I thought I would quickly highlight this Realclimate post which is intended to be the best description of the Greenhouse effect. It’s written by Ramus Benestad, the author of this paper. The main reason I wanted to highlight it was for the animation, that I’ll include at the end of this post.

The basic idea is, I think, similar to what I was trying to describe in this post. If we had no atmosphere (or it was radiatively inactive) then all the energy radiated from the Earth would come from the surface and the surface temperature would be set by the radiated energy matching the energy received – about 255K if we reflect 30% of the incoming solar radiation (an albedo of 0.3). If there is a raditively active atmosphere (one with greenhouse gases) then these would act to increase the opacity of the atmosphere (making it opaque to some of the radiation coming from the surface) and some of the energy would be radiated from the within atmosphere, rather than all of it from the surface.

This allows us to consider the energy as being radiated from an effective emission height in the atmosphere, with the temperature at this height being essentially the same as the surface temperature in the absence of an atmosphere (about 255K assuming the same albedo). The temperature on the surface would then be set by whatever processes transport energy from the surface to this effective emission height. Since the greenhouse gases have increased the opacity in the atmosphere, this cannot all be via radiation. In the case of the Earth’s atmosphere, the dominant process is convection.

In a dry atmosphere, the temperature gradient to which the lower atmosphere would tend would be around 10K/km. The effective emission height in the Earth’s atmosphere is at around 6-7km, which suggests that if the only transport mechanism were convection, the surface temperature would be 60-70K warmer than it would be in the absence of a greenhouse effect. It clearly is not. The reason is that convection is not the only energy transport mechanism. Another is evporation. Water evaporates at the surface and is carried up into the atmosphere. When it condenses it releases energy and heats the atmosphere. It, however, doesn’t do this uniformly and so it acts to reduce the temperature gradient relative to what it would be in the presence of convection only.

Consequently, the typical temperature gradient in the Earth’s atmosphere is 5-6K/km which, with an effective emission height of 6-7km, gives a surface temperature about 33K warmer than it would be in the absence of the greenhouse effect. I should stress, however, that this is a fairly simple picture. The energy is not actually radiated from a single height in the atmosphere, and the temperature gradient is not always what you would expect from convection and evaporation, but it is quite a nice illustration of the general process.

It also allows one to consider what would happen if we add more greenhouse gases. As we do so, we increase the opacity and raise the effective emission height. The surface temperature then rises because the temperature gradient is largely set by non-radiative energy transport processes (convection and evaporation). An interesting point, though, is that the amount by which the surface warms depends on the relationship between convection and evaporation (which drives the hydrological cycle). The more the hydrological cycle intensifies, the less the surface actually warms. This, however, implies that lower surface warming would be associated with more dramatic changes to rainfall patterns, which – as the Realclimate post suggests – shouldn’t necessarily be interpreted as the climate being less sensitive to changes in external forcings.

In the past when I’ve written about this there have ended up being lengthy discussions in the comments about some of the details. Hopefully I’ve got it roughly right this time, but even the Realclimate post suggests that even among experts there are disagreements about how best to decribes this, so maybe I shouldn’t be surprised if not everyone agrees. Anyway, I think it’s nicely illustrated in the figure below.