CO2 Radiative Properties

[blizzard1024]

I have been digging deeper into the radiative effects of CO2 to better understand emission

spectra like the one below. 

 

 

Initially, I erroneously used Wein's Displacement Law to conclude that CO2 has it main effect

on temperatures around -50C or colder because of its absorption band centered on 15 microns.  The Earth radiates in the IR close to a blackbody, but not the atmosphere as was pointed out on this forum.  So my use of Wein's Law was in error. Thanks to whoever pointed this out.  

 

After further reading and study, the above CO2 emission spectra is seen at around -50C (223K) because this is approximately the temperature in the upper troposphere where CO2 is well mixed. Hence, the spectra clearly shows the dominance of CO2 in absorping IR in this spectral band. 

 

Since the upper troposphere has so little water vapor, CO2 is dominate in the upper troposphere. At lower levels, there is much more water vapor and you get overlap of the absorption bands. But when CO2 increases it does add radiation to the lower atmosphere which does absorb at a span of wavelengths.  

 

Thus based on HITRAN and other line by line radiative models, there is little doubt that doubling CO2 will lead to extra radiative forcing...the latest models state  3.7 w/m2 for a doubling. This equates to 1.2C of warming assuming no feedbacks. The feedbacks is where people tend to disagree and where the uncertainties lie.  

 

I am especially interested in how the water vapor vs CO2 bands overlap and what the effect of this in the lower atmosphere. If anyone has any other insights on this or other rad tran I am all ears. This stuff is incredibly complex and I think there are a lot of misconceptions among people and also a lot of people who don't try to understand this stuff due to the complexity. I am not debating the feedbacks on this topic.