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1 Water vapor. 2  The most abundant greenhouse gas, but importantly, it acts as a feedback to the climate. 3  Water vapor increases as the Earth's atmosphere warms, but so does the possibility of clouds and precipitation, making these some of the most important feedback mechanisms to the greenhouse effect.
Water vapor is the most abundant green house gas but it is the most ineffective at holding heat. CO2 is the second most abundant and is relatively good at holding heat but not as good as methane but methane is only present in trace amounts and is not increasing in amount as greatly as CO2.