The Western Hudson Bay polar bear population represents one of nature's most intensively studied sentinels of climate change. For half a century, researchers have documented the lives of these bears through a long-term research program that began in 1980 when Dr. Ian Stirling established a study site near Churchill 2 .
50+ Years
Of continuous research
Southernmost
Polar bear population
4x Faster
Arctic warming rate
What began as fundamental ecological research has evolved into a critical climate change warning system. The Western Hudson Bay bears, representing the southernmost polar bear population globally, experience the longest annual fasting period of any polar bears 1 . As the Arctic warms at nearly four times the global average rate, this population has become a living barometer of ecosystem health—their declining fortunes offering a sobering preview of challenges facing polar bears across their range 4 7 .
The Churchill Natural Laboratory
Churchill's unique geography made it ideally suited for polar bear research. Each summer, when Hudson Bay's sea ice completely melts, polar bears are forced ashore where they gather in a relatively small area along the coast. Rather than tracking bears across vast expanses of sea ice, researchers could systematically study them concentrated in an area less than one percent of the Bay's total size 2 .
Systematic Data Collection
The research program developed consistent, systematic data collection methods. Each handled bear was uniquely identified, with body measurements, health assessments, and biological samples meticulously recorded.
Long-term Record
The growing database of marked individuals created an incredible record of changes over time, capturing the life histories of generations of polar bears.
"Individuals handled in previous years and later recaptured told interesting stories about where and when they were encountered and when they had cubs."
The Climate Connection Emerges
Late 1990s
Analysis of the first two decades of data revealed troubling trends: declining body condition in both male and female bears, fewer cubs being produced, and fewer yearling bears in capture samples 2 .
Critical Connection
The critical connection emerged when researchers connected these declines to shrinking sea ice cover. Analysis revealed a significant relationship between earlier spring sea ice breakup and poorer body condition when bears came ashore.
Natural Experiment
Following the 1991 eruption of Mount Pinatubo, which cooled global temperatures, sea ice breakup in Hudson Bay was significantly delayed in 1992. That year, polar bear cubs were in better condition and had higher survival rates—directly linking sea ice availability to polar bear well-being 2 .
The Bioenergetic Breakthrough: Counting Calories for Conservation
In January 2025, researchers unveiled a groundbreaking bioenergetic model that finally quantified the precise mechanism driving polar bear declines. Published in Science, the study analyzed energy as currency—calculating calories acquired through hunting versus calories expended on survival needs 7 .
The research team developed an "energy budget model" that tracked the energy balance of individual polar bears across their entire lifetimes. "The amount of energy ingested versus the amount of energy used determines how fat an individual is, which in turn decides whether a bear can successfully reproduce, rear offspring, and even survive," explained lead author Dr. Louise Archer 7 .
Western Hudson Bay Polar Bear Population Decline
| Year | Population Estimate | Percentage Decline from 1980s |
|---|---|---|
| 1980s | 1,185 | Baseline |
| 2011 | 949 | 20% |
| 2016 | 842 | 29% |
| 2021 | 618 | 48% |
Data compiled from multiple surveys showing consistent downward trend 1
The model explained observed declines with remarkable precision: with diminishing hunting periods on sea ice, mothers struggle to produce sufficient milk, jeopardizing cub survival; cubs face reduced survival during their first fasting period; and mothers have fewer cubs over their lifetime 7 . Most significantly, the model demonstrated that the Western Hudson Bay population's approximate 50% decline between 1979-2021 was directly caused by climate-driven sea ice loss reducing hunting opportunities 7 .
The Ice-Free Period: A Critical Survival Threshold
Recent research has identified 183 days as the critical fasting threshold for Western Hudson Bay polar bears—beyond this point, starvation rates increase dramatically 1 . As of July 2025, the ice-free period has already reached 42 days and continues to grow longer each year 1 .
Current Ice-Free Period
Critical Threshold
Beyond 183 days of fasting, starvation rates increase dramatically for polar bears in Western Hudson Bay.
