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Greenhouse effect

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   A schematic representation of the exchanges of energy between outer
   space, the Earth's atmosphere, and the Earth surface. The ability of
   the atmosphere to capture and recycle energy emitted by the Earth
   surface is the defining characteristic of the greenhouse effect.
   A schematic representation of the exchanges of energy between outer
   space, the Earth's atmosphere, and the Earth 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, discovered by Joseph Fourier in 1824 and first
   investigated quantitatively by Svante Arrhenius in 1896, is the process
   in which the emission of infrared radiation by an atmosphere warms a
   planet's surface. The name comes from an incorrect analogy with the
   warming of air inside a greenhouse compared to the air outside the
   greenhouse. The Earth's average surface temperature is about 20-30°C
   warmer than it would be without the greenhouse effect . In addition to
   the Earth, Mars and especially Venus have greenhouse effects.

   In common usage, "greenhouse effect" may refer either to the natural
   greenhouse effect due to naturally occurring greenhouse gases, or to
   the enhanced ( anthropogenic) greenhouse effect which results from
   gases emitted as a result of human activities (see also global warming,
   scientific opinion on climate change and attribution of recent climate
   change).

The basic mechanism

   The Earth receives energy from the Sun in the form of radiation. The
   Earth reflects about 30% of the incident solar flux; the remaining 70%
   is absorbed, warming the land, atmosphere and oceans.

   To the extent that the Earth is in a steady state, the energy stored in
   the atmosphere and ocean does not change in time, so energy equal to
   the absorbed solar radiation must be radiated back to space. Earth
   radiates energy into space as black-body radiation, which maintains a
   thermal equilibrium. This thermal, infrared radiation increases with
   increasing temperature. One can think of the Earth's temperature as
   being determined by the infrared flux needed to balance the absorbed
   solar flux.
   Solar radiation at top of atmosphere and at Earth's surface.
   Solar radiation at top of atmosphere and at Earth's surface.

   The visible solar radiation heats the surface, not the atmosphere,
   whereas most of the infrared radiation escaping to space is emitted
   from the upper atmosphere, not the surface. The infrared photons
   emitted by the surface are mostly absorbed by the atmosphere and do not
   escape directly to space.
   Atmospheric transmittance of various wavelengths of electromagnetic
   radiation (measured along sea level).
   Atmospheric transmittance of various wavelengths of electromagnetic
   radiation (measured along sea level).

   The reason this warms the surface is most easily understood by starting
   with a simplified model of a purely radiative greenhouse effect that
   ignores energy transfer in the atmosphere by convection (sensible heat
   transport) and by the evaporation and condensation of water vapor (
   latent heat transport). In this purely radiative case, one can think of
   the atmosphere as emitting infrared radiation both upwards and
   downwards. The upward infrared flux emitted by the surface must balance
   not only the absorbed solar flux but also this downward infrared flux
   emitted by the atmosphere. The surface temperature will rise until it
   generates thermal radiation equivalent to the sum of these two incident
   radiation streams.

   A more realistic picture taking into account the convective and latent
   heat fluxes is somewhat more complex. But the following simple model
   captures the essence. The starting point is to note that the opacity of
   the atmosphere to infrared radiation determines the height in the
   atmosphere from which most of the photons emitted to space are emitted.
   If the atmosphere is more opaque, the typical photon escaping to space
   will be emitted from higher in the atmosphere, because one then has to
   go to higher altitudes to see out to space in the infrared. Since the
   emission of infrared radiation is a function of temperature, it is the
   temperature of the atmosphere at this emission level that is
   effectively determined by the requirement that the emitted flux balance
   the absorbed solar flux.

