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1. Greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's
Sun goes through its atmosphere and warms the planet's surface, but the atmosphere
prevents the heat from returning directly to space, resulting in a warmer planet. Light
arriving from our Sun passes through Earth's atmosphere and warms its surface. The
warmed surface then radiates heat, absorbed by greenhouse gasses such as carbon
dioxide. Earth's average temperature would be well below freezing without the natural
greenhouse effect. Current human-caused increases in greenhouse gasses trap greater
amounts of heat, causing the Earth to grow warmer over time.
Anything warmed radiates energy related to its temperature – the Sun at about 5,500 °C
(9,930 °F) sends most as visible and near-infrared light, while Earth's average surface
temperature about 15 °C (59 °F) emits longer wavelength infrared radiant heat. The
atmosphere is transparent to most incoming sunlight and allows its energy to the
surface. The term greenhouse effect comes from a flawed analogy comparing this to
transparent glass allowing sunlight into greenhouses, but greenhouses mainly retain
heat by restricting air movement, unlike this effect. Most of the atmosphere is
transparent to infrared, but a tiny proportion of greenhouse gasses makes it almost
completely opaque to wavelengths emitted by the surface. Greenhouse gas molecules
absorb and emit this infrared, so heat up and emit radiant heat in all directions, warming
other greenhouse gas molecules and passing heat on to the surrounding air. Radiant
heat going downwards further increases the surface temperature, adding to energy
going up into the atmosphere. Without Earth's natural greenhouse effect, the Earth
would be more than 30 °C (54 °F) colder.
Sunlight varies day and night, by season, and distance from the equator. Half of the
available sunlight is reflected from clouds and the Earth's surface, depending on their
reflectivity. Greenhouse gasses vary in effect, time in the atmosphere, and altitude,
leading to positive feedback. Variations are evened out by Earth's heat engine, causing
energy flows. Eventually, higher layers of the atmosphere tend to emit about as much
energy into space as is arriving from the Sun, forming Earth's energy balance. A
runaway greenhouse effect occurs if positive feedback leads to the evaporation of
greenhouse gasses into the atmosphere, as happened with carbon dioxide and water
vapour on Venus.
History
The existence of the greenhouse effect, while not named as such, was proposed by
Joseph Fourier in 1824. Claude Pouillet further strengthened the argument and the
evidence in 1827 and 1838. John Tyndall was the first to measure various gasses and
vapours' infrared absorption and emission. From 1859 onwards, he showed that the
effect was due to a very small proportion of the atmosphere, with the main gasses
having no effect, and was largely due to water vapour, though small percentages of

2. hydrocarbons and carbon dioxide had a significant effect. The effect was more fully
quantified by Svante Arrhenius in 1896, who made the first quantitative prediction of
global warming due to a hypothetical doubling of atmospheric carbon dioxide. However,
the term "greenhouse" was not used to refer to this effect by any of these scientists; the
term was first used in this way by Nils Gustaf Ekholm in 1901
Description
The infrared radiative effect of all infrared-absorbing constituents in the atmosphere.
Greenhouse gasses (GHGs), clouds, and aerosols absorb terrestrial radiation emitted
by the Earth's surface and elsewhere in the atmosphere. These substances emit
infrared radiation in all directions, but everything else is equal. The net amount emitted
to space is normally less than would have been emitted without these absorbers
because of the decline of temperature with altitude in the troposphere and the
consequent weakening of emission. An increase in the concentration of GHGs
increases the magnitude of this effect; the difference is sometimes called the enhanced
greenhouse effect. Because of anthropogenic emissions, the change in GHG
concentration contributes to an instantaneous radiative forcing. Earth's surface
temperature and troposphere warm in response to this forcing, gradually restoring the
radiative balance at the top of the atmosphere.
Earth receives energy from the Sun in ultraviolet, visible, and near-infrared radiation.
About 26% of the incoming solar energy is reflected in space by the atmosphere and
clouds, and 19% is absorbed. Most of the remaining energy is absorbed in the surface
of Earth. Because the Earth's surface is colder than the Sun, it radiates at wavelengths
much longer than the wavelengths absorbed. Most of this thermal radiation is absorbed
by the atmosphere and warms it. The atmosphere also gains heat by sensible and
latent heat fluxes from the surface. The atmosphere radiates energy both upwards and
downwards; the part radiated downwards is absorbed by the surface of Earth. This
leads to a higher equilibrium temperature than if the atmosphere did not radiate.
An ideal thermally conductive blackbody at the same distance from the Sun as Earth
would have a temperature of about 5.3 °C (41.5 °F). However, because Earth reflects
about 30% of the incoming sunlight, this idealized planet's effective temperature (the
temperature of a blackbody that would emit the same amount of radiation) would be
about −18 °C (0 °F). The surface temperature of this hypothetical planet is 33 °C (59 °F)
below Earth's actual surface temperature of approximately 14 °C (57 °F). The
greenhouse effect contributes greenhouse gasses and aerosols to this difference, with
imperfect modelling of clouds being the main uncertainty.
Details
The idealized greenhouse model is a simplification. In reality, the atmosphere near the
Earth's surface is largely opaque to thermal radiation, and most heat loss from the
surface is by convection. However, radiative energy losses become increasingly
important in the atmosphere, largely because of the decreasing concentration of water

