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How does global warming affect polar bears?

Posted on 30 September 2010 by Anne-Marie Blackburn

Polar bears are found in the Arctic circle and surrounding land masses. There are 19 recognised subpopulations, and estimates place their numbers at about 20,000 to 25,000. Polar bears are classed as vulnerable by the World Conservation Union (IUCN) and listed as a threatened species under the US Endangered Species Act. Yet some claim that polar bear numbers have increased since the 1950s and are now stable. So what is the situation for this species?

First of all, a few points need to be made about polar bear numbers:

  • Nobody really knows how many bears there were in the 1950s and 1960s. Estimates then were based on anecdotal evidence provided by hunters or explorers and not by scientific surveys.
  • Polar bears are affected by several factors, including hunting, pollution and oil extraction. Most notably, hunting, particularly following the introduction of snowmobiles, airplanes and ice breakers, led to a huge decline in certain subpopulations. The introduction of the International Agreement on the Conservation of Polar Bears in 1973, which restricted or even banned hunting in some circumstances, consequently resulted in an increase in polar bear numbers.
  • Not all subpopulations are affected to the same degree by climate change, and while some subpopulations are well studied, for others there is insufficient data to make broad statements about current and past numbers.

With this caveat in mind, what do the figures actually say? According to a 2009 report by the IUCN Polar Bear Specialist Group, of the 19 recognised subpopulations of polar bears, 8 are in decline, 1 is increasing, 3 are stable and 7 don’t have enough data to draw any conclusions. Figure 1 below compares the data for 2005 and 2009.


Figure 1: Subpopulation status of polar bears for 2005 and 2009 (Source: Polar Bear Specialist Group)

Both habitat degradation and over-harvesting are responsible for the decline in some subpopulations. To understand why the IUCN and US Endangered Species Act consider polar bears to be at risk, it is important to look at how rising temperatures are likely to affect their habitat in the future. Polar bears are highly specialised mammals which rely heavily on sea ice for food and other aspects of their life cycle. Satellite data show that Arctic sea ice has been decreasing for the past 30 years, and projections show that this trend will continue as temperatures carry on rising. The changes in sea ice affect polar bears in several ways:

  • The early retreat of summer sea ice means that bears have less time to hunt and therefore less time to build up fat reserves.
  • The fragmentation and reduction in sea ice has several impacts. It forces the bears to swim longer distances, using up some of their fat reserves. It also reduces the number of seals, which are the bears’ main source of food, and impedes travelling and den making. And it also forces the bears to spend more time on land, with increased interactions with humans potentially leading to higher mortality.

To get an idea of the potential impacts of future climate change on polar bears, we can look at subpopulations found at the bears’ southern range, where habitat changes have been most noticeable so far. A good example is the western Hudson Bay subpopulation, which is one of the best studied. Here, ice floe break-up is taking place earlier than 30 years ago, effectively reducing the feeding period by about three weeks. As a result, the average weight of female polar bears has dropped by about 21% between 1980 and 2004, and the population declined by 22% between 1987 and 2004. In Alaska, there is evidence of increased cub mortality caused by a decline in sea ice.

In conclusion, the reason polar bears have been classed as threatened comes from the impacts of future climate change on the bears’ habitat. Current analysis of subpopulations where data is sufficient clearly shows that those subpopulations are mainly in decline. Further habitat degradation will increase the threats to polar bears.

This post is the Basic Version (written by Anne-Marie Blackburn) of the skeptic argument "Polar bear numbers are increasing". 

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Comments 1 to 7:

  1. The headlines one reads these days... Polar Bears Unlikely to Survive in Warmer World, Biologists Say. (Source study available here) In a hundred years the headlines will say: The Last Polar Bear Died Today In The San Diego Zoo; Cause Of Death = Air-Conditioning Failure Story Subheading: Now They Exist Only In Our Memories The Yooper
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  2. This is so sad that the polar bears are badly affecting by this climate change. One of the reason of melting these cold region is human technology advancement. Due to which there are many natural calamities are taking place worldwide. Polar bears and global warming
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  3. Continuing from here. Someone asked for research: Durner et al 2011 Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat Amstrup et al 2010 Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence Polar Bear Abundance and vital rates Survival of juvenile, sub adult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than at the beginning of the study in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival. Mortality During 24 years of research on polar bears in the southern Beaufort Sea region of northern Alaska and 34 years in northwestern Canada, we have not seen other incidents of polar bears stalking, killing, and eating other polar bears. We hypothesize that nutritional stresses related to the longer ice-free seasons that have occurred in the Beaufort Sea in recent years may have led to the cannibalism incidents we observed in 2004. And what of the poor walrus, who seems to get no respect? Walrus Tracking and Sea Ice Decline in the Chukchi Sea Let the skeptic arguments come forth! Correlation isn't causation; no specific cause of death attribution; not a long enough time sample; swimming is great exercise; they would have died anyway ...
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  4. 85 CB Dunkerson. Think about your logic here. If Polar bear are endangered, why is it OK to shoot so many per year? If you think shooting 700 bears per year in just one country is OK, it rather undermines your concern for the welfare of an animal you claim is endangered, especially when you say hunting is nowhere near as much of a problem as climate change when you freely admit there is no reliable data to support such a claim at present. I’m afraid it’s opinions like this which give climate science a bad name. Your views on climate change appear to be based on good science,I am puzzled as to why you are so blinkered on this issue. Bears may well be threatened in the longer term by the loss of Arctic ice, but the reality is that they are threatened here and now by shooting. Or is it OK to reduce the population because they are going to suffer under climate change regardless? I must confess, your logic on this one is very strange. . If a species is endangered it’s OK to shoot them and reduce the population by many thousands, as long as we understand why they are endangered in the long run ? The only way out of such an impasse is to say, well they are not endangered at present, so it’s OK to shoot them. But does that not then impact on your original idea of bears starving and drowning with a fall in population due to climate change? You cannot have it both ways.Or are you saying the effects of climate change are bad, but lets shoot them anyway? Mullers claim about not a single bear having died from the effects of climate change is odd. How can he know that? We don’t know how every bear dies, so we don’t know whether that is correct or not. We just know that there is no direct evidence at present. It is a recurring theme that just because we know that there are long term negative effects from climate change, we tend to overlook the here and now dangers impacting our environment. ps, Neither of your links “disprove” anything, or make statements “blatantly false” What they do is draw strong correlations between a reduction in some populations and melting ice, which I think you will agree, is different. By the way, the links also contradict some of the points in your own posts. Apologies for the length of the post
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  5. As CB stated on a separate thread, the polar bear population increased from ~5000 in 1964 to ~25,000 today, largely due to hunting restriction. The decrease prior to 1964 was due to unrestricted hunting. This has clearly been a much more significant influence on polar bear populations than the recent warming, especially since polar bear populations are currently stable (within measurement uncertainty). Unfortunately for CB, his claim that just because there is no evidence that climate change has had a negative impact means that climate change has not impacted polar bears. The problem is that human interactions have swamped any potential climatic effect. Neither Muller's claim of no polar bears dying due to global warming, nor other claims of polar bears dying due to global warming can be verified at present. That does not immediately falsify either of those arguments, as some have contended on this and other threads. Even the studies to which muon linked show no statistical difference in polar bear numbers in their studies. The only correlation was the decreased survival of sub-adult bears, although increased human contact could not be ruled out as a cause. In short, we do not know if the recent warming has affected the polar bears in a positive or negative way. Contact with man is known to have negative consequences.
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  6. Etr#5: "we do not know if the recent warming has affected the polar bears in a positive or negative way. " No one has proposed any ways that warming benefits polar bears (wait until oil exploration is big time in the Arctic; that'll sure be good for 'em). The question is no effect or a growing negative one. It's looking like we'll be finding out very soon. Now you have to choose: Do nothing or do something. Choose wisely.
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  7. garethman, see my response to your comments in the other thread. You seem to have a rather inaccurate picture of the amount of hunting going on, what hunting levels polar bear scientists have estimated as sustainable, and the relative importance (i.e. very little) hunting has compared to sea ice loss. "Mullers claim about not a single bear having died from the effects of climate change is odd. How can he know that?" Which was precisely my point. He can't. But beyond his lack of any basis for making the statement, logic also says it must be wrong. Polar bears that drowned in areas which were previously always covered by ice died due to climate change. The condition which led to their deaths (i.e. large areas of open water) did not exist before. Ergo, the thing which caused that condition caused their deaths. Otherwise, his argument comes down to the equivalent of, 'not a single polar bear has been killed by hunting' - you just have to ignore that the reason their heart, brain, or other vital organ stopped working (the DIRECT cause of death) was the bullet they were shot with (the INdirect cause of death) the same way you ignore the sea ice retreat. EtR wrote: "Unfortunately for CB, his claim that just because there is no evidence that climate change has had a negative impact means that climate change has not impacted polar bears." I can't make out what you are trying to say here... or even what you think I said. However, you go on to claim that polar bears dying due to global warming cannot be verified. THAT is not true. Just above I presented the very simple logic that polar bears cannot drown if they are not in the water... which also proves (global warming -> less sea ice -> more polar bear hours spent swimming -> more polar bears swimming when storms come up or beyond the limits of exhaustion -> more drowned polar bears) that global warming is killing bears. You have also been pointed at studies which extend this inescapable logic into the realm of observed reality.
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