   But the temperature of the atmosphere generally decreases with height
   above the surface, at a rate of roughly 6.5 °C per kilometer on
   average, until one reaches the stratosphere 10-15 km above the surface.
   (Most infrared photons escaping to space are emitted by the
   troposphere, the region bounded by the surface and the stratosphere, so
   we can ignore the stratosphere in this simple picture.) A very simple
   model, but one that proves to be remarkably useful, involves the
   assumption that this temperature profile is simply fixed, by the
   non-radiative energy fluxes. Given the temperature at the emission
   level of the infrared flux escaping to space, one then computes the
   surface temperature by increasing temperature at the rate of 6.5 °C per
   kilometer, the environmental lapse rate, until one reaches the surface.
   The more opaque the atmosphere, and the higher the emission level of
   the escaping infrared radiation, the warmer the surface, since one then
   needs to follow this lapse rate over a larger distance in the vertical.
   While less intuitive than the purely radiative greenhouse effect, this
   less familiar radiative-convective picture is the starting point for
   most discussions of the greenhouse effect in the climate modeling
   literature.

   The term "greenhouse effect" is a source of confusion in that actual
   greenhouses do not warm by this same mechanism (e.g. ).

The greenhouse gases

   Quantum mechanics provides the basis for computing the interactions
   between molecules and radiation. Most of this interaction occurs when
   the frequency of the radiation closely matches that of the spectral
   lines of the molecule, determined by the quantization of the modes of
   vibration and rotation of the molecule. (The electronic excitations are
   generally not relevant for infrared radiation, as they require energy
   larger than that in an infrared photon.)

   The width of a spectral line is an important element in understanding
   its importance for the absorption of radiation. In the Earth’s
   atmosphere these spectral widths are primarily determined by “pressure
   broadening”, which is the distortion of the spectrum due to the
   collision with another molecule. Most of the infrared absorption in the
   atmosphere can be thought of as occurring while two molecules are
   colliding. The absorption due to a photon interacting with a lone
   molecule is relatively small. This three-body aspect of the problem,
   one photon and two molecules, makes direct quantum mechanical
   computation for molecules of interest more challenging. Careful
   laboratory spectroscopic measurements, rather than ab initio quantum
   mechanical computations, provide the basis for most of the radiative
   transfer calculations used in studies of the atmosphere.

   The molecules/atoms that constitute the bulk of the atmosphere; oxygen
   (O[2]), nitrogen (N[2]) and argon; do not interact with infrared
   radiation significantly. While the oxygen and nitrogen molecules can
   vibrate, because of their symmetry these vibrations do not create any
   transient charge separation that enhances the interaction with
   radiation. In the Earth’s atmosphere, the dominant infrared absorbing
   gases are water vapor, carbon dioxide, and ozone (O[3]), these
   molecules being “floppier” so that their rotation/vibration modes are
   more easily excited. For example, carbon dioxide is a linear molecule,
   but it has an important vibrational mode in which the molecule bends
   with the carbon in the middle moving one way and the oxygens on the
   ends moving the other way, creating some charge separation, a dipole
   moment. A substantial part of the greenhouse effect due to carbon
   dioxide exists because this vibration is easily excited by infrared
   radiation. Clouds are also very important infrared absorbers.
   Therefore, water has multiple effects on infrared radiation, through
   its vapor phase and through its condensed phases. Other absorbers of
   significance include methane, nitrous oxide and the
   chlorofluorocarbons.

   Discussion of the relative importance of different infrared absorbers
   is confused by the overlap between the spectral lines due to different
   gases, widened by pressure broadening. As a result, the absorption due
   to one gas cannot be thought of as independent of the presence of other
   gases. One convenient approach is to remove the chosen constituent,
   leaving all other absorbers, and the temperatures, untouched, and
   monitoring the infrared radiation escaping to space. The reduction in
   infrared absorbtion is then a measure of the importance of that
   constituent. More precisely, define the greenhouse effect (GE) to be
   the difference between the infrared radiation that the surface would
   radiate to space if there were no atmosphere and the actual infrared
   radiation escaping to space. Then compute the percentage reduction in
   GE when a constituent is removed. The table below is computed by this
   method, using a particular 1-dimensional model of the atmosphere. More
   recent 3D computations lead to similar results.

                     Gas removed
                                percent reduction in GE
                     H[2]O      36%
                     CO[2]      12%
                     O[3]       3%

    (Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465
                             (1978)); see also .

