Guest article by Dominik Kreil and Martin Hoffmann.

The Paris Agreement had its anniversary on December 12th. On December 16th, a committee meeting of the parliament took place. This meeting will discuss the demands of the Klimavolksbegehren. All of them share one central topic, the reduction of a specific molecule in our atmosphere. This molecule is almost everywhere and its amount is still increasing. It contains an important ingredient of life: carbon. It is the basic substance of plants, plants which become our food, finally. But why has this mysterious substance, carbon dioxide (CO2), such a bad image and why is it seen as an allegory for climate change? Follow us through a short story of the history of gases.

CO2 is the symbol for the climate crisis.

1st Act: At the beginning there was light, or 2everything radiates"

Every object, independent of size, shape or color radiates energy. In the 19th century, Josef Stephan and Ludwig Boltzmann successfully described the radiation of black bodys. Scientifically speaking, radiation of black bodies can be understood solely through inner processes. Reflexion, transmission and surface characteristics must not play a role. Stephan and Boltzmann studied the dependence of emitted radiation on the temperature of the object. Earth and sun can be seen as a black body in good approximation. Both are in a so-called radiation equilibrium; meaning that the earth heats up until its radiated energy equals the absorbed one. Without the natural greenhouse effect in the atmosphere, the average temperature on earth would be minus 18 degrees centigrade.

Visible light is only a small part in the whole electromagnetic spectrum. A useful classification of this light is the wavelength; the distance between two oscillating wave peaks. Visible light has a wavelength between 380 and 780 nanometer. Smaller wavelengths contain more energetic radiation like ultraviolet or X-rays up to the very dangerous gamma rays. Going to larger wavelengths, one finds the infrared radiation (heat radiation), the microwaves and further up to the very long-wavelength radio signals.

The different spectral regimes of light, from short to long wavelengths.
Dominik Kreil

At the beginning of the last century, Max Planck was able to show the connection between the spectrum of a black body and its temperature. Light which is coming from the sun (about 6000 K surface temperature, where K stands for Kelvin) contains mostly visible light, infrared and ultraviolet radiation. The vast majority is absorbed by the surface of the earth which is then radiating mostly in the infrared spectrum at about 300K. Here, the greenhouse gases become important. They absorb this infrared radiation and start to heat up. This absorption is shifting the radiation equilibrium point to higher temperatures.

2nd Act: CO2 and the greenhouse effect, or “not everything contributes equally”

The average temperature (over day/night, summer/winter, equator/pole, …) on earth is about 15 degrees centigrade. A very unusual value considering equilibrium of earth, sun and the universe (approximately minus 270 degree centigrade). Without an atmosphere we freeze with negative temperatures which makes life impossible. The gases which cover our planet play an important role; each and every one of them with it’s one characteristic. In the visible light spectrum, most of them are transparent. However, some absorb larger and smaller wavelengths. One prominent compound is ozone, which absorbs an essential part of ultraviolet light, before it hits the surface of the earth. For longer wavelengths, molecules like methane, water vapour or carbon dioxide are strong absorbers. Leading to an increase in global temperature.These climate warming gases are not abundant, they make up less than 1% of the earth's atmosphere. Nonetheless, they are 100 percent responsible for global warming. How is that possible?

The main part of our atmosphere consists of nitrogen with almost 78 percent, followed by oxygen (O2) with approximately 21 percent. Both gases are abundant, but do not contribute at all to the infrared absorption. They are not active in the region of heat radiation. Rays can propagate right through them, as if they were not there. Furthermore, they do not have any possibility to reduce the effect of carbon dioxide. Only the total amount of CO2 plays an important role. To understand this, we will have to proceed to the next act!

3rd Act 3: Resonance and the structure of climate gases, or “everything vibrates”

Atoms and molecules are electrically neutral in most of the cases. This means they carry the same amount of positive and negative charges. However, these charges do not have to be distributed homogeneously; which is the case in carbon dioxide (O-C-O). The oxygen atoms (O) tend to attract electrons stronger than carbon (C) and leave it with a slightly positive charge; resulting in a so-called dipole moment. This dipole moment is the basic requirement to ensure coupling to infrared light. Also water vapor (H2O) and methane (CH4) are able to absorb heat radiation in this way. Other gases like oxygen (O2) and nitrogen (N2) do not have a dipole moment, and thus, are not able to interact with infrared light. This is the reason why the relative proportion of CO2 in the atmosphere (approx. 0.04 percent) is misleading. From a infrared point of view, O2 and N2 are transparent and do not dilute the effect of carbon dioxide.

The charge distribution and the electric dipole moment of CO2 in comparison to O2 and N2.
Dominik Kreil

Everyone can contribute

These three acts were discussed at the Science Holidays at JKU. This was an initiative of the Bundesministeriums für Bildung, Wissenschaft und Forschung in Summer 2020, wherein we discussed the implications of CO2 with young pupils. The theories were underpinned with various experiments. We have seen that the absolute amount of CO2 in the atmosphere is a direct cause of the greenhouse effect. Since the industrialisation, its amount has increased by about 50 percent. We have to reduce our netto emission to zero until 2040 to prevent major natural catastrophes. Besides governmental restrictions, each and every one of us can contribute to this goal. One can also contribute by being a part of organisations like Fridays for Future, Klimabündnis, Extinction Rebellion, Klimavolksbegehren, and help to make the visions of the future a reality. For those interested in the theory, you may find scientific results for this locally, for example at the Wegener Institute in Graz or Boku in Vienna. (Dominik Kreil, Martin Hoffmann, 22.12.2020)

Dominik Kreil and Martin Hoffman are researchers at the faculty of Theoretical Physics. 

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