The age of polar bears

In a paper recently published in PNAS entitled “Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear” Charlotte Lindqvist and coauthors performed genomic sequencing and analysis on the oldest polar bear fossil discovered to date, finding that it was surprisingly genetically and temporally proximate to the brown and polar bear’s last common ancestor:

Intriguingly… this ancient polar bear, which exhibits a very short branch length, lies almost directly at the branching point between polar bear and the genetically unique clade of ABC [from the Admiralty, Baranof, and Chichagof islands -TB] brown bears (Fig. 3B). Thus, both cladistically and anagenetically, this ancient specimen existed very close to the most recent common ancestor of polar bears and brown bears.

Phylogenetic network of complete mt genomes (excluding the VNTR repeat) of 11 polar and brown bears based on Neighbor-Net analysis with LogDet distances (see scale bar).

Along with stratigraphic dating of the fossil to ~110-130 kya (thousand years ago), their work enabled them to peg the “birth” of polar bears at ~150 kya, meaning that they survived the Eemian (AKA Riss-Würm, Ipswichian) interglacial:

The discovery of this jawbone confirms that the polar bear was already a distinct species at least 110 kya…

…we estimated the mean age of the split between the ABC bears and the polar bears to be 152 ky, and the mean age for all polar bears as 134 ky, near the end of the Eemian interglacial period and completely in line with the stratigraphically determined age of [this fossil].

This finding, which should have been celebrated as a coup for our knowledge of polar bear evolution, has instead become yet another target of global warming denialism.

It has been touted by those who refute the reality of and/or problems posed by man-made (anthropogenic) warming of the climate as evidence that anthropogenic warming poses no threat to polar bears, as they had survived previous episodes of warming before. And some of these claims actually go back to the jawbone’s initial discovery, years earlier.

A similar claim has been made that anthropogenic warming poses no threat to polar bears because their numbers are actually increasing.

Let’s take these in reverse order. UCN/SSC Polar Bear Specialist Group Chair, Dr. Andrew Derocher:

The various presentations of biased reporting ignore, or are ignorant of, the different reasons for changes in populations. If I thought that there were more bears now than 50 years ago and a reasonable basis to assume this would not change, then no worries. This is not the case.

The bottom line here is that it is an apples and oranges issue. The early estimates of polar bear abundance are a guess. There is no data at all for the 1950-60s. Nothing but guesses. We are sure the populations were being negatively affected by excess harvest (e.g., aircraft hunting, ship hunting,self-killing guns, traps, and no harvest limits). The harvest levels were huge and growing. The resulting low numbers of bears were due only to excess harvest but, again, it was simply a guess as to the number of bears…

Comparing declines caused by harvest followed by recovery from harvest controls to declines from loss of habitat and climate warming are apples and oranges. Ignorant people write ignorant things.

Derocher is saying that such claims are at heart a combination of two forms of fallacious reasoning, the cherry pick (polar bear populations are increasing during a certain interval) and the non sequitur (therefore they are not threatened by anthropogenic warming). The claim that polar bear populations are rising only holds true for a subset of populations, and only does so then because the period is selected to begin at a low point brought about by unregulated hunting. Once hunting restrictions were put in place, populations began returning to their historic norms. This dynamic can be seen in recent assessment reports of the two Alaskan polar bear stocks-

US Fish and Wildlife Service’s January 2010 Southern Beaufort Sea (SBS) Polar Bear Stock Assessment Report:

Prior to the 20th century, when Alaska’s polar bears were hunted primarily by Natives, both the Chukchi/Bering seas and Southern Beaufort Sea stocks probably existed near carrying capacity (K). Once harvest by non-Natives became common in the Southern Beaufort Sea in the early 1960s, the size of these stocks declined substantially (Amstrup et al. 1986, Amstrup 1995). Since passage of the Marine Mammal Protection Act (MMPA) in 1972, both Alaska polar bear stocks seem to have increased…

The January 2010 Chukchi/Bering Seas (CBS) Polar Bear Stock Assessment Report similarly states:

Prior to the 20th century, when Alaska’s polar bears were hunted primarily by Alaskan Natives, both stocks probably existed at near carrying capacity (K). The size of the Beaufort Sea stock declined substantially in the late 1960’s and early 1970’s (Amstrup et al. 1986) due to excessive sport harvest. Similar declines could have occurred in the Chukchi Sea, although there are no population data to support this assumption. Since passage of the MMPA, the southern Beaufort Sea population grew during the late 1970’s and 1980’s and then stabilized during the 1990’s (Amstrup et al. 2001b).

