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ICE ACROSS BIOMES

Photo: sea ice © iStock.com/jwebb

Sea ice

Loss of sea ice has major ecological consequences for biodiversity. Open water has lower refl ectivity than ice and holds more heat, increasing fog cover and reducing sunlight to near-shore plant and animal communities. Reduction of sea ice can expose shorelines to wave action and storms, leading to increased coastal erosion, as observed along the coast of the Beaufort Sea.9, 10 Species such as seals, polar bears,11 Arctic foxes,12 and some caribou herds13 that rely on ice for breeding or feeding habitat, and/or for movement across the landscape are profoundly affected by changes in sea-ice distribution and extent. Some seabirds and gulls – for example, the ivory gull, which has declined dramatically since the 1980s – depend on ice-edge habitat for survival.14, 15 Earlier break-up has been linked to shifts in trophic dynamics in some species assemblages – for example, reduced abundance of Arctic cod along with an increase in capelin.16 Earlier break-up has also been linked to a shift to earlier breeding in seabirds such as thick-billed murres and glaucous gulls.17-19 An emerging issue for Arctic marine biodiversity is the anticipated increase in shipping through an ice-free Arctic, which will expose sensitive marine ecosystems and biota to risk from invasive species released in ballast, increasing noise and contact with ships, and oil spills.11, 20

Changes in sea-ice extent in the northern hemisphere

Extent of sea ice in September (millions km2), 1979 to 2010
Graph: changes in sea ice extent in the northern hemisphere. Click for graphic description (new window).
Source: data from Fetterer et al., 20104

The average of sea-ice extent for September (the month with the least ice cover) has declined over the Northern Hemisphere by 11.5% per decade since satellite measurements began in 1979.4, 5 Average ice extent declined for all seasons over this period.5 Ice is melting earlier in the year,6 and its age and distribution are changing. Multi-year ice is being lost, meaning that a greater proportion of ice is younger, thinner, and more subject to rapid break-up.7, 8

These changes in sea ice vary regionally. In the Canadian Arctic Archipelago, September ice extent declined by 9% per decade from 1979 to 2008, but the rate of decline varied from about 2% to 25% for different sub-regions.7 In Hudson Bay, summer ice (July through September) declined by almost 20% per decade from 1979 to 2006.5 For the Newfoundland and Labrador Shelves, ice extent declined in all seasons from 1979 to 2006, despite a period of greater ice cover in the 1990s.5 The Gulf of St. Lawrence, with no summertime ice, has experienced less change.5

Deterioration in southern Hudson Bay polar bear body condition

Mean body condition index
Graph: deteroriation in southern Hudson Bay polar bear body condition. Click for graphic description (new window). Photo: Polar bears © iStock.com/Visual Communications.
Source: adapted from Obbard et al., 200621

Some 4,000 polar bears, or about 20% of the total world population, range over sea ice of Hudson and James bays in the winter, feeding mainly on seals.22 When ice on these bays melts completely each summer, the bears come ashore where they spend up to five months (eight months for pregnant females) before the sea ice re-forms.23 The annual ice-free period has increased by almost three weeks since the mid-1970s.24 This has reduced the time that polar bears have on the ice to feed on seals and store fat for the summer.

The Southern Hudson Bay subpopulation is showing significant declines in body condition21 as well as declines in survival rates of all age and sex classes.25 Together these observations suggest that this subpopulation, which has been stable from the mid-1980s until at least 2003-2005, may decline in abundance in the future.25 The adjacent Western Hudson Bay subpopulation of polar bears has already declined from about 1,194 bears in 1987 to 935 in 2004, a decline of 22%.26 Coincident with this population decline were indications of declining body condition and reduced survival rates in some age classes.26, 27 The impacts on polar bears documented in Hudson Bay are not yet occurring throughout the polar bear’s range, though they may be a harbinger of changes to come as sea ice declines around the circumpolar Arctic. Currently polar bear trends are variable, with some subpopulations being stable, some increasing, and some not known.28