Why are the polar bears getting thinner? The answer to that question appears to be tied to global warming and the melting of Earth's Arctic — or northern— ice cap. Polar bears do most of their feeding while wandering the sea ice. These "bears of the sea" are terrific swimmers and can spend hours in the cold water, hunting for the seals they eat. However, much of their time is spent on the sea ice. A normal year has a small period of time when the sea ice breaks up and the bears must retreat to land and live off their fat. What appears to be happening now is that increased temperatures are causing the sea ice to break apart sooner and refreeze again later, lengthening the amount of time the bears are on land and shortening the amount of time the polar bears are free to hunt.

The consequences for the polar bears of thinning or disappearing sea ice are dire. It is the loss of a habitat. Already researchers in the Hudson Bay area are seeing female bears who are thinner and less healthy as they enter their season for having cubs. The females appear to be running out of their stored fat which they and their cubs live on. The female bears are needing to leave their dens earlier than they used to; in more and more cases, the smaller of a female bear's two cubs will starve, as there isn't enough food.

• Read more about the endangered polar bears in The Shrinking Polar Bears from the CBC/The National Online.
  
  
• For more general information about polar bears, visit this informational site from Sea World.

The Arctic ice cap covers an area roughly the size of the United States. Or rather, it did cover an area the size of the United States. But the Arctic ice cap is shrinking dramatically. Every year since 1978, the ice cap has lost an average of 33,800 square kilometers (13,000 square miles) — that's a loss each year of an area the size of Massachusetts and Connecticut combined. And it's not just area being lost. Measurements made by submarines show that the average thickness of the ice cap decreased from 3 meters (10 feet) during the 1960s to 1.8 meters (6 feet) in the 1990s.

• Approximately how many square kilometers (or square miles) has the ice cap lost in the 22 years since 1978?
  
  
• By what percentage has the thickness of the ice decreased from the 1960s?

Data collected on the thickness of the ice below the ocean's surface — known as "sea ice draft" — paints a similar picture. Recently researchers compared data collected in the mid-1990s with data gathered from the 1950s to 1970s. They were startled to find that Arctic sea ice draft had decreased by 1 meter (3 1/2 feet) since the earlier time period.

The graph below compares the sea ice draft at various Arctic locations. The reduction of thickness at all the measured points shows that the melting ice in the Arctic is widespread.

• Estimate the percentage decline of sea ice draft from 1958-1976 (blue bars) to 1993-1997 (red bars) in each of these Arctic regions: Chukchi Cap, Beaufort Sea, Canada Basin, North Pole, and Eastern Arctic.
  
  
• Which region had the greatest percentage loss?
  
  
• What was the percentage decline for all regions?

Antarctica, which contains about 90% of Earth's ice, is melting too. So far, most of the melting has been at the continent's edges. This year, icebergs larger than Rhode Island and Delaware have broken loose from the ice shelves surrounding the Antarctic continent. In many places, there is little ice shelf left to break free.

However, things in Antarctica may not be as bleak as in the Arctic. New measurements show that ice in parts of Antarctica is actually thickening, not thinning. Temperatures in Antarctica's desert valleys have dropped sine the mid-1980s.

Evidence of Earth's warming trend continues to accumulate. A recent study of ice-core samples taken high in the Himalayas reveals that both the last decade and the last 50 years were the warmest in 1,000 years. 2001 was the second warmest year on record. Nine of the ten warmest years on record have occurred since 1990. This includes 1999 and 2000, when the La Niña weather pattern would have mitigated temperatures with its cooling effect.

If predictions are right, then the warming trend will continue. Some computer models predict an additional rise of 5° to 10° F over the next century. To put that number in perspective, Earth is now an estimated 5° to 9° warmer than it was during the last Ice Age about 18,000 to 20,000 years ago. At this rate of temperature increase, scientists worry that environmental conditions in the biosphere will change faster than plant and animal species can adapt or migrate. Large scale extinction would be the sad, unavoidable result.

• Look at the graph of average global temperature change from 1861 to 1996. The y-axis shows the amount that temperature was greater or less than a baseline value.
  
  
• Calculate how much average global temperature has increased since: a. 1901 b. 1921 c. 1941 d. 1961 e. 1981
  
  
• Which twentieth century decade was the warmest?
  
  
• How does the rate of temperature change in the nineteenth (from 1861 onward) and twentieth centuries compare?

Global sea level has risen by 10 to 25 centimeters (4 to 10 inches) over the last century. Melting ice caps and glaciers account for about one-fifth of the increase. But as global ice melting speeds up in coming decades, the sea level rise is predicted to grow significantly.

Some climate models predict global sea level to rise by .9 meters (3 feet during) this century. And if the massive ice sheet covering Greenland, the sea level worldwide would increase by a staggering 7 meters (23 feet). Scientists emphasize that such a development is not likely in the coming century or two, but it's definitely a possible scenario for the distant — or perhaps not-so-distant — future.

In 1997, 39 industrial nations signed the Kyoto Protocol, a treaty calling for a 5.2% reduction in greenhouse gas emissions below 1990 levels by the year 2112. But there's a catch. The agreement would not kick in until it was ratified by 55% of the countries emitting at least 55% of the gases. President George Bush pulled the United States out of the Kyoto agreement because of concerns over the economic impact on American industry. The United States is the world's largest producer of greenhouse gases and, in spite of President Bush's proposed voluntary incentives for reducing greenhouse gases, many are worried that the approach will be inadequate for slowing global warming.

Not everyone agrees that greenhouse gases are to blame for rising global temperatures. Some scientists point out that Earth's temperature has warmed for more than 300 years, before coal and oil were burned as fuels. Instead, they suggest the planet may be in the middle of a long-term climate shift that would be happening with or without the rise of greenhouse gases. Some even say that the warming trend could eventually reverse itself. This is another reason advisors to President Bush suggested doing more studies rather than rushing to curb greenhouse emissions now.

With more time and research, the debate will ultimately be settled. However, how much time do the world's polar bears have? If the world's political and scientific leaders are wrong and take too much time, will Earth's largest land predators slowly starve and become extinct, victims of inaction and the shrinking ice?