Sea ice extent
(million km
2
)
30
World total
20
Southern Hemisphere
10
Northern Hemisphere
0
JFMAMJJASOND
Figure 5.1: Monthly average variations in sea-ice extent in total
and in both hemispheres.
Source: Based on Thomas 2004
1
(amended from original by J. Comiso,
NASA)
around the Antarctic continent are approximately 3 million atmosphere from the huge heat source in the ocean, its
km
2
in February during the Antarctic summer and 18 mil- role in the formation of bottom water (the densest water
lion km
2
at the height of winter in September (Figure 5.1). found in the ocean, which is extremely important in the
In regions with seasonal sea ice, the ice cover achieves a circulation of the ocean), and the part it plays in feed-
thickness varying from less than 1 metre to more than 2 back and amplification processes. Snow-covered sea ice
metres, depending on air and water temperatures and oth- is highly reflective and returns a lot of sunlight back to
er conditions. In regions where ice survives the summer, space. In contrast, when sea ice is not present the dark
thicker, multi-year ice is formed. But these conditions are ocean can absorb this heat from the Sun.
changing. Sea ice has decreased in the Arctic and is project-
ed to decline much more in both polar regions, with conse- Sea ice is home to many ice-associated organisms, from
quences to climate, ecosystems and human livelihoods. tiny algae and crustaceans to penguins, polar bears and
whales. Many organisms in Arctic and Antarctic marine
Sea ice is extremely important to the climates of the po- food webs depend on the ice itself or on processes con-
lar regions because of the part it plays in insulating the nected with sea ice. And sea ice is important to humans.
CHAPTER 5 ICE IN THE SEA 65