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Fossil Results Indicate Polar Bears Survived Last Global Warming Deglaciation In Siberian And Canadian Refugia
Polar bears are a familiar sight to many through the media as we see evocative images of singular bears floating on isolated ice rafts as they face the harsh realities of climate change shrinking sea ice in the Arctic. Their range is significantly impacted by the presence of perennial sea ice—that which survives at least one summer melt season. Simulations suggest polar ice cover is predicted to disappear during summer months by 2050, meaning this apex predator is increasingly being threatened by extinction due to habitat loss.
The response of this majestic beast to climate change during the last deglaciation is the focus of new research published in Quaternary Science Reviews to determine the effect on their global range and population size.
The Late Glacial, occurring 12,000–15,000 years ago, experienced significant melting of the expansive Scandinavian and North American ice sheets in the northern hemisphere that helped moderate the planet's temperature. This occurs due to ice albedo, whereby incoming solar radiation from the sun melts the "white" ice and exposes more of the comparatively "dark" land and sea, which absorbs the radiation rather than reflecting it back out to space. Consequently, more of the neighboring snow and ice melts, revealing more of the "dark" surface to absorb heat and so the feedback loop continues.
Professor Heikki Seppä, from the University of Helsinki, Finland, and colleagues turned to fossil bones (including forearms, upper hindlimbs and skulls) and teeth previously reported from various Scandinavian countries and islands off the north coast of Russia. These were found in coastal sections as well as archaeological sites, where they likely represent the aftermath of hunting by humans.
The research team determined that polar bears existed on the southwestern margin of the Scandinavian Ice Sheet through the Late Glacial, surviving into the earliest Holocene (the present interglacial over the last 12,000 years). Paleoenvironmental proxies from marine sediments, such as single-celled foraminifera and diatomaceous algae, suggest summer temperatures in the region were 6–10°C.
Continuing into the interglacial, sea ice cover in the arctic 8,000–10,000 years ago is modeled to be approximately comparable to present day. During the Holocene, this supported an exceptional assemblage of polar bears on Zhokhov Island off the coast of Russia, where at least 130 polar bear remains have been found, and is inferred to represent the maximum perennial sea ice extent at this time.
This archaeological site also offers insights into the hunter-gather human population migrating between here and the Siberian mainland, with polar bears and reindeer being a substantial component of their diet.
However, the warmest period of the Holocene, the Holocene Thermal Maximum between 5,000–9,000 years ago experienced polar temperatures 1.5–2.5°C higher than today and coincides with a notable absence of polar bears in the fossil record of this region.
Instead, the research team note that polar bear remains from adjacent areas radiocarbon-dated to 5,000–6,000 and 8,000–9,000 years ago are indicative of these mammals seeking out cold refugia which were persistently cooler than surrounding environments, located near the East Siberian Sea, northern Greenland and the Canadian Archipelago.
Fossils found on Alaska's Aleutian Islands and Pribilof Islands likely represent the southernmost extent of polar bears during these considerably warmer intervals, and the researchers deduced that the surrounding Bering Sea only experienced sea ice during winter, if at all.
Therefore, the research team suggest that the polar bears found ways to survive at the seasonal limits of sea ice, seeking out seal breeding grounds to hunt their prey, evidenced by the preservation of ringed seal fossils in association with polar bears on these islands.
Yet, as sea ice-limited populations of seals are also likely to decline, polar bears turning to alternative food sources is not something that current researchers have observed to sufficiently support their continued survival, nor is terrestrial food nutritionally complete, as seen in declining polar bear body mass.
While a general cooling trend and expanding sea ice has been predominant over the last 4,000 years, the effects of anthropogenic warming pivoting this trend to exacerbate polar bear habitat decline may see Siberia and Canada once more become refugia for these majestic creatures. This in turn necessitates further resources to help protect such refugia in the decades to come to ensure they too survive.
More information: Heikki Seppä et al, Polar bear's range dynamics and survival in the Holocene, Quaternary Science Reviews (2023). DOI: 10.1016/j.Quascirev.2023.108277
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Citation: Fossil results indicate polar bears survived last global warming deglaciation in Siberian and Canadian refugia (2023, September 27) retrieved 30 September 2023 from https://phys.Org/news/2023-09-fossil-results-polar-survived-global.Html
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Scientists Say They've Found A Direct Link Between Planet-warming Pollution And Polar Bear Survival
CNN —
Scientists say they have found a link between human-related greenhouse gas emissions and polar bear reproduction and survival rates for the first time in a new study, potentially overcoming a barrier to protecting the species.
