The Polar Ice Cap
The Earth’s northern polar region is covered by floating pack ice (sea ice) over the Arctic Ocean, which for the purposes of this book are considered terrestrial. Portions of the ice that do not melt seasonally can become very thick, up to 3–4 m over large areas, with ridges of up to 20. One-year ice is usually around 1 m thick. With global warming, the extent of arctic ice has decreased about 4.2 per cent per decade since the 1980s.
Species and subspecies
The polar bear (Ursus maritimus) is the largest living bear species, as well as the world’s largest land carnivore. It is distributed in low densities throughout the Arctic Circle. Although some occur in the permanent multi-year pack ice of the central Arctic basin, they are most common in the annual ice over the continental shelf and inter-island archipelagos that surround it. The southern limit of the range extends to the coast of Newfoundland in the north-western Atlantic, while the northernmost record is just 25 km from the geographic North Pole. At least some populations seem to be nomadic, moving on the ice from east to west in a large circumpolar loop that includes Greenland and Baffin Island and in the Old World runs chiefly inside the large islands of arctic Eurasia. Polar bears move south in winter and north in summer, following the food supply – mainly seals – as the ice breaks up and shifts. This movement helps to explain the failure until relatively recently of any country to take responsibility for the welfare of polar bear populations. The species has probably always been confined to arctic areas, although from time-to-time individuals have strayed to Iceland, the Norwegian mainland, Manchuria, and Japan. Those that have continuous access to sea ice are able to hunt throughout the year. However, those living in areas where the sea ice melts completely each summer are forced to spend several months on land, where they primarily fast on stored fat reserves until freeze-up. This use of land by polar bears during the ice-free season is increasing, at least in some areas. Intensive hunting of the species did not begin until the early seventeenth century, but then increased to such a degree that by 1850 it had been seriously depleted, particularly in the Spitsbergen area and on Novaya Zemlya. In addition, when most of the arctic whales had been exterminated men began to hunt the seals, which increased the hunting pressure on the polar bears as well. It soon became evident that polar bears were declining in number all over their range, including the New World. With increased protection populations began to recover, although ‘subsistence harvesting’ is still allowed in Alaska, Canada, and Greenland (but prohibited in Norway and Russia). Currently, it is the loss of arctic sea ice due to climate change that is the most serious threat.