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A schematic representation of the exchanges of energy between outer space, the Earth's atmosphere, and the Earth's surface. The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect.

The greenhouse effect is caused by an atmosphere containing gases that absorb and emit infrared radiation. Greenhouse gases trap heat within the surface-troposphere system, causing heating at the surface of the planet or moon.^[1][2] This mechanism is fundamentally different from that of an actual greenhouse, which works by isolating warm air inside the structure so that heat is not lost by convection. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.^[3]

The black body temperature of the Earth is 5.5 °C.^[4] Since the Earth's surface reflects about 28% of incoming sunlight^[5], in the absence of the greenhouse effect the planet's mean temperature would be far lower - about -18 or -19 °C ^[6][7] instead of the much higher current mean temperature, about 14 °C.^[8]

Global warming, a recent warming of the Earth's surface and lower atmosphere,^[9] is believed to be the result of an "enhanced greenhouse effect" mostly due to human-produced increases in atmospheric greenhouse gases.^[10] This human induced part is referred to as anthropogenic global warming (AGW).

Basic mechanism

The Earth receives energy from the Sun mostly in the form of visible light and nearby wavelengths. About 50% of the sun's energy is absorbed at the Earth's surface, the rest is reflected or absorbed by the atmosphere. The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as the Idealized greenhouse model, show this heat being lost as thermal radiation. The reality is somewhat more complex: the atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for "window" bands), and most heat loss from the surface is by sensible heat and latent heat transport. However, radiative effects become increasingly important higher in the atmosphere as the higher levels become progressively more transparent to thermal radiation, largely because the atmosphere is drier and water vapor - an important greenhouse gas - becomes less. It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid-troposphere, which is effectively coupled to the surface by a lapse rate.

Within the region where radiative effects are important the description given by the idealized greenhouse model becomes realistic: a layer of atmosphere with greenhouses gases will absorb heat being radiated upwards from lower layers, and re-radiate it in all directions, both upwards and downwards. In order to achieve thermal equilibrium, this results in a warmer surface below, in order still to radiate enough heat back out into deep space from the upper layers. Increasing the concentration of the gases increases the amount of absorption and re-radiation, and thereby leads to a still warmer surface.^[7]

Greenhouse gases

By their percentage contribution to the greenhouse effect^[11][12] the four major gases are:

• water vapor, 36–70%
• carbon dioxide, 9–26%
• methane, 4–9%
• ozone, 3–7%

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the atmosphere.^[12]

Enhanced greenhouse effect

When it comes to the physical processes that produce the greenhouse effect, increases that are caused by human activities are known as the enhanced (or anthropogenic) greenhouse effect.^[13] This increase in radiative forcing from human activity is contributed to mostly by increased atmospheric carbon dioxide levels.^[14]

CO₂ is produced by fossil fuel burning and other activities such as cement production and tropical deforestation.^[15] Measurements of CO₂ from the Mauna Loa observatory show that concentrations have increased from about 313 ppm ^[16] in 1960 to about 383 ppm in 2009. The current observed amount of CO₂ exceeds the geological record maxima (~300 ppm) from ice core data.^[17] The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first described in 1896 by Svante Arrhenius, has also been called the Callendar effect.

Because it is a greenhouse gas, elevated CO₂ levels contribute to additional absorption and emission of thermal infrared in the atmosphere, which could contribute to net warming. In fact, according to Assessment Reports from the Intergovernmental Panel on Climate Change, "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".^[18]

Over the past 800,000 years,^[19] ice core data shows unambiguously that carbon dioxide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm.^[20] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.^[21]

The distinction between the greenhouse effect and real greenhouses

A modern Greenhouse in RHS Wisley

The "greenhouse effect" is named by analogy to greenhouses but this is a misnomer. The greenhouse effect and a real greenhouse are similar in that they both limit the rate of thermal energy flowing out of the system, but the mechanisms by which heat is retained are different. A greenhouse works primarily by preventing absorbed heat from leaving the structure through convection, i.e. sensible heat transport. The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.

A greenhouse is built of any material that passes 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. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (R. W. Wood, 1909) that a "greenhouse" with a cover of rock salt (which is transparent to infra red) heats up an enclosure similarly to one with a glass cover.^[22] Thus greenhouses work primarily by preventing convective cooling.^[23][24]

In the greenhouse effect, rather than retaining (sensible) heat by physically preventing movement of the air, greenhouse gases act to warm the Earth by re-radiating some of the energy back towards the surface. This process exists in real greenhouses, but is comparatively unimportant there.

Bodies other than Earth

In our solar system, Mars, Venus, and the moon Titan also exhibit greenhouse effects. Titan has an anti-greenhouse effect, in that its atmosphere absorbs solar radiation but is relatively transparent to infrared radiation. Pluto also exhibits behavior similar to the anti-greenhouse effect.^[25][26][27]

A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.^[28] A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.^[29]

See also

• Anti-greenhouse effect
• Climate change
• Climate forcing
• Earth's energy budget
• Earth's radiation balance
• Global dimming
• Global warming
• Idealized greenhouse model

Literature

• Earth Radiation Budget, http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
• Fleagle, RG and Businger, JA: An introduction to atmospheric physics, 2nd edition, 1980
• IPCC assessment reports, see http://www.ipcc.ch/
• Ann Henderson-Sellers and McGuffie, K: A climate modelling primer (quote: Greenhouse effect: the effect of the atmosphere in re-readiating longwave radiation back to the surface of the Earth. It has nothing to do with glasshouses, which trap warm air at the surface).
• Idso, S.B.: "Carbon Dioxide: friend or foe," 1982 (quote: ...the phraseology is somewhat in appropriate, since CO₂ does not warm the planet in a manner analogous to the way in which a greenhouse keeps its interior warm).
• Kiehl, J.T., and Trenberth, K. (1997). "Earth's annual mean global energy budget," Bulletin of the American Meteorological Society '78' (2), 197–208.

References