Polar bears live in 19 populations across the Arctic and are found in Canada, the United States, Russia, Greenland and Norway, according to conservation organization Polar Bears International.
The populations live under distinct and varying circumstances, but all depend upon ice sheets to access their main prey, two species of seal, said study coauthor Steven Amstrup, chief scientist emeritus at Polar Bears International.
When sea ice melts, polar bears are forced onto land where they are deprived of food and must survive on fat reserves that they have accumulated beforehand.
Climate change caused by human activity is accelerating sea ice loss, giving polar bears less time to feed and build up their fat reserves, and more days where they are forced to go without food. This leads ultimately to a decline in their population.
Researchers from Polar Bears International, the University of Washington and the University of Wyoming have quantified the connection between the number of ice-free days a population of polar bears has to endure and the amount of planet-warming pollution released into the atmosphere, as well as corresponding polar bear survival rates in some populations, according to the study published Thursday in the journal Science.
Polar bears were listed as "threatened" due to human-caused climate warming under the US Endangered Species Act, or ESA, in 2008. But the US Department of Interior said at the time that, because threat to a particular species couldn't be directly linked to a specific source of greenhouse gases, federal agencies don't have to consider emissions when approving projects.
The researchers said the new study provides evidence of that direct link.
Researchers looked at polar bear subpopulations that had experienced at least 10 years with ice-free seasons from 1979 — when satellite imagery of sea ice first became available — to 2020.
They found that the number of days polar bears were forced to go without food increased as greenhouse gas emissions accumulated.
For example, polar bears in the Chukchi Sea in the Arctic Ocean were forced to go without food for about 12 days in 1979. It increased to about 137 days in 2020, with another day of fasting being added for every 14 gigatons of greenhouse gas emissions released into the atmosphere.
Subpopulations in areas where ice sheets would typically melt entirely during the summer — such as in Hudson Bay — saw slower increases in the days they were forced to go without food over time, compared with subpopulations in areas where ice sheets would usually remain during the summer, such as in the South Beaufort Sea.
Bears in areas where ice sheets used to persist for longer in the summer are now experiencing "a sudden shift" in their environment, requiring a significant adjustment where they are having forced fast days that they did not have before, Amstrup told CNN.
A 2020 study that Amstrup was involved in found that the number of days a bear can survive without food varies by region and the condition of the animal, but the more ice-free days experienced, the steeper the decline in reproduction and survival.
The researchers behind the new study combined the relationship they found between the number of forced fast days polar bears had to endure and cumulative greenhouse gas emissions with the connection found in 2020 between the number of forced fast days and declining survival rates. That allowed them to calculate the impact of cumulative emissions on the rate of polar bear survival.
"When the (Department of the Interior) memo was written in 2008, we could not say how greenhouse gas emissions equated to a decline in polar bear populations. But within a few years we could directly relate the quantity of emissions to climate warming and later to Arctic sea ice loss as well," said study coauthor Cecilia Bitz, professor of atmospheric sciences at the University of Washington, in the news release. "Our study shows that not only sea ice, but polar bear survival, can be directly related to greenhouse gas emissions."
For example, the study noted that each of the hundreds of power plants in the US might make a relatively small contribution to emissions, but collectively the power plants emit nearly 2 gigatons of greenhouse gases each year. This would be about 60-plus gigatons over the 30-plus year life span of a polar bear in the southern Beaufort Sea.
In this subpopulation, bears endure an extra forced fasting day for every 23 gigatons of emissions added to the atmosphere, so their survival rate would be reduced by about 4% by the power plant emissions over that period, according to the study.
The US Department of the Interior declined to comment on the findings.
Kirsten Zickfeld, distinguished professor of climate science at Simon Fraser University in Canada, said the new research shows a shift in what's possible when it comes to demonstrating the effects of climate change.
"Previous research showed that the more carbon dioxide we produce from human activities, the more our climate changes. The new study by Amstrup and Bitz takes this idea further by showing that this direct link also applies to ecosystem impacts," said Zickfeld, who was not involved in the study. "They found that when we emit more carbon dioxide, it directly affects how many polar bear cubs can survive. This finding allows us to connect the decline in polar bear populations to individual sources of carbon emissions."
Amstrup said he hopes the study can be used as a template by other researchers to get other species affected by human-caused climate change protected by the ESA.
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