Saturday, June 16, 2007

Carbon Dioxide, It's Role in Atmospheric Heating....A Detailed Look

Here we are, getting down to the "nitty-gritty". The big question is, by adding carbon dioxide to the atmosphere are we humans "cooking our own goose"? By that, I mean are we causing global warming and all of the crazy weather we see every night on the TV news? It seems there are differing opinions on the subject. It is complex, but I think we can understand, if we try. Certainly we must not rely on politicians and celebrities to tell us what to believe.

The following is one educated opinion.


Greenhouse Gas Effect and Carbon Dioxide
This is a revised version of my previous post. When in energy balance, the Earth radiates from the top of the atmosphere at 235 Watts per square meter (1). Radiation from the greenhouse gases goes in all directions, and so, effectively, half is radiated out into space, and half is returned to the Earth’s surface and so helps to increase the surface temperature up to a value for which the radiated emission is twice that from the ToA to outer space. ie. the Earth’s surface radiates at 470 W.m^ -2.

Carbon dioxide has an absorption peak at almost 14 micrometres, but very little at longer wavelengths. The energy of a 14 micron infrared photon is 1.34×10^ -20 Joules. Therefore, the emission from the surface is about 3.5×10^22 photons/second per square metre. Given a relaxation time of 10 microseconds for the CO2 molecule to decay from its excited state, then a 1 second time interval can be considered as 10^5 periods of 10 microseconds each. Assuming an even distribution of emission across the area of 1m^2, and a uniform time rate of emission, then the number of photons emitted in each 10 microsecond period is 3.5×10^17 photons in 1m^2. In order to ensure 100% absorption of these photons, the 1m^2 area must be “covered” by sufficient molecules of CO2.

Now, the absorption cross section of a CO2 molecule for a 14 micron photon is about 5×10^ -22 m^2 per molecule, and so the number of molecules required to cover the area is 1.0 / 5×10^ -22, ie. 2×10^21 molecules per square metre. Now consider a vertical column of the Earth’s atmosphere based on a square of area 1 m^2.This air column has a mass of 1.01×10^4 Kg.m^ -2.The mass of the neutron (& proton) is approximately 1.67×10^ -27 Kg.So the mass of the nitrogen molecule is 4.68×10^ -26 Kg. Therefore, the number of N2 molecules in the column is approximately 2.15×10^29.Now, carbon dioxide is currently present at the level of about 380 ppm by volume, and so the number of CO2 molecules in our 1 m^2 column is about 8×10^25. Therefore, the 100% cover can be provided 8×10^25 / 2×10^21 times over, ie. 4×10^4 times.

So the carbon dioxide is grossly oversubscribed. This means that although the natural GHG effect works very well, the idea of anthropogenic “enhanced” greenhouse effect is totally non-viable as far as carbon dioxide is concerned, because there has long been far, far more carbon dioxide in the atmosphere than required, and so another 30% ( or several orders ) extra will have absolutely no effect. Reference(1)Kiehl and Trenberth, 1997
Aubrey E Banner, Sale, Cheshire, UK19th June 2006

No comments: