Greenhouse effect

The greenhouse effect is the eponymous effect of greenhouse gases in atmospheres on the temperature at the bottom. This stand on planetary surfaces higher temperatures than would arise without the greenhouse effect. The effect arises from the fact that the atmosphere is substantially transparent for incoming radiation from the sun short wavelength, but is not very transparent to long-wave infrared radiation which is emitted by the hot surface and by the heated air. The most important greenhouse gas of the earth is water vapor.

The greenhouse effect was discovered by Joseph Fourier in 1824 and 1896 by Svante Arrhenius first described quantitatively accurate. The systematic study of the greenhouse effect began in 1958 by Charles D. Keeling, a student student of Roger Revelle. By Keeling a variety of stations for carbon dioxide has been established; the most famous is located on Mauna Loa in Hawaii.

The caused by human intervention share of the atmospheric greenhouse effect is called the anthropogenic greenhouse effect, see global warming.

Unlike the name suggests, the heating of a greenhouse is not based by incident solar radiation on the greenhouse effect. The underlying this effect is called greenhouse effect. However, sometimes both names - the greenhouse effect and greenhouse effect - also used interchangeably for both effects.

• 2.1 Selective transparency
• 2.2 Use

• 3.1 GHG

• 4.1 speed
• 4.2 mechanism

Physical Basics

Comparisons with data from other planets and (relatively) simple calculations show that there are not only here on earth a greenhouse effect. From the albedo of the earth, one can calculate the equilibrium temperature that would exist if there were no atmosphere: it would in this case amount to an average of -18 ° C ( see also Examples of radiation exchange ). This is considerably less than the one indicated by measurements and interpolation value of 14 ° C. The difference is much more powerful when neighboring planet Venus: instead of the calculated 141 ° C is about 440 ° C was actually measured. In both cases there is one cause: the greenhouse effect.

A sample calculation for an idealized in several respects planet can be found in the article Idealized greenhouse model.

Compact representation of the mechanism

The greenhouse effect can be explained in the following steps:

Elaboration

When radiation passes through matter, it is absorbed by matter partially and partially transmitted. The strength of absorption and transmittance depends on the wavelength ( in the visible spectrum = color) of the radiation. The greenhouse effect occurs when the permeability and absorption coefficients of the boundaries of the volume is dependent on the wavelength. Here in the volume of the external radiation occurs relatively unhindered and is supplemented by radiation emanating from the boundary. Closely associated with the radiation emitted by the limitation is that the internal beam is largely absorbed by the boundary of the volume ( Kirchhoff's law ). Here, a substantial portion of the inner radiation in the enclosed volume of the boundaries is absorbed (mainly) or reflected. The reflection in the atmospheric greenhouse effect plays no role and also at the Glass House the importance of reflection is very low, because in the relevant wavelength range have the glass almost as a black body. At this inner radiation comes another radiation (mainly from the sun), which penetrates a part of the boundary ( glass or the layer of greenhouse gases) due to the different wavelength almost effortlessly (selective transparency) and from another part of the boundary surface (eg, ground ) is absorbed. By the sum of the two beams (internal radiation of a cavity that emanates from all of the interfaces, plus the transmitted radiation ) the steps taken locations are heated more than without disc or greenhouse.

Greenhouse effect

As greenhouse effect (derived from the French by effet de serre as it was first named by Joseph Fourier ), the effect is called when in an interior by glazed window openings or roofs incident sunlight is absorbed by the materials of the interior. Thus, the interior can heat up considerably above the level of the ambient temperature. Of the heated wall surfaces is heated by thermal conduction and convection, the air in the cabin. Since in a closed greenhouse little heated air is replaced by cold outside air, the cooling effect is not large due to cold outside air. Sometimes it is the greenhouse effect by the large, architecturally stylish greenhouses of botanical gardens and palace gardens - the orangeries - also called orangery effect.

Selective transparency

Window glass is transparent to visible light and short-wavelength infrared, such as it is emitted by the sun. For long-wave infrared ( the range of thermal radiation in the glasshouse temperatures) it is impermeable. This difference is often depicted as the cause of the warming effect. Such an effect of the difference in the transparency of the glass cover of a greenhouse some consider too small and relate to a misunderstood publication. Wood writes at the end of its release, that he has the problem not been studied in detail and just wanted to give a thought. The heating of the greenhouse is therefore based mainly in the fact that the incident solar radiation acting as a furnace, and that the exchange of air is prevented with the outside air - analogous to that hardly anyone opens the window time when heated.

