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The term greenhouse effect may be used to refer to two different things in common parlance: the natural greenhouse effect, which refers to the greenhouse effect which occurs naturally on earth, and the enhanced (anthropogenic) greenhouse effect, which results from human activities (see also global warming). The former is accepted by all; the latter is a matter of some dispute.
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.
The solar power hitting earth must be continually balanced by an equal amount of power radiating from the earth, or the earth's temperature would increase without limit. The radiation leaving the earth takes two forms: reflected solar radiation and thermal blackbody radiation.
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.
The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave UV 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 and land.
Like the sun, the earth is a thermal blackbody radiator. But 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.
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.
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.
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 compensates for the atmospheric absorption and a new equilibrium temperature is reached.
Any change to the earth's atmosphere that impedes or facilitates the transmission of longwave infrared radiation will upset this equilibrium, and the earth's surface will warm or cool until a new equilibrium temperature is reached.
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.
It is commonplace for over-simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. [1]), but this is an incorrect oversimplification: see below.
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.
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.
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