Gases that trap heat in the atmosphere are often called greenhouse gases.  Some greenhouse gases such as carbon dioxide occur naturally and are emitted to the atmosphere through natural processes and human activities.  The international Panel on Climate Change (IPCC) recognizes four primary greenhouse gases (GHGs).  These are water vapor (H2O), carbon dioxides (CO2), nitrous oxide (N2O) and methane (CH4).  However, the IPCC and other leading climate change agencies include a number of entirely human-made gases in their GHG inventory which by category, include halocarbons and other chlorine and bromine containing substances, also known as fluorinated gases, which are dealt with under the Montreal Protocol, and sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) dealt with under the Kyoto Protocol.  Carbon dioxide has a global warming potential of one, while methane and nitrous oxide have warming potentials of 25 and 310 respectively.  Fluorinated gases can be intense greenhouse gasses with the multiplier for sulfur hexafluoride (SF6) being 23900 times as potent as carbon dioxide.  A complete list of pertinent GHGs can be found on the IPCC website.  The principal greenhouse gases that enter the atmosphere because of human activities are:

• Carbon Dioxide (CO2):  Carbon Dioxide enters the atmosphere through the burning of fossil fuels (oil, natural gas and coal), solid waste, trees and wood products, and as a result of other chemical reactions (e.g., manufacturing cement).
• Methane (CH4):  Methane is emitted during the production and transport of coal, natural gas and oil.  Methane emissions also result from waste elimination by livestock, some agricultural processes and by the decay of organic waste in municipal solid waste landfills.
• Nitrous Oxide (N2O):  Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.
• Fluorinated Gases:  Hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes.  Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons).  These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases ("High GWP gases").