A climate change laboratory
The world's most southerly population of polar bears (Ursus maritimus) lives along the shores of Western Hudson Bay. This unique location forces the bears onto land for three to four months every summer when the sea ice completely melts. This predictable, extended fasting period makes them a sensitive measure of the effects of a warming climate. Since 1980, a long-term research program, primarily led by scientists from Environment and Climate Change Canada, has monitored this specific population, creating one of the most detailed datasets for any large mammal.
The research was initiated by Dr. Ian Stirling to understand the basic ecology of the species, long before global warming was a widespread concern. The accessibility of the bears near the town of Churchill, Manitoba, allowed for consistent year-over-year data collection, a logistical advantage over more remote Arctic populations. Researchers use capture-recapture methods, satellite tracking, and physiological sampling to track the health, movement, and demographics of over 4,600 individual bears over five decades. This continuous monitoring has transformed the Western Hudson Bay bears into a real-time case study of climate change's biological impact.
The evidence on the ice
The data collected near Churchill shows a direct link between rising temperatures, shrinking sea ice, and the bears' physical condition. The ice-free period in Western Hudson Bay is now three to four weeks longer than it was in the early 1980s. This matters because sea ice is the platform from which polar bears hunt their primary food source: energy-rich ringed and bearded seals. Less time on the ice means less time hunting.
The consequences are measurable and severe. Each day a polar bear fasts on land, it loses approximately one kilogram (2.2 pounds) of body mass. The extended ice-free season has led to significant declines in the bears' body condition. Recent studies confirm that the body mass of adult females has dropped by an average of 39 kg (86 lbs) over a 37-year period.
This nutritional stress directly affects reproduction and survival. Fewer females are successfully giving birth, and the survival rates for cubs and older bears have fallen. The population of the Western Hudson Bay polar bears has declined by nearly 50% between 1979 and 2021. A 2021 aerial survey estimated the population at just 618 individuals, down from approximately 1,200 in the late 1980s. These numbers provide direct evidence of how changes in the Arctic environment are affecting its apex predator.