The greenhouse effect is a process by which radiative energy leaving a planetary surface is absorbed by some atmospheric gases, called greenhouse gases. They transfer this energy to other components of the atmosphere, and it is re-radiated in all directions, including back down towards the surface. This transfers energy to the surface and lower atmosphere, so the temperature there is higher than it would be if direct heating by solar radiation were the only warming mechanism [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 byJohn Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.[3]
If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth, it would have an expected blackbody temperature of 5.3 °C. However, since the Earth reflects about 30%[4] (or 28%[5]) of the incoming sunlight, the planet's actual blackbody temperature is about -18 or -19 °C [6][7], about 33°C below the actual surface temperature of about 14 °C or 15 °C.[8] The mechanism that produces this difference between the actual temperature and the blackbody temperature is due to the atmosphere and is known as the greenhouse effect.
Global warming, a recent warming of the Earth's surface and lower atmosphere,[9] is believed to be the result of a strengthening of the greenhouse effect mostly due to human-produced increases in atmospheric greenhouse gases.[10]
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Basic mechanism
The Earth receives energy from the Sun in the form of visible light. This light is absorbed at the Earth's surface, and re-radiated as thermal radiation. Some of this thermal radiation is absorbed by the atmosphere, and re-radiated both upwards and downwards; that radiated downwards is absorbed by the Earth's surface. Thus the presence of the atmosphere results in the surface receiving more radiation than it would were the atmosphere absent; and it is thus warmer than it would otherwise be.
This highly simplified picture of the basic mechanism needs to be qualified in a number of ways, none of which affect the fundamental process.
- The incoming radiation from the Sun is mostly in the form of visible light and nearby wavelengths, largely in the range 0.2 - 4 μm, corresponding to the Sun's radiative temperature of 6,000 K.[11]. This is mostly "visible" light; our eyes are adapted to use the most common radiation.
- About 50% of the Sun's energy is absorbed at the Earth's surface and the rest is reflected or absorbed by the atmosphere. The reflection of light back into space - largely by clouds - does not much affect the basic mechanism; this light, effectively, is lost to the system.
- 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 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 heattransport. Radiative energy losses become increasingly important higher in the atmosphere largely because of the decreasing concentration of water vapor, an important greenhouse gas. 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: The surface of the Earth, warmed to a temperature around 255 K, radiates long-wavelength, infrared heat in the range 4 - 100 μm.[11] At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are more absorbent.[11] Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers. To maintain its own equilibrium, it re-radiates the absorbed heat in all directions, both upwards and downwards. This results in more warmth below, while still radiating enough heat back out into deep space from the upper layers to maintain overall thermal equilibrium. Increasing the concentration of the gases increases the amount of absorption and re-radiation, and thereby further warms the layers and ultimately the surface below.[7]
- The majority of the atmosphere—in particular, O2 and N2 which together form more than 99% of the dry atmosphere—is transparent to infrared radiation. Only triatomic (and higher) gases interact with infrared. However, due to intermolecular collisions, the energy absorbed and emitted by the greenhouse gases is effectively shared by the non-radiatively active gases.
- The simple picture assumes equilibrium. In the real world there is the diurnal cycle as well as seasonal cycles and weather. Solar heating only applies during daytime. During the night, the atmosphere cools somewhat, but not greatly, because its emissivity is low, and during the day the atmosphere warms. Diurnal temperature changes decrease with height in the atmosphere.
Greenhouse gases
By their percentage contribution to the greenhouse effect on Earth the four major gases are:[12][13]
- 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.[13]
Role in climate change
Strengthening of the greenhouse effect through human activities is known as the enhanced (oranthropogenic) greenhouse effect.[14] This increase in radiative forcing from human activity is attributable mainly to increased atmospheric carbon dioxide levels.[15]
CO2 is produced by fossil fuel burning and other activities such as cement production and tropical deforestation.[16] Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm [17] in 1960 to about 389 ppm in 2010. The current observed amount of CO2 exceeds the geological record maxima (~300 ppm) from ice core data.[18] 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 emissionof thermal infrared in the atmosphere, which produce net warming. According to the latest Assessment Report 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".[19]
Over the past 800,000 years,[20] 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.[21] Paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.[22][23]
The distinction between the greenhouse effect and real greenhouses
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 throughconvection, 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.[24] Thus greenhouses work primarily by preventingconvective cooling.[25][26]
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 may exist 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.[27] Titan has an anti-greenhouse effect, in that its atmosphere absorbs solar radiation but is relatively transparent to infrared radiation. Pluto also exhibits behavior superficially similar to the anti-greenhouse effect.[28][29]
A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.[30] A runaway greenhouse effect involving carbon dioxide and water vapor is thought to have occurred on venus
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