The evidence comes from the Nimbus 4 Michelson interferometer/infrared thermometer viewing the atmosphere from above and recording a temperature of 220 K of outgoing radiation around wavenumber 667, which is attributed to the presence of the trace gas CO2 at the tropopause.
But is it possible that the trace gas CO2 can have such a major impact on the radiation balance of the Earth? Is the ditch in the radiance spectrum reality or fiction? How is the radiance spectrum constructed? By directly measuring the radiance of different frequencies?
No, it is constructed from measurements of temperature according to Planck's law of radiation assuming an emissivity = 1. This is because the Nimbus 4 infrared thermometer measures temperature and not radiance, and the emissivity is not measured.
An infrared thermometer is in principle a blackbody BB which can measure the temperature of a given object O through radiative equilibrium at distance, as illustrated below in analog situation with two communicating vessels with the water level representing temperature: The level/temperature of O can be measured at distance by the reference BB through equilibration, while the widths of the vessels represent different emissivities. Evidently the wide vessel has much greater capacity of delivering water than the narrow vessel, thus has a much bigger (potential) emissivity, as indicated by the red arrows.
The Nimbus 4 infrared thermometer thus reports the temperature as function of wavenumber with in particular a temperature of 220 K from the presence of the trace gas CO2 in the tropopause, from which the above radiance spectrum is constructed according to Planck's law assuming emissivity = 1.
But is the emissivity of atmospheric CO2 = 1? No, it is not: Nasif Nahle computes using experimental data by Hottel and Leckner, presented in many books on heat transfer, that the emissivity of atmospheric CO2 is less than 0.002, thus very small.
But this means that the above radiance spectrum presented as the visual evidence of the greenhouse effect, is misleading. The ditch appears to be fiction.