May 16, 2008

Where Are All The Drowning Polar Bears?

Filed under: Adaptation, Animals, Arctic, Polar

The Interior Department just announced its decision to list the polar bear as “threatened” under the U.S Endangered Species Act (ESA). The justification behind the decision is that polar bears are highly dependent on sea ice in the Arctic for their livelihood—hunting, mating, birthing, family rearing, etc.—and thus if sea ice declines, so will the overall health of the species. While this may, in fact, be true in some sense, it also gives short-shrift to the bears adaptive abilities, which must be large, given that they survived the previous interglacial warm period as well as an extended period of warmer-than-present conditions in the Arctic (which undoubtedly were associated with reduced sea ice levels) about 5,000 to 7,000 years ago (give or take a thousand years) (see here fore example). If the bears fare worse this time around, it will mostly likely be because their natural adaptive response may run up against a human roadblock in the form of habitat disruption or other types of difficulties that an increased human presence may pose to the adapting bears. It seems that this is what the intent of the ESA is aimed at tempering, not trying to alter the climate—precisely how the Act should have be applied, despite all the criticism surrounding the decision.

All this renewed attention to polar bears has piqued our interest in just how the bears have been faring recently. Al Gore made movie stars out of drowning bears in his 2006 film An Inconvenient Truth with an animation sequence depicting a small patch of floating ice disintegrating under a struggling polar bear until it was left swimming alone in a vast expanse of open ocean. One couldn’t help to get a little teary-eyed at the notion.

And as the public just can’t get enough of cute, cuddly, slightly aggressive movie stars who are a little down on their luck, the paparazzi are never too far behind to document their each and every move. Pictures of Paris Hilton partaking in every activity imaginable abound and Britney can’t even pull out of a parking lot without running over a photographer’s foot. So where are all the pictures of drowned and drowning polar bears?

Last fall, as a massive media campaign reminded us, the extent of Arctic ice was at an all-time (since 1979) low, yet we cannot recall a single report of a drowned polar bear as a result. Surely, with all the attention on polar bear well-being that arose as the Interior Department considered its ESA decision, if there were evidence of polar bears drowning last summer, it would have been held up front and center. But it wasn’t. Because they weren’t.

So where does this now omnipresent notion come from that polar bears—famously strong swimmers—will perish in droves under the warming waves as the distance between the ice edge and the shore becomes too great to overcome? Let’s have a look-see.

The original source of the drowning polar bear story is a series of studies conducted by Charles Monnett and colleagues from the U.S. Minerals Management Service (MMS) out of Alaska which as been observing and counting polar bears on Alaska’s north shore for the past 30 years or so as part of a broader efforts to survey bowhead whale populations in the region and assess any impacts that oil and gas exploration activities may be having on them. Since the late 1970s, aerial surveys have been conducted from small airplanes flown during the late summer/early fall documenting the numbers of whales, polar bears, and other large marine mammals.

In December 2005, Monnett et al. presented a poster at the Marine Mammals Conference in San Diego (followed soon thereafter by a publication in the journal Polar Biology in early 2006) in which they documented a change in the patterns of late-summer polar bear sightings. During the first part of the record, polar bears were usually spotted on ice floes lying off the Alaskan coast, between say Barrow and Demarcation Point, near the Alaska/Canada border. During the latter part of the record, from 1992-2005, most of the bears were spotted on land as there was little ice to be found within tens to hundreds of kilometers of the coast. Alone, these observations indicated that the behavior of the polar bears was changing as the environmental conditions around them were changing. Hardly newsworthy in and of itself—polar bears adapting as best they could to climate change.

But the part of the study that garnered the press attention so much so that it has become ingrained in global warming lore was that Monnett et al. reported the sighting of four polar bear carcasses floating in the sea several kilometers from shore, presumably having drowned. All four dead bears were spotted from the plane a few days after a strong storm had struck the area, with high winds and two meter high waves. Since polar bears are strong swimmers, the authors concluded that it was not just the swimming that caused the bears to drown, but that the swimming in association with high winds and waves, which made the exertion rate much greater, sapping the bears of their energy and leading to their deaths. The authors also suggested that the frequency and intensity of late summer and early fall storms should increase (as would the wave heights) because of global warming and thus the risk to swimming bears will increase along with the number of bears swimming (since there will be less ice) and subsequently more bears will drown. But they didn’t stop there—they suggested that the increased risk will not be borne by all bears equally, but that lone females and females with cubs will be most at risk—putting even more downward pressure of future polar bear populations. And thus a global warming poster child (or cub) is born.

But does all of this follow from the data? Again, we haven’t heard of any reports of polar bear drownings in Alaska in 2005, 2006, or 2007—all years with about the same, or even less late-summer sea ice off the north coast of Alaska than in 2004, the year of the documented drownings.

In 2004, the researchers saw four, that’s right 4, polar bear carcasses floating at sea where they had never seen any in previous surveys. The 4 dead bears, coupled with 10 other bears that were observed to be swimming in open water, more than 2 km from land, led them to conclude that global warming was making the bears swim long distances and then drowning as the exertion overcame them when they got caught in a storm.

But is this really true? This NASA web site shows the minimum extent of Arctic sea ice each summer since 1979. As you scroll down through the list of years, notice that in many if not most late summers, the edge of the sea ice is quite a ways from the north coast of Alaska. So, the sea ice conditions along the northern coast of Alaska were hardly that unusual during September 2004. No more so than they were in the years since or in many prior. So bears weren’t encountering unusual ice conditions in 2004. In fact, in the period 1992-2004, more than 50% of bear sightings were in regions of no ice (Monnett et al., 2005). Why an elevated number of bears were observed swimming in open water in 2004 is unclear, but it could be from any number of reasons, sampling effort, bear population dynamics, bear food dynamics, to name a few—but an unusual expanse of open water doesn’t seem to be one of them.

What was potentially unusual was a big storm that caught them off guard. But even that seems unlikely. True it was windy for a several day stretch in mid-September 2004, but such a windy stretch is not particularly unusual there during that time of year.

What all of this means is that the number of drowning polar bears is not very significant in terms of the overall population of bears, which number in the low thousands in Alaska. In fact, polar bears drowning seems to be quite rare and unusual events, perhaps brought about by a confluence of ice free ocean waters and an especially strong storm. However, as summer ice conditions off the north Alaskan coast couldn’t get much worse than they were in 2007, when there was hardly at all, and since there has been no evidence yet presented that a large number (if any) bears drowned as a result, it would seem that death by drowning is not putting any meaningful downward pressure on the population of Alaskan polar bears.

But, truth be told, we have been withholding a piece of information this whole time—there were reports of drowning polar bears in 2007, and they were directly attributable to human activities. But they didn’t drown because of global warming, instead, they drowned because they had first been shot with tranquilizer darts and then slipped into the sea and were unable to be recovered.

This goes to show what we have been proclaiming all along—the real reason polar bears may suffer under climate warming is their increased encounters with humans as the bears change their adaptive behavior.

And this is where the application of the ESA to polar bears could prove most effective.

References:

Monnett, C., Gleason, J. S., and L. M. Rotterman, 2005. Potential effects of diminished sea ice on open-water swimming, mortality, and distribution of polar bears during fall in the Alaskan Beaufort Sea. 16th Biennial Conference on the Biology of Marine Mammals, 12-16 December 2005, San Diego, CA.

Monnett, C., and J. S. Gleason, 2006. Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea. Polar Biology, 29, 681-687.




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