Polar bears in a warmer world

Polar bears are entirely dependent on sea ice. This is where they find their prey and also prospective mates in the breeding season. Throughout its evolution, the polar bear has been perfectly adapted to cope with the ice, and it only needs to stay on land to hibernate – or in the summer, when the ice has melted.

Receding ice a major problem
Over the past 30 years or so, the sea ice in the Arctic has receded due to the warmer climate, and this has reduced the polar bear’s habitat. In some places, the retreat of the ice is very noticeable, e.g. in eastern and western Greenland, in Hudson Bay in Canada, the Beaufort Sea off northern Alaska and in the Barents Sea.

The retreat of the ice cap looks set to continue, and things will become progressively more difficult for polar bears. Moreover, the ice forms later in the autumn and, unlike in the past, it does not reach a great thickness during winter before it breaks up again in the spring. This means that, in many places, the spring break-up of the ice is happening earlier than it used to.

Over the past 30 years or so, Canadian research scientists have been studying a group of polar bears living in the south-western parts of Hudson Bay. These days, the spring break-up of the ice in this region occurs about three weeks earlier than it did 30 years ago. The ice melts away completely in Hudson Bay in the summer and the bears have to spend many months on land, where they have nothing to eat, before the ice re-forms in the autumn.

Diminishing state of health
The seals that the polar bears live on will also have a harder time. These are in particular ringed seals, but also the bearded seal, hooded seal and harp seal, all of which are closely associated with the sea ice.
As far as polar bears are concerned, this is a serious trend, and in several places, the negative consequences for the bears have already been ascertained.

The longer a polar bear can spend in the spring catching ringed seals before the ice breaks up and it has to go ashore, the larger the deposits of fat it will manage to build up. The result of the earlier break-up of the ice is that polar bears in Hudson Bay have been forced to go ashore earlier, and now they are in poorer physical condition than they used to be. This is especially a problem for pregnant females, because the ability to carry a pregnancy through to term diminishes if the she-bear is too thin. If the retreat of the ice in Hudson Bay continues, it may be that she-bears will be so far reduced in their body weight that they will be unable to reproduce.

The cubs’ chance of survival also depends on the mother being well fed. In Hudson Bay, increased mortality among cubs and young polar bears and in old animals has been ascertained. Increased mortality has also been observed in polar bears in the southern part of the Beaufort Sea.

An increase in cannibalism among polar bears
Cannibalism is a natural phenomenon among polar bears. However, concurrent with the polar bears being forced to stay on the coasts more, where there is not much for them to eat, an increase in intra-species killing and cannibalism will no doubt occur.

In 2004, research scientists saw four instances of polar bears that had fallen victim to cannibalism in an area of Alaska where, despite many years of intensive bear studies, this had not been observed before.

Open waters: a challenge
In areas where there is pack ice in summer – e.g. eastern Greenland – the retreat of the ice means there are larger expanses of water between the floes and there is more movement of the ice. Accordingly, polar bears will have to expend more energy moving around among the pack ice. This can make it difficult for very small cubs to keep up with the mother bear. They do not have a thick, insulating layer of blubber, so they are not as well equipped to cope with the cooling if they have to swim longer distances in the icy waters.

In 2004, four drowned polar bears were found in the Beaufort Sea. It is thought that they must have given up the attempt to reach the pack ice, which was unusually far north of the coast that year. It is anticipated that more polar bears will drown in the future. Bears that have to swim across large distances to reach ice or land can be caught off guard by storms and bad weather.

Reduction in population
The polar bear population in the southern part of Hudson Bay has declined by approximately 25% since the early 1980s. The population in the southern part of the Beaufort Sea has also diminished, and here, too, this is attributed to the reduction in the ice, and hence also in the ability of the polar bear to get enough to eat.

In 2004, the International Union for Conservation of Nature, IUCN Polar Bear Specialist Group maintained that the number of polar bears would fall by more than 30% over the next 45–50 years – corresponding to three generations of polar bears. When the polar bear nations – the US, Canada, Greenland, Norway and Russia – met in March 2009 in Tromsø, Norway, to assess the situation of the polar bear, the conclusion was clear: the greatest threat to the polar bear is global warming and the receding of the sea ice, the key habitat of the polar bear.

By senior research fellow Erik W Born
Greenland Institute of Natural Resources, www.natur.gl