The impressive size of the ditch is supposed to express the power of the trace gas CO2 as "greenhouse gas" capable of changing the radiative balance of the Earth and thus the Earth surface temperature.
The picture is complemented by leading exam questions, to be answered by "the ditch for clear sky" as the evidence of the existence of a powerful GHE as the way to pass the exam in the course Introduction to Atmospheric Science at Dalhousie University.
On the other hand, the lower curve for cloudy sky has no ditch (but rather the opposite as an effect of the stratosphere, which however is insignificant here). Obviously, the GHE is supposed to be operational only for clear sky.
To understand if GHE is real or fiction, we have to understand how the radiance curves are constructed in order to understand the relevance of the crucial ditch for clear sky, which connects to the previous post and to How to Fool Yourself with a Pyrgeometer.
The essential insight is that radiance is computed using Planck's law from temperature measured by satellite. But the temperature varies through the atmosphere, and so which temperature is then measured from the satellite using which thermometer?
In earlier posts we have seen that the temperature of a given body can be measured through radiative equilibrium with a blackbody acting as thermometer. The satellite thus is equipped with a blackbody thermometer capable of measuring the temperature of a portion of the atmosphere through radiative equilibrium. This is how the temperature of the cloud tops of the cloudy sky is measured to be 200 - 220 K as the temperature of the tropopause.
In the case of clear sky we see the Earth surface temperature being measured for wave numbers larger than 800 including the atmospheric window, and for wave numbers smaller than 550 the temperature of the top of the lower troposphere with water vapor.
The temperature in the crucial dip between 550 and 800 is measured to be 220 K, which is the temperature of the tropopause. The blackbody thermometer in the satellite is thus in radiative equilibrium with the tropopause, and not with a lower layer with higher temperature.
The presence of the trace gas CO2 thus makes it possible for the tropopause to be in radiative equilibrium with the satellite thermometer in the range 550 - 800, and the thermometer can thus report a temperature of 220 K, if the thermometer is sensible enough to react to a trace gas.
But this does not mean that the trace gas CO2 has absorbed all radiation from warmer lower layers of the atmosphere, only that the satellite thermometer is sensible enough to report the temperature of the trace gas CO2 with an emission spectrum in the range 550 - 800. It is then natural to expect the thermometer to report the CO2 temperature in the tropopause as the outer boundary of the troposphere.
To sum up:
- The radiance spectrum is computed using Planck's law from temperature measurements using a thermometer which is sensible to the trace gas CO2.
- This does not mean that the trace gas CO2 acts as blackbody absorbing/emitting radiation according to Planck's law causing a major dip in the outgoing radiance spectrum expressing a powerful GHE.
- It only means that the thermometer is sensible to the trace gas CO2.
- A sensible instrument can measure a small effect, which can be plotted in a big diagram, but it does not make the effect big.
- The dip in the spectrum supporting GHE is fiction and not reality.