Greenhouse effect

The greenhouse effect, first discovered by Joseph Fourier in 1824, is the process by which an atmosphere warms a planet.

The natural greenhouse effect

Process

The earth receives an enormous amount of solar radiation. Just above the atmosphere, the solar power flux density averages about 1367 watts/m2, or 1.28 * 1014 watts over the entire earth. This figure vastly exceeds the power generated by human activities.

Related Topics:
Earth - Solar radiation

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The solar power hitting earth is balanced over time by a roughly equal amount of power radiating from the earth (as the amount of energy from the sun that is stored is small). Almost all radiation leaving the earth takes two forms: reflected solar radiation and thermal blackbody radiation.

Related Topics:
Solar radiation - Blackbody

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Reflected solar radiation accounts for 30% of the earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface.

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The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave ultraviolet over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans. This absorbed energy heats the atmosphere, oceans, land and powers life on the planet.

Related Topics:
Ultraviolet - Ozone layer

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Like the sun, the earth is a thermal blackbody radiator. So because the earth's surface is much cooler than the sun (287 K vs 5780 K), Wien's displacement law dictates that the earth must radiate its thermal energy at much longer wavelengths than the sun. While the sun's radiation peaks at a visible wavelength of 500 nanometers, earth's radiation peak is in the longwave (far) infrared at about 10 micrometres.

Related Topics:
Wien's displacement law - Infrared

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The earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres. Only about 6% of the earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a blackbody radiator in the far infrared, so it re-radiates this energy.

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The earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of the earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor.

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Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over the earth's surface. Visible and near-visible radiation from the sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, the earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption.

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The surface of the Earth is in constant flux with daily, yearly, and ages long cycles and trends in temperature and other variables from a variety of causes.

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The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the earth's surface temperature were determined solely by the albedo and blackbody properties of the surface.

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It is commonplace for simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. http://www.epa.gov/globalwarming/kids/greenhouse.html), but this is an incorrect oversimplification: see below.

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Limiting factors

The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth creates habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.

Related Topics:
Greenhouse gas - Carbon dioxide - Venus - Lead - Earth - Mars

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The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO2 perturbation is on the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect.

Related Topics:
Feedback cycles - Water - Carbon dioxide - Hydrosphere - Biosphere - Carbon - Warming of an aging Sun

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The average surface temperature would be -18°C without a greenhouse effect or 72°C with just the greenhouse effect and no convection, but in reality this temperature is closer to 15°C due to convective flow of heat energy within the atmosphere and partly above much of the thermal IR absorbence of the atmosphere. http://eaps.mit.edu/faculty/lindzen/cooglobwrm.pdf

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The greenhouse gases

Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%) http://www.geo.utexas.edu/courses/388G/KiehlTrenbertth97.pdf. Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect.

Related Topics:
Water vapor - Carbon dioxide - Methane - Nitrous oxide - Ozone - Greenhouse gas - Callendar effect

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The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on molecular properties of the different gas molecules. It so happens that heteronuclear diatomic molecules and tri- (and more) atomic gases absorb at infrared wavelengths but homonuclear diatomic molecules do not absorb infrared light. This is why H2O and CO2 are greenhouse gases but the major atmospheric constituents (N2 and O2) are not.

Related Topics:
Light - Gas - Quantum mechanics - Molecules

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Between the absorptions of water vapor and those of carbon dioxide, there is an atmospheric window where, prior to the industrial era, no infrared radiation was trapped, lying between 8 and 15 micrometres. Compounds such as perflurocarbons (CF4, C2F6 etc.), chlorofluorocarbons, halons and SF6 absorb very strongly in this window. This means that they are extremely potent greenhouse gases, especially given the absence of natural sinks to remove them. Perfluorocarbons can have a lifetime of 50,000 years, possibly longer.

Related Topics:
Atmospheric window - Micrometres - Chlorofluorocarbons - Halons

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