The Arctic is changing. As temperatures slowly rise and the ice recedes, the shore is getting further away. Food sources that the polar bear have come to rely on are becoming less plentiful. In this changing climate, the polar bear is already showing signs of distress.

While polar bears struggle, opportunistic grizzlies fare well, as they benefit from the melting Arctic’s effects on its ecosystem. While the grizzly moves into newly opened territories, the polar bear can only wait for the freezing of its one-time kingdom. If the changing world proclaims the grizzly the new king of the Arctic, what will become of the polar bear?

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Online content for Arctic Bears was originally posted February 2008.

There’s no doubt the Arctic is warming. In fact, this extreme region has warmed faster than any other on earth, with the Arctic temperature increasing three to five times faster than the Earth as a whole over the past 100 years. Climate models predict that the Arctic will become an additional 7 to 12 degrees Fahrenheit warmer during the next century.

Experts predict these rising temperatures are likely to cause the melting of at least half the Arctic sea ice by the end of the century. Melting ice is expected to lead to even higher Arctic temperatures as bright white ice plays a significant role in reflecting the sun’s radiation. As ice melts, more of the dark ocean and land are exposed to absorb the radiation, thus further warming the climate. Research has found that over a major portion of the ice-covered Arctic Ocean, sea ice is 40 percent thinner than it used to be. And some climate models predict that by 2070, there may be no summer ice cover in the Arctic at all.

The disappearance of sea ice is a particularly dire threat to the polar bear, a super specialist in the Arctic environment. Polar bears rely on the ice to hunt seals, their main food source, and also to rest between hunts out on the ice. Trapped, drowning and starving bears have become such a concern that in September 2007, the U.S. Geological Survey (USGS) released a series of studies that led its scientists to conclude that “future reduction of sea ice in the Arctic could result in a loss of two-thirds of the world’s polar bear population within 50 years.” As sad as such a fate for this majestic creature would be, the consequences would extend far beyond the bear. A polar bear decline could trigger what biologists call a “trophic cascade,” or a complete uncoupling of the Arctic food chain.

As much attention as the effects of melting sea ice on polar bears has drawn of late, it is important to understand that the thaw will have immediate effects on everything in the marine food chain, from benthic invertebrates to marine mammals. What will be the fate of the ringed seal, bearded seal, bowhead, beluga, and walrus — all creatures that depend on the ice for habitat or food?

Warming not only affects sea ice, it alters the Arctic’s terrestrial landscape. Melted permafrost means a proliferation of low-lying shrubs. These northern-spreading shrubs establish a new order of plant life, shading out low-growing lichens and plants like ground willow and cotton grass, all favorite foraging items for grazers like caribou. Without their main summer and winter foods, caribou are vulnerable to starvation.

Another species affected by changing Arctic vegetation is the lemming. Drastic changes in its food supply of sedges and mosses, along with a lack of snow tunnels to burrow in, will challenge the survival of these rodents and may cause their populations to crash beyond repair. When lemming numbers drop, so do the numbers of predators like snowy owls and Arctic foxes. Researchers have already observed warning signs in the Arctic fox population in the form of an invasion of red foxes into the traditional range of Arctic foxes.

With a warming Arctic comes an earlier spring and a proliferation of parasitic insects such as flies and mosquitoes. For Arctic dwellers as disparate as caribou and guillemots, a population explosion of insects is at best a nuisance, and at worst, life-threatening. Grazers spend less feeding time and more energy just trying to escape these pests. Arctic-nesting Brunnich’s guillemots in Canada have been such victims of relentless mosquito attack, they have been observed abandoning their nests.

A changed season for insects also has a different effect on some other species of birds, which time their migrations to coincide with insect swells. Northern Alaska dunlins, for example, migrate from Asia and lay their eggs to take advantage of peak insect populations in order to feed their young. Warmer temperatures may cause the insects to hatch earlier, throwing off the carefully timed breeding and nesting season of the dunlins.

With the Arctic experiencing the most rapid and severe climate change on Earth, the plants and animals that have evolved to survive in this extreme habitat come increasingly under threat. Like the canary in the coal mine, the Arctic can serve as our early warning sign of impending climate change. Observing the tumultuous change its inhabitants are experiencing can be a lesson to us about the changes in store for the rest of the world.

Two decades ago, ecologist Andrew Trites of the University of British Columbia in Vancouver started wondering why populations of northern fur seals, a once economically important marine mammal that lives in the North Pacific, were declining. That work eventually got him involved in trying to solve another major ecological mystery: Why have some populations of Steller’s sea lions declined so dramatically off the coast of Alaska over the last few decades?

Some researchers blame giant trawlers, which scoop up tons of fish, and have supported government moves to ban or limit catches in order to keep food supplies available to sea lions. Others believe that uncontrolled whaling decades ago sparked an ecological domino effect that is just now affecting the sea lions. Still others, including Trites, say many factors may be to blame including climate change, predation by killer whales, and a natural ecosystem shift that altered the nutritional quality of prey consumed by sea lions.

NATURE recently spoke with Trites, whose work is featured in NATURE’s A Mystery in Alaska, about the complexities of Alaska’s offshore ecosystems. For instance, he notes that while sea lion populations in Southeastern Alaska tend be stable or increasing, populations elsewhere have shrunk with few signs of recovery. “You can’t assume the factors that are influencing one region are the same somewhere else,” he says.

Where did you start in your own thinking about this problem?

I originally thought, as did most people, that there was probably a pretty simple cause-and-effect relationship between fisheries and sea lions. An open-and-shut case: There was a shortage of pollock because of fishing, and sea lions could not find enough to eat. But when I looked at the numbers, I couldn’t find a relationship between fish catches and sea lion populations. That led me to dig deeper.

