Gases that trap heat in the atmosphere are often called greenhouse gases. Some, such as carbon dioxide (CO2), occur naturally and are emitted to the atmosphere through natural processes and human activities.
Others (e.g., fluorinated gases) are created and emitted solely through human activities. The principal greenhouse gases that enter the atmosphere because of human activities are: (1) CO2 — Enters the atmosphere through the burning of fossil fuels (oil, natural gas, coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). It is also removed from the atmosphere ("sequestered") when it is absorbed by plants as part of the biological carbon cycle. (2) Methane (CH4) — Emitted during the production and transport of coal, natural gas and oil. CH4 emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills. (3) Nitrous Oxide (N2O) — Emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste. (4) Fluorinated Gases — Synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. These are sometimes used as substitutes for ozone-depleting substances (CFCs, HCFCs and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases. (Source: EPA / Photo: Flickr)