What are Greenhouse Gases?
Greenhouse gases are atmospheric gases that trap infrared radiation emitted from the earth, lower atmosphere, or clouds or aerosols and, as a result, cause positive radiative forcing (which means they enhance global warming). Many of these gases are naturally occurring and are essential to life on earth by providing a blanket for marine and terrestrial organisms. Without them, temperatures on earth would be intolerably cold. However, when their concentrations become too high, they may contribute to global warming to such an extent that they would make the earth intolerably hot. Nature has maintained these gases in the 'tolerable' range for billions of years, but human activities have been changing the concentrations of these gases since the start of the industrial revolution, around 1850.
Greenhouse gases may be described as
- Long-lived, which means they are stable and, therefore, last years in the atmosphere
- Well-mixed, which means they are pretty evenly distributed in the atmosphere.
Most of the significant greenhouse gases are long-lived and well-mixed. They include carbon dioxide, methane gas, oxides of nitrogen, and halocarbons. Ozone is long-lived but is found in higher concentrations in cities, especially in the northern hemisphere. Therefore, it is not well-mixed.
Water vapor is a greenhouse gas that is neither well-mixed nor long-lived. Because of this, its overall effect on global warming is the least understood. Data being collected by the Atmospheric Radiation Measurement (ARM) Program sites are continually improving our understanding of water vapor, which actually is the dominant greenhouse gas. There are other greenhouse gases that collectively contribute little to global warming at this time, and these will not be discussed on this page.
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