   By this particular measure, water vapor can be thought of as providing
   36% of the greenhouse effect, and carbon dioxide 12%, but the effect of
   removal of both of these constituents will be greater than 48%. An
   additional proviso is that these numbers are computed holding the cloud
   distribution fixed. But removing water vapor from the atmosphere while
   holding clouds fixed is not likely to be physically relevant. In
   addition, the effects of a given gas are typically nonlinear in the
   amount of that gas, since the absorption by the gas at one level in the
   atmosphere can remove photons that would otherwise interact with the
   gas at another altitude. The kinds of estimates presented in the table,
   while often encountered in the controversies surrounding global
   warming, must be treated with caution. Different estimates found in
   different sources typically result from different definitions and do
   not reflect uncertainties in the underlying radiative transfer.

Positive feedback and runaway greenhouse effect

   When the concentration of a greenhouse gas (A) is itself a function of
   temperature, there is a positive feedback from the increase in another
   greenhouse gas (B), whereby increase in B increases the temperature
   which, in turn, increases the concentration of A, which increases
   temperatures further, and so on. This feedback is bound to stop, since
   the overall supply of the gas A must be finite. If this feedback ends
   after producing a major temperature increase, it is called a runaway
   greenhouse effect.

   According to some climate models ( Clathrate gun hypothesis), such a
   runaway greenhouse effect, involving liberation of methane gas from
   hydrates by global warming, caused the Permian-Triassic extinction
   event. It is also thought that large quantities of methane could be
   released from the Siberian tundra as it begins to thaw, methane being
   21-times more potent a greenhouse gas than carbon dioxide .

   A runaway greenhouse effect involving CO[2] and water vapor may have
   occurred on Venus. On Venus today there is little water vapor in the
   atmosphere. If water vapor did contribute to the warmth of Venus at one
   time, this water is thought to have escaped to space. Venus is
   sufficiently strongly heated by the Sun that water vapor can rise much
   higher in the atmosphere and is split into hydrogen and oxygen by
   ultraviolet light. The hydrogen can then escape from the atmosphere and
   the oxygen recombines. Carbon dioxide, the dominant greenhouse gas in
   the current Venusian atmosphere, likely owes its larger concentration
   to the weakness of carbon recycling as compared to Earth, where the
   carbon dioxide emitted from volcanoes is efficiently subducted into the
   Earth by plate tectonics on geologic time scales , .

Anthropogenic greenhouse effect

   CO[2] production from increased industrial activity (fossil fuel
   burning) and other human activities such as cement production and
   tropical deforestation has increased the CO[2] concentrations in the
   atmosphere. Measurements of carbon dioxide amounts from Mauna Loa
   observatory show that CO[2] has increased from about 313 ppm (parts per
   million) in 1960 to about 375 ppm in 2005. The current observed amount
   of CO[2] exceeds the geological record of CO[2] maxima (~300 ppm) from
   ice core data (Hansen, J., Climatic Change, 68, 269, 2005 ISSN
   0165-0009).

   Because it is a greenhouse gas, elevated CO[2] levels will increase
   global mean temperature; based on an extensive review of the scientific
   literature, the Intergovernmental Panel on Climate Change concludes
   that "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" .

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

Real greenhouses

   The term 'greenhouse effect' originally came from the greenhouses used
   for gardening, but it is a misnomer since greenhouses operate
   differently . A greenhouse is built of glass; it heats up primarily
   because the Sun warms the ground inside it, which warms the air near
   the ground, and this air is prevented from rising and flowing away. The
   warming inside a greenhouse thus occurs by suppressing convection and
   turbulent mixing. 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 (Wood, 1909): a
   "greenhouse" built of rock salt (which is transparent to IR) heats up
   just as one built of glass does. Greenhouses thus work primarily by
   preventing convection; the atmospheric greenhouse effect however
   reduces radiation loss, not convection. It is quite common, however, to
   find sources (e.g., ) that make the "greenhouse" analogy. Although the
   primary mechanism for warming greenhouses is the prevention of mixing
   with the free atmosphere, the radiative properties of the glazing can
   still be important to commercial growers. With the modern development
   of new plastic surfaces and glazings for greenhouses, this has
   permitted construction of greenhouses which selectively control
   radiation transmittance in order to better control the growing
   environment PDF (271  KiB).

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