Evidence that CO2 is a mild greenhouse gas and that water vapour is far more important
About one half of the solar energy that reaches the atmosphere’s outer limits from space actually hits the surface of the earth. The other half of solar insolation is already reflected or absorbed earlier on its way through the atmosphere. It it thus by the remaining half that reaches the ground that the surface of the earth is heated. Every heated body, though, radiates by itself, proportional to its temperature. According to the laws of physics, one is able to calculate the range of wavelengths in which radiation is emitted at a certain temperature of the stove plate or, more generally, a heated body. The many-colored area in Figure 6 shows us how the radiation of heat is distributed if the temperature of a body is of the order of 280 Kelvin (+7°C). This almost corresponds to the earth’s mean global temperature at its surface. The illustration shows a spectrum of radiation approximately between 400 and 1800 cm-1.
Earth’s Albedo: the energy reflected back into space from earth’s surface.
In this figure, red + yellow + blue represents the total spectrum of energy reflected by the Earth at 7° C, in the range between 400 and 1800 cm-1. Red represents the absence of an absorption spectrum due to technical reasons concerning the measurements. Blue denotes the radiation that is absorbed by greenhouse gases. And yellow represents the radiation that is allowed to pass by greenhouse gases. Above the curve are horizontal bars that represent the absorption spectrum of the various gases. If readers are not familiar with the idea of absorption spectrum, it can be explained thus: Molecules are able to oscillate, in many ways. Each different mode of oscillation requires a very specific amount of energy to stimulate the oscillation. That is why molecules absorb at different radiation energies. Also, there are major differences in the absorption spectra of different gases, and these must be factored in if one wants to understand how energy from the earth’s reflected energy is captured. Water vapour (H20) absorbs energy over most of the spectrum: from 400-900 microns and from 1130-1800 microns. CO 2 absorbs energy from about 650 to 750 microns. In a rough approximation the following trace gases contribute to the greenhouse effect: 60% water vapor; 20% carbon dioxide (CO2);the rest (~20%) is caused by ozone (O3), nitrous oxide (N2O), methane (CH4 ), and several other gases.
However, supporters of cuts from human emissions may object that the bar for CO2in Figure 6 understates the absorption spectrum of CO2, so let us consider the absorption spectrum preferred by those supporters (Available at http://www.iitap.iastate.edu/gccourse/forcing/spectrum.html Click on Figure 3).This is reproduced as Figure 7. Note that the absorption spectra in
Absorption spectra of major greenhouse gases
Figure 7 assumes a uniform input of radiation at all frequencies. As Figure 7 shows, this is incorrect for the frequencies in earth’s reflected energy. And although Figure 7 shows CO2 as having a higher absorption level at around 1400-1500 microns than water vapour, according to to Figure 6, the reflected energy of earth at this range contains only about 27 W·m−2·sr−1 of radiation energy, whereas at about 600 microns, where H2 O has very strong absorption properties, and CO2 weak absorption properties, the total energy level is about 124 W·m−2·sr−1 . Furthermore, a comparison of the absorption spectra of H2 O and the lowest spectrum shows that they are overwhelmingly similar, which is exactly what one would expect if H2 O is the dominant greenhouse gas. The only range where the whole atmosphere spectrum matches that of CO2 is in the upper range above about 1300 microns. But as we pointed out, the earth’s reflected radiance in this micron range is very weak. Taking all these factors into account, one suspects that the 20% of the energy of earth’s albedo assumed above for CO2 is probably over-generous. Still, let us work with the 20% figure for the time being.
A point made by the IPCC is that as a result of the human-caused increase in CO2energy normally lost from the earth because of infra-red radiation to the atmosphere is instead reabsorbed by the atmosphere, resulting in a greenhouse effect. It is also claimed that this absorption occurs in the lower atmosphere. This is the troposphere, which contains approximately 75% of the atmosphere’s mass and 99% of its H2 O and aerosols. The percentage H2 O in surface air varies from a trace in desert regions to about 4% over oceans. But we are interested here in the troposphere, which, according to the IPCC, is where greenhouse gases do their work. And since the troposphere contains 99% of the total H2 O, it makes sense to assume a conservative average for water vapour of 2% for the whole troposphere. Contrast that with the 0.039% presence of CO2. What this means is that at the equator water vapour molecules are 51.28 times more common than CO2 molecules. So to calculate the contribution to greenhouse warming by CO2 vis-a-vis water vapor, we need to multiply the absorption proportions of each molecule (1/3) by the frequency of those molecules in the troposphere: 1/3 X 1/51.28 which comes to 0.0065. But remember that the absorption contributed by CO2 is the result of all the CO2 in the atmosphere -780 Gigatonnes. The total annual CO2 budget is 191.6-224.1 Gt/yr, which is 24.6% of the total. Above we pointed out that human emissions represent 7.0-7.5 Gigatonnes per year to the CO2 budget. That is, they constitute only 3.5% to 3.9% of all carbon emissions (mean 3.7%). So to calculate the annual greenhouse effect of human emissions vis -a-vis water vapour, we need to take 3.7% of the 0.0065% contribution of all CO2, which means that annual contribution of all human emissions are responsible for 0.0002405% of greenhouse warming. Since Australia is reponsible for about 1% of those emissions, we need finally to calculate Australia’s annual contribution to warming by CO2 is 0.000002405%. Hardly a good reason to seriously damage either Australia’s or the world’s economy.