Changes

A '''"greenhouse gas"''' traps heat in the atmosphere by absorbing radiated heat, in the form of [[infrared]]. Any gas which contributes to the greenhouse effect is called a greenhouse gas, although those produced by human activity have gotten more attention in the past few decades. The most abundant greenhouse gas is [[water vapor]];<ref>Water vapor has the largest greenhouse effect, though since the quantity of water vapor in the atmosphere is determined primarily by the temperature it is not regarded as a climate forcing agent. [http://earthobservatory.nasa.gov/Library/glossary.php3?mode=all (NASA)] </ref> see [[cloud cover]]. Also important are [[carbon dioxide]], [[methane]], [[nitrous oxide]], and [[ozone]].<ref>http://yosemite.epa.gov/oar/globalwarming.nsf/content/emissions.html</ref> The level of carbon dioxide in the atmosphere has increased by about 37 percent since the start of industrialization. It is now at its highest level in at least 650,000 years.<ref>[[http://www.nature.com/nature/journal/v453/n7193/abs/nature06949.html (Nature)]] </ref>  

A '''"greenhouse gas"''' traps heat in the atmosphere by absorbing radiated heat, in the form of [[infrared]]. Any gas which contributes to the greenhouse effect is called a greenhouse gas, although those produced by human activity have gotten more attention in the past few decades. The most abundant greenhouse gas is [[water vapor]];<ref>Water vapor has the largest greenhouse effect, though since the quantity of water vapor in the atmosphere is determined primarily by the temperature it is not regarded as a climate forcing agent. [http://earthobservatory.nasa.gov/Library/glossary.php3?mode=all (NASA)]</ref> see [[cloud cover]]. Also important are [[carbon dioxide]], [[methane]], [[nitrous oxide]], and [[ozone]].<ref>http://yosemite.epa.gov/oar/globalwarming.nsf/content/emissions.html</ref> The level of carbon dioxide in the atmosphere has increased by about 37 percent since the start of industrialization. It is now at its highest level in at least 650,000 years.<ref>[http://www.nature.com/nature/journal/v453/n7193/abs/nature06949.html (Nature)]</ref>  

The nature of a greenhouse gas is to pass [[visible light]], i.e., incoming energy from the sun, but to be opaque to the [[infrared]], i.e., heat energy being re-radiated outward from the earth's surface. Greenhouse gases, by definition, are good absorbers at these wavelengths. This functions not at all like the [[glass]] on a [[man-made]] [[greenhouse]], and serves to trap warmth at the surface and in the lower [[atmosphere]]. Some extent of greenhouse gases are necessary to maintain life at a livable temperature on Earth. These greenhouse gas molecules (principally [[water vapor]], [[carbon dioxide]], and [[methane]]) absorb and re-emit long-wave [[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).  

The nature of a greenhouse gas is to pass [[visible light]], i.e., incoming energy from the sun, but to be opaque to the [[infrared]], i.e., heat energy being re-radiated outward from the earth's surface. Greenhouse gases, by definition, are good absorbers at these wavelengths. This functions not at all like the [[glass]] on a [[man-made]] [[greenhouse]], and serves to trap warmth at the surface and in the lower [[atmosphere]]. Some extent of greenhouse gases are necessary to maintain life at a livable temperature on Earth. These greenhouse gas molecules (principally [[water vapor]], [[carbon dioxide]], and [[methane]]) absorb and re-emit long-wave [[infrared rays]] emitted from the earth's surface into the atmosphere (see [[radiational cooling]]).  

==Solar radiation==

==Solar radiation==

[[Image:158224main earth.jpg|thumb|right|300px|A diagram illustrating the so-called "greenhouse effect."]]

[[Image:158224main earth.jpg|thumb|right|300px|A diagram illustrating the so-called "greenhouse effect."]]

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. Anything with a temperature radiates according to [[Planck's law]],<ref> Thermodynamics says that any body with a temperature above absolute zero will radiate its energy away. [http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BlackHoleThermo/BlackHoleThermo.html] </ref> 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, emitted from the Earth's surface.  

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. Anything with a temperature radiates according to [[Planck's law]],<ref>Thermodynamics says that any body with a temperature above absolute zero will radiate its energy away. [http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BlackHoleThermo/BlackHoleThermo.html]</ref> 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, emitted from the Earth's surface.  

