Thursday, October 14, 2010

Receding Sea Ice Puts Polar Bears At Risk

Polar bears have been facing the challenges resulting from increasing global temperatures for many years and there has been a corresponding decrease in their populations worldwide (Molnar et al. 2010). A recent study by Molnar et al. 2010 adds insight to this connection between polar bear population and global warming.

Polar bears rely heavily on sea ice as a solid platform to catch ringed and bearded seals, their main source of food (Molnar et al. 2010). But, as global temperatures continue to increase, the percentage of the year that sea ice is present decreases. For example, in the Western Hudson Bay area the spring breakup of ice has been occurring earlier every year and the fasting period of the polar bears in the area has been prolonged (Molnar et al. 2010).

As global temperature have increased, the polar bears’ fasting period has started earlier and lasted longer, with the fasting period increasing by seven days each decade since the early 1980’s (Molnar et al. 2010). Molnar et al. 2010 derived mathematical models to estimate the effect of these increased fasting periods on polar bears. In the 1980’s the fasting period was roughly 120 days and 3-6 percent of all male polar bears died due to starvation (Molnar et al. 2010). If the trend of increased fasting periods continues, when the fasting period reaches 180 days, 38-48 percent of all males will die due to starvation (Molnar et al. 2010).

As the sea ice continues to dissipate earlier polar bears have been recorded having less body mass and being in poorer body conditions for the animals. This is a major issue as pregnant female polar bears must be above 189 kg to produce viable offspring (Molnar et al. 2010). As the amount of food decreases and as polar bears continue to become lighter and lighter as a consequence, this could become a major issue in the future.

Polar bears also use sea ice as an alternative mode of transportation to swimming (Molnar et al. 2010). As the ice recedes earlier every year and more stretches of ocean are left uncovered, polar bears will have to expend more energy swimming between ice flows. The bears will also have to expend energy following the flows along with their food source and scrounging for food on land when the sea ice retreats (Molnar et al. 2010).

Pregnant polar bears will also face additional challenges as the size and mass of their litter directly corresponds to female body condition. Female body condition will decrease as food becomes more difficult to find with decreasing levels of sea ice. After the litter is born, decreased body conditions will also cause poorer quality milk to be produced which will cause decreased cub growth and increased mortality rates (Molnar et al. 2010).

The impact of decreased sea ice (or increased global temperatures) on the reproductive success of polar bears has also been examined. The study reports mathematical models that show that reproduction rates could rapidly drop as travel distances increase and it becomes more difficult to find mates (Molnar et al. 2010). This compounds the difficulties facing the polar bears as not only will their numbers decrease due to starvation but also as a result of decreased fertility rates across the population.

The study also mentions that two thirds of the polar bear population may be extinct by the mid-century due to the shrinking sea ice (Molnar et al. 2010). Specifically, ten out of the nineteen subspecies of polar bears are expected to have died out in this time period, and all other subspecies are expected to have decreased (Molnar et al. 2010).

Another issue the study mentions is pollutants in our oceans which are also negatively affecting the polar bear population (Molnar et al. 2010). Hormones found in the oceans caused by pollution negatively affect the hormone cycle of polar bears and impacts their reproduction in negative ways (Molnar et al. 2010). These pollutants can also decrease the effectiveness of the immune systems in polar bears making them more susceptible to disease (Molnar et al. 2010).

The findings from this study suggest that polar bear populations will continue to decline across the world. As global temperatures continue to rise and sea ice continues to recede polar bears will face many challenges including starvation and decreased reproduction. This is compounded by the effects of increasing pollutants in the oceans. Further research on the impact of environmental change on the health of the polar bear population will be critical to our developing strategies to protect their habitats.


Molnar PK, Derocher AE, Thiemann GW, Lewis MA. Predicting survival, reproduction and abundance of polar bears under climate change. Biological Conservation [Internet]. [cited 2010 Oct 12]; 143(7); 1612-1622. Available from:

- Devin Vriezen, Student I.D. #0709727

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