Is CO2 Saturated in the Atmosphere?

A very common objection to climate science goes something like this - CO2 already absorbs all the IR radiation it can possibly absorb, so the greenhouse effect is saturated and adding more CO2 will not cause any more warming. Sometimes this objection is accompanied by an analogy. If you apply translucent tape to a clear window, you obstruct some but not all light passing through the window, but the more tape you add, the more lite is obstructed, and eventually you reach a thickness of tape beyond which no light can pass through. The tape has made the window opaque, and so adding more tape can't do anything more. Of course, if this were true, and if we are either at or near that saturation point, there would be no need to stop emitting carbon, since any additional carbon we add to the atmosphere won't change the Earth's temperature (it would continue to cause ocean acidification, though, but let's leave that for another day).

The reasoning behind this objection is flawed in that it misrepresents how increasing CO2 causes warming in at least two ways.

1. Surface Temperatures Increase with Emission Altitude. Whenever CO2 molecules absorb upwelling longwave radiation (LWR), they don't hold onto this energy for very long. They either 1) collide with other molecules and give up their energy by collision, thus adding to the total kinetic energy of the molecules in the atmosphere, or 2) they emit this energy in all directions, with about half headed towards space and about half headed back to the surface. The GHE works essentially by slowing down the rate at which LWR escapes into space, and the amount of warming is set by the emission altitude, not at the surface. As CO2 concentrations increase LWR emitted towards space must travel to higher altitudes, where the atmosphere is thinner, in order to finally escape the atmosphere without any more absorption. A perturbation of the Earth's energy imbalance by increasing CO2 pushes that emitting altitude higher into the troposphere where it's colder, and colder air is not as efficient at emitting, so more energy stays in the climate system and the surface warms according to its lapse rate. Once equilibrium is reached, the temperature of that effective emitting layer will be the effective temperature that the surface of the earth would have if their were no greenhouse gases (about 255K). It's not the amount of absorption at the surface that determines the amount of warming (it's not how many layers of tape are applied to the window); it's the altitude at which LWR can finally escape to space. The higher this emission altitude becomes, the more the surface warms in response.

At the surface, we can actually observe downwelling LWR radiated from CO2 in the atmosphere. In a recent post, I described an experiment that set up IR detectors in two locations that were capable of detecting increases in LWR at wavelengths absorbed by CO2 and emitted towards the surface. The clear-sky radiative forcing they measured showed "statistically significant trends of 0.2 W m−2 per decade (with respective uncertainties of ±0.06 W m−2 per decade and ±0.07 W m−2 per decade) and have seasonal ranges of 0.1–0.2 W m−2."[1]

If CO2 saturation at the surface was preventing the Earth's surface from experiencing more warming, this increase in additional LWR emitted from the atmosphere towards the earth would not occur.

2. Pressure Broadening at Higher CO2 Concentrations. Another factor this objection fails to consider is that as CO2 concentrations increase, the range of wavelengths of LWR absorbed by CO2 broadens. The central wavelength absorbed by CO2 is 15 μm (or a wavenumber of about 650/cm), and there is certainly a limit to to how much LWR can be absorbed at that wavelength (you can see this depicted at the 220K curve on the graph below). 

Yet this graph shows a broadening of absorption bands as the absorption coefficient (α) increases. There is an increase in energy retained within the climate system at the edges. So as CO2 levels increase, the "wings" of this absorption band broaden, so that increasing CO2 levels still traps more LWR within the climate system. The effect can be described as pressure and doppler broadening. This has been understood at least as early as 1956; see the following from Gilbert Plass:

One further objection has been raised to the carbon dioxide theory: the atmosphere is completely opaque at the center of the carbon dioxide band and therefore there is no change in the absorption as the carbon dioxide amount varies. This is entirely true for a spectral interval about one micron wide on either side of the center of the carbon dioxide band. However, the argument neglects the hundreds of spectral lines from carbon dioxide that are outside this interval of complete absorption. The change in absorption for a given variation in carbon dioxide amount is greatest for a spectral interval that is only partially opaque; the temperature variation at the surface of the Earth is determined by the change in absorption of such intervals.[2]

But even in the 19th century, the basic logic behind this was already largely understood. John Tyndall wrote back in 1862 that "As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial [infrared] rays, produces a local heightening of the temperature at the Earth’s surface." In other words, increasing CO2 levels always increases the strength of the greenhouse effect, as more heat is dammed up in the climate system, and the average surface temperature of the Earth must warm. 

Conclusion

The results above are non-controversial, and to my knowledge have been established since the 1950s at the latest. While the effect does explain why the relationship between increasing CO2 concentrations and temperature is logarithmic (2xCO2 causes ~3 C warming with feedbacks), there is no evidence that we're approaching any saturation limit above which no more warming can occur. And this is not something that climate scientists disagree about. Both proponents of and detractors to AGW agree on this point. Here is an explanation from Roy Spencer, affirming what I've offered above.

It doesn’t matter even if the CO2 absorption bands are 100% opaque to the transmission of IR radiation from the surface to the top of the atmosphere…adding more CO2 still causes a warming tendency in the lower atmosphere (and cooling in the upper atmosphere).

See the rest of the post from Spencer for more details. While Dr. Spencer agrees that increasing CO2 pushes climate towards warming; he disagrees with how much, so much so that he takes many "contrarian" positions within climate science. There are others that simply deny well-established physics, so much so that even contrarians like Spencer feel compelled to write against their claims;  they make contrarians like Spencer look bad. But these claims about saturation are simply opposed to physics; they are extremist positions outside the realm of science. My rule of thumb here is that, if even Spencer calls you out for holding a position that makes contrarians look bad, it is fair to say that your claims are extremist claims.


References:

[1] Feldman DR, Collins WD, Gero PJ, Torn MS, Mlawer EJ, Shippert TR. Observational determination of surface radiative forcing by CO2 from 2000 to 2010. Nature. 2015;519(7543):339‐343. doi:10.1038/nature14240
https://escholarship.org/content/qt3428v1r6/qt3428v1r6_noSplash_b5903aebfe105b4071103e11197138f8.pdf

[2] James Rodger Fleming, Gavin Schmidt, Gilbert Plass. "Carbon Dioxide and the Climate: A 1956 American Scientist article explores climate change; two contemporary commentaries illuminate its relevance to the present." American Scientist. Vol. 98.1, page 58.
https://www.americanscientist.org/article/carbon-dioxide-and-the-climate

See also, Plass, G. N. 1956. The influence of the 15 micron ozone band on the atmospheric infra-red cooling rate. Quarterly Journal of the Royal Meteorological Society 82.