Which wavelength produces the most warming effect

Greenhouse effect

The natural greenhouse effect

The earth's climate system is powered by solar radiation. Half of the solar radiation is absorbed by the earth's surface, 30% is reflected by the earth's surface and the atmosphere, and 20% is absorbed by the atmosphere. The heated earth's surface emits long-wave heat rays, a large part of which is absorbed by components of the atmosphere (greenhouse gases and clouds) and radiated in all directions. The part of this radiation directed downwards, towards the earth's surface, is responsible for the greenhouse effect. It warms the lower layers of the air and the ground.

1 The natural greenhouse effect

left (a): theoretical radiation (heat radiation) of the earth's surface without greenhouse effect (red curve) and actual radiation (heat radiation) due to the effect of greenhouse gases (blue area);
right (b): IR spectra of greenhouse gases; the respective IR spectrum shows the wavelength ranges in which the greenhouse gases mentioned absorb the thermal radiation. The absorption coefficient indicates the intensity of this absorption.

1.1 "Heat accumulation" in the lower atmosphere

With regard to the radiation processes in the atmosphere, it is of decisive climatic importance that the long-wave thermal radiation from the heated earth's surface does not leave the atmosphere for the most part directly, but is first absorbed by atmospheric trace gases, the natural greenhouse gases such as water vapor, carbon dioxide or methane, and clouds. Trace gases and clouds then emit this energy on the one hand towards space and on the other hand radiate it back towards the earth's surface, which is thereby additionally heated and in turn emits long-wave radiation to the atmosphere, which radiates it back towards the earth's surface, etc.

This creates a kind of "heat build-up" in the lower atmosphere, which - compared to the case without greenhouse gases - causes a temperature difference of +33 ° C or a warming of -18 ° C to a mean global temperature of +15 ° C and this is what makes life on earth possible in the first place.

1.2 Radiation budget

The earth's surface receives a total of 492 W / m of energy from solar radiation and the greenhouse effect2 (168 W / m2 Solar radiation + 324 W / m2 atmospheric heat radiation) and gives the atmosphere 350 W / m2 as heat radiation (terrestrial radiation). The resulting excess energy of 142 W / m2 is balanced on the one hand by the fact that a small part of the heat radiation (40 W / m2) is not absorbed by the greenhouse gases and escapes into space through the so-called absorption window. On the other hand, the earth's surface gives an average of about 24 W / m2 as sensible warmth and 78 W / m2 as latent heat to the atmosphere. The flow of sensible heat transports energy from the warmed earth through the rising of warm air into the lower atmosphere. Latent heat is transported into the atmosphere by water vapor, in that energy is first extracted from the environment through evaporation of water, which is then released again during condensation at a higher altitude (see also air temperature).

1.3 "greenhouse effect"

Based on the garden greenhouse, the build-up of heat in the lower atmosphere is called the "greenhouse effect". However, the comparability between the two 'greenhouses' is limited. Like the atmosphere, the glass cover of the real greenhouse largely lets short-wave sun rays pass through. The interior of the greenhouse is heated and emits long-wave thermal radiation, which is absorbed by the glass in a similar way to the greenhouse gases in the atmosphere. In contrast to the greenhouse gases in the atmosphere, glass also prevents air and water vapor transport and thus largely the flow of sensible and latent heat.

