Tuesday, 8 July 2008

Radiation in the atmosphere

This diagram from Global Warming Art helps show the contribution of various gases to heat retention in the atmosphere.


On the spectral intensity graph the red line is the radiation coming from the sun. It is stylised by using a blackbox assumption feeding in the temperature at the surface of the sun. The red area is the radiation that is transmitted thru the atmosphere to the surface of the earth. The purple, blue and black lines are the radiation transmitted from the earth into space; again with a blackbox assumption based on temperature. The 3 lines represent 3 different temperatures from 210–310 K (–63 to +37 °C) due to the variation of the surface and atmospheric temperature over the earth. The blue area is what escapes into space.

The next graph shows the absorption of the radiation (energy) at various wavelengths. Both for inward and outward radiation; these are predominantly non-overlapping. A level of 100% means that no radiation at that frequency gets transmitted thru the atmosphere (ie. from space to earth on the left side of the graph or from earth to space on the right side of the graph). A level of 0% means that all radiation is transmitted.

Note that this is as a percentage of incident radiation, but as can be seen from the first graph, there is not the same amount of radiation for all frequencies. Thus high absorption near peaks of radiation is more important than high absorption at nadirs. Thus the resultant effect is seen in the first graph in the solid red and blue areas. The white area of the second graph is nearly a mirror image of the first graph (though stretched at low spectral intensities due to the first being an absolute scale and the second being a relative scale as mentioned).

The bottom graph shows the spectral absorption of various gases and the effect of Rayleigh scattering. There is no scale on the y-axis but it is a percentage scale like the second graph. Note it goes from 0–100% for each of the 5 gases.

Several things can be appreciated here. The most obvious being how important water is to heat absorption. Water reacts moderately to the incident solar radiation and significantly with the outgoing earth radiation. Any modelling of the climate that neglects to incorporate the effect of water, or treats it as an unchanging constant is virtually guaranteed to be incorrect. And this is just water vapour! The cloud effect is much more complicated.

The interesting thing about CO2 is that several of the absorption lines correspond to wavelengths that are of minor consequence. They lie at the tails of the solar and earth spectral intensities. So the CO2 intensity band of most interest is the far right one. This band is important. It is wide and it is absorbs 100% of the incident radiation. It is also in an area where water absorbs a significant amount of radiation and given that there is far more water than CO2 in the atmosphere, the contribution from water at this frequency is not negligible.

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