Greenhouse gases are gases in the Earths atmosphere that trap heat and raise the surface temperature of planets such as Earth. They absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. Greenhouse gases are infrared active gases that absorb and emit infrared radiation in the wavelength range emitted by Earth. The most abundant greenhouse gases in Earths atmosphere, listed in decreasing order of average global mole fraction, are:

• Water vapor (H2O)
• Carbon dioxide (CO2)
• Methane (CH4)
• Nitrous oxide (N2O)
• Ozone (O3)
• Chlorofluorocarbons (CFCs and HCFCs)
• Hydrofluorocarbons (HFCs)
• Perfluorocarbons (CF4, C2F6, etc.), SF6, and NF3

Carbon dioxide is the primary greenhouse gas emitted through human activities, and it accounts for 79% of all U.S. greenhouse gas emissions from human activities. Methane and nitrous oxide are also significant greenhouse gases. Water vapor is the most abundant greenhouse gas, but it is not included in U.S. or international greenhouse gas inventories because it is not directly produced by human activity. Greenhouse gases act like the glass walls of a greenhouse, trapping heat in the atmosphere and warming the planet. Without the greenhouse effect, temperatures would drop to as low as -18˚C (-0.4˚F), which is too cold to sustain life on Earth. The release of greenhouse gases associated with human activities and climate change is referred to as greenhouse gas emissions or climate pollution.