The Earth maintains an average 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 keep the earth at that temperature. They capture the sun's energy by allowing solar radiation to enter the earth's atmosphere, but preventing some from escaping back into space. That keeps the earth warm enough to sustain life -- without greenhouse gases, scientists estimate temperatures would plunge to zero degrees Fahrenheit.
But humans produce much more greenhouse gas than would occur naturally in earth's atmosphere. 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 caused much of the rise in temperature over the past few decades. The increased emission of greenhouse gases, such as carbon dioxide from 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 36 percent while methane is up 148 percent and nitrous oxide has risen 18 percent.
In 2007, the U.N. Intergovernmental Panel on Climate Change -- made up of more than 2,500 scientists from 130 countries -- issued a series of reports that attached the strongest wording yet to humans' impact on the climate.
The first report said greenhouse gasses, including carbon dioxide, released by human activity were almost certainly causing most of the Earth's warming since 1950. The second report said the average global temperature could rise between 3.2 degrees and 7.1 degrees Fahrenheit by the year 2100.
The panel also found that global warming is already impacting weather around the world, and that hundreds of millions of people face potential droughts, floods and other natural disasters should the warming trend continue.
A third report said the earth's citizens must significantly change its energy use and cut back on greenhouse gases to stem rising global temperatures. But if the rise in temperature is held to less than 3.6 degrees Fahrenheit, the report said, the increase in temperature would reduce the world's average annual GDP growth by about 0.12 percent per year, or 3 percent by 2030.
The fourth and final report outlined scenarios if greenhouse gasses are not reduced, including competition for scarce water supplies in Africa, a higher risk of flooding in coastal areas, and less snow pack and more intense heat waves in North America.
While nearly all scientists agree the earth is warming, the debate continues over what should be done to reduce greenhouse gas emissions. Ben Lieberman, a senior policy analyst at the Roe Institute for Economic Policy Studies at the Heritage Foundation, a conservative think tank, wrote in a paper released Nov. 24, 2008: "Concern that carbon dioxide and other greenhouse gases are gradually warming the planet has emerged as the major environmental issue of the day, and certainly the most hyped one. Carbon dioxide is a naturally occurring component of the air, but is also the ubiquitous and unavoidable by-product of fossil fuel combustion, which currently provides 85 percent of America's energy. Thus, any effort to substantially curtail such emissions would have extremely costly and disruptive impacts on the economy and on living standards," he wrote.
But according to the World Wildlife Fund, "Climate change is impossible to hide and ought to be impossible to ignore. The 10 hottest years on record have occurred since 1990. Arctic sea ice has declined to the lowest levels on record and studies suggest that two-thirds of the world's polar bear population will be gone by 2050. But more than polar bears and ice caps are at risk -- climate change endangers all life on our planet."
Natural disasters like Hurricane Katrina, which destroyed major parts of the U.S. Gulf Coast in 2005, have been attributed in part to global warming. Worries about low-lying coastal countries were raised again when a cyclone hit Bangladesh in 2007 and killed several thousand people.
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. Discussions are underway on a new global warming treaty.
To date, the United States -- the world's largest greenhouse gas emitter -- has not signed the protocol. The Bush administration has 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. But in July 2008, leaders of the Group of Eight nations with the world's largest economies -- including the United States -- agreed in principle to cut greenhouse gas emissions in half by 2050.