A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. These structures range in size from small sheds to very large buildings.

Greenhouses can be divided into glass greenhouses and plastic greenhouses. Plastics mostly used are PEfilm and multiwall sheet in PC or PMMA. Commercial glass greenhouses are often high tech production facilities for vegetables or flowers. The glass greenhouses are filled with equipment like screening installations, heating, cooling, lighting and may be automatically controlled by a computer.

The glass used for a greenhouse works as a selective transmission medium for different spectral frequencies, and its effect is to trap energy within the greenhouse, which heats both the plants and the ground inside it. This warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature drops considerably. This principle is the basis of the autovent automatic cooling system. Greenhouses thus work by trapping electromagnetic radiation and preventing convection. A miniature greenhouse is known as a cold frame.

The greenhouse effect is a natural phenomenon that traps radiation within the earth's atmosphere. Natural greenhouse gases include water vapor, carbon dioxide, nitrous oxide, and methane which are essential to support life.

How does the greenhouse effect work? As is the case with anything involving Earth's atmosphere, the full story is complicated. However, the basic idea is fairly straightforward. Let's take a look at two descriptions of the greenhouse effect; the first is simpler than the second. You can decide which is most suitable for your students.

Visible light from the Sun arrives at the top of Earth's atmosphere. As the light enters the atmosphere, some of it is scattered by air molecules or reflected from white clouds back into space. Since air is mostly transparent to visible light, much of the light that isn't reflected back into space goes through the atmosphere to Earth's surface. Some of the light that makes it to the surface is also reflected back into space (especially if the surface is bright, as is the case when snow or ice covers the ground). However, since the average albedo of Earth's surface is around 15%, most of the light that makes it to the surfaces is absorbed, warming our planet. Overall, slightly less than half of the the sunlight at the top of our atmosphere is absorbed by Earth's surface.

These two simple cartoons show the multiple paths sunlight takes as it enters Earth's atmosphere (left) and the basic mechanism of the greenhouse effect (right). The portion of incoming sunlight that is absorbed by Earth is re-emitted as infrared radiation. Some IR energy escapes directly to space, but most is absorbed by greenhouse gases in the atmosphere. This warms Earth's atmosphere; our atmosphere would be roughly 30° C (54° F) colder if it contained no greenhouse gases!
Credit: The anas program

Greenhouse effect

The greenhouse effect is a natural process that warms the Earth’s surface. When the Sun’s energy reaches the Earth’s atmosphere, some of it is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases.

Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, ozone and some artificial chemicals such as chlorofluorocarbons (CFCs).

The absorbed energy warms the atmosphere and the surface of the Earth. This process maintains the Earth’s temperature at around 33 degrees Celsius warmer than it would otherwise be, allowing life on Earth to exist.

Enhanced greenhouse effect

The problem we now face is that human activities – particularly burning fossil fuels (coal, oil and natural gas), agriculture and land clearing – are increasing the concentrations of greenhouse gases. This is the enhanced greenhouse effect, which is contributing to warming of the Earth.

The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere.^[1][2]

Radiatively active gases (i.e., greenhouse gases) in a planet's atmosphere radiate energy in all directions. Part of this radiation is directed towards the surface, warming it. ^[3] The intensity of the downward radiation – that is, the strength of the greenhouse effect – will depend on the atmosphere's temperature and on the amount of greenhouse gases that the atmosphere contains.

Earth’s natural greenhouse effect is critical to supporting life, and initially was a precursor to life moving out of the ocean onto land. Human activities, however, mainly the burning of fossil fuels and clearcutting of forests, have accelerated the greenhouse effect and caused global warming.^[4]

The planet Venus experienced runaway greenhouse effect, resulting in an atmosphere which is 96% carbon dioxide, with 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 735 K (462 °C; 863 °F) ^[5][6] [7]

The term "greenhouse effect" continues to see use in scientific circles and the media despite being a slight misnomer, as ^[8] an atmosphere reduces radiative heat loss while ^[2] a greenhouse blocks convective heat loss. The result, however, is an increase in temperature in both cases. ^[9][10]

Contents

• 1History
• 2Description
• 3Mechanism
• 4Greenhouse gases
• 5Role in climate change
• 6Real greenhouses
• 7Related effects
  • 7.1Anti-greenhouse effect
  • 7.2Runaway greenhouse effect
• 8Bodies other than Earth
• 9See also
• 10References
• 11Further reading
• 12External links

History

The existence of the greenhouse effect was argued for by Joseph Fourier in 1824. The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838 and reasoned from experimental observations by Eunice Newton Foote in 1856.^[11] John Tyndall expanded her work in 1859 by measuring radiative properties of a wider spectrum of greenhouse gases.^[12] 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.^[13] 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.^[14][15]

Description

Earth receives energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation. About 26% of the incoming solar energy is reflected to space by the atmosphere and clouds, and 19% is absorbed by the atmosphere and clouds. Most of the remaining energy is absorbed at the surface of Earth. Because the Earth's surface is colder than the Sun, it radiates at wavelengths that are much longer than the wavelengths that were 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%^[16][17] 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).^[18][19] The surface temperature of this hypothetical planet is 33 °C (59 °F) below Earth's actual surface temperature of approximately 14 °C (57 °F).^[20] The greenhouse effect is the contribution of greenhouse gases to this difference.

Mechanism

The basic mechanism can be qualified in a number of ways, none of which affect the fundamental process. 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 heat transport. 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 layer in the mid-troposphere, which is effectively coupled to the surface by a lapse rate. The simple picture also assumes a steady state, but in the real world, the diurnal cycle as well as the seasonal cycle and weather disturbances complicate matters. Solar heating applies only during daytime. During the night, the atmosphere cools somewhat, but not greatly, 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 a temperature around 255 K, radiates long-wavelength, infrared heat in the range of 4–100 μm.^[21] At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are more absorbent.^[21] Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers. It reradiates in all directions, both upwards and downwards; in equilibrium (by definition) the same amount as it has absorbed. This results in more warmth below. Increasing the concentration of the gases increases the amount of absorption and reradiation, and thereby further warms the layers and ultimately the surface below.^[19]

Greenhouse gases—including most diatomic gases with two different atoms (such as carbon monoxide, CO) and all gases with three or more atoms—are able to absorb and emit infrared radiation. Though more than 99% of the dry atmosphere is IR transparent (because the main constituents—N
₂, O
₂, and Ar—are not able to directly absorb or emit infrared radiation), intermolecular collisions cause the energy absorbed and emitted by the greenhouse gases to be shared with the other, non-IR-active, gases.