Joseph E. Postma and the Greenhouse Effect Part 2
Posted on 17 August 2011 by Chris Colose
In part 1, Joseph E. Postma did not like the use of uniform solar absorption over the planetary sphere. It was discussed that this is mainly just a bash against simple models which everyone knows is not how Earth works, but nonetheless gives useful answers. Postma goes well beyond this however, and then proceeds to really show that he does not understand the very model he is complaining about, or much else about atmospheric physics.
Venus is More Optically Thick Than a One Layer Model Can Give You
Postma starts by using Venus as a template for where the greenhouse model he is using breaks down. And indeed, he is right. His argument is that f (the emissivity) cannot possibly be greater than 1 (which is correct), and yet it must be in order to produce the Venus surface temperature in his Equation 29) Based on this, he then states that the standard greenhouse model does not work in general. The problem is that his Equation 29 assumes a one-layer atmosphere, which is an absurd assumption when you approach the extremely high optical thickness of Venus. Venus has a 90 bar atmosphere that has well over 90% carbon dioxide, some water vapor, and a greenhouse effect generated by suluric acid droplets and SO2. The radiative transfer on Venus works much differently than on Earth, owing in part to intense collisional broadening of CO2 molecules. A photon has an extremely difficult time escaping Venus, unable to do so until it reaches the very outer parts of its atmosphere.
Using the layer model, you would need many atmospheric layers to produce something close to Venus; with enough layers you would find that you could produce the surface temperature of Venus without violating conservation of energy. With just one perfect absorbing atmospheric layer, the surface temperature cannot exceed 21/4 times the emission temperature (Te=~230 K on Venus). But with two perfectly absorbing atmospheric layers, it can rise to 31/4Te. With three layers, the maximum temperature is 41/4Te, and so on. The reason the surface temperature is capped in this way is because the atmosphere itself must be emitting radiation and heats up when it absorbs photons from the surface, which in turn increases emission. If the atmospheric layer were instead a good infrared reflector (i.e., it has a high thermal albedo), then you could delay heat loss to space that way and increase the surface temperature well beyond this value. This could happen with CO2 clouds instead of H2O clouds, the latter are much more effective IR absorbers than IR scatterers, whereas the former could raise the IR albedo.
In essence, Postma stretches a simplified model to areas that it was never designed to go to, and then declares that its failure to work means the whole paradigm of the greenhouse effect is wrong. The incompetence is overwhelming. Postma is not done though, and decides to dig in further. His next argument is amusing, but perhaps a bit strange to follow, so I will try to explain.
Lapse Rate Confusion
He claims that observations of the atmospheric lapse rate (the rate at which temperature declines with height) disallow the greenhouse effect. His reasoning is that the atmosphere is at a fixed height. When greenhouse gases warm the surface, and cool the upper atmosphere, that height still remains fixed, but obviously the temperature difference between the bottom and top of the atmosphere must increase. Postma then claims that this necessarily implies that the lapse rate must have a greater slope than the theoretical value that he derived of about -10 K per kilometer (which is about right for a dry air parcel ascending). That is, if the atmospheric height remains fixed, and the temperature difference between bottom and top is increased, then the rate at which air cools with height must increase. Since this is not observed, then we have a problem, right?
In actuality, the atmospheric height is a distraction. The adiabatic lapse rate does not extend beyond the point where convection breaks down, which is the tropopause. The whole point of the greenhouse effect is that increasing atmospheric greenhouse gases does increase the “average” height at which emission to space takes place (and the tropopause increases in height too), so one IS allowed to extrapolate further down the adiabat to reach a higher surface temperature. On Venus, the optical thickness forces the tropopause to some 60 km altitude. Additionally, it is worth pointing out that greenhouse gases warm the upper troposphere, not cool it, but they do cool the stratosphere.
Figure 1: Qualitative schematic of the old (blue) and new (e.g., after CO2 increase) temperature with height in a dry atmosphere. Moisture tends to enhance the tropical upper atmosphere warming relative to surface. Temperature increases to the right.
TOA vs. Surface
Perhaps just as crucial to all of this, Postma cannot get around the surface energy budget fallacy, which says that increased CO2 causes surface warming by just increasing the downward infrared flux to the surface. This problem is described in standard treatments of the greenhouse effect, which he does not seem to know exist, such as in Ray Pierrehumbert’s recent textbook. The primacy of the top of the atmosphere budget, rather than the surface energy budget, has been known at least since the work of Manabe in the 1960s (see also Miller, 2011 submitted)
In reality, the top of the atmosphere budget controls the surface temperature even more than the surface forcing, because the atmosphere itself is adjusting its outgoing radiation to space (and much of the radiation to space is originating in the upper atmosphere, owing to its IR opacity). Where the atmosphere is well-stirred by convection, the adjustment in temperature at this layer is communicated to the surface. I described this in more detail here. (As a side note, I hope people can bookmark the home page to that blog, which is run by a team of meteorologists, climatologists, and grad students in atmospheric science, at the University of Albany in NY, and we will be posting periodically on many different issues from ENSO to climate change to recent weather around the country).
Postma runs into this mistake again when he claims that the low water vapor in hot deserts is a problem for greenhouse theory, but this is largely due to the lack of evaporation cooling, which is just one component of the surface energy budget, and nearly absent in a desert. This is one scenario where a detailed consideration of the surface budget is critical, as well as in other weakly coupled regimes.
The way CO2-induced warming really works in a well mixed atmosphere is by reducing the rate of infrared radiation loss to space. Virtually all of the surface fluxes, not just the radiative ones, should change in a warming climate, and act to keep the surface and overlying air temperature relatively similar. The back-radiation will indeed increase in part because of more CO2 and water vapor, but also simply because the atmosphere is now at a higher temperature. But if the lower atmosphere was already filled with water vapor or clouds to the point where it emitted like a blackbody (at its temperature), increasing CO2 would not directly increase downward emission before temperature adjustment, but would nonetheless warm the planet by throwing the TOA energy budget out of whack.
In summary, Joseph Postma published an article criticizing a very simple model that nonetheless produces useful results. He made several very simple errors along the way, none of which are very technical in nature. More sophisticated models are obviously designed to handle the uneven distribution of solar heating (which is why we have weather!); nonetheless, the educational tools are useful for their purpose, and in no way does Postma undermine the existence or necessity of the greenhouse effect. Without a greenhouse effect, multiple studies have shown that the Earth collapses into a frozen iceball (Pierrehumbert et al., 2007; Voigt and Marotzke 2009, Lacis et al 2010) and indeed, after an ice-albedo feedback, plummets below the modern effective temperature of 255 K. This work makes extraordinary claims and yet no effort was made to put it in a real climate science journal, since it was never intended to educate climate scientists or improve the field; it is a sham, intended only to confuse casual readers and provide a citation on blogs. The author should be ashamed.