Greenland polar bear populations have been retreating for 20 000 years | Polarjournal
Classified as vulnerable, the polar bear is suffering the effects of global warming head-on. The sea ice, on which it depends for food, is shrinking as sea water temperatures increase.
Image: Michael Wenger

Greenland’s polar bear populations have been retreating for 20 000 years, according to a recently published study which also reveals that a slight rise in sea temperature can have significant consequences for the Arctic king.

It’s a well-known fact: polar bears are threatened by global warming and melting sea ice. What was less known, however, was that even a slight increase in sea temperature could significantly affect the number of polar bear. A discovery that calls into question the very ability of bears to survive in an environment largely affected by human-induced climate change.

To reach this conclusion, the authors, who published their results in Science Advances on November 8 analyzed the bears’ DNA, their food choices, as well as historical climate data and the amount of habitat available since the end of the last ice age: “Through our genomic analyses, we can open a window into the past to gain insight into the species’ development and population history. Our analyses revealed that the number of Greenlandic polar bears significantly reduced several times since the last ice age,” explains Eline Lorenzen, professor and head of research at the Globe Institute, University of Copenhagen, and co-author of the study, in a press release issued by the university.

To estimate the genetic diversity of polar bears in Greenland over the past 20,000 years, researchers carried out genetic analyzes on samples of polar bear skulls collected over the past four decades by the University of Aarhus in Denmark. The analysis of the bones also revealed this large carnivore’s diet. Image: Klaus Rassinger und Gerhard Cammerer, Museum Wiesbaden, Wikimedia Commons

The study shows that over the last 20 000 years, the sea around Greenland has repeatedly experienced a temperature rise of between 0.2 and 0.5 degrees. This has led to a 20-40% reduction in the polar bear population.

This came as a surprise to the researchers, who had not imagined that a relatively modest increase could have such a significant impact on bears. “Looking ahead, we are potentially facing a 2° to 5° increase in sea temperatures around Greenland. So, it’s a 10-fold increase in temperature changes compared to the last 20,000 years.”, says Pr Lorenzen.

However, the authors emphasize the adaptability of bears in their food choices. These mammals can, if necessary, seek out food different from that to which they are accustomed.

Female and male bears in East Greenland seem to differentiate their food choices so as not to compete with each other. While males devour all types of seals, females consume ringed seals. A strategy a species can adopt when resources are scarce: “We don’t see that in West Greenland, where all bears primarily eat their favorite prey species, ringed seals. Due to the ocean currents, West Greenland has a significantly higher primary production than in the east, therefore more food is available for the polar bears and they can eat what they like”, explains Michael Westbury, assistant professor at the University of Copenhagen and co-director of this interdisciplinary study, fruit of six years’ work by twenty researchers from different countries.

A predator at the top of the chain. The polar bear’s diet consists mainly of seals, which it hunts on the sea ice. Image: Julia Hager

While the impact of global warming on polar bears is obvious, the authors emphasize that it is a symptom of something much wider.

The polar bear is a predator at the top of the food chain and represents Arctic ecosystems. The changes affecting it show that the world is changing, and that these changes don’t stop at Greenland or the Arctic. “It’s all of us. The Earth is a large, interconnected ecosystem, of which we are an integrated and deeply dependent part. Nature knows no boundaries,” stresses Pr Lorenzen.

Link to the study: Michael V. Westbury et al, “Impact of Holocene environmental change on the evolutionary ecology of an Artic top predator”, Science Advances (2023). DOI: 10.1126/sciadv.adf3326

Mirjana Binggeli, PolarJournal

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