Polar bears depend on sea ice for survival. According to the analysis of passive-microwave satellite imagery beginning in the late 1970s, the sea ice has been breaking up at earlier dates. Stirling and Parkinson (2006) explore five populations of polar bears and predict the effect of climate change on these populations. They hypothesize that the polar bears will become increasingly food-stressed, and their numbers will decrease significantly due to earlier ice break up. — Clara Lyashevsky
Stirling, I., Parkinson, C., 2006. Possible effects of climate warming on selected populations of polar bears (Urus martimus) in the Canadian Arctic. Environmental Toxicology and Chemistry 59, 261–275.
In 2005, the Department of Environment of the Government of Nunavut, Canada announced an overall increase in polar bear quotas. The estimates of population size were determined from mark-recapture, survival rates, and reproductive rates. However in Western Hudon Bay, Foxe Basin, Baffin Bay, and Davis Strait, the population increase was based on traditional Inuit knowledge. They had reported seeing more bears in recent years around settlements, hunting camps, and in other locations where they had not been seen before. These observations lead to the conclusion that populations were growing. Although the Inuit population did not document the actual numbers of bears, the dates or locations, or the approximate age and sex of bears, their observations are generally regarded as accurate.
The authors analyzed the patterns of ice breakup in five regions. The boundaries were made using studies of movements of tagged bears of all ages and sexes, the annual movements of adult females wearing satellite radio collars, and genetic studies. Satellites have collected multichannel passive-microwave data on the Arctic sea-ice cover since late 1978. These data show the monitoring of the ice cover to a resolution of about 25 km. The data are collected day and night, in all seasons of the year, and under all weather conditions. The authors used two satellite passive-microwave data sets, one from NASA’s Nimbus 7 SMMR and the other from the DMSP Special Sensor Microwave Imager. They used the SSMI record every day through the end of 2004 and the SMMR every other day from October 1978 to August 1987. The SMMR and SSMI radioactive data were converted to sea-ice concentrations (percent areal coverages of sea ice) through a multichannel algorithm. Ice extents were calculated by summing the areas of all pixels with at least 15% ice concentration. Ice areas were calculated by summing the products of the area and ice concentration of all pixels with at least 15% ice concentration.
The authors calculated ice areas for the five polar bear regions and divided those ice areas by the area of the respective region, which gave them the daily percent ice coverage. For the SMMR years, the authors performed temporal interpolation to provide values for the days without data. This way, they were able to produce a complete daily data set. They determined the data in each year when the ice cover fell below 50% (break up), and then calculated the trend over the course of the 1979–2004 record.
Every fall, the authors measured the body length and axillary girth of 100–300 immobilized polar bears to estimate their weights and overall body condition. They used a Pearson product-moment correlation to test for statistical significance between the number of problem bears handled and the date of break up. They used a linear regression to test for significance between weights of pregnant females in fall and year.
The data showed an overall trend toward progressively earlier sea-ice breakup. On average, breakup occurred about 7–8 days earlier per decade. In Western Hudson Bay, all bears must fast for at least four months during the ice-free season whereas the pregnant females must fast for eight months because they give birth to cubs in dens at about the time the rest of the population can return to the ice. As a result of the earlier breakup, the polar bears are forced to come ashore earlier to begin fasting and for a longer period of time.
Although the number of polar bears seen by people has increased, the results of their study show that this is not indicative of an increase in population, but in fact a decrease. The decline is due to earlier breakup of the sea ice, due to climate warming.