input to surface waters in locations where sea ice melts. the ice cover during the melt season as the snow-covered
This means that a significant volume of fresh water is ice is replaced by a mix of melting snow, bare ice, and
exported as sea ice through Fram Strait and the Canadi- ponded ice
30
. As the melt season progresses, the bare-
an Archipelago – this is a major component of the North ice albedo remains fairly stable, but the pond albedo
Atlantic Ocean’s salt balance. Changes in sea-ice export decreases. During summer the ice cover retreats, expos-
will impact both the thermohaline circulation and the ing more of the ocean, and the albedo of the remaining
location where the warm but relatively salty northward ice decreases as the snow cover melts and melt ponds
flowing North Atlantic Current sinks beneath the cold form and evolve. These processes combine to form the
but relatively fresh surface water and flows into the Arc- ice–albedo feedback mechanism (Figure 5.11b).
tic basin (see Figure 2.1 in Chapter 2).
Changes in sea-ice cover are also significant on a global Impacts of changes in sea ice
scale because of the potential to amplify climate change
through positive feedback mechanisms
25,26
. A key mecha- Overview
nism is the ice–albedo feedback
27
. Albedo is a simple but
powerful geophysical parameter. It is simple because it is Changes in ice within the Arctic Ocean will also have
just the fraction of the incident sunlight that is reflected impacts on Arctic marine ecosystems and three ‘tipping
by a surface. If all the sunlight is reflected the albedo is 1 points’ can be hypothesized
31
: the first would occur if and
(or 100 per cent reflection), if none is reflected the albedo when the seasonal ice routinely retreats past the edge
equals zero. It is powerful because sunlight is the prima- of the continental shelf, thus allowing wind-driven up-
ry planetary heat source and how much of that sunlight welling which would result in increases in primary pro-
is reflected is a key factor determining climate. ductivity; the second would occur if and when the Arctic
becomes ice-free in summer, thus eliminating multi-year
Aerial photographs of Arctic sea-ice cover in spring and ice and associated ecosystems; the third would occur if
in summer are shown in Figure 5.10. The spring photo and when significant regions within the Arctic basin re-
is representative of much of the year when the surface is main ice-free in winter, thus impacting the distribution
a combination of highly reflecting snow-covered ice and of seasonally migrating marine mammals.
highly absorbing dark areas of open water. Conditions
become more complex in the summer with a mixture of Reductions in ice-cover thickness, extent and duration,
melting snow, bare ice, ponds, and an overall increase in and changes in current patterns and fronts will likely
the amount of open water. have both gradual (predictable) and catastrophic (sur-
prise) consequences
32
:
The albedos for these different surface conditions are bottom-up controls (such as stratification, mixing and
plotted in Figure 5.11a. They range widely, from roughly upwelling of seawater) will certainly change;
85 per cent of radiation reflected for snow-covered ice keystone predators within a given region may move
to 7 per cent for open water
28,29
. These two surfaces cov- into the region, move away from the region, or become
er the range from the largest to the smallest albedo on extinct; and
earth. Melting snow, bare ice and ponded ice lie within linkages between the open ocean ecoystems and the
this range. There is a general decrease in the albedo of ocean bottom ecosystems may weaken.
CHAPTER 5 ICE IN THE SEA 75