The physical process of radiative forcing is calculable with radiative transfer codes, such as the CRM I have dabbled with in previous posts. Also, there is a truism associated with radiative forcing that the only significant way in which earth sheds energy is through radiation.
With these thoughts in mind, I set out to consider the ways other than surface warming which could possibly restore balance to the global disequilibrium imposed by a doubling of carbon dioxide.
Below, I ran runs of the CRM for the Northern Hemisphere TREX sounding with the following variations of profile:
- warming the upper troposphere but not the surface
- lowering the ‘top of the water vapor’ without reducing water vapor
- changing the fraction of a cloud layer at 700millibars
- lowering the top of a cloud layer
Here is the comparison of 1x CO2 with the same profile but 2x CO2 and various amounts of warming aloft, but no warming in the lower levels. The net radiance at the tropopause ( RF ) decreases as warming aloft increases:
Below is the comparison of a 1x CO2 atmosphere and the same profile but with 2xCO2 and various ‘top-of-the-water-vapor’ levels. The dew point depression of profiles with lower ‘tops’ is reduced such that the absolute amount of water vapor does not decrease from the original sounding. This is indicated by the ‘Precipitable Water’ calculation, labelled ‘PW’ for each profile. As the ‘top-of-the-water-vapor’ lowers for each profile, the RF decreases ( and becomes negative ):
The following runs indicate the effect of changing the cloud fraction at 700mb. Increasing cloud fraction greatly reduces net radiance, but this sounding is at mid-day:
Here is the same effect of cloud fraction at 700mb, but at mid night instead of mid day. For this location (45N, 0E) and time (Julian 180) the shortwave albedo is of the opposite sign and exceeds the longwave forcing such that at mid night, increasing cloud fraction increases RF:
Finally, below is the effect of varying the height of a cloud layer of a constant cloud fraction, but of a cloud liquid water content proportional to the humidity of the level. This evidently makes the effect of lower clouds to have much greater albedo:
The same experiment but at night yields a different relationship than a varying cloud fraction at a given height. The clouds are more emissive, not only because they are lower ( and thus warmer ) but also because they contain more liquid water droplets with which to emit. Lowering cloud levels for this time and profile can reverse a 2x CO2, but only by greatly lowering the cloud top from 400mb to 850mb:
I don’t believe any of these cases means that CO2 does not likely ’cause warming’. To the contrary, while this is by no means exhaustive, it’s a reminder that whatever happens, the only response that can matter to reverse CO2 forcing is radiative.
Never the less, these cases do represent what is possible though improbable means of increased radiative transfer to space.