Many chemical compounds found in the Earth’s atmosphere act as "greenhouse gases." These include carbon dioxide (CO2), methane, nitrous oxide and water vapor. These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth’s surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth’s surface roughly constant. However, as atmospheric levels of greenhouse gases increase, more heat is trapped in the Earth’s atmosphere than is radiated back into space, causing the temperature of the Earth’s surface to rise.

Why are atmospheric levels of greenhouse gases increasing?

Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively know as the "carbon cycle." The movement ("flux") of carbon between the atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. While these natural processes can absorb some of the net 6.1 billion metric tons of anthropogenic [made or generated by a human or caused by human activity] carbon dioxide emissions produced each year, an estimated 3.2 billion metric tons is added to the atmosphere annually. The Earth’s positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.

With the beginning of large-scale industrialization around 150 years ago, human activities have been dramatically adding to the carbon dioxide levels in the atmosphere. The burning of fossil fuels, such as coal and oil to power our cars, factories, businesses and homes, is increasing carbon dioxide beyond natural levels. According to the American Association for the Advancement of Science, "today’s rising atmospheric carbon dioxide concentration, at 380 parts per million by volume, is already 27 percent higher than its highest recorded level during the last 650,000 years."

References:

• Energy Information Administration, Greenhouse Gases, Climate Change and Energy ( modified 4/2/04), found at: http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html
• American Association for the Advancement of Science, 2005. New Research in Science Shows Highest CO2 levels in 650,000 Years. (Press Release), found at: http://www.aaas.org/news/releases/2005/1128ice.shtml