I and other researchers started comparing different populations of sea lions (some of which are declining, some of which are stable and increasing). We wanted to know what’s different about what they eat and how long it takes them to find food. We’ve also looked at the possible effects of climate change and predation. Eventually, we concluded that no single factor can explain why the numbers have gone down, but there are some underlying factors that tie things together. For instance, the composition of the fish community has changed since the 1960s and ’70s, and is now dominated by pollock and flatfish. These species were there 25 to 50 years ago, but were not as dominant. There were more crabs and fish such as herring and sandlance. So there has been a major shift in the types of fish now available to sea lions.

Ecologist Andrew Trites believes many factors are at play in the sea lion mystery.

And you’ve found that some fish are better food for sea lions than others, right?

When we’ve done feeding experiments with trained sea lions, we’ve found that while pollock is now a major part of the sea lion’s diet, it may not be the healthiest thing for them to eat. The fattier fish such as herring that they used to eat more of, had a lot more calories than pollock. So a sea lion has to eat a lot more pollock to get enough energy to do well.

Now, an adult probably can do perfectly well on a pollock diet. But we suspect a young sea lion that needs more energy to grow doesn’t have the stomach capacity to hold all the pollock it needs to meet its daily requirements. So, while sea lions may not be starving to death, some appear to be nutritionally compromised. And that could make them more susceptible to disease, stunt their growth, or make them more vulnerable to killer whales.

Poor nutrition works in very subtle ways. It removes one animal here and there, but you don’t see massive die-offs. That’s what’s been so perplexing about getting a quick answer to why sea lions have declined.

So does that mean areas closed to pollock fishing should be reopened?

Right now, fishing is closed around the [sea lion] rookeries because there was a fear that fishing was behind the sea lion decline. But there was never any data and still isn’t any evidence that links the two. We therefore need to go the next step and re-open fishing in a few areas, but not in others, and then compare what happens. We need to think about this as a carefully controlled experiment and an opportunity to learn once and for all whether fishing has an effect on Steller’s sea lions. Just because sea lions and fisheries target the same species, doesn’t necessarily mean that they compete with each other. It is like two people who breathe the same air in a room. They do not compete with each other unless the room is sealed and their supply of oxygen is limited.

What about the role of killer whales?

From the ecological modeling we’ve done, it appears unlikely that killer whales caused the decline. But sea lion numbers are so low now that predation by killer whales could be preventing them from recovering. The sea lions may be caught in a “predator-pit.”

What have you learned from trying to tackle this mystery?

That things are not always what they first appear to be. That it takes very careful sleuthing to follow a line of evidence and not jump to conclusions without having the facts to support them. Many people jumped to the conclusion that people must be behind the sea lion decline. But research is showing that a suite of changes occurred in the North Pacific over the past 50 years including a shift in ocean climate, a flip in the dominance of key species in the ecosystem, and a corresponding change in sea lion diets and the nutritional value of their prey.

Also, you have to think about your value system when passing judgment over ecological changes. There is a lot of rhetoric about how unhealthy the Bering Sea is because some species that people value have declined. But it appears that the Bering Sea and Gulf of Alaska can exist in more than one state, and that you probably can’t have all of the species there all of the time in high numbers. If your favorite species in the world were pollock or flatfish, you’d be ecstatic right now. Our perceptions of ecosystem health tend to be based on which species we most value, which are a very small subset of the whole ecosystem.

For the layperson, trying to understand the essence of climate change research can be exasperating — particularly when it comes to the potential impact of global warming on the Antarctic. On the one hand it is reported that the Antarctic Peninsula, which extends north toward Chile, is warming; that a massive ice shelf the size of Delaware broke off from the Larsen B peninsula in 2002, a collapse that scientists say was of a scale unprecedented in the last 12,000 years; and that significant Antarctic ice melting could result in a perhaps disastrous rise in sea levels. On the other hand research indicates that, over the last thirty-five years, the Antarctic as a whole has cooled, and that the levels of sea ice are actually increasing.

Such ostensible contradictions have been used to suggest that global warming is, in fact, not happening. Yet part of the confusion is due to the lack of a broader context that can only come from more research. According to the results of a 2004 study by NASA’s Goddard Institute of Space and Studies (GISS), the recent cooling trend is an effect of the depletion in the ozone layer over Antarctica. “In the coming decades, ozone levels are expected to recover due to international treaties that banned ozone-depleting chemicals,” states a NASA press release. Once it does, Antarctica’s cooling trend is likely to rapidly reverse. The increase in sea ice levels, meanwhile, is also the result of global warming. A 2005 satellite study funded by NASA suggests that warming of the climate has led to an increase in precipitation rates and deeper snow in the Southern Ocean, which becomes so heavy that it pushes Antarctic ice below sea level, resulting in more and thicker ice.

There is still a lot of uncertainty about what could happen to Antarctica, and this sense of doubt — about the climate in general — has been used by politicians on both the left and the right to bolster their respective positions. President George W. Bush, for instance, has declined to cut carbon dioxide emissions from power plants, “given the incomplete state of scientific knowledge of the causes of, and solutions to, global climate change.” And in 2006, former Vice President Al Gore returned to the public eye when An Inconvenient Truth, a successful documentary film based on Gore’s lecture series about climate change, raised concerns that an immediate response to global warming was required to prevent catastrophe.

Perhaps the best a concerned nature-lover can do is turn to an outside body. According to a 2001 report by the Intergovernmental Panel on Climate Change (IPCC), an assessment panel established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), a few things, at least, seem clear:

• The Earth’s climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities.
• Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.
• More hot days and heat waves and fewer cold and frost days are very likely over nearly all land areas.
• The projected rate and magnitude of warming and sea-level rise can be lessened by reducing greenhouse gas emissions.