A Geological Perspective of Polar Bears

Guest “geological perspective” by David Middleton

A Popular Narrative

Polar bears (Ursus maritimus) are the poster child for the impacts of climate change on species, and justifiably so. To date, global warming has been most pronounced in the Arctic, and this trend is projected to continue. There are suggestions that before mid-century we could have a nearly ice-free Arctic in the summer. This increases the urgency with which we must act to reduce our greenhouse gas emissions to delay or avoid some of the worst consequences of climate change.


Figure 1. Change.org

A Geological Perspective

Figure 2. “Global polar bear population size estimates to 2018. From Chapter 10 of The Polar Bear Catastrophe That Never Happened (Crockford 2019).” Polar Bear Science.

Effects of declining ice on polar bear behavior and habitat selection

Among all polar bear subpopulations, those occurring in the Beaufort and the Chukchi seas have experienced some of the greatest losses of sea ice in the entire Arctic Ocean. The mechanisms that link habitat to population trend, however, have not been quantitatively described. The role of habitat preference on behavior (i.e., movement and activity) or whether some habitats are important for life history requirements is uncertain. Movements of polar bears are linked to foraging success and energetic costs. Hence, movements, as a proxy for behavior and energetics, may be indicative of habitat value and provide a quantifiable means to elucidate mechanisms of polar bear population response to a changing Arctic. We are using polar bear location data collected by the USGS since 1985 to develop models of polar bear distribution that account for long-term changes in behavior and habitat function. This research will provide products that can be used by managers to mitigate adverse impacts to polar bears and polar bear habitats from anthropogenic activities.


Key passage:

Among all polar bear subpopulations, those occurring in the Beaufort and the Chukchi seas have experienced some of the greatest losses of sea ice in the entire Arctic Ocean. The mechanisms that link habitat to population trend, however, have not been quantitatively described. The role of habitat preference on behavior (i.e., movement and activity) or whether some habitats are important for life history requirements is uncertain.


“The mechanisms that link habitat to population trend” cannot be “quantitatively described.”

November 14, 2018
First tally of U.S.-Russia polar bears finds a healthy population
Hannah Hickey
UW News

Not all polar bears are in the same dire situation due to retreating sea ice, at least not right now. Off the western coast of Alaska, the Chukchi Sea is rich in marine life, but the number of polar bears in the area had never been counted. The first formal study of this population suggests that it’s been healthy and relatively abundant in recent years, numbering about 3,000 animals.

The study by researchers at the University of Washington and federal agencies is published Nov. 14 in Scientific Reports, an open-access journal from the Nature Publishing Group.

“This work represents a decade of research that gives us a first estimate of the abundance and status of the Chukchi Sea subpopulation,” said first author Eric Regehr, a researcher with the UW’s Polar Science Center who started the project as a biologist in Alaska with the U.S. Fish and Wildlife Service. “Despite having about one month less time on preferred sea ice habitats to hunt compared with 25 years ago, we found that the Chukchi Sea subpopulation was doing well from 2008 to 2016.


Of the world’s 19 subpopulations of polar bears, the U.S. shares two with neighboring countries. The other U.S. subpopulation — the southern Beaufort Sea polar bears, whose territory overlaps with Canada — is showing signs of stress.

“The southern Beaufort Sea subpopulation is well-studied, and a growing body of evidence suggests it’s doing poorly due to sea-ice loss,” Regehr said.


Recent ecological observations had suggested that Chukchi Sea bears are doing well. A study led by co-author Karyn Rode, at the U.S. Geological Survey, showed the top predators have similar amounts of body fat as 25 years ago, a good indicator of their overall health.

The current study is the first assessment of the subpopulation size using modern methods. It estimates just under 3,000 animals, with generally good reproductive rates and cub survival.


For the first time, the model also considered local and traditional ecological knowledge collected by the North Slope Borough of Alaska from Native hunters and community members who have generations of experience with polar bears.

