On thin ice: drowning polar bears? Stranded walrus calves? A Science World editor travels north to report on the effects of global warming.

Ring. Ring. I'm awakened by the tinny sound of a phone. Disoriented, I look around in an attempt to figure out where I am. A faint light shines through a nearby porthole, gently illuminating my surroundings--metal bunk beds that are crammed inside a small room. At that moment, it all comes back to me: I am on the U.S. Coast Guard icebreaker Healy.

I've been aboard the cutter for a couple of weeks now, plowing through the icy waters of the northern Bering Sea between Alaska and Russia. I'm here with a team of scientists who are studying the ecological changes taking place in the region. The scientists say these changes are mainly due to global warming, or an average increase in Earth's temperature (see Nuts & Bolts, p. 15).

Ring. Ring.

The caller is persistent. I look at my watch. It's three a.m. Who would be calling me so early? There was only one way to find out: "Hello?" "Come up to the ship's bridge," replies Ruth Cooper, a middle school student from St. John Neumann Catholic School in Knoxville, Tennessee. Ruth is onboard the Coast Guard cutter with her parents--scientists Lee Cooper and Jackie Grebmeier of the University of Tennessee. "We've just spotted a polar bear!" Ruth says.

ON THE LOOKOUT

Hoping to catch a glimpse of the bear before it disappears from view, I hang up the phone and sprint up four flights of steep, narrow steps to the ship's bridge. The Healy's crew members navigate from this room perched high on top of the ship, so I have a clear view of the ice that's ahead. I grab a nearby pair of binoculars and join the dozen or so onlookers who are scanning the thick ice in search of the bear.

Ursus maritimus, the "bear of the sea," is well-adapted to life in this icy environment. Its off-white fur blends with the jumbled chunks of ice that surround the ship, making it impossible for me to locate it. After a half hour of scouring the horizon, I realize that it's hopeless. The carnivore, or meat eater, is probably long gone by now, prowling the ice in search of ringed seals--its primary prey--to eat.

Polar bears like this one use sea ice as a platform from which to hunt. The bear will search for breathing holes in the ice made by seals, and wait patiently by a hole until a seal pops up to breathe.

Then, BAM! The polar bear uses its enormous paw to deliver a solid blow to the seal. The bear's dinner is ready.

POLAR PROBLEMS

Polar bears are massive predators: A male bear can grow to be a hefty 770 kilograms (1,700 pounds). Only thick ice can support that much weight. From my vantage point on the Healy's bridge, the ice where the polar bear was first spotted looks solid and expansive. But according to scientists onboard the Healy, sea ice is melting due to global warming (see SW 10/9/06).

In recent history, one of the biggest sea-ice retreats in summer occurred in 2004. That year, a pulse of unusually warm Bering Sea water traveled up along the Alaska coast, causing the seasonal ice north of Alaska to melt. The area's polar bears felt the heat: Scientists came across four polar bears that had drowned, apparently exhausted from swimming because there was no thick sea ice on which they could rest.

Reduced sea ice threatens more than just polar bears. Any animal that depends on the floating platforms for survival is at risk, says Cooper. He goes on to tell me that when he was on the Healy in 2004--the same year and in the general location where the polar bears were found drowned--he and others on the ship spotted baby walruses swimming by themselves in deep waters.

WALRUS WOES

Normally, for the first one to three months of a walrus calf's life, the calf and its mother are like glue, sticking together in waters that are just a couple of hundred meters deep. After that initial period, the walrus room separates from her calf only to dive to the seafloor to search for clams and other foods. This nourishment gives her the energy she needs to produce milk and nurse her baby for the first two years of its life. For these foraging trips, the room is careful to leave her baby on a chunk of ice where it can rest--protected by other walruses in the herd.

But Cooper says he saw a total of nine baby walruses [each alone] in water as deep as 3,000 meters (1.8 miles). Their mothers were nowhere in sight--and the sea ice on which the calves had been resting was gone. The calves were doomed to die, says Cooper.

How to explain the stranded walrus calves? Violent storms could have caused the moms and babies to become separated. Or perhaps the mother walruses died, orphaning the calves. But another suspect is global warming. Some scientists speculate that the unusually warm Bering Sea water current that occurred in the summer of 2004 may have been to blame. This warm water may have caused the ice on which the calves were resting to recede north toward the much-deeper Arctic Ocean. "If sea ice [were to] recede too far north, then the [adult] walruses could not feed," says Carleton Ray, a walrus expert from the University of Virginia who is, also on the Healy. Walruses do not dive in deep water. They can't dive much deeper than approximately 100 m (328 ft)." Cooper speculates that as the walrus mothers traveled to shallower waters to feed, their calves drifted north with the receding ice. Gradually, the growing distance between moms and calves may have permanently separated them from each other.