===Infrared radiation===

===Infrared radiation===

==Runaway greenhouse effect==

==Runaway greenhouse effect==

A runaway greenhouse effect has been claimed by some climate scientists, many of whom are involved in [[Climategate]], and have argued that slight increases in trace gases in the Earth's atmosphere would lead to higher air temperatures. These higher temperatures would in turn cause "greenhouse gases" such as [[carbon dioxide]] to have higher concentrations, which in turn would accelerate the [[warming trend]]. Alarming increases of 3 to 15 degrees Fahrenheit have been predicted, as modeled in [[Al Gore|Al Gore's]] [[hockey stick graph]]. Outside of the political sphere, few academics give any credence to this theory, and many scientists have been [[Climategate#Critique_of_the_Hockey_Stick_Reconstruction|critical of the hockey stick reconstruction]].  

A runaway greenhouse effect has been claimed by some climate scientists, many of whom are involved in [[Climategate]], and have argued that slight increases in trace gases in the Earth's atmosphere would lead to higher air temperatures. These higher temperatures would in turn cause "greenhouse gases" such as [[carbon dioxide]] to have higher concentrations, which in turn would accelerate the [[warming trend]]. Alarming increases of 3 to 15 degrees Fahrenheit have been predicted, as modeled in [[Al Gore|Al Gore's]] [[hockey stick graph]]. Outside of the political sphere, few academics give any credence to this theory, and many scientists have been [[Climategate#Critique of the Hockey Stick Reconstruction|critical of the hockey stick reconstruction]].  

MIT professor [[Richard Lindzen]] wrote:

MIT professor [[Richard Lindzen]] wrote:

{{cquote|[[Computer model]]s suggest that a doubling of the concentration of the main greenhouse gas, carbon dioxide, may raise the [[average global temperature]] between 1 and 3° Celsius. However, the numeric equations of computer models do not accurately simulate the effects of a number of possible [[negative feedback]]s. For example, many of the models cannot properly simulate the negative effects that increased [[cloud cover]] would have on the [[radiation balance]] of a warmer Earth. Increasing the Earth's temperature would cause the [[ocean]]s to evaporate greater amounts of water, causing the atmosphere to become cloudier. These extra clouds would then reflect a greater proportion of the Sun's energy back to space reducing the amount of [[solar radiation]] absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the effects of an enhanced greenhouse effect may be counteracted.<ref>http://www.physicalgeography.net/fundamentals/7h.html</ref>}}

{{cquote|[[Computer model]]s suggest that a doubling of the concentration of the main greenhouse gas, carbon dioxide, may raise the [[average global temperature]] between 1 and 3° Celsius. However, the numeric equations of computer models do not accurately simulate the effects of a number of possible [[negative feedback]]s. For example, many of the models cannot properly simulate the negative effects that increased [[cloud cover]] would have on the [[radiation balance]] of a warmer Earth. Increasing the Earth's temperature would cause the [[ocean]]s to evaporate greater amounts of water, causing the atmosphere to become cloudier. These extra clouds would then reflect a greater proportion of the Sun's energy back to space reducing the amount of [[solar radiation]] absorbed by the atmosphere and the Earth's surface. With less solar energy being absorbed at the surface, the effects of an enhanced greenhouse effect may be counteracted.<ref>http://www.physicalgeography.net/fundamentals/7h.html</ref>}}

Some scientists have noted a [[Global_warming#Natural_Climate_Change_on_Other_Planets|natural climate change on other planets]]; others have pointed to [[Venus]] as an example of a planet where a runaway greenhouse effect has already occurred. However, Venus has many differences with Earth, including a lack of plate tectonics which make the comparison faulty.  

Some scientists have noted a [[Global warming#Natural Climate Change on Other Planets|natural climate change on other planets]]; others have pointed to [[Venus]] as an example of a planet where a runaway greenhouse effect has already occurred. However, Venus has many differences with Earth, including a lack of plate tectonics which make the comparison faulty.  

==References==

==References==

[[Category:Earth Sciences]]

[[Category:Earth Sciences]]

[[Category:Environmentalism]]

[[Category:Environmentalism]]

[[Category : Gardening Glossary]]

[[Category:Gardening Glossary]]