Nitrous oxide is added from nitrogen fertilizers used in agriculture. Methane is emitted by rice paddies and cattle feedlots. CFC's not only trap heat but also destroy earth's stratospheric ozone layer. This is due, in part, to the growing number of people on earth, along with the growth of energy use, industrial activity, and agriculture. According to the Washington-based Environmental Protection Agency, humans presently spew six billion tons of carbon dioxide and other greenhouse gases into the atmosphere each year.
Scientists are generally quite confident about two things.
First, in recent decades and centuries, the amount of carbon dioxide and other greenhouse gases in the atmosphere has increased. Second, over the last hundred years, the average surface temperature of the earth has increased between 0.5 and 1.1 degrees Fahrenheit. Some scientists say probably not, pointing out that the increase in temperature falls within the range natural variation and that the sun may be responsible. However, many climate experts agree with wording of a report by the Intergovernmental Panel on Climate Change. When climatologists predict a future greenhouse effect, they rely on climate models run on the world's fastest and most powerful computers.