UNCERTAIN FUTURE

Although Cooper saw nine stranded walrus calves, odds are likely that there were more that he did not observe. Still, the events in 2004 did not threaten the overall walrus population, which is estimated to be between 150,000 and 250,000.

But global warming is a constant and growing problem. In fact, in the summers of 2002, 2005, and 2006, sea ice retreated at a similar rate as in 2004. So Cooper and other scientists fear that the continued loss of sea ice due to a warming world could make walrus strandings more common.

All of the walruses that I see from the Healy's bridge are safely perched on thick slabs of ice--mothers and calves together. But Cooper warns that if global warming doesn't stop, tough times are ahead for the animals of the Bering Sea and other polar regions. "Whether they can adapt or not is uncertain," says Cooper. "I feel optimistic that there's still time to [reverse global warming], but the clock is definitely ticking."

MELTING AWAY

Much of the southeastern Bering Sea used to freeze in winter. That is no longer the case. In the 1970s, the average extent of southeastern sea ice in late April receded to near St. George Island (see map). Three decades later, the average extent of southeastern sea ice was just south of Nunivak Island. Last spring, although sea ice extended farther south than in recent years, it also melted away quickly.

Cool tips

Everyone can fight global warming! Here's how you can limit the burning of fossil fuels.

1. For short trips, leave the car at home--bike or walk instead.

2. It takes energy to produce paper: reduce and recycle.

3. If you have a dishwasher, save energy by running it only when it is full.

EARTH: Global Warming

On Thin Ice

PRE-READING PROMPTS

Jump-start your lesson with these pre-reading questions:

* Polar bears are excellent swimmers. They can swim up to 40 miles across the sea without resting. These giant mammals often climb onto ice platforms in the Bering Sea and other polar waters to hunt for prey such as seals. How might global warming affect the bears' hunting methods?

* Walrus mothers usually nurse their young in the water. The females can even sleep while nursing. How? The walrus has air sacs in its neck, which allow it to sleep with its head above water. When it is asleep, the walrus looks as if it's standing in the water. Female walruses use this position when feeding their newborn calves. Walruses are very protective of their calves. A mother will fast so she does not have to leave her nursing calf. When the calf is old enough, mom will leave her baby on ice floes while she goes on short foraging trips. How might global warming affect how walruses raise their young?

* Last year, three environmental groups jointly filed a lawsuit to compel the Bush administration to protect the world's polar bears from extinction under the Endangered Species Act. The Bush administration has promised to make a decision by December 27, 2006. What might have prompted the environmental groups--Center for Biological Diversity, the Natural Resources Defense Council, and Greenpeace--to file the lawsuit?

CRITICAL THINKING:

* According to NASA scientists, 75 percent of Earth's freshwater is stored in glaciers and ice sheets. Researchers believe that as Earth gets warmer, its frozen freshwater source will keep melting. Can you think of some problems this may cause for humans and other land animals?

CROSS-CURRICULAR CONNECTIONS:

MATH: According to NASA, sea level has risen on average approximately 3 millimeters (0.12 inches) each year for the past 12 years. If sea level keeps increasing at this rate for the next 25 years, how much will it rise in total? Answer: 75 mm (about 3 in.)

RESOURCES

* To learn more about the research performed aboard the Healy and how teachers can become involved in expeditions, visit the Teachers and Researchers Exploring & Collaborating (TREC) program's Web site at: www.arcus.org/trec

* Learn more about global warming from Science World's special Web site on the topic: www.scholastic.com/globalwarming

* To find out more information about global warming, visit this U.S. Environmental Protection Agency Web site: http://epa.gov/climatechange/kids/index.html

On Thin Ice

DIRECTIONS: Fill in the blanks to complete the following sentences.

1. Sea ice is receding due to--, or an average increase in Earth's--

2. Polar bears are--, or meat eaters. They primarily prey on--

3. Normally, for the first--to--months of a walrus calf's life, the calf and its mother stick together in waters that are just a couple of hundred meters deep. A walrus mom nurses her baby for the first--of its life.

4. Many animals rely on sea ice for survival. For instance: Ribbon seals--and--their pups on the ice. Without sea ice, ribbon seals would likely become extinct.

5.--are plantlike organisms that make their food using a process called--

ANSWERS

ON THIN ICE

1. global warming, temperature

2. carnivores; ringed seals

3. one, three; two years

4. give birth, rear

5. algae, photosynthesis