Canada’s Hudson Bay is part of the seasonal sea ice ecoregion, where the sea ice melts and disappears in the summer and returns in the cooler fall and winter months. Over the last 30 years, Hudson Bay has warmed by approximately 1 degree Celsius, leading to an earlier melt and a later freeze-up, with the sea ice absent for on average around a month longer now compared to the 1980s. These shifts in the seasonal sea ice patterns pose a challenge for the polar bears in the region, which are increasingly faced with longer stretches on land and shorter time on the sea ice hunting seals, their most important prey. 

Earlier research identified the polar bear populations in Hudson Bay as among those most vulnerable to collapse with continued warming. A recently published collaboration between climate scientists and polar bear and seal biologists considered these risks in light of the climate goals laid out in the Paris Agreement, a legally binding international treaty on limiting climate change that was adopted in 2015. Their findings show a troubled future for Hudson Bay bears under a breach of the Paris Agreement.

Predicting the future for sea ice 

To understand future scenarios of climate warming, scientists rely on climate models – sophisticated combinations of models that simulate the physics, chemistry, and biology of our world. The new study –  led by Prof. Julienne Stroeve of the University of Manitoba – used outputs from these state-of-the-art climate models to project the ice-free period in Hudson Bay using the thickness of sea ice, recognizing that polar bears need a minimum amount of sea ice to successfully hunt. This new approach provides valuable insights into the conditions polar bears in the region will likely face in the near future.

If we hit 2° C degrees of warming ­– the upper limit to warming laid out by the Paris Agreement ­– projections from the sea ice models indicate that the ice-free period will last for around 165–170 days in Western Hudson Bay and 175–180 days in Southern Hudson Bay – considerably longer than the 1980s average for the region of ~120 days without sea ice. Although reproduction and cub survival prospects are already being impacted by the extended periods on land that we are seeing today, keeping warming below 2 degrees remains vital for the survival prospects of adult bears. In fact, the new study finds that if the Paris Agreement is breached, and the world surpasses 2°C of warming, the Southern Hudson Bay population could disappear, and most of Hudson Bay’s polar bears will be at or near their survival limit. Even more worryingly, if we continue fossil fuel use at current levels and global temperatures reach 4 degrees C above pre-industrial levels by the end of the century, the projected loss of sea ice is expected to surpass what polar bears are capable of withstanding and both populations will be lost.

Snow and seals

Of course, polar bears will not be the only species affected by changing Arctic temperatures. The study was also able to predict how warming will influence snow depth in Hudson Bay and impacts on seal populations. Future losses in snow accumulation projected by the study in the spring could compromise the snow-covered lairs where ringed seals give birth, and which provide shelter and protection to their newborn pups. Negative impacts on seals may further jeopardize the long-term persistence of polar bears in the region.

With more refined models comes a more detailed understanding of the prospects for polar bears and for the wider Arctic ecosystem, painting a stark picture: If polar bears are to have any future in Hudson Bay, it is critical that we keep warming within the targets of the Paris Agreement.

Dr. Louise Archer is a Mitacs Elevate Postdoctoral Fellow at the University of Toronto Scarborough, supported by Polar Bears International.