When scientists learned that the earth's temperature reached its highest average last year, they renewed the debate over whether global warming is the cause.

Meteorologists have recorded yearly global temperatures for 110 years. The records show that the earth's annual average temperature is about 59 degrees Fahrenheit, said James F. Kasting, associate professor of geosciences and meteorology. In 1990, the National Aeronautic and Space Association reported that the earth's average temperature -- 59.8 degrees Fahrenheit -- surpassed the average of any other recorded year.

But most scientists agree that the temperature record does not necessarily indicate that the greenhouse effect is causing a warming trend, said Lee Kump, assisant professor of geosciences.

The greenhouse effect is not a new phenomenon, Kump said. Without it, the earth's temperature would never rise above freezing.

Carbon dioxide molecules in the earth's atmosphere trap the sun's energy and prevent it from radiating back to outer space. This is called the greenhouse effect, because greenhouse windows use the same principle to trap heat for the plants inside.

When humans burn fossil fuels for energy, carbon dioxide is emitted into the atmosphere, adding to the natural warming, he said.

Scientists debate if greenhouse warming is affecting the earth's climate.

"It's a difficult problem. People are bound to argue until something definite happens," Kump said.

Bob Grumbine, a post doctoral research fellow in the meteorology department, said he believes that global warming hasn't been proven. The data collected by temperature recording centers in North America do not provide an accurate representation of the earth's climate, Grumbine said.

Many of these centers have changed locations over the past century, he said, and in many cases the change in location has affected the temperature recordings.

"I'm not entirely sure that we've recorded a warming trend," Grumbine said.

Some scientists are skeptical because the natural variability of the planet's climate causes the temperature to vary many degrees from year to year. Because of these fluctuations, the true temperature trend is difficult to determine, Kasting said.

"You're not going to convince everyone that there is a greenhouse until the climate has changed enough that it goes beyond the natural variability," Alley said.

If the world's climate does significantly warm, Alley said, disaster is certain. Since the world's population is so large and humans have so much invested in the earth, any greenhouse warming would cause people to suffer. Whether it be wheat fields turning to deserts or Philadelphia becoming part of the Atlantic Ocean, some sacrifices will be made, he said.

Scientists also worry that about 300 feet of ice on Greenland and Antarctica will melt if the temperature increases, causing a rise in sea level and forcing people to abandon many coastal areas, Kasting said.

"It's going to be very expensive because a lot of our big cities and population centers are along the coasts," Kasting said.

Global warming's effects, however, may not hurt everyone.

"If we are going to warm up the climate and you live in Siberia, you'd probably be happy," said Richard Alley, an assistant professor of geosciences and affiliate of the University's Earth Systems Science Center.

But nobody really knows what will happen, Alley said, adding that global warming may cause anything from mass disaster to mild improvement on the earth.

Because the possible disasters are so large, Alley suggested that people do something to slow the warming process. He said he is afraid these disasters may be triggered before scientists know how to stop them.

"If they made me king for a day I'd put a $1 a gallon tax on gasoline and on all other fossil fuel sources," he said.

Kasting agreed with an increased gasoline tax, saying that Europe and Japan already charge a $1 to $2 tax per gallon. Such a tax would encourage energy conservation, he said, as well as decrease the United States' dependency on foreign oil sources.

"Energy independence is something we really need," Kasting said. "So we don't have to go and play policeman in the Middle East."

Alternate energy sources are also an important consideration, he added, such as nuclear and solar power. In order to discontinue carbon dioxide emissions, is to discontinue all use of fossil fuels, he said.