Greenhouse gases are gases that allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth's surface, some of it is reflected back towards space as infrared radiation. Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Five of the main greenhouse gases that are natural are water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Other greenhouse gases are exclusively man-made, such as gases used for aerosols.
Most greenhouse gases can be described as long-lived and well-mixed. To be long-lived means that the gas is stable and they will last years in the atmosphere. Well-mixed means that they are evenly distributed within the atmosphere. Carbon dioxide, methane, and nitrous oxide are both long-lived and well-mixed. However, ozone is only long-lived because it is found in higher concentrations in cities, making it not well-mixed. Water vapour is neither well-mixed or long-lived. It is because of this that its contribution to global warming is the least understood.
Sources:
http://www.eia.gov/oiaf/1605/ggccebro/chapter1.html
http://education.arm.gov/studyhall/globalwarming/experts/expertgas
This is so fascinating
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