3. vapour, an important greenhouse gas. Rather than the surface itself, it is more realistic
to think of the greenhouse effect as applying to a layer in the mid-troposphere, which is
effectively coupled to the surface by a lapse rate. A simple picture also assumes a
steady state, but the diurnal and seasonal cycles and weather disturbances complicate
matters in the real world. Solar heating applies only during the daytime. During the
night, the atmosphere cools somewhat, but not great, because its emissivity is low.
Diurnal temperature changes decrease with height in the atmosphere.
Within the region where radiative effects are important, the description given by the
idealized greenhouse model becomes realistic. Earth's surface, warmed to an "effective
temperature" around −18 °C (0 °F), radiates long-wavelength infrared heat in the range
of 4–100 μm. Greenhouse gasses largely transparent to incoming solar radiation are
more absorbent at these wavelengths. With greenhouse gasses, each layer of the
atmosphere absorbs some heat radiated upwards from lower layers. It reradiates in all
directions, upwards and downwards, in equilibrium (by definition) the same amount it
has absorbed. This results in more warmth below. Increasing the concentration of the
gasses increases the amount of absorption and re-radiation and thereby further warms
the layers and ultimately the surface below. Greenhouse gasses can absorb infrared
radiation, including most diatomic glasses with two different atoms (such as carbon
monoxide, CO) and all gasses with three or more atoms. Though more than 99% of the
dry atmosphere is IR transparent (because the main constituents—N
2, O
2, and Ar—cannot directly absorb or emit infrared radiation), intermolecular collisions
cause the energy absorbed and emitted by the greenhouse gasses to be shared with
the other, non-IR-active gasses.
Greenhouse gasses
By their percentage contribution to the greenhouse effect on Earth, the four major
gasses are:
Atmospheric gasses only absorb some wavelengths of energy but are transparent to
others. The absorption patterns of water vapour (blue) and carbon dioxide (pink) overlap
in some wavelengths. Carbon dioxide is not as strong a greenhouse gas as water
vapour, but it absorbs energy in longer wavelengths (12–15 micrometres) than water
vapour does not, partially closing the "window" through which heat radiated by the
surface would normally escape to space. (Illustration NASA, Robert Rohde)
● water vapour, ~50% (~75% including clouds)
● carbon dioxide, 9–26%
● methane, 4–9%
● ozone, 3–7%
It is impossible to assign a specific percentage to each gas because the absorption and
emission bands of the gasses overlap (hence the ranges given above). Also, a water

4. molecule only stays in the atmosphere for an average of 8 to 10 days, which
corresponds with high variability in the contribution from clouds and humidity at any
particular time and location.
The other most important is nitrous oxide (N2O), perfluorocarbons (PFCs),
chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), sulfur hexafluoride (SF6).
Clouds
Clouds are special forms of water that are highly influential to the Earth's energy budget.
Clouds absorb and emit infrared radiation and thus affect the atmosphere's radiative
properties. The effect of clouds is dependent on the type of clouds. Specific types of
clouds can have great contributions to the greenhouse effect. Higher clouds usually
have a larger greenhouse effect, and there is tropical high-cloud altitude feedback.
Aerosols
A few aerosols absorb solar radiation, the most important being black carbon, on which
research is ongoing as it causes several effects, not just the greenhouse effect.
Role in climate change
The strengthening of the greenhouse effect through human activities is known as the
enhanced (or anthropogenic) greenhouse effect. As well as being inferred from
measurements by the CERES satellite throughout the 21st century, this increase in
radiative forcing from human activity has been observed directly and is attributable
mainly to increased atmospheric carbon dioxide levels. According to the 2014
Assessment Report from the Intergovernmental Panel on Climate Change,
"atmospheric concentrations of carbon dioxide, methane and nitrous oxide are
unprecedented in at least the last 800,000 years. Their effects, together with those of
other anthropogenic drivers, have been detected throughout the climate system and are
extremely likely to have been the dominant cause of the observed warming since the
mid-20th century."
CO2 is produced by fossil fuel burning and other activities such as cement production
and tropical deforestation Measurements of CO2 from the Mauna Loa Observatory
show that concentrations have increased from about 313 parts per million (ppm) in
1960, passing the 400 ppm milestone in 2013. The current observed amount of CO2
exceeds the geological record maxima (≈300 ppm) from ice core data. 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.

