Solar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface, which then emits this energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which in turn re-radiate much of the energy to the surface and lower atmosphere. The mechanism is named after the effect of solar radiation passing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by reducing air flow, isolating the warm air inside the structure so that heat is not lost by convection.
If an ideal thermally conductive blackbody was the same distance from the sun as the Earth is, it would have a temperature of about 5.3 °C. However, since the Earth reflects about 30% of the incoming sunlight, this idealized planet's effective temperature (the temperature of a black body that would emit the same amount of radiation) would be about -18 °C. the surface temperature of this hypothetical planet is 33 °C below the Earth's actual surface temperature of approximately 14 °C. The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect.
Earth's natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil fuels and clearing of forests, have intensified the natural greenhouse effect, causing global warming.
By their percentage contribution to the greenhouse effect on Earth, the four major gases are:
- water vapor, 36-70%
- carbon dioxide, 9-26%
- methane, 4-9%
- ozone, 3-7%
Greenhouse gas
A greenhouse (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases greatly affect the temperature of the Earth; without them, the Earth's surface would average about 33 °C colder than the present average of 14 °C (57 °F).
Since the beginning of the Industrial Revolution, the burning of fossil fuels has contributed to a 40% increase in the concentration of carbon dioxide in the atmosphere from 280ppm to 397ppm, despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle. Anthropogenic carbon dioxide (CO²) emissions (i.e., emissions produced by human activities) come from combustion of carbon-based fuels, principally wood, coal, oil, and natural gas.
Greenhouse
A greenhouse is a structural building with different types of covering materials that pass sunlight, usually glass, or plastic. It mainly heats up because the sun warms the ground inside, which then warms the air in the greenhouse. The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. In addition, the warmed structures and plants inside the greenhouse re-radiate some of their thermal energy in the infrared spectrum, to which glass is partly opaque, so some of this energy is also trapped inside the greenhouse. However, this latter process is a minor player compared with the former (convective) process. Thus, the primary heating mechanism of a greenhouse is convection. This can be demonstrated by opening a small window near the roof of a greenhouse; the temperature drops considerably. This principle is the basis of the autovent automatic cooling system. Thus, the glass used for a greenhouse works as a barrier to air flow, and its effect is to trap energy within the greenhouse. The air that is warmed near the ground is prevented from rising indefinitely and flowing away.
Although heat loss due to thermal conduction through the glass and other building materials occurs, net energy (and therefore temperature) increases inside the greenhouse.
Convection
Convection is the concerted, collective movement of ensembles of molecules within fluids (e.g., liquids, gases) and rheids. It is the circulatory motion that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity.
Blackbody
A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.
Real materials emit energy at a fraction ‒ called the emissivity ‒ blackbody energy levels. By definition, a blackbody in thermal equilibrium has an emissivity of ε = 1.0. A source with lower emissivity independent of frequency often is referred to as a gray body. Construction of blackbodies with emissivity as close to one as possible remains a topic of current interest. A white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a blackbody that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's temperature when the body's emissivity curve (as a function of wavelength) is not known.
When the star's or planet's net emissivity in the relevant wavelength band is less than unity (less than that of a blackbody), the actual temperature of the body will be higher than the effective temperature. The net emissivity may be low due to surface or atmospheric properties, including the greenhouse effect.
Emissivity
The emissivity of the material (usually written ε or e) is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a blackbody at the same temperature. A true blackbody would have an ε = 1 while any real object would have ε < 1. Emissivity is a dimensionless quantity.
In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02.
Sources:
http://en.wikipedia.org/wiki/Greenhouse_effect
http://en.wikipedia.org/wiki/Greenhouse_gas
http://en.wikipedia.org/wiki/Greenhouse
http://en.wikipedia.org/wiki/Convection
http://en.wikipedia.org/wiki/Blackbody
http://en.wikipedia.org/wiki/Effective_temperature
http://en.wikipedia.org/wiki/Emissivity
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