Projected Ice-Free Period Under Different Warming Scenarios
| Global Warming Above Pre-industrial Levels | Projected Ice-Free Period (Western Hudson Bay) | Impact on Polar Bears |
|---|---|---|
| +1.0°C | 140 days | Stress begins |
| +1.5°C | 150-155 days | Reproductive impacts |
| +2.0°C | 165-170 days | Survival threatened |
| +4.0°C | 220-240 days | Localized extinction likely |
Based on CMIP6 climate model projections 6
"If we fail to limit global warming to less than two degrees Celsius, we will lose Hudson Bay's populations of polar bears," warns lead author Professor Julienne Stroeve. Southern Hudson Bay polar bears face imminent disappearance, with Western Hudson Bay not far behind 9 .
The Scientist's Toolkit: Research Reagent Solutions
Polar bear research requires specialized equipment and methodologies adapted to harsh Arctic conditions and the formidable size of the study species.
Satellite GPS Collars
Modern research uses Vertex Plus collars with camera options from Vectronic Aerospace, which collect location data and can record video documentation of bear behavior 3 .
Biological Samples
Systematic collection of blood, fat, and hair samples during hands-on research provides data on physiology, genetics, contaminant loads, and diet composition 2 .
Body Measurements
Standardized tools and techniques for recording length, girth, and weight assessments allow for tracking body condition changes over time 2 .
Bioelectrical Impedance
This technology, adapted from human medicine, estimates body composition (fat versus lean mass) in polar bears, providing more precise health assessment than weight alone .
Den Detection
Polar Bears International is testing and developing new technologies, including aerial reconnaissance and potentially satellite imagery, to locate snow-covered maternal dens 5 .
A Community of Coexistence
The concentration of fasting polar bears near human settlements has necessitated innovative conflict prevention programs. Churchill's Polar Bear Alert Program, established in 1983, represents a successful model of human-bear coexistence. Dedicated wildlife officers respond to bear sightings around town, initially using hazing techniques to scare bears away 1 .
Polar Bear Jail
Persistent offenders are held in a unique "Polar Bear Jail" on the town's outskirts—a former military building where bears are housed temporarily without food until they can be released back onto the sea ice.
Community Practices
This approach, combined with community "bear awareness" practices like leaving car and house doors unlocked for emergency shelter, has proven remarkably effective.
Successful Coexistence
Only one person has been killed by a polar bear in Churchill since 1968, and none since the Polar Bear Alert Program's inception 1 .
Fasting Impact on Polar Bear Physiology
| Fasting Duration | Physiological Consequences | Population Impact |
|---|---|---|
| 120 days (historical average) | Bears maintain healthy weight | Stable population |
| 150 days (current average) | Average weight loss of 1 kg/day | Declining body condition |
| 180 days (critical threshold) | 21% of adult males, 63% of subadults starve | Rapid population decline |
| 210+ days (projected future) | Reproductive failure | Localized extinction |
Based on research into fasting physiology and population impacts 1 6
The Path Forward
The five decades of research in Western Hudson Bay have yielded an unambiguous conclusion: the future of polar bears depends on climate action. As Polar Bears International's senior director of research and policy, Geoff York, emphasizes, "This report is more detailed and alarming than previous studies, reinforcing that Hudson Bay's polar bears are not on a good trajectory unless significant emissions mitigation measures are implemented" 9 .
Paris Agreement Targets
The research shows that meeting Paris Agreement targets is essential for polar bear survival. While the most ambitious goal of limiting warming to 1.5°C appears increasingly challenging, staying below 2°C global warming could prevent ice-free periods from exceeding the critical 183-day threshold in Western Hudson Bay 6 .
Timeline for Action
Current policies putting the world on track for approximately 2.5-3°C warming would likely lead to the extirpation of both Western and Southern Hudson Bay polar bear populations between the 2030s and 2060s 9 .
"I hope that this research will lead to concrete actions to conserve and secure polar bears in the wild for many future generations to come."
The story of Western Hudson Bay's polar bears is both a warning and an opportunity. These five decades of research have provided an unprecedented window into climate change impacts—and a clear roadmap for action. The fate of the Arctic's iconic predator now rests in human hands.