greenhouse effect

The greenhouse effect, first discovered by Joseph Fourier in 1824, and first investigated quantitatively by Svante Arrhenius in 1896, is the process in which the absorption of infrared radiation by an atmosphere warms a planet. Without these greenhouse gases, the Earth's surface would be up to 30 °C cooler. The name comes from an incorrect analogy with the way in which greenhouses are heated by the sun in order to facilitate plant growth. In addition to the Earth, Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects.

In common parlance, the term "greenhouse effect" may be used to refer either to the natural greenhouse effect, due to naturally occuring 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. 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 incident solar radiation must be radiated back to space. Radiation leaving the Earth takes two forms: reflected solar radiation and emitted thermal infrared radiation. The Earth reflects about 30% of the incident solar flux; the remaining 70% is absorbed, warms the land, atmosphere and oceans, and powers life on this planet. Eventually this energy is reradiated to space as infrared photons. This thermal, infrared radiation increases with increasing temperature. One can think of the Earth's temperature as being determined by the requirement that it produce the infrared flux needed to balance the absorbed solar flux.

The key to the greenhouse effect is the fact that the atmosphere is relatively transparent to visible solar radiation but strongly absorbing at the wavelengths of the thermal infrared radiation emitted by the surface and the atmosphere. The visible solar radiation heats the surface, not the atmosphere. Whereas most of the infrared radiation escaping to space is being 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.

The reason that this results in a warming of the surface is most easily understood by starting with a model of a purely radiative greenhouse effect, in which one ignores the fact that a large part of the energy transfer in the atmosphere is not in fact radiative, but associated with 1) convection, (sensible heat transport), and 2) the evaporation and condensation of water vapor, or 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 must rise until the surface generates enough thermal radiation to balance 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 temperatues 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. [1]).

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 (O₂), nitrogen (N₂) 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, 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 of 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 are 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 absorption 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 consituent 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.

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 absorprtion 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

Runaway greenhouse

The strength of the greenhouse effect is dependent on the concentration of greenhouse gases in the planetary atmosphere. The deep and carbon dioxide-rich atmosphere of Venus (combined with an orbit closer to the sun than that of Earth) causes 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.

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. Water vapor is thought to provide a positive feedback of this type in response to increase in carbon dioxide. If a change in temperature of 1 degree causes an increase in water vapor by an amount that, in isolation, caused a further increase in temperature of x degrees, then the final warming will be enhanced by the factor 1/(1-x) = 1 +x +x² +x³ + .... . If x is larger than unity, this series diverges and temperatures increase until the source of the gas is exhausted or some other nonlinearity intervenes. On Earth, x for water vapor is thought to lie in the range 0.3-0.4, so the Earth is far from this runaway condition, as is also self-evident from the stability of the climate through geological time.

A runaway greenhouse effect may, however, 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 vapour 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. [3],[4].

Anthropogenic greenhouse effect

Main article: global warming

CO₂ production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestation has increased the CO₂concentrations in the atmosphere. Measurements of carbon dioxide amounts from Mauna Loa observatory show that CO₂ has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO₂ exceeds the geological record of CO₂ maxima (~300 ppm) from ice core data (Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009).

Because it is a greenhouse gas, elevated CO₂ levels will increase global mean temperature. There has been an observed global average temperature increase of about 0.5^oC since 1960 (Science 308, 1431, 2005), . Quantitative understanding of climate sensitivity to CO₂ concentration remains elusive due to uncertainties in a variety of feedbacks, especially those related to clouds, but there is little doubt that a substantial portion of the warming in the last half century was caused by the incresase in CO₂ concentrations in the atmosphere.

Over the past 800,000 years [5], 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 [6]. Paleoclimatogists 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[7] [8]. 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 greenhouse effect however reduces radiation loss, not convection. It is quite common, however, to find sources (e.g. [9] [10]) 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 [11].

See also

• Global warming
• Climate forcing
• Climate sensitivity
• Emissions trading
• Kyoto Protocol
• Greenhouse

References

• 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
• Fraser, Alistair B., Bad Greenhouse http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
• Giacomelli, Gene A. and William J. Roberts1, Greenhouse Covering Systems, Rutgers University, downloaded from: http://ag.arizona.edu/ceac/research/archive/HortGlazing.pdf on 3-30-2005.
• 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.
• Piexoto, JP and Oort, AH: Physics of Climate, American Institute of Physics, 1992 (quote: ...the name water vapor-greenhouse effect is actually a misnomer since heating in the usual greenhouse is due to the reduction of convection)
• Wood, R.W. (1909). Note on the Theory of the Greenhouse, Philosophical Magazine 17, p319–320. For the text of this online, see http://www.wmconnolley.org.uk/sci/wood_rw.1909.html
• IPCC assessment reports, see http://www.ipcc.ch/