The controlled ventilation via roof hatches is used in greenhouses, especially during the day to dissipate in the hot summer months, excess thermal energy not by radiation, but by exchange of air masses and thus to keep inside a tolerable temperature level (below 40 ° C). At night, however, can be quite cool even in summer. Cause are losses by conduction (low), air mass exchange through ventilation ( larger if possible), but especially the emission of the greenhouse outside into space (especially with a starry sky ), governing the entire earth's crust at night. Because the glass house walls are usually built so that the heat flow from the inside of the glass house wall is impeded only very marginally to the outside, it cools down and the interior of the Glass House. However, If you hold at night the hatches closed, the heat loss is attenuated, the temperature fluctuations between day and night low and you therefore causes a more balanced and relatively warm microclimate in the greenhouse.

Use

The effect has long been used in crops under glass / greenhouses. Except in the greenhouses greenhouse effect is used specifically as a passive solar gain in the architecture in order to save fuel for heating of living spaces. This is achieved by a south facing large glass fronts and / or conservatories of the building, over which the air of the house is heated. In particular, so-called low-energy houses and passive houses use this effect to a drastic reduction in the use of an additional heating. Particularly intensively can be observed / touch it in a parked car in the sun this phenomenon.

Also solar panels this effect. Here the selective transparency of the glass can be supplemented with a selective absorption of the collector, in order to reduce the heat radiation. In vacuum panels also convection between the glass and the collector is prevented.

Atmospheric greenhouse effect

Greenhouse gas

In the Earth's atmosphere causing greenhouse gases such as water vapor, carbon dioxide, methane and ozone in the history of the earth a greenhouse effect, the decisive influence on the climate history of the past and the present-day climate. The role of the glass is here taken over by the said greenhouse gases, which are continuous for the short-wavelength portion of the solar radiation, on the other hand absorb long-wave heat radiation, depending on the greenhouse gas in different wavelengths and emit.

Most of the greenhouse effect is caused with a share of about 36-70 % ( excluding the effects of clouds) by water vapor in the atmosphere. Carbon dioxide in the atmosphere contributes about 9-26 % to the greenhouse effect, methane approximately 4-9 %, and ozone, approximately 3-7 %. The ozone content in the stratosphere plays in particular a very important role for the climate. It is influenced by the people directly, but indirectly on fluorinated greenhouse gases. In the Kyoto Protocol are therefore hydrogenous (HFC or HFC) has been added to the list of greenhouse gas perfluorinated fluorocarbons (PFCs or HFCs) and sulfur hexafluoride ( SF6).

An exact percentage effect contribution of each greenhouse gas to the greenhouse effect can not be specified, since the influence of the individual gases varies depending on latitude and mixing ( respectively higher percentage values ​​indicate the approximate proportion of the gas itself, the lower values ​​are obtained from the mixtures of gases).

Are driven, the heat processes at the surface and in the atmosphere by the sun. The strength of the solar radiation in the Earth's orbit is called the solar constant and has a value of approximately 1367 W / m², which varies depending on Erdentfernung and solar activity between 1325 W / m and 1420 W / m².

For the great mass of earth heat storage plays a significant role, which is to recognize the fact that in the northern hemisphere enters the warmest period in the summer after high sun ( about June 22 ). Because of this large memory effect is always calculated in the energy balance in the atmosphere with the average over the whole surface of the earth.

So-called " energy balances " are expected to average radiation on the surface of the earth: The earth receives solar radiation at the surface of Erdquerschnitts and has an area of ​​. These two surfaces have a ratio of 1:4. That is, if 1 367 W / m² would radiate to the earth and converted into earth surface temperature, the surface could average 342 W / m radiate again.

Through clouds, air and soil (especially ice and snow, see albedo) is reflected a share of about 30 % of the incident solar energy back into space - that is about 410 W / m². The remaining 70 % is absorbed - that is about 957 W / m². Would be the only radiation that would be absorbed by the earth, the earth's surface would assume a temperature of about -18 ° C in the middle, when the heat would be distributed evenly about the earth. Noting that temperature differences exist, the average temperature is far below this upper limit (see Hölder's inequality).

But there is another exposure to the heated greenhouse gases with about 150 W / m², the atmospheric counter-radiation called. This absorbs the earth's surface consists of 389 W / m² - and which are delivered at the actual average surface temperature of 14 ° C in several ways. A portion thereof is discharged through radiation, which will again be described by the Planck's law of radiation. The energy output is done but also by other operations such as the convection. The surface temperature is also the surface air temperature.

Radiated from the surface energy has a different spectral ( color ) distribution, as the incident sunlight, which has a spectral distribution in accordance with a color temperature of about 6000 K and is hardly absorbed from the atmospheric gases. The spectral distribution of the radiated energy from the surface is determined by the 14 ° C of the surface, so that only about 90 W / m² to be radiated directly from the surface into space. The remaining 299 W / m², partly by radiation to the opaque for this wavelength portion atmosphere ( caused by the greenhouse gases) emitted by absorption, partly also by other processes, such as convection. Wherein the atmosphere is heated. The atmosphere has two surfaces: one to space out and go to earth. The radiation from the atmosphere on each side of the same, if the temperature of the soil is constant. The absorbed energy of 299 W / m² is so on each side half - radiated ie 150 W / m². For comparison, a black body with a radiation of 150 W / m² has a temperature of about -40 ° C. The radiation is greater in one direction than in the other, it comes to heating or cooling of the earth. The difference is the radiative forcing. With this size just can resulting from the amended balance sheet, new equilibrium temperature of the Earth are calculated.

By radiation into space from the atmosphere at 150 W / m², the direct 90 W / m² of the surface and the albedo portion of 103 W / m which is about equal to the average irradiance of 342 W / m, ie. , radiation is about the same charisma. This is also reflected in the fact that the Earth's temperature changes only slowly - resulting necessarily follows that the earth is the absorbed solar energy delivers again - but because of the low temperature of the earth, the energy is mainly emitted as long-wave infrared radiation ( Wien's displacement law ).

The heat flow from the Earth's interior plays practically no role ( about 0.06 W / m²). From the world's energy consumption ( in 2004) in the amount of 432 EJ of energy and the size of the Earth's surface to around 510 million square kilometers, a zurückzuführender on the use of non-renewable energy sources for heat flow ( power) of around 0,026 watts per square meter calculated.

Problems with some of the energy that emit greenhouse gases towards the surface (150 W / m² - as already mentioned above), since this energy from a cooler body ( about -40 ° C) to the warmer body ( the earth's surface about 14 ° C) flows and this supposedly contradicts the second law of thermodynamics. But this is a wrong interpretation, because it lets the solar radiation ignored ( even by 6000 K), in the balance sheet is again the second law met (see also radiation exchange ).

In summary, we have: The backscatter from the atmosphere to the earth leads to additional warming of the Earth's surface by at least 33 ° C. So that the average overall temperature is held at 14 ° C significantly lower than -18 ° C.

Also important is the distribution of height, where the heat radiation reaches the earth's surface. For the greenhouse effect is directly significant, only the portion of the radiation from low altitudes, because only this radiation reaches the ground without being previously absorbed by the greenhouse gases again ( see next paragraph). Here is the " low " very wavelength-dependent, because the length after which the radiation is absorbed again ( absorption length ) is wavelength-and concentration-dependent. Is the absorption length is greater than the thickness of the atmosphere, the atmosphere is almost transparent at these wavelengths. Since the intensity of radiation on the temperature of the source is dependent upon the beam strength increases if the absorption length is shorter: because of the temperature decrease with the height, the average temperature rise on the shorter absorption length. Thus, the atmospheric counter-radiation in a wavelength region with increasing amounts of GHG may also be even stronger if the atmosphere in this wavelength range is already as good as opaque.

The temperature profile up to an altitude of about 11 km is practically only adiabatically due to lost passing through the emission of the greenhouse gas energy is replaced by convection and radiation absorption. The absorbed radiation comes from different sources:

• Solar radiation ( very small portion )
• Emission from the surface
• Radiation from deeper layers
• Radiation from higher layers

The proportion of the emission of long-wave heat radiation by greenhouse gases such as

• Carbon dioxide ( CO2),
• Methane (CH4),
• Nitrous oxide ( N2O)

And other gases is called dry greenhouse effect. The inclusion of water vapor leads to the moist greenhouse effect. About 62 % of the greenhouse effect caused by water vapor, about 22 % of carbon dioxide.

Interestingly, the temperature profile as a function of pressure altitude ( at the surface is the highest pressure 1.013 bar). Back to top of the pressure decreases because the mass of gas is lower. Peer Pressure changes correspond to an equal number of gas particles. In the troposphere, the temperature profile is best described by an adiabatic curve with the exponent 0.19. Above the troposphere, the gas mass is low and there is no adiabatic course more ago. The top of the Real atmosphere at low pressure is caused by the UV - absorption of oxygen ( ozone formation and decomposition ). The curvature of the curve in the troposphere, the existence of the troposphere is understandable: If the curve is a straight line, it would on average absorbed by the greenhouse gases emitted energy equal to the energy - due to the curvature and its nature, however, the emitted energy is greater than the absorbed energy, the air is thus cooled and drops down. Thus, a vertical circulation is started after the gas laws with constant heat content ( the radiation loss is small for the heat content ) leads to the adiabatic gradient.

The importance of the global greenhouse effect can thus be seen also in the extremely different surface temperatures of the planet Venus, Earth and Mars. These temperature differences depend not only on the distance to the sun, but also from the ( due to various causes ) different atmospheres.

Anthropogenic greenhouse effect

The anthropogenic greenhouse effect is not to be confused with the also man-made damage to the stratospheric ozone layer, which leads to the so-called ozone hole.

Speed

In contrast to the application running on geological time scales, natural climate change of anthropogenic climate change, which is caused primarily by an increase in the concentration of carbon dioxide is a extremely short time. The global warming of the last ice age to the current interglacial period was a warming of about one degree per 1000 years. Increasing the concentration of all greenhouse gases over the last 100 years has led to an increase in the average global temperature by about 0.8 K in the last 100 years. This man has a 2% share of the total greenhouse effect of 33 ° C.

Mechanism

Net heat radiation from the Earth into space is only a smaller part of near-ground atmospheric layers, as in the lower layers of the atmosphere, infrared radiation is mostly absorbed by overlying layers of air again. It also does not take place in a tightly circumscribed area, but in a region ranging from near-surface areas up to an altitude of about 15 km and an average from a height of 5.5 km. The radiative equilibrium temperature of the Earth would be without the effect of greenhouse gases at -18 ° C. By increasing the concentration of greenhouse gases, the height of the air layer increases, in a temperature of -18 ° C prevails. For the sake of thermodynamics the temperature falls to the earth by 6.5 ° C / km, as one moves upward. An increase in the concentration of greenhouse gases causes the layer in which there is the -18 ° C radiation equilibrium temperature moves upwards. Per kilometer increase of the layer there is in radiative equilibrium, the temperature at the surface is increased by 6.5 ° C. Consequently, The British meteorologist Ernest Gold had published in 1908, that was to be expected that the tropopause increases with increasing CO2 concentration trough the enhanced greenhouse effect higher. This was confirmed by measurement beginning of the 21st century.

Contrary to some media representation of the greenhouse effect is far from saturated, because, as already described, the emission takes place to a large extent not close to the ground, but several thousand meters height. There is much colder than near the ground. The water vapor content of air is highly temperature dependent, so that cold air can contain significantly less of this greenhouse gas than warm air. An increase in the concentration of carbon dioxide has a greater effect as if it can at sea level measurements suspect because where the radiation energy of the earth into space mainly takes place, hardly is water vapor. The effect of the greenhouse effect by changing the concentration of carbon dioxide would therefore even increase if no change in absorbance was measured at sea level.

The effect of increasing the concentration of greenhouse gases has been correctly identified already in 1901 by Nils Ekholm. He wrote: " radiation from the Earth into space is not directly from the ground there, but from a layer located at a considerable height above the ground. This layer is higher, the greater the force with which the air emitted from the floor can absorb radiation. With increasing elevation, however, decreases the temperature of this layer. As colder air can radiate less heat, the soil warms even more, the higher these radiating layer. "