1.4 greenhouse gases

The actual cause of the greenhouse effect is water vapor (H.2O) and a number of trace gases such as carbon dioxide (CO2), Methane (CH4), Nitrous oxide (N.2O), tropospheric ozone (O3) and others, which together make up less than 1% of the total mass of the atmosphere. These greenhouse gases largely allow the short-wave solar radiation to pass through, but absorb the long-wave heat radiation from the earth's surface in the infrared range; they do this from a wavelength of approx. 3 µm. The individual trace gases absorb in different absorption bands, these are certain wavelength ranges in which the absorption is strong. The reason is that the molecules of these gases can vibrate and rotate. If the energy of the incident light corresponds exactly to the energy difference of one of its oscillations and rotations, the light is absorbed and emitted again. The molecules act like a small antenna that only reacts to certain wavelengths (like a radio that has to be tuned to a station). Between the absorption bands (which consist of many individual lines, one line for each energy of the vibrations and rotations) are wavelength ranges to which these gases do not react. Since the radiation can pass through there unhindered, these areas are also called "windows", only that infrared radiation is meant and invisible radiation as it can get through an actual window. A significant window in the atmosphere lies between about 8 and 12 µm wavelength, only interrupted by an ozone absorption band at 9.6 µm.

The most important natural greenhouse gas is water vapor, which is responsible for almost two thirds of the natural greenhouse effect. It absorbs almost completely in broad spectral ranges around 3 µm, 5 µm and 20 µm. In other wavelength ranges such as around 4 µm and around 10 µm, however, it allows the infrared radiation to pass almost completely. In contrast, the other greenhouse gases act in these areas. The second most important natural greenhouse gas, carbon dioxide, absorbs just 4 µm and 15 µm. Ozone, nitrous oxide and methane fill further gaps in the wavelength spectrum (cf. the IR spectra in the graphic above on the right (b) (file: Absorption.gif) - In the literature, the formulation CO is found relatively often for the effect just described2 close "the radiation window".[1]) Of course, there is also overlap, i.e. areas in the spectrum where several gases absorb at the same time. If this happens or if a gas is already present, it can mean that the radiation of such a wavelength can no longer pass through the atmosphere. Additional gases then no longer lead to stronger absorption. Nevertheless, the greenhouse effect increases a bit because the absorption increases at the edges of the lines. The idea that additional carbon dioxide in the atmosphere no longer has any effect because of this "saturation" is therefore wrong.

2 The anthropogenic greenhouse effect

Change in mean global and annual radiative forcing due to anthropogenic greenhouse gases, anthropogenic aerosols and solar variability between 1750 and 2000

Since the beginning of the industrial age, humans have influenced the climatic effectiveness of the atmosphere through an additional greenhouse effect. Different human activities increase the concentration of natural greenhouse gases such as carbon dioxide, methane, nitrous oxide etc. on the one hand, and on the other hand, CFCs emit new greenhouse gases into the atmosphere. The effectiveness of the anthropogenic contributions depends, among other things, on how strongly the respective absorption bands are already saturated by the effects of the natural greenhouse gases.

Since a rise in temperature also leads to higher evaporation, the water vapor content of the atmosphere also increases as a result of human climatic influences. However, the temperature effectiveness of the additional water vapor is relatively low, since the absorption bands of water vapor are almost saturated. The increase in CO has a slightly larger temperature effect2Content from the burning of fossil fuels and changes in land use. But here, too, the most important absorption band at 15 µm is largely saturated and only the enormous amount of anthropogenic CO2-Input of over 36 billion tons per year causes CO2 is responsible for well over half of the anthropogenic greenhouse effect.

In the case of the other anthropogenic greenhouse gases, on the other hand, the natural absorption bands are only saturated to a low degree or (in the case of CFCs) not at all. In addition, different types of molecules (i.e. different gases) react differently to the incident radiation. For example, a molecule of FCWK-12 absorbs about 23,000 times more than a molecule of carbon dioxide. For the greenhouse effect, however, it is also decisive how many molecules are present in the atmosphere and how long they remain there, i.e. how long the "service life" of the gas is. If you take this service life into account, one kilogram of methane, for example, is 28 times as long[2] and one kilogram of the CFC-12 molecule mentioned is 10900 times the global warming potential of one kg of CO2. These figures are referred to as the Global Warming Potential (GWP).

