If you could stick a big thermometer in Earth's mouth, you would find that she's running a serious fever, scientists say. Our planet is warming up.

Polar ice has been melting. Glaciers have been shrinking. Many plants and animals are starting to live closer to the poles and at higher elevations, where it's cooler.

Why does this matter to the average kid, who probably hates wearing a coat and loves T-shirt weather?

Global warming means more than just a few extra-hot summer days. A seemingly small shift in global temperature can trigger large-scale changes in the climate and affect animal and plant life. The changes could cause rising sea levels and, in some areas, poor crop yields due to droughts or increased rainfall, a United Nations report said.

Global warming also could drive many species to extinction, recent studies have concluded. The Edith's checkerspot butterfly is an example. It moved its range from Mexico about 60 miles north in the direction of busy San Diego and Los Angeles, where the temperatures are cooler but the habitat is not as good for butterflies. The checkerspot is now endangered.

How Much Hotter?

Earth's average temperature has risen a little more than 1 degree Fahrenheit in the past 100 years.

By the end of this century, the average will rise 2 to 10 more degrees, the United Nations-sponsored Intergovernmental Panel on Climate Change predicted in 2001.

These numbers may sound small, but to climate scientists they're huge. Consider this: At the end of the last Ice Age, more than 10,000 years ago, when ice covered large areas of North America, the average temperature probably was only about 5 to 9 degrees lower than today.

Changes also are happening more quickly than usual. Climate shifts of the past generally took place over thousands of years, not decades. But more than half of the 20th century temperature rise happened in the last 30 years.

Surf's Up!

During the past 100 years, sea levels have risen four to eight inches. Water expands as it gets hotter, so the warmer oceans have increased in volume. Another probable cause of rising tides is rapidly melting ice.

Weird Warmth! Why?

Most scientist agree that human beings are a major cause. Cars, factories and power plants all produce gases -- called greenhouse gases -- that trap heat close to Earth.

Earth now has higher concentrations of greenhouse gases than at any time in the past 400,000 years. Could the warming be part of some natural cycle? Climate has varied since prehistoric times. But in 2001 the National Academy of Sciences reported that "the changes observed over the last several decades are likely mostly due to human activities."

The Forecast: Hazy

So, what to do? Some scientists and environmentalists say we should stop putting so much greenhouse gas into the atmosphere. Cleaner factories and more efficient cars would help, they say, as would switching to energy sources such as solar or wind power.

But these changes can be expensive, and getting governments and companies to make them is difficult. Countries met in 1997 in Japan and agreed on goals to lower emissions of these gases.

More than 120 nations promised to sign the treaty, but President Bush pulled the United States out in 2001, saying that the treaty would cost American jobs, hurt businesses and let some big polluter countries off the hook. This was a blow to the treaty because the United States produces more greenhouse gases than any other country.

How strongly should government crack down on greenhouse gas polluters? Some say the science is not strong enough to get tough. Others say the warning signs are so serious we would be foolish not to act.

One prediction seems safe: This topic is likely to stay hot for a long time.

-- Fern Shen