“It was important to bring our science together with the observations and expertise of people who live in polar bear country year-round and understand the animals in different ways,” Regehr said.

University of Washington

If the loss of sea ice due to global warming is stressing out the Beaufort Sea polar bears, why are they doing so well in the Chukchi Sea?

The Chukchi Sea is more ice-free than the Beaufort Sea…

Figure 3. Arctic sea ice. NASA
Figure 4. Arctic sea ice extent. NASA

The “core study area” was the between Lisburne and Seward Peninsulas…

Figure 5. “The striped area on the left shows the Chukchi Sea polar bear subpopulation’s range. Sea ice reaches south to the dotted line in winter, and retreats to the solid black line in summer. The right shows a closeup of the study area off Alaska’s coast. White circles show where polar bears were tagged between 2008 and 2016.Regehr et al./Scientific Reports” (UW)

The “core study area” has been especially hard-hit by sea ice loss…

Figure 6. June 1979
Figure 7. June 2019
Figure 8. Animation

Setting aside the fact that overall polar bear populations don’t seem to be declining, how did polar bears manage to survive the vast majority of the Holocene Epoch, when sea ice was far less extensive than today? For that matter, how did they survive the Eemian (Sangamonian) interglacial stage, when the Arctic was 5-10 °C warmer than it is today?

The Holocene Epoch

The Chukchi Sea, the place where the polar bear population appears stable and healthy was nearly ice-free for much of the Holocene.

Figure 9. “Modern sea-ice cover in the study area, expressed here as the number of months/year with >50% coverage, averages 10.6 ±1.2 months/year… Present day SST and SSS in August are 1.1 ± 2.4 8C and 28.5 ±1.3, respectively… In the Holocene record of core HLY0501-05, sea-ice cover has ranged between 5.5 and 9 months/year, summer SSS has varied between 22 and 30, and summer SST has ranged from 3 to 7.5 8C (Fig. 7). (McKay et al., 2008)

Over most of the Holocene, >50% sea ice coverage occurred from 5.5 to 9 months each year.  During the “Anthropocene”, >50% sea ice coverage has ranged from 9 to 12 months each year.

Figure 10. How did the polar bears survive with so little sea ice?

Stein et al., 2017 (H/T tty) provides a great description of a rather novel method of determining paleo sea ice extent.

In a pioneering study by Belt et al. (2007), the ability to (semi-)quantitatively reconstruct paleo-sea ice distributions has been significantly improved by a biomarker approach based on determination of a highly branched isoprenoid (HBI) with 25 carbons (C25 HBI monoene = IP25). This biomarker is only biosynthesized by specific diatoms living within the Arctic sea ice (Brown et al., 2014) and appears to be a specific, sensitive and stable proxy for Arctic sea ice in sedimentary sections representing Late Miocene to Recent times (Stein et al., 2012, 2016; Belt and Müller, 2013; Stein and Fahl, 2013; Knies et al., 2014). The presence of IP25 in the studied sediments is direct evidence for the presence of sea ice.


For more semi-quantitative estimates of present and past sea ice coverage, M€uller et al. (2011) combined the sea-ice proxy IP25 and phytoplankton biomarkers in a phytoplankton- IP25 index, the so-called ‘PIP25 index’:

PIP25 = [IP25]/([IP25] + ([phytoplankton marker] x c))

with c is the mean IP25 concentration/mean phytoplankton biomarker concentration for a specific data set or core.

Stein et al., 2017

This schematic diagram from Belt et al., 2013 relates the PIP25 index to sea ice conditions:

Figure 11. Relationship of sea ice conditions to PIP25 index (Belt ea al., 2013). Click to enlarge.

Generally speaking, the PIP25 index correlates to sea ice extent as follows:

  • >0.7 = Extended, perennial (year-round) ice cover
  • 0.5-0.7 = Seasonal ice cover/ice edge situation
  • 0.1-0.3 = Reduced ice cover
  • <0.1 = Ice-free year-round

Stein et al. 2017, constructed a cross-section of PIP25 curves across the Arctic from the Fram Strait to the Chukchi Sea.

