Greenhouse gases are chemical compounds found in the Earth’s atmosphere that allow sunlight to enter and trap the reflected radiation and therefore heat, keeping the planet warm and habitable. Greenhouse gases, which occur in nature, include water vapor, carbon dioxide, methane and nitrous oxide. These gases, among others, are also produced by human activities. If the amount of radiation the Earth receives from the sun and the concentration of greenhouse gases remain relatively constant, the temperature of the Earth’s surface should also remain constant.
In the United States, most human-produced greenhouse gases are the result of energy-related combustion emissions from petroleum, coal and natural gas. This represents 82 percent of the total emissions for the country.
Carbon dioxide is one of the most widely recognized and also most widely released greenhouse gases. It has a significant impact on the climate of the planet as it absorbs and emits infrared radiation and plays a role in maintaining the heat of the planet. The current measurement of carbon dioxide in the atmosphere is 387 parts per million (ppm) of carbon dioxide – for every million of other molecules, there are 387 carbon dioxide molecules. For most of human history, the amount of carbon dioxide in the air has been at about 275 ppm, but since the Industrial Revolution, it has been increasing steadily each year. Many researchers believe there is a tipping point in the ppm number beyond which it will be impossible to reverse the effects of global climate change. Recent research and calls from the scientific community insist that we must reduce carbon dioxide in the atmosphere below 350 ppm.
http://www.eia.doe.gov/bookshelf/brochures/greenhouse/Chapter1.htm