Greenhouse gases are found in the Earth's atmosphere; they absorb and re-emit infrared radiation, causing the earth to heat. These gases are primarily water vapor, carbon dioxide, methane, ozone and nitrous oxide. The three main greenhouse gases produced by cars are carbon dioxide, methane and nitrous oxide.
Up to 95 percent of passenger vehicle emissions are carbon dioxide gases, according to the Environmental Protection Agency. Also known as CO2, carbon dioxide is colorless, odorless and incombustible. The typical passenger vehicle emits 5.48 metric tons CO2e (carbon dioxide equivalent.) Carbon dioxide is not classified as a pollutant because it is a natural component of the air and needed by plants for photosynthesis. Because of increasing automobile emissions and deforestation, the atmosphere cannot absorb all of the CO2. This causes more heat to be trapped and sent back down to Earth.
Methane is a colorless, odorless, flammable gas. It remains in the atmosphere for approximately nine to 15 years. According to the EPA, "Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO2) over a 100-year period." It is emitted from a variety of natural and human-influenced sources, including landfills, agricultural activities, natural gas and petroleum systems, and coal mining.
Together with methane, and in a very small part HFCs (hydrofluorocarbons), nitrous oxide emissions account for about 5 to 6 percent of greenhouse gas emissions from passenger vehicles. Nitrous oxide has an atmospheric lifetime of roughly 120 years (about 310 times more effective in trapping heat than CO2 over 100 years.) Nitrous oxide comes from biological sources as well as human-influenced emissions, such as soil and animal manure management, sewage treatment, and mobile and stationary fuel combustion.
