Greenhouse effect
The greenhouse effect, first discovered by Joseph Fourier in 1824, and quantified by Svante Arrhenius in 1896, is the process by which an atmosphere warms a planet.
Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects, but for simplicity the rest of this article will refer to the case of Earth.
The term greenhouse effect may be used to refer to two different things in common parlance: the natural greenhouse effect, which refers to the greenhouse effect which occurs naturally on Earth, and the enhanced (anthropogenic) greenhouse effect, which results from human activities (see also global warming). The former is accepted by all; the latter is accepted by most scientists, although there is some dispute.
The natural greenhouse effect
Process
The Earth receives an enormous amount of solar radiation. Just above the atmosphere, the solar power flux density averages about 1366 watts per square metre, or 1.740×1017 W over the entire Earth. This figure vastly exceeds the power generated by human activities.
The solar power hitting Earth is balanced over time by a roughly equal amount of power radiating from the Earth (as the amount of energy from the Sun that is stored is small). Almost all radiation leaving the Earth takes two forms: reflected solar radiation and thermal blackbody radiation.
Reflected solar radiation accounts for 30% of the Earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface.
The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave ultraviolet over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans. This absorbed energy heats the atmosphere, oceans, land and powers life on the planet.
Like the Sun, the Earth is a thermal blackbody radiator. So because the Earth's surface is much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates that Earth must radiate its thermal energy at much longer wavelengths than the Sun. While the Sun's radiation peaks at a visible wavelength of 500 nanometers, Earth's radiation peak is in the longwave (far) infrared at about 10 micrometres.
The Earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres. Only about 6% of the Earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a blackbody radiator in the far infrared, so it re-radiates this energy.
The Earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of Earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor.
Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over Earth's surface. Visible and near-visible radiation from the Sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, Earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption.
The surface of the Earth is in constant flux with daily, yearly, and ages long cycles and trends in temperature and other variables from a variety of causes.
The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the Earth's surface temperature were determined solely by the albedo and blackbody properties of the surface.
It is commonplace for simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. [1]), but this is an incorrect oversimplification: see below.
Limiting factors
The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The deep and carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with 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.
The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO2 perturbation is on the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect.
The average surface temperature would be -18°C without a greenhouse effect or 72°C with just the greenhouse effect and no convection, but in reality this temperature is closer to 15°C due to convective flow of heat energy within the atmosphere and partly above much of the thermal IR absorbence of the atmosphere. [2]
Recent 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., Climate Change, 61, 269, 2005). This suggests that the CO2 production rate from increased industrial activity (automobile use and fossil fuel generation) and other human activities such as land-use changes has overwhelmed the normal feedback control mechanisms. Global climate model calculations indicate that the elevated CO2 levels are likely to lead to global warming. There has been an observed global average temperature increase of about 0.5oC since 1960 (Science 308, 1431, 2005). There is still some public controversy about the role of human activities and that of CO2 and other greenhouse gas increases for global warming.
The greenhouse gases
Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%). Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect.
The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on molecular properties of the different gas molecules. It so happens that heteronuclear diatomic molecules and tri- (and more) atomic gases absorb at infrared wavelengths but homonuclear diatomic molecules do not absorb infrared light. This is why H2O and CO2 are greenhouse gases but the major atmospheric constituents (N2 and O2) are not.
Between the absorptions of water vapor and those of carbon dioxide, there is an atmospheric window where, prior to the industrial era, no infrared radiation was trapped, lying between 8 and 15 micrometres. Compounds such as perflurocarbons (CF4, C2F6 etc.), chlorofluorocarbons, halons and SF6 absorb very strongly in this window. This means that they are extremely potent greenhouse gases, especially given the absence of natural sinks to remove them. Perfluorocarbons can have a lifetime of 50,000 years, possibly longer.
Effects of various gases
It is hard to disentangle the percentage contributions to the greenhouse effect by different gases, because their respective infrared spectrums overlap. However, one can calculate the percentage of trapped radiation remaining, and discover:
| Species removed |
% trapped
radiation remaining |
|---|---|
| All | 0 |
| H2O, CO2, O3 | 50 |
| H2O | 64 |
| Clouds | 86 |
| CO2 | 88 |
| O3 | 97 |
| None | 100 |
(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978)); see also [3].
Water vapor effects
Water vapor is the major contributor to Earth's greenhouse effect. Its effects vary due to localized concentrations, mixture with other gases, frequencies of light, different behaviour in different levels of the atmosphere, and whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature but is distinct from water vapor gas.
The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effects and difficulties in assessing the quality of the data, water vapor has generally increased over the 20th Century.
Estimates of the percentage of Earth's greenhouse effect due to water vapor:
- 36% (table above)
- 60-70% Nova. Greenhouse - Green Planet [4]
Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47-48 estimate water vapor at 60-70% whereas Baliunas & Soon estimate 88% [5] considering only H2O and CO2. Water vapor in the troposphere, unlike the better-known greenhouse gases such as CO2, is essentially passive in terms of climate: the residence time for water vapor in the atmosphere is short (about a week) so perturbations to water vapor rapidly re-equilibriate. In contrast, the lifetimes of CO2, methane, etc, are long (hundreds of years) and hence perturbations remain. Thus, in response to a temperature perturbation caused by enhanced CO2, water vapor would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapor, the atmosphere would re-equilibriate due to clouds causing reflective cooling and water-removing rain. The contrails of high-flying aircraft sometimes form high clouds which seem to slightly alter the local weather.
Real greenhouses
The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently [6] [7]. 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. [8] [9]) 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. [10].