A mother bear standing on melting ice with a cub bear

Photo: Kt Miller / Polar Bears International

New Study Quantifies Link Between Sea Ice and Polar Bears

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30 Jan 2025

Climate Change Drives Polar Bear Population Decline: New Study Quantifies Link Between Sea Ice and Polar Bears

Bioenergetic Model and Analysis of Four Decades of Data Explains Halving of Western Hudson Bay Polar Bear Subpopulation; Report Underscores Need for Immediate Climate Action

Churchill, Canada – January 30, 2025 – Researchers from leading institutions including the University of Toronto Scarborough, Environment and Climate Change Canada, and Polar Bears International have established a mechanistic link between climate change-driven sea ice loss and polar bear population declines. Published today in Science, the report “Energetic Constraints Drive the Decline of a Sentinel Polar Bear Population” introduces an individual-based bioenergetic model and analyses of long-term data grounded in 42 years of observations and monitoring. The researchers use energy as a currency, analyzing energy acquired and energy expended by polar bears to link environmental conditions to individuals and populations. Polar bears require sea ice to hunt and therefore survive, and this rigorous analysis of the Western Hudson Bay polar bears from 1979 - 2021 demonstrates that this subpopulation’s approximate 50% decline was due to climate-driven sea ice loss that reduced hunting opportunities for polar bears. The research highlights the urgent need for climate action to protect polar bears, especially vulnerable cubs, as the Arctic warms over four times faster than the global average.

“We know that polar bear populations are already declining in some regions, at the same time as climate warming has accelerated sea ice loss across the Arctic,” says Dr. Louise Archer, lead author of the study and Polar Bears International Research Fellow at the University of Toronto Scarborough, adding, “We wanted to take an approach that would allow us to quantify the underlying causes that are driving these changes from individual bears right up to entire populations.”

Highlighting Vulnerabilities for Polar Bear Moms and Cubs: This model fills gaps in monitoring data and provides a way to find the root of the causes that are driving changes in polar bear populations. The authors emphasize that cubs are most vulnerable to climate change and that their survival impacts the population's survival. This study highlights ways in which climate change is negatively impacting polar bear cubs, including: 

  1. With diminishing hunting periods on sea ice, mothers struggle to produce sufficient milk, jeopardizing cub survival. 

  2. Cubs also face reduced survival rates during their first fasting period if they fail to gain enough weight during shorter hunting seasons. 

  3. Polar bear mothers are having fewer cubs over their lifetime, as their litter size is decreasing and they’re keeping cubs longer due to stress from sea ice conditions.

A Quantifiable Link, with Broad Implications: Compared to previous studies that focused on polar bears during the ice-free period or future-looking predictions, this research modeled the full lifecycle of polar bears—from reproduction to hunting on sea ice and fasting on land— and compared model simulations to four decades of historical data from the Western Hudson Bay polar bear population. Western Hudson Bay is one of the world’s southernmost and best-studied polar bear populations, serving as a bellwether for the changes coming to polar bears across the Arctic. Because all animals require energy to reproduce and survive, this “calorie-counting approach” can be used across the Arctic to assess how changing landscapes can affect animal populations, even in remote data-deficient areas. This tool could prove invaluable for adaptive management and conservation efforts as the Arctic faces rapid warming and industrialization. The study combined mathematical modeling, physiology, and analysis of long-term polar bear monitoring data.

“We built this model for polar bears, but since we use energy as the currency, it can be adapted to analyze other species,” notes Dr. Péter Molnár, Associate Professor at the University of Toronto Scarborough and senior author of this study, adding “By demonstrating how energetic limitations resulting from sea ice loss have driven this population to decline, we add further weight to projections that continued climate warming risks the survival of polar bears in all but a few high Arctic regions.”  

Bioenergetics Model: The researchers developed an “energy budget model” for polar bears, which tracks the energy balance of an individual polar bear across its entire lifetime, accounting for incoming energy from feeding and energy spent on movement, growth, and reproduction, amongst other processes. “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”, explains Dr. Louise Archer, adding “Loss of sea ice across the years means bears spend less time roaming the sea ice where they hunt seals, and more time fasting on land. This negatively affects their energy balance, with knock-on effects on reproduction and survival. We applied this approach to thousands of bears to see if we could capture trends that have been observed in Western Hudson Bay across the last four decades. Our model was able to explain observed declines in body mass, number of cubs, and survival rates, and an overall decline in population size across this period.” 

“Long-term monitoring programs in Western Hudson Bay allowed us to account for short-term variability while capturing long-term trends, and to create predictive models that are vital as the Arctic warms four times faster than the global average,” adds co-author Dr. Stephen Atkinson. 

As nations look to meet climate targets, decision-makers need to have robust evidence grounded in long-term observational datasets. This study provides rigorous mathematical evidence that further supports scientists’ reports that meeting the Paris Climate Agreements and updating the Endangered Species Act are required to better protect species from climate change.

“Polar bears are an iconic Arctic species, but with accelerating sea ice loss, we will likely see more populations decline into the future. It is crucial to understand exactly how and why some populations are already struggling if we are to revert this trend. With this study, we’ve been able to directly quantify the driving cause of the decline in a region that has been experiencing some of the most rapid change, and in doing so, underscore the consequences of continued warming for polar bears across the Arctic,” said Dr. Archer.

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About Polar Bears International

Polar Bears International’s mission is to conserve polar bears and the sea ice they depend on. The organization works to inspire people to care about the Arctic, the threats to its future, and the connection between this remote region and our global climate. Polar Bears International is the only nonprofit organization dedicated solely to wild polar bears and Arctic sea ice, and the staff includes scientists who study wild polar bears. The organization is a recognized leader in polar bear conservation. For more information, visit www.polarbearsinternational.org.

Media Contact

Annie Edwards, for Polar Bears International 
annie@fabricmedia.net
+44 0 7307 139782

About the University of Toronto Scarborough

Founded in 1964, the University of Toronto Scarborough is an anchor institution in the eastern Greater Toronto Area. Situated in one of Toronto’s most diverse and multicultural neighbourhoods, the campus fosters a vibrant culture of community engagement. Here, students cross academic and geographic boundaries in their pursuit of knowledge. Experiential learning is one of the hallmarks of our approach to education. We are proud to be part of the University of Toronto, recognized as the most sustainable university in the world and Canada’s top university. U of T has a long history of challenging the impossible and transforming society through the ingenuity and resolve of its faculty, students, alumni and supporters. We are part of one of the top research-intensive universities, bringing together top minds from every conceivable background and discipline to collaborate on the world’s most pressing challenges. Together, we continue to defy gravity by taking on what might seem unattainable today and generating the ideas and talent needed to build a more equitable, sustainable and prosperous future.

About Environment and Climate Change Canada

ECCC is the lead federal department for a wide range of environmental issues, including taking action on clean growth and climate change. The Department is also engaged in activities aimed at preventing and managing pollution, conserving nature, and predicting weather and environmental conditions.