What exactly is the greenhouse effect? And what does it look like if we view it from a new angle? Of course, we know the answer, and Raymond Pierrehumbert has written an excellent paper about it (Infrared radiation and planetary temperature). Computer code used in climate models contain all the details.
But is it possible to provide a simple description that is physically meaningful and more sophisticated than the ‘blanket around earth’ concept? I wanted a description that could be grasped by physicists. Without the clutter of too much details – just the essentials. A ‘back-of-the-envelope’ type derivation of the greenhouse effect.
The starting point was to look at the bulk – the average – heat radiation and the total energy flow. I searched the publications back in time, and found a paper on the greenhouse effect from 1931 by the American physicist Edward Olson Hulburt (1890-1982) that provided a nice description. The greenhouse effect involves more than just radiation. Convection also plays a crucial role.
How does the understanding from 1931 stand up in the modern times? I evaluated the old model with modern state-of-the-art data: reanalyses and satellite observations.
With an increased greenhouse effect, the optical depth increases. Hence, one would expect that earth’s heat loss (also known as the outgoing longwave radiation, OLR) becomes more diffuse and less similar to the temperature pattern at the surface.
An analysis of spatial correlation between heat radiation estimated for the surface temperatures and that at the top of the atmosphere suggests that the OLR has become more diffuse over time.
The depth in the atmosphere from which the earth’s heat loss to space takes place is often referred to as the emission height. For simplicity, we can assume that the emission height is where the temperature is 254K in order for the associated black body radiation to match the incoming flow of energy from the sun.
Additionally, as the infrared light which makes up the OLR is subject to more absorption with higher concentrations of greenhouse gases (Beer-Lambert’s law), the mean emission height for the OLR escaping out to space must increase as the atmosphere gets more opaque.
There has been an upward trend in the simple proxy for the emission height in the reanalyses. This trend seems to be consistent with the surface warming with the observed lapse rate (approximately -5K/km on a global scale). One caveat is, however, that trends in reanalyses may be misleading due to introduction of new observational instruments over time (Thorne & Vose, 2010).
Finally, the energy flow from the surface to the emission height must be the same as the total OLR emitted back to space, and if increased absorption inhibits the radiative flow between earth’s surface and the emission height, then it must be compensated by other means.
The energy flow is like the water in a river: it cannot just appear or disappear; it flows from place to place. In this case, the vertical energy flow is influenced by deep convection, which also plays a role in maintaining the lapse rate.
A popular picture of the greenhouse effect emphasises the radiation transfer but does not explicitly account for convection. As a result, it fails to explain the observed climate change.
Hulburt’s old model from 1931 included both radiative energy transfer and convection. It has now been validated against state-of-the-art data, and non-traditional diagnostics show a physically consistent picture.
An increased overturning can even explain a hypothetical slowdown in the global warming, and the association between these aspects can be interpreted as an entanglement between the greenhouse effect and the hydrological cycle, in which a reduced energy transfer associated with increased opacity is compensated by an acceleration of the hydrological cycle. This also makes a link with clouds.
The old conceptual model also explains why the so-called ‘saturation’ (which doesn’t exist on Venus) is a red herring, which is also explained in the report by the Copenhagen Diagnosis. I think those who present this argument have a poor understanding of what the greenhouse effect is all about.
A bold proposal: One way to view the greenhouse effect is the vertical distance between the place where incoming energy is deposited and where the average outgoing heat loss takes place. This distance depends on the concentration of greenhouse gases, and at what height the OLR can escape to space without being reabsorbed by air above.
The graphics below provides a crude illustration: the OLR is determined by Stephan-Boltzman’s law and the temperature at the same height, and the surface temperature is then given by the emission temperature, the emission height, and the lapse rate.
Not all of my colleagues may agree with my description of the greenhouse effect; it was a struggle to get this paper published. To my surprise, I realised that there are scholars with different ideas about it. However, I hope that my description will lead to more discussions and debate about the over-arching principles and our basic understanding of this phenomenon.
This also touches upon the question of climate sensitivity which is merely defined in terms of temperature change. A response to increased greenhouse gases could involve both a global warming and a speed-up of the hydrological cycle if the greenhouse effect and the hydrological cycle are intertwined. In other words, there could be more dramatic changes to the rainfall patterns than the temperature, but this doesn’t necessarily imply that the climate is less sensitive to the forcings.
- P.W. Thorne, and R.S. Vose, "Reanalyses Suitable for Characterizing Long-Term Trends", Bulletin of the American Meteorological Society, vol. 91, pp. 353-362, 2010. http://dx.doi.org/10.1175/2009BAMS2858.1
- R.E. Benestad, "A mental picture of the greenhouse effect", Theoretical and Applied Climatology, vol. 128, pp. 679-688, 2016. http://dx.doi.org/10.1007/s00704-016-1732-y