Polar bears are suffering from an "energy deficit" that is devastating their populations

A recent study has confirmed what scientists have long suspected – the devastating link between climate change, melting sea ice, and the decline of polar bears.

Unlike previous research that only highlighted correlations, this study presents a direct, measurable connection.

For the first time, researchers have developed a model that precisely demonstrates how shrinking sea ice leads to polar bear population collapse in Western Hudson Bay.

Polar bears are starving

A changing climate has dramatically altered the Arctic landscape, forcing polar bears to spend more time on land instead of hunting seals on the ice.

As their primary food source becomes less accessible, the bears enter longer fasting periods, leading to declining health and population numbers.

“A loss of sea ice means bears spend less time hunting seals and more time fasting on land,” said Louise Archer, a postdoctoral researcher and lead author of the study.

This prolonged fasting drains polar bears’ energy reserves, reducing their ability to reproduce and raise cubs. Without enough stored fat, female bears struggle to give birth and nurse their young. Over time, this energy deficit has led to a sharp population decline.

Energy loss in polar bears

To understand the impact of climate change on polar bears, researchers developed a “bio-energetic” model that measures how much energy the bears gain from hunting and how much they need to grow and reproduce.

This model is unique because it tracks individual polar bears throughout their lives, from cubhood to adulthood.

The model uses over four decades of data from monitoring the Western Hudson Bay polar bear population between 1979 and 2021.

This long-term dataset allows researchers to analyze trends and predict how continued sea ice loss will affect polar bears in the future.

Dramatic polar bear population decline

The findings are alarming. The polar bear population in this region has dropped by nearly 50% over the past four decades.

Body mass has also declined significantly, with adult females weighing 39 kg (86 lbs) less than before and one-year-old cubs losing an average of 26 kg (47 lbs).

“Our model goes one step further than saying there’s a correlation between declining sea ice and population decline,” noted Péter Molnár, associate professor at University of Toronto Scarborough and co-author of the study.

“It provides a mechanism that shows what happens when there is less ice, less feeding time and less energy overall. When we run the numbers, we get a near one-to-one match to what we’re seeing in real life.”

This level of accuracy confirms that climate change is the primary driver of polar bear decline, rather than other potential factors.

Cubs face the greatest risk

The research team, which includes experts from Environment and Climate Change Canada, found that cubs are the most vulnerable to the changing conditions.

Their survival depends on their ability to gain enough weight before their first fasting period, but shorter hunting seasons make this increasingly difficult.

Archer explained that when hunting time is cut short, mother bears cannot produce enough milk, which puts cubs at greater risk. If a cub does not gain enough weight before winter, it has a much lower chance of survival.

Mothers are also giving birth to fewer cubs. Monitoring data shows that litter sizes have decreased by 11% over the past 40 years. Additionally, many mothers are now keeping their cubs for longer periods because the young bears are not strong enough to survive on their own.

“It’s pretty simple – the survival of cubs directly impacts the survival of the population,” said Archer. Her research is supported by a Mitacs Elevate postdoctoral fellowship and the non-profit organization Polar Bears International.

The reduced cub survival rate signals a troubling future for polar bears in Western Hudson Bay. If these trends continue, the population could face an even sharper decline in the coming decades.

Warning for polar bears worldwide

Western Hudson Bay has long been seen as an indicator of how climate change affects polar bear populations globally.

The Arctic is warming at a rate four times faster than the global average, making the survival of these bears even more uncertain.

“This is one of the southernmost populations of polar bears, and it’s been monitored for a long time, so we have very good data to work with,” said Molnár, who specializes in how global warming affects large mammals.

“There’s every reason to believe what is happening to polar bears in this region will also happen to polar bears in other regions, based on projected sea ice loss trajectories. This model basically describes their future.”

With sea ice disappearing at an increasing rate, other polar bear populations will likely face the same challenges as those in Western Hudson Bay.

Experts warn that unless significant action is taken to address climate change, these majestic animals may struggle to survive in the coming decades.

Need for conservation

The researchers hope that their findings will contribute to ongoing conservation efforts and encourage policymakers to take stronger action against climate change.

The bio-energetic model developed for this study could be applied to other polar bear populations to predict future trends and inform conservation strategies.

By tracking energy intake and expenditure, scientists can gain a clearer picture of how climate change will impact different regions.

Polar bears have long been a symbol of the Arctic, but their future depends on how quickly humanity responds to the challenges posed by global warming. Without intervention, these animals will continue to struggle as their habitat disappears.

The study received funding from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation. 

The research is published in the journal Science.

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