. However, off Labrador, the opposite effect
is evident. This correlation fits the pattern, associated
with the NAO, of warmer temperatures in the
north-east Atlantic and cooler ones in eastern North
America. Concomitant with this retreat, sea-ice
thinned by 0.13 m between 1970 and 1992. In the
eastern Siberian Sea, ice thickness decreased from
3.1 m in the 1960s to 1.8 m in the 1990s. In some
locations, especially between Greenland and Norway,
ice cover has thinned by 40 per cent over the past three
decades. More dramatic has been the disappearance of
iceshelves around the Antarctic Peninsula. Five
iceshelves, amounting to 5000 km
2
of iceshelf, have
collapsed due to regional warming of 2.5°C in the last
(Defant, 1961; Gross, 1972; Vinje, 2001; Anisimov &
Fitzharris, 2001; Long et al., 2002)
Sea-ice has two origins: through the freezing of
seawater and by the discharge of ice from glaciers into
the ocean. The former mechanism is the more
common method of forming sea-ice. Because the
ocean has a salinity of 31-35 ‰ (parts per thousand),
it freezes at temperatures around -1.9°C. New sea-ice
does not contain pure water but has a salinity of
5-15 ‰. The faster ice forms, the more saline it is
likely to become. As ice ages, the brine drains away,
and the ice becomes salt-free and clear. These charac-
teristics can be used to identify the age and origin of
sea-ice. At -30°C, sea-ice can form at the rate of
10 cm day
-1
. Typically in the Canadian Arctic, a winter's
freezing cycle will produce ice 2-3 m thick. At this
point, ice insulates the underlying water and freezing is
substantially reduced. The process is similar for the
freezing of ice in lakes and rivers; but because of low
salinities, ice forms faster in these latter environments.
The density of seawater is 1027 kg m
-3
. With an air
