Emission spectra as seen from satellites, or modelled with modtran, are probably among the most profound evidence for Earth's greenhouse effect. They visualize not just how greenhouse gases reduce emissions, but also which GHGs do that and to what extent. However, these data are highly deceptive as they get matched against the erroneous benchmark of a perfectly emitting surface. And it seems everyone falls for the trick.
A straight forward answer would be, we do not really know. There are a couple of issues which make it hard to simply measure emissivity, and that is not just true for Earth, but also for the Moon for instance. Just like within the visible light, the optical properties of the surface within the LWIR range can be very diverse. Just think of the different colors you see when you look outside. What we perceive as colors are effectively different wavelengths of electromagnetic radiation within the relatively tight spectrum of visible light. The fact that surfaces have all these different colors…
I was a bit reluctant to write this article, since it is certainly a horribly annoying issue. There are no insights to be gained here, at least not with regard to climate science. It is only about cleaning up a horrible mess. The only thing making it worthwhile is in pointing out how stupid people are. And with people I mean "experts".
The position of the IPCC on the cloud radiative effect (CRE, also frequently named cloud forcing CF) is simple and straight forward: By enhancing the planetary albedo, cloudy conditions exert a global and annual short-wave cloud radiative effect (SWCRE) of approximately –50 W m–2 and, by contributing to the greenhouse effect, exert a mean longwave effect (LWCRE) of approximately +30 W m–2, with a range of 10% or less between published satellite estimates