5. Over the past 800,000 years, ice core data shows that carbon dioxide has varied from
values as low as 180 ppm to the pre-industrial level of 270 ppm. Paleoclimatologists
consider variations in carbon dioxide concentration to be a fundamental factor
influencing climate variations over this time scale.
Real greenhouses
The "greenhouse effect" of the atmosphere is named by analogy to greenhouses that
become warmer in sunlight. However, a greenhouse is not primarily warmed by the
"greenhouse effect. Greenhouse effect" is a misnomer since heating in the usual
greenhouse is due to the reduction of convection, while the "greenhouse effect" works
by preventing absorbed heat from leaving the structure through radiative transfer.
A greenhouse is built of any material that passes sunlight: usually glass or plastic. The
Sun warms the ground and contents inside just like the outside, warming the air.
Outside, the warm air near the surface rises and mixes with cooler air aloft, keeping the
temperature lower than inside, where the air continues to heat up because it is confined
within the greenhouse. This can be demonstrated by opening a small window near the
roof of a greenhouse: the temperature will drop considerably. It was demonstrated
experimentally (R. W. Wood, 1909) that a (not heated) "greenhouse" with a cover of
rock salt (which is transparent to infrared) heats an enclosure similar to one with a glass
cover.] Thus greenhouses work primarily by preventing convective cooling
Heated greenhouses are yet another matter: as they have an internal heating source, it
is desirable to minimize the amount of heat leaking out by radiative cooling. This can be
done through the use of adequate glazing.
In theory, it is possible to build a greenhouse that lowers its thermal emissivity during
dark hours; such a greenhouse would trap heat by two different physical mechanisms,
combining multiple greenhouse effects, one of which more closely resembles the
atmospheric mechanism, rendering the misnomer debate moot.
Related effects
Anti-greenhouse effect
The anti-greenhouse effect is a mechanism similar and symmetrical to the greenhouse
effect: in the greenhouse effect, the atmosphere lets radiation in a while not letting
thermal radiation out, thus warming the body surface; in the anti-greenhouse effect, the
atmosphere keeps radiation out while letting thermal radiation out, which lowers the
equilibrium surface temperature. Such an effect has been proposed for Saturn's moon
Titan.
Runaway greenhouse effect
A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all
greenhouse gasses into the atmosphere. A runaway greenhouse effect involving carbon
dioxide and water vapour has long ago been hypothesized to have occurred on Venus;

6. this idea is still largely accepted. The planet Venus experienced a runaway greenhouse
effect, resulting in an atmosphere of 96% carbon dioxide and a surface atmospheric
pressure roughly the same as found 900 m (3,000 ft) underwater on Earth. Venus may
have had water oceans, but they would have boiled off as the mean surface
temperature rose to the current 735 K (462 °C; 863 °F).
Bodies other than Earth
The 'greenhouse effect' on Venus is particularly large for several reasons:
1. It is nearer to the Sun than Earth by about 30%.
2. It's very dense atmosphere consists mainly of carbon dioxide.
"Venus experienced a runaway greenhouse in the past, and we expect that Earth will in
about 2 billion years as solar luminosity increases. "Titan is a body with both a
greenhouse effect and an anti-greenhouse effect. The presence of N2, CH4, and H2 in
the atmosphere contribute to a greenhouse effect, increasing the surface temperature
by 21K over the expected temperature of the body with no atmosphere. The existence
of a high-altitude haze, which absorbs wavelengths of solar radiation but is transparent
to infrared, contributes to an anti-greenhouse effect of approximately 9K. The net effect
of these two phenomena results in a net warming of 21K- 9K= 12K, so Titan is 12 K
warmer than it would be if there were no atmosphere.