Difference between revisions of "Greenhouse effect"
(Not necessarily earth's atmosphere. Venus experiences the greenhouse effect far more than earth) |
(gas molecules absorb some of the heat radiating from the ground (see infrared radiation), so that it all doesn't escape into outer space) |
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{{merge|greenhouse gas}} | {{merge|greenhouse gas}} | ||
| − | The '''greenhouse effect''' refers to the retention of heat by a planet's [[atmosphere]] | + | The '''greenhouse effect''' refers to the retention of heat by a planet's [[atmosphere]]. |
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| + | It happens because gas molecules absorb some of the heat radiating from the ground (see [[infrared radiation]]), so that it all doesn't escape into [[outer space]]. It was discovered in 1824 by [[Joseph Fourier]]. | ||
The effect has been compared to that of the glass roof and walls of a [[greenhouse]], although the process is different. A gardener's greenhouse simply traps warm air inside the building. | The effect has been compared to that of the glass roof and walls of a [[greenhouse]], although the process is different. A gardener's greenhouse simply traps warm air inside the building. | ||
Revision as of 17:15, March 11, 2008
The greenhouse effect refers to the retention of heat by a planet's atmosphere.
It happens because gas molecules absorb some of the heat radiating from the ground (see infrared radiation), so that it all doesn't escape into outer space. It was discovered in 1824 by Joseph Fourier.
The effect has been compared to that of the glass roof and walls of a greenhouse, although the process is different. A gardener's greenhouse simply traps warm air inside the building.
These greenhouse gas molecules (principally water, carbon dioxide, and methane) absorb and re-emit longwave infrared rays emitted from the earth's surface into the atmosphere (see radiational cooling).
Sunlight falling on the surface of a planet is primarily in the visible part of the spectrum. The surface absorbs some of the incident sunlight, and some is reflected, depending on the reflectivity of the surface (see albedo). The reflected light mostly passes through the atmosphere back to space. The absorbed light warms the surface.
Any body with a temperature radiates according to Planck's law, [1] and the Earth is no exception. Due to its relatively low temperature, the emission from the ground is in the infrared (IR) part of the electromagnetic spectrum. Greenhouse gases, by definition, are good absorbers at these wavelengths. The gas absorbs the IR radiation, and emits IR radiation, both up and down. Because these gases are above the surface, they are at a lower temperature than the surface, so they emit at a different frequency. The net effect is that energy is retained by the Earth system, warming it.[2] This is the natural greenhouse effect, which allows the Earth to be habitable. When the atmospheric composition is changed by human activity resulting in increases in greenhouse gases, the warming associated is called the anthropogenic greenhouse effect.
Richard Lindzen wrote (in 1992):
- The main absorbers of infrared in the atmosphere are water vapor and clouds. Even if all other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 percent of the current greenhouse effect. [2]
However, this figure has been disputed. The textbook Chemistry of Atmospheres puts the contribution of water vapor and clouds at less than 88 percent of the greenhouse effect, while carbon dioxide is at least 12 percent.[3]
This insulating effect has been compared to a blanket or to the glass walls and roof of a greenhouse, but the process is not the same.