Figure 12. Location map of sediment cores and cross-section A-A’. (modified after Stein et al., 2017)

All four core locations currently reflect seasonal ice cover/ice edge situations (PIP25 index 0.5-0.7), with the Fram Strait being an ice edge situation and the other three reflecting seasonal ice cover.

Figure 13. Cross-section A-A’. High and low refer to Northern Hemisphere insolation.

Three key takeaways:

  1. Maximum Holocene sea ice extent occurred within the past 500-1,000 years at every location.
  2. The current sea ice extent is higher at all of the locations than over 50% to 85% of the Holocene.
  3. Polar bears survived low sea ice extent over most of the Holocene.

When I plot the cross-section on Kinnard’s probability map, we can see that the entire area of low ice extent larger than present day, has been seasonal throughout most of the Holocene.

Figure 14. Probability of sea ice occurrence (1870-2003) A = maximum, B= minimum. (Kinnard et al., 2008)

A significant reduction in Arctic summer sea ice relative to today, would be returning to Early Holocene conditions. If we currently have an “Anthropocene in the Arctic,” it’s actually icier than most of the Holocene’s “Goldilocks conditions.”

When did polar bears first evolve?

No one really knows for sure.

Polar Bear Evolution Was Fast and Furious

By Elizabeth Pennisi May. 8, 2014 , 12:00 PM

For polar bears, being tubby is a way of life. Fat can make up 50% of their body weight; the blubber-laden seals they eat make bacon look downright healthy. Now, a new, extensive comparison of the genomes of polar bears and their closest relative, the brown bear, has revealed how polar bears survive such unhealthy diets.

The work also suggests that the bears evolved these changes relatively quickly, likely because they had to adapt to extreme conditions that forced them to switch to a diet that would be toxic to other mammals. “It’s a schoolbook example of evolution,” says Eske Willerslev, an evolutionary geneticist at the University of Copenhagen who helped the lead the research.

Brown bears—some of which are called grizzlies—and polar bears are closely related and are even able to interbreed. In the past few years, researchers have used genetic information to sort out this relationship and to understand how polar bears thrive in the frigid Arctic, feeding primarily on seals and other marine life captured from holes in the ice. This work has included sequencing the animals’ genomes, which has indicated that polar bears are truly a distinct species that at times lived apart from brown bears and at times intermingled and interbred with them. But researchers disagree about when the polar bear began to split off from brown bears, with estimates ranging from about 600,000 years to as much as 5 million years ago.

In the latest sequencing effort, Willerslev and researchers from Denmark, China, and the United States analyzed the genomes of 80 polar bears from Greenland and 10 brown bears from North America and Europe. “[It’s] the most comprehensive genomic data set to date, as far as bears are concerned,” says Frank Hailer, an evolutionary biologist from Goethe University Frankfurt in Germany.

Drawing on that data, Willerslev and his colleagues conclude that polar bears split off from brown bears between 343,000 and 479,000 years ago. Although little more than a blink in time from an evolutionary perspective, that was long enough for key genetic differences to evolve, they note in a report today in Cell.



Estimates have ranged from 70,000 to 5,000,000 years ago. The oldest confirmed polar bear fossil dates to 110,000 to 130,000 years ago… Meaning that polar bears survived the Eemian interglacial stage.

The peak warmth of the Eemian interglacial stage marks the boundary between the Late Pleistocene Tarantian Age and the Middle Pleistocene Ionian Age.

