Climates are changing worldwide. Average global temperatures have increased in the past several hundred years—and the increase is occurring faster than ever before. How are scientists studying these changes?

Average temperatures on Earth's surface have gone up about one degree Fahrenheit over the past century. That increase may not seem like much, but it is a change that deserves attention, says Janine Bloomfield, a staff scientist at Environmental Defense in Boston, Massachusetts.

Bloomfield points out that until the past century, average temperatures hadn't varied by more than a half a degree in 1,000 years. And recently, the rate of warming has accelerated. The 10 warmest years in the twentieth century all occurred within the past 15 years.

"In the next 100 years," Bloomfield warns, "average temperatures could increase by 5 or 10 degrees Fahrenheit. We're playing an experiment. An uncontrolled experiment. We don't have any way of turning back if the experiment doesn't work out so well."

Temperatures rise when greenhouse gases—mainly in the form of carbon dioxide (CO 2 )—trap heat in Earth's atmosphere. This phenomenon is generally a good thing, since without it the planet would not be able to support life.

Light comes in from the Sun, heating Earth's surface. Earth radiates energy back into space. Greenhouse gases in the atmosphere prevent some of the energy from escaping back into space. Glass panels in a greenhouse act in a similar way, preventing heat from escaping.

However, as greenhouse gas accumulates, it traps more heat. The oil we burn to power factories, the gasoline we use to run our cars, the coal we burn to heat our homes, and the use of firewood for cooking in many countries all put CO 2 into the atmosphere. It can stay there for at least 150 years, resulting in a continuous and dangerous buildup.

• Some of the carbon dixoide produced on Earth is captured and turned into oxygen during the process of photosynthesis. Explore how this process works in the Biology Gateways activity Fueling Photosynthesis with Carbon Dioxide and Water and the Middle School Gateways activity of the same name. (Requires Logal Express. Get a free trial subscription.)

Scientists who study global warming have tracked changes that have occurred over the past decades and centuries. Their data includes the following phenomena, all of which they believe are due to rising temperatures:

• Reduced numbers of butterflies, birds, frogs, and other species

• Ponds freezing later in the winter and thawing earlier in the spring

• Trees leafing out earlier in the spring

• Melting polar ice caps and glaciers

• Rising sea levels

• Bleaching and death of the coral reefs

• Changing weather patterns

Almost all scientists agree that global temperature and CO 2 rates are rising. However, not all of them agree on the extent of the problem and its causes. Some scientists question the data that is being studied. Some believe that we need to look at data that covers a longer period of time.

Some question the degree to which humans are contributing to the problem. And there are some that question the severity of the consequences. Still others suggest that the recent increase in temperatures is merely the high point of a natural warming and cooling cycle.

The science of climate change has also become a controversial political issue. Oil and gas companies believe that their industries will be threatened by regulations to control emissions. Emission controls, which require new technology, would increase energy costs for both countries and individuals.

But most scientists conclude that if we take no action today, the effects in 50 years—when today's students near retirement—will be far worse. "The rate and direction of change is mostly due to greenhouse gases," says Anne Waple, at the University of Massachusetts' Climate System Research Center in Amherst. "Whether we can arrest that change is another question."

Ray Bradley measures Arctic lake sediments. Lisa Sorenson counts ducks. Both are contributing to our understanding of climate change. Scientists from such diverse disciplines as geology, biology, and meteorology are studying global warming.

Bradley and his group at the Climate System Research Center in Amherst, Massachusetts, study paleoclimatology, the history of climate in prehistoric times. These scientists spend summers north of the Arctic Circle studying sediment that has collected in a lake in the Canadian High Arctic.

By studying sediment deposits, the scientists can determine how much rain runoff there was during a given summer. The amount of runoff provides clues to the air temperature of a given year.

In Greenland, the Antarctic, and atop high mountains such as Kilimanjaro in Africa, scientists are collecting ice core samples. Bubbles exist in the ice and these bubbles contain atmospheric gases. The ice cores contain samples of CO 2 levels from before scientists began taking instrument readings.

By collecting samples and analyzing the gas composition of the bubbles, scientists obtain atmospheric information for the Earth going back 400,000 years. In addition to studying CO 2 levels in the atmosphere, scientists also look at the amount of CO 2 that occurs naturally. They separate out the amount of CO 2 that is introduced into the atmosphere by the burning of fossil fuels.

World population, CO 2 emissions from fossil fuels, and automobile production have all been increasing. The chart below shows data for 1955 to 1995.

Sources: American Automobile Manufacturers Association; U.S. Census Bureau; Carbon Dioxide Information Analysis Center

• By approximately how many times did the total CO 2 emissions from fossil fuels increase between 1955 and 1995?

• Which 10-year span in the above chart shows the greatest increase in CO 2 emissions? What might explain the reduction of emissions in the following years?

• Using the figures from 1965-1995, predict the number of automobiles in the world in 2005. What solutions could reverse this trend?

Bradley sees a relationship between the increasing standard of living and energy usage. "Everybody wants to have their own car. Everybody wants to have lots of electrical appliances," he says.

"The demand for energy is increasing. So far, we haven't come to grips with using energy more efficiently. We're still wasting a lot of energy. So the demand for fossil fuel is increasing."

Meanwhile, Dr. Lisa Sorenson, a staff scientist with the Union of Concerned Scientists in Boston, Massachusetts, studies duck populations in the Northern Great Plains. The area of Minnesota, North and South Dakota, as well as the Canadian provinces of Manitoba, Alberta, and Saskatchewan, are filled with wetlands, where the majority of the North American duck population breeds.

According to Sorenson, the wetter the weather, the more wetland acreage and the more successful ducks are at breeding. According to the computer climate models she has used, if temperatures increase as expected, there will be less rain and fewer wetlands in the Northern Great Plains. This change will have a direct effect on the ducks of North America.

Sorenson studied data on the duck populations of the U.S. and Canada over a 35-year period. The chart below contains data for 1993 through 1996 in southern Canada.

Source: Dr. Lisa Sorenson

• By approximately what percentage did the number of wetland acres change from 1994 to 1995?

• How many more ducks were recorded in 1996 than in 1993?

• In the years with the most ducks, what factor seemed to affect the size of the population the most? What might be surprising about the data recorded for 1994?