Greenhouse effect – The warming of the Earth produced by the presence of certain gases in the atmosphere. The greenhouse effect is a natural phenomenon necessary for life on earth; without it the planet’s average temperature would be 0 degrees Fahrenheit, instead of 60 degrees. Greenhouse gases include nitrous oxides, chlorofluorocarbons, methane, tropospheric ozone and carbon dioxide. They trap heat that would otherwise escape into space and return some of it to the earth’s surface, causing a rise in temperature. These gases result from both natural (oceans, wetlands, lakes, volcanoes, tundra and decaying organic matter) and human (the burning of fossil fuels, deforestation, etc.) sources.