Figure 15. Pleistocene stratigraphic nomenclature, SQS.
Figure 16. “The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today.” North Greenland Ice Core Project members, 2004

If sea ice is so crucial to polar bear survival, how did they survive the Eemian? Well it appears that despite this:

The last time that Arctic temperatures were significantly higher than today was the Early Holocene Thermal Maximum910. The Holocene, however, is an interglacial cycle not concluded yet. This certainly justifies climatic evaluations of older, concluded warm interglacial cycles such as the last interglacial (LIG), i.e., Marine Isotope Stage (MIS) 5e (Eemian), lasting from about 130 to 115 ka and often proposed as a possible analog for our near-future climatic conditions on Earth1112. Based on proxy records from ice, terrestrial and marine archives, the LIG is characterized by an atmospheric CO2 concentration of about 290 ppm, i.e., similar to the pre-industrial (PI) value13, mean air temperatures in Northeast Siberia that were about 9 °C higher than today14, air temperatures above the Greenland NEEM ice core site of about 8 ± 4 °C above the mean of the past millennium15, North Atlantic sea-surface temperatures of about 2 °C higher than the modern (PI) temperatures1216, and a global sea level 5–9 m above the present sea level17. In the Nordic Seas, on the other hand, the Eemian might have been cooler than the Holocene due to a reduction in the northward flow of Atlantic surface water towards Fram Strait and the Arctic Ocean, indicating the complexity of the interglacial climate system and its evolution in the northern high latitudes121819.

Stein et al., 2017

Arctic sea ice didn’t totally vanish…

Figure 17. “Simulation of Arctic sea ice cover of the Last Interglacial and the pre-industrial climate. Last Interglacial (LIG) conditions were simulated for three time slices: LIG-130 (130 ka), LIG-125 (125 ka), and LIG-120 (120 ka). White circles indicate locations of the four studied sediment cores. ” Stein et al., 2017
Figure 18. Too fracking funny!


Belt S.T., Müller J.  “The Arctic sea ice biomarker IP25: A review of current understanding, recommendations for future research and applications in palaeo sea ice reconstructions”. (2013)  Quaternary Science Reviews,  79 , pp. 9-25. Belt_2013

Crockford, Susan J, and Global Warming Policy Foundation. The Polar Bear Catastrophe That Never Happened. London, The Global Warming Policy Foundation, 2019.

Fetterer, F., K. Knowles, W. N. Meier, M. Savoie, and A. K. Windnagel. 2017, updated daily. Sea Ice Index, Version 3. [Sea Ice Monthly By Year]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: https://doi.org/10.7265/N5K072F8. [Accessed October 16, 2019].

Kinnard, C., Zdanowicz,C.M., Koerner,R .,Fisher,D.A., 2008. “A changing Arctic seasonal ice zone–observations from 1870–2003 and possible oceanographic consequences”. 35, L02507. Kinnard_2008

McKay, J.L., A. de Vernal, C. Hillaire-Marcel, C. Not, L. Polyak, and D. Darby. 2008. Holocene fluctuations in Arctic sea-ice cover: dinocyst-based reconstructions for the eastern Chukchi Sea. Can. J. Earth Sci. 45: 1377–1397

Middleton, David. “Back to the Anthropocene! Arctic Sea Ice Edition.” Watts Up With That?, 17 Oct. 2019, wattsupwiththat.com/2019/10/17/back-to-the-anthropocene-arctic-sea-ice-edition/.

North Greenland Ice Core Project members. 2004. “High-resolution record of Northern Hemisphere climate extending into the last interglacial period”. Nature 431(7005):147-151.

Stein, R., Fahl, K., Gierz, P. et al. Arctic Ocean sea ice cover during the penultimate glacial and the last interglacial. Nat Commun 8, 373 (2017). https://ift.tt/3jQiAZl

Stein, R. , Fahl, K. , Schade, I. , Manerung, A. , Wassmuth, S. , Niessen, F. and Nam, S. (2017), Holocene variability in sea ice cover, primary production, and Pacific‐Water inflow and climate change in the Chukchi and East Siberian Seas (Arctic Ocean). J. Quaternary Sci., 32: 362-379. doi:10.1002/jqs.2929 stein2017

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October 11, 2020 at 08:37PM

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