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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.
A greenhouse gas (or GHG for short) is any gas in the atmosphere which absorbs heat, and thereby keeps the planetâs atmosphere warmer than it otherwise would be. The main GHGs in the Earthâs atmosphere are water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone.