Outlook for sea ice centage changes in late summer (Figure 5.9 upper and
lower right) and an increase in the amplitude of the an-
Northern Hemisphere nual cycle. However, in Antarctica the projected change
in sea-ice volume of around 30 per cent is about half the
Climate models project a continuing decrease of ice ex- value projected for the Arctic, and the increase in ampli-
tent in the Arctic
19,20
accompanied by thinning of the ice. tude of the seasonal cycle is also less pronounced
21
. The
The most dramatic change, projected by about half the difference in sea-ice volume change can be explained by
current climate models developed as part of the IPCC the finding that the most rapid thinning of sea ice oc-
assessment report 4 (AR4), is a mainly ice-free Arctic curs in regions of thicker ice. On average the sea ice over
Ocean in late summer by 2100 (Figure 5.9 upper right). the Arctic is thicker than around Antarctica at present.
The projected change in the winter is smaller: 15 per
cent decrease in sea-ice extent (Figure 5.9 upper left). Sea-ice retreat: potential for tipping points and
The annual average decrease projected is 25 per cent by enhanced rates of change
2100. These seasonal differences will result in increased
amplitude of the seasonal cycle of sea-ice extent (greater There is evidence for the occurrence of tipping points in
differences between seasons). A model that examines the future, manifested as periods of abrupt decrease of
sea-ice volume projects that it will decrease even more Arctic sea-ice occurrence
22
. These abrupt changes may
than the ice extent, with reductions of annual means of result when the ice thins and the rate of retreat becomes
about 60 per cent by 2100
21
. more rapid for a given melt rate. Typically they would
In the transition zone between high Arctic and subarctic,
and in the subarctic, where seasonal ice dominates now
Figure 5.9: Sea-ice concentration change over the 21st cen-
(including the Barents, Baltic, Bering and Okhotsk Seas),
tury as projected by climate models. The data are taken from
climate model experiments of 12 (out of 24) different models
expected trends are: reduced ice extent, shorter ice seasons
that were conducted for the IPCC Assessment Report 4 using
and thinner ice. More frequent winter warm spells may also
the SRES A1B greenhouse gas emission scenario. Plots on the
result in snow melting and refreezing as superimposed
right show changes in late summer and those on the left show
ice. The two most recent northern winters (2005/2006 and changes in late winter.
2006/2007) were warmer than normal in the European
Notes:
Arctic and several of these effects were clearly visible in,
1) Sea-ice extent is the area in which a defined minimum of
for example, the Barents Sea and the Baltic Sea.
sea ice can be found. Sea-ice concentration is the proportion
of the ocean area actually covered by ice in the area of the total
sea-ice extent.
Southern Hemisphere
2) Small ocean inlets, such as those in the Canadian Archipel-
ago, while not showing a decrease on these plots, are also ex-
Around Antarctica the projected annual average decrease pected to experience a decrease in sea-ice concentration – this
of sea-ice extent is similar to the Arctic, at around 25 per
is an issue related to the resolution of the climate models.
cent by 2100. Both polar regions show the largest per- Source: Based on data from T. Bracegirdle, British Antarctic Survey
72 GLOBAL OUTLOOK FOR ICE AND SNOW