The Earth maintains a temperature of about 60 degrees Fahrenheit or 16 degrees Celsius, temperatures that enable people, plants and animals to live safely within its atmosphere. Naturally occurring gases known as greenhouse gases help capture the sun's energy, keeping the Earth warm enough to sustain life. Without greenhouse gases, scientists estimate temperatures would plunge to zero degrees Fahrenheit, making it impossible for life to exist.

In the last century, the Earth has warmed by 1 degree Fahrenheit -- a change significant enough to cause atmospheric disturbances, some scientists believe -- with the highest increases occurring within the past 50 years, the height of the industrial revolution, according to the U.S. Environmental Protection Agency.

Most scientists believe human activity has contributed to much of the rise in temperature over the past few decades. The increased emission of greenhouse gases, such as carbon dioxide caused by car exhaust, and methane and nitrous oxide from industrial plants and agricultural activities, has trapped more heat close to the planet's surface.

The EPA estimates that in the last 50 years, the presence of carbon dioxide in the atmosphere has increased 30 percent, methane 50 percent and nitrous oxide 15 percent. The higher concentrations of these gases counter the effects of gases in the atmosphere that help keep the Earth cool.

Many scientists and policy-makers see this trend, commonly referred to as "global warming," as a threat to the environment and its inhabitants, while others -- including advocates for large industry -- say the danger is overstated. They say policies aimed at stemming high levels of greenhouse gas emissions harm consumers, who would bear the brunt of the associated costs.

The argument over who or what is to blame for global warming and to what extent the environment could be damaged by its effect has become one of the most urgent -- and hotly debated -- environmental issues of the 21st century.

Evidence that the Earth is warming can be found in the melting of Arctic glaciers, changes in weather patterns such as the increase of hurricanes and droughts, and in rising sea levels -- globally the sea level has risen 4 inches to 8 inches in the past century, according to the EPA.

Some advocacy groups, criticized by their opponents as apocalyptic, attribute these changes to poor environmental policy and predict dire consequences for future generations.

"We will experience extreme temperatures, rises in sea levels, and storms of unimaginable destructive fury," the nonprofit organization stopglobalwarming.org predicts on its Web site. "Recently, alarming events that are consistent with scientific predictions about the effects of climate change have become more and more commonplace."

Natural disasters like the 2004 tsunami that hit South Asia and Hurricane Katrina, which destroyed major parts of the U.S. Gulf Coast in 2005, have been attributed in part to global warming.

Because other factors affect the Earth's temperature and because the science of global warming is not exact, some groups, who disagree with efforts to reduce emissions, have discounted not only the phenomenon itself, but also its effects on the environment.

"While the planet is indeed warming - probably due in no small part to industrial greenhouse gas emissions - the warming has been modest, benign, and largely confined to northern latitudes during winter nights," Jerry Taylor, director of natural resource studies at the Cato Institute, wrote in a 2004 commentary. "There are good reasons to expect that warming pattern to continue. And that warming pattern does not threaten to usher in the convulsive climatic events we are warned about in the press or in the movie theaters. In fact, some scientists and economists can make a pretty good case that global warming will prove a net plus to both the economy and the global environment."

Most scientists do agree, however, that global warming is occurring and that along with other factors including natural changes in the climate, human activity is accelerating it.

"Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise," a 2001 report commissioned by the National Research Council said. "Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability. Human-induced warming and associated sea level rises are expected to continue through the 21st century."

To counter the effects of global warming, 140 countries in 2005 ratified the Kyoto Protocol, an international agreement made in 1997 that forces developed nations to reduce their emission of greenhouse gases to pre-1990 levels by 2012.

As of early 2006, the United States -- the world's largest contributor to greenhouse gas emissions -- had not signed the agreement. The Bush administration said that placing mandatory restrictions on emissions could hurt large industry and damage the U.S. economy by making it less competitive with countries such as China, which is exempt because of its status as a developing nation.

-- Compiled by Kristina Nwazota for the Online NewsHour