This is actually any of a number of gases, accumulating in the Earth's atmosphere, that act just like the panels of glass in a greenhouse roof: They allow sunlight to enter, but then trap the heat that is reflected back. Rather than nurturing seedlings in those cozy conditions, however, greenhouse gases contribute significantly to global warming. Many of the gases, including carbon dioxide, methane, ozone, nitrous oxide, and water vapor, are released naturally as a result of respiration, evaporation, and the decay of organic matter. Others, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are man-made. Over the past 150 years, human activity-particularly the burning of fossil fuels, deforestation, growth of landfills, and increase in agriculture-have led to a dramatic increase in the levels of both natural and man-made greenhouse gases in the atmosphere.

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