The length of stay results from the amount itself and from the sinks that control how quickly a substance is removed from the atmosphere. Carbon dioxide is removed from the atmosphere through very different processes, e.g. through photosynthesis in plants, solution in the ocean or absorption in the soil, and therefore has no clear mean residence time in the atmosphere. In contrast, the atmospheric lifespan of methane is almost exclusively controlled by the oxidation with OH in the atmosphere, which results in an average stay in the atmosphere of 12 years. The long residence time of nitrous oxide of 114 years is explained by the fact that this greenhouse gas is removed almost exclusively by photolysis in the stratosphere.

In contrast to the long-lived greenhouse gases, which remain in the atmosphere for decades and longer and are therefore well mixed around the globe, the lifespan of ground-level ozone is only a few hours to days. Ozone is created by photo-oxidation of carbon monoxide, methane and other hydrocarbonates with the participation of NOx and is destroyed by ultraviolet photolysis and reaction with OH radicals. Its concentration therefore varies greatly, both spatially and temporally. While the anthropogenic increase in ozone in the troposphere has a (local) warming effect, the stratospheric ozone destruction through anthropogenic CFC emissions has a cooling effect.

The amount of emissions, the relative global warming potential and the atmospheric dwell time determine the share of the individual gases in the overall additional greenhouse effect. The increase in the concentration of trace gases with a greenhouse effect, which has been observed since the beginning of industrialization, leads to a change in the radiation equilibrium of the atmosphere and thus to climate change. The disturbance of the radiation budget or the radiation forcing (English "radiative forcing") by the anthropogenic greenhouse effect since approx. 1750 is due to the change of the net radiation flux density at the tropopause in watts per m2 specified. The long-lived and evenly distributed greenhouse gases have a radiative forcing of around 3.0 W / m by 20172 since the beginning of the industrial age. This shows the increase in carbon dioxide by 2.0 W / m2 involved that of methane with 0.5 W / m2that of nitrous oxide with 0.2 W / m2 and with 0.2 W / m2 that of halogenated hydrocarbons (CFC's) and other long-lived greenhouse gases.[3] The radiative forcing of tropospheric ozone is very difficult to quantify because of the uneven distribution and short life of this greenhouse gas and is estimated by the Intergovernmental Panel on Climate Change in its 5th assessment report from 2013 at 0.4 W / m2 estimated, with the stratospheric ozone depletion caused by CFCs having an effect of -0.05 W / m2 owns.[4]

The anthropogenic greenhouse effect is also counteracted by the man-made increase in the aerosol concentration in the atmosphere, which is mainly caused by the combustion of fossil fuels and is subject to strong spatial and temporal variation, as the anthropogenic aerosols only for a few days in the vicinity of the centers of origin float in the air and then sink again or be washed away with the rain. Aerosols are firstly directly active by radiation, in that they reflect or absorb sunlight, and secondly, indirectly, because they have an influence on cloud formation. Both effects, especially the last one, are difficult to assess and the IPCC estimates the total to be -0.9 W / m2 estimated.[5] Compared to the anthropogenically caused changes in the radiation balance, the effect of the increase in solar radiation on the radiative forcing decreases by approx. 0.05 Wm-2 very modest since 1750.[6]

3 individual proofs

  1. ↑ cf. also: Electromagnetic spectrum, radiation energy and absorption (H. Kehl, TU-Berlin, Inst. f. Ecology)
  2. ↑ IPCC (2013): Climate Change 2013, Working Group I: The Science of Climate Change, Table 8.7
  3. ↑ Butler, J.H, and S.A. Montzka (2018): The NOAA Annual Greenhouse Gas Index (AGGI)
  4. ↑ IPCC (2013): Climate Change 2013, Working Group I: The Science of Climate Change, Technical Summary, TS.3.2
  5. ↑ IPCC (2013): Climate Change 2013, Working Group I: The Science of Climate Change, Technical Summary, TS.3.3
  6. ↑ IPCC (2013): Climate Change 2013, Working Group I: The Science of Climate Change, Technical Summary, TS.3.5

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