# How does extra carbon dioxide in the atmosphere contribute to radiative forcing?

The CO2 absorption spectrum taken at 200 mmHg partial pressure, beam path length = 10 cm, resolution = 4 cm^-1 shows that light can only travel a couple hundred meters before being completely absorbed by atmospheric CO2 at its absorption peaks. How does extra CO2 increase absorption?

Answers here discussed several mechanisms of spectral broadening and minor absorption bands. But the major absorption bands seem to be wider beyond what a few nanometers of broadening could affect. Would these effects have a non-negligible contribution to carbon dioxide radiative forcing?

As seen in the outgoing spectral radiance taken at the top of the atmosphere, the CO2 absorption peak is only half as high as the blackbody spectrum at the same wavelength. This is contradictory to the argument that the atmosphere is completely opaque in CO2 absorption bands. Does that mean that the thermal emission of CO2 accounts for most of the outgoing radiance at 15 µm? Is there a quick way to see that, and what implications does it have on the radiative forcing of CO2?

• Isn't the too low height of the peak more a statement about how much $CO_2$ is between the $\tau=2/3$-surface and the observer in space? – AtmosphericPrisonEscape Oct 12 '19 at 12:53
• It's also a statement about the effective temperature of the layer doing most of the emission observed from space. In this case a broad region around the tropopause - overplot the Planck curve for T=220 K and it'll roughly pass through that CO2 trough. – Deditos Oct 12 '19 at 17:44
• @Dedios Would't the rest of the spectrum be too high if the effective temperature is 220K? Or is it that the atmospheric emission temperature is bimodal? – arax Oct 31 '19 at 22:14