Anthropogenic emissions (those caused by humans) consist primarily of CO₂, CH₄, CO, and N₂O. With the aid of sunlight, CH₄, CO, N₂O, and other hydrocarbons help form O₃ in the lower levels of the atmosphere. The amount of water vapor in the atmosphere is not directly affected by human activity. However, as the temperature increases due to increasing levels of the other gases, it is expected that water vapor levels will also increase.

CO₂ emissions constitute over 90% of all anthropogenic emissions, so the other gases are generally referred to in terms of their CO₂ equivalent effect. While CO₂ may be the dominant gas by amount, the other greenhouse gases are more efficient at trapping outgoing IR radiation. The amount of time the gas spends in the atmosphere is also important: the longer the time, the more energy trapped. A factor called the Global Warming Potential (GWP) is calculated for each gas, based on the above two effects: efficiency at trapping IR radiation and time in the atmosphere. CO₂ has a GWP = 1. CH₄ has a GWP = 25, and CFCs and HCFCs have GWPs ranging from 100 to 12,000. The GWP tells us that one molecule of CH₄ has the same effect as twenty-five CO₂ molecules. It may be more beneficial, in certain situations, to control emissions of CH₄ or CFCs depending on the amount of these emissions relative to CO₂.

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