söndag 17 februari 2013

Low Emissivity of Atmospheric CO2

Recent posts suggest a low emissivity of atmospheric CO2 away from its main resonance at wave number 667.  To check let us compute the transmittance of a transparent atmosphere at 225 K after adding 400 ppm CO2 over a distance of 100 m at a pressure of 200 mb using the free version of the commercial software SpectralCalc to get the following spectrum with a blowup around 667:

We see that the transmittance is zero in the narrow interval 664 - 668, where 400 ppm CO2 makes the atmosphere opaque, while outside this interval the transmittance is low only in a small portion of the spectrum. In other words, the total emissivity of 400 ppm CO2 is very small which means that CO2 has a very limited capability to "block radiation" from the Earth surface, thus contradicting typical OLR spectra with full blocking in the interval 600 - 800.

Further, changing to 600 ppm gives almost the same transmittance spectrum, signaling small radiative forcing from doubling of the preindustrial concentration of CO2:

PS Further evidence is given by Ed Caryl.

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