So in these stocks, polar bear numbers are up from a low brought about by unregulated, non-Native hunting.  But what are the current population trends? SBS Stock Assessment Report [following emphases mine]:

The Southern Beaufort Sea stock experienced little or no growth during the 1990’s (Amstrup et al. 2001b). Declining survival, recruitment, and body size (Regehr et al. 2006, Regehr et al. 2007), and low growth rates (λ) during years of reduced sea ice during the summer and fall (2004 and 2005), and an overall declining growth rate of 3% per year from 2001-2005 (Hunter et al. 2007) indicates that the Southern Beaufort Sea population is now declining.

CBS Polar Bear Stock Assessment Report:

Based on demographic data 2001 to 2006, the overall population growth rate in the Southern Beaufort Sea population declined approximately 0.3% per year (Hunter et al. 2007). Until 1992 it is likely that the Chukchi/Bering seas stock mimicked the growth pattern and later stability of Southern Beaufort Sea stock, since both stocks experienced similar management and harvest histories. However, since 1992 the CBS population has faced different stressors than the SBS population. These include increased harvest in Russia (150 – 250 bears/yr) (Kochnev 2006, Ovsyanikov 2006, Eduard Zdor personal communication) and greater loss of summer sea ice habitat from global warming (Overland and Wang 2007), which suggest that using the growth rate for the Southern Beaufort Sea may not be applicable. The status of the Chukchi/Bering seas stock was listed as data deficient (Aars et al. 2006) due to the lack of abundance estimates with measurable levels of precision. The population is believed to be declining and the status relative to historical levels is believed to be reduced based on harvest levels that were demonstrated to be unsustainable in the past.

Moreover, the long term concern about polar bear survival is not based solely on whatever the current population trend happens to be (whether one of increase or decrease), nor only on the effects of the anthropogenic warming that they have already experienced. The concern is based upon not only existing stressors but also the additional threat posed by future anthropogenic warming on top of those already in play.

A similar case of fallacious reasoning is at work in claims that polar bears surviving the Eemian interglacial means that they aren’t threatened by anthropogenic warming- i.e., “If polar bears survived the warming of the previous interglacial, they aren’t threatened by anthropogenic warming.”

How does the Eemian compare in terms of climate? At its warmest, modeling and paleoclimatic evidence (e.g. here) suggest that the Eemian was as much as 1-3°C warmer in the Northern Hemisphere vs. the preindustrial Holocene climate, with even higher seasonal values occurring at high latitudes- perhaps 4-5°C warmer in the Arctic summer, with a concomitant substantial reduction in seasonal sea ice (e.g. here).

So, the thinking apparently goes, if polar bears survived that, a little man-made global warming should be a walk in the park, right?

Not so much. As in the Eemian, expected anthropogenic warming will not be globally uniform, and polar amplification will ensure that the Arctic warms up more and more quickly than the global average. The mid-range of the fossil fuel intensive end (A1FI) of the AR4 projections for the global average by the end of this century is ~4°C, which translates to July Arctic temperatures ranging in some areas more than 5°C higher than their modern (2000 CE) average values. Seasonal sea ice reductions equivalent to or greater than Eemian maximums are expected by the end of this century, and perhaps in as little as 30-50 years (e.g. here, here, here).

Whereas Eemian polar bears had thousands-to-tens of thousands of years to adapt to changes in their environment, today’s polar bears are facing equivalent or greater changes on time scales at least an order of magnitude more rapidly, on top of their reduced population levels and other modern stressors like pollutants.

As in many aspects of the physical world, it’s not simply the amount of something, it’s also the rate and other variables that make a difference. I drink about 2 liters of water a day. If I were stupid enough to increase my rate of water intake by an order of magnitude, I’d be risking death. Expecting polar bears to shrug off unchecked global warming because they survived the Eemian is no less foolish- and may have an equally lethal result.

Ipswichian

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2 responses to “The age of polar bears

  1. Though the polar bear lineage split from the ancestral line 130-150 kya that does not mean they were polar bears then as we know them now.
    Did the bears survive the Eemian, or did they adapt to fill the new niche created by the expanding ice fields that signalled the end of the interglacial period?

    • Indeed, that’s a great question, Chris. I was taking the “best case” scenario in favor of the denialist position to make my argument, but there’s no reason to suspect that this was the case. I believe I’ve read that there is evidence that modern polar bears didn’t assume their present form (in terms of dentition, for example) until sometime during the Last Glacial Maximum, but I’d have to go back and check.

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