The physical environment of the arctic is harsh. Winters are long, dark, windy, and biting cold. Summers are short and often wet, with 24 hours of continuous sunlight. The ground is frozen and is mostly covered by permanent ice and snow, except for a thin layer that thaws in some lowland areas during summer. Few species have adapted to these harsh conditions, but among those few, some have flourished.

This is particularly true for the relatively shallow shelf seas that surround the deep basin of the Arctic Ocean. The arctic is, in fact, mostly sea covered by a thick layer of ice year round. During winter the sea ice expands southward; in the spring and summer it melts and retreats north. This melting, combined with the continuous sunlight, spawns a bloom of algae and phytoplankton along the ice edge. Zooplankton and small crustaceans, such as krill, feed on this bloom. These in turn, serve as food for an abundance of fish, seals, and seabirds.

At the top of the arctic marine food chain reigns the polar bear. These largest of the land carnivores are spread throughout the arctic in 20 relatively distinct populations that vary in size from a few hundred to a few thousand individuals. There are at least 22,000 polar bears in the wild, with about 60% of these living in Canada.

The sea ice is where polar bears spend most of their lives. Here they hunt their primary prey: ringed and bearded seals. Bears also prey upon harp seals as well as young walrus and beluga whale, narwhal, fish, and seabirds and their eggs.

During summer, the sea ice melts in all or part of the range of some polar bear populations. These bears are forced to spend several months on land waiting for the autumn refreezing. Otherwise, polar bears are usually only on land when moving between feeding areas, when in search of a mate, when resting, or when having cubs.

Pregnant females dig dens in deep snowdrifts on land in the late autumn, while other bears remain active throughout the winter. Cubs, usually two, are born in the den during November or December. Each cub weighs around 600 grams and is about the size of a guinea pig. The cubs are nursed until they are large enough to venture out onto the sea ice, usually in March or April.

A polar bear eats most of its food out on the sea ice between late April and mid-July. Access to abundant food in this period is critical, particularly for pregnant females. When food is unavailable, e.g. when bears are stranded on land during the ice-free season, they fast for long periods. Fasting can last three to four months and up to eight months for pregnant females in some populations. Fasting for extended periods is demanding and can weaken a bear's body condition dramatically.

Due to its position at the top of the arctic marine food chain, the polar bear is an ideal species through which to monitor the effects of human-caused impacts to this ecosystem, such as climate change.

In the southern range of polar bears, for example the Hudson and James Bays of Canada, sea ice is now melting earlier in the spring and forming later in the autumn. The time bears have on the ice, storing up energy for the summer and autumn when there is little available food, is becoming shorter. As the periods without food are extended, the overall body condition of these bears declines. This is particularly serious for pregnant or nursing females, and young cubs. In Hudson Bay, scientists have found the main cause of death for cubs to be either an absence of food or lack of fat on nursing mothers.

The situation currently observed in western Hudson Bay is indicative of what is expected in other parts of the arctic in the near future. In addition to less sea ice, climate change in the arctic is expected to bring increased precipitation. Ringed seals, the polar bears' primary prey, keep their pups in snow lairs on the sea ice. Spring rains destroy such lairs and expose the pups to the elements and to predators. If climate change leads to increased spring rains, ringed seal populations will be devastated. This means a substantial loss of food for polar bears, most likely leading to local extinction.

Late winter rains can have a more direct effect on polar bear survival by causing polar bear maternity dens to collapse. Warm spring temperatures can also thaw out a den, exposing its occupants to the elements and to predators.

The trend toward stronger winds and increasing ice drift observed in parts of the arctic over the last five decades will also likely increase energy expenditures and stress for polar bears that spend most of their time on the drifting sea ice.

Large carnivores are sensitive indicators of ecosystem health and can be used to define the minimum area necessary to preserve intact ecosystems. WWF has identified the polar bear as a unique symbol of the complexities and inter-dependencies of the arctic marine ecosystem as it works toward its goal of preserving biodiversity for future generations.

WWF Climate Change Program

Global warming and climate change pose serious threats to the survival of many species and to the well-being of people around the world.

WWF's campaign has three main aims:

• to ensure that industrialized nations make substantial reductions in their domestic emissions of carbon dioxide - the main global warming gas -by 2010;
• to promote the use of clean, renewable energy in the developing world;
• to reduce the vulnerability of nature and economies to the impacts of climate change.
  

Source: VANASHING KINGDOM, The Melting Realm of the Polar Bear. A publication of WWF Climate Change Program. © 2002