15 Jan, 2010
Does the greenhouse effect exist?
I'm interested in climate change research in general, and I'd like to think that I know something about radiation transfer, too. I know there are plenty of people out there who don't believe the science about climate change, I think it's fundamentally sound and when I come across references to this in what I regard as reputable media, I feel I have to respond. In the Jan. 4, 2010 issue of ""From the view of a trained physicist, Gore's movie is rather grotesque, since it is shockingly wrong. Every licensed radio amateur knows that what is depicted in Figure 16 would be true only [if the radiation is short-wave radio]."They then devote some space to an irrelevant discussion the ionosphere and the reflection of radio waves, and goes on to accuse Gore of confusing the physical processes of reflection and absorption/re-emission. Never mind that the figure in question never actually mentions reflection, but more importantly, they never actually complete the argument to explain how this distinction is relevant to whether the greenhouse effect works or not. (It is not: the greenhouse effect works regardless of whether the medium is absorbing or purely scattering.) As another example, section 3.7.10 calculates how long it will take a body to radiate away its heat, assuming it keeps uniform temperature. By equating the Stefan-Boltzmann emission power with the drop in thermal energy, they conclude that a cube of air 1 meter on a side at 300K will radiate enough energy to drop to half that temperature in 3s. They go on to say:
"For air, even if only one of the cube's planes were allowed to radiate, one would get a fall in temperatures of seventy degrees within the first three seconds, and almost 290 degrees within ten hours - a totally unrealistic cooling process."While they don't specify why this cooling would be unrealistic (is it too fast or too slow?) it is of course totally unrealistic: First it assumes the air is a perfect blackbody, and second that it is placed in intergalactic space so there is no corresponding influx of radiation. But they instead conclude that blackbody radiation is inapplicable to gases, and go so far as to (in section 3.7.3) to say that "crude approximations based on T^4 expressions need to be taken with great care. ... they prove nothing." But this is after they themselves have pointed out that for "graybodies", the Stefan-Boltzmann law must be modified with a temperature-dependent emissivity (or as we would say, a wavelength-dependent opacity). So they are using an expression they have already said must be modified to rail against the use of blackbody radiation in general, another sleight of hand. There are only two core arguments I can find: First, that for determining the global radiation balance, a properly weighted "average" temperature is needed. This supposedly removes the empirical need for a greenhouse effect to explain the global temperature. Second, that the greenhouse effect violates the second law of thermodynamics. The first has some merit, the second is ludicrous. As for the average temperature, they point out that since the outgoing radiation is (roughly) proportional to T^4, for the purpose of determining the total radiated power into space, the relevant average is the "radiative" average temperature, ie <T^4>^0.25. Since regions with higher temperature contribute disproportionally to the emitted radiation, this radiative average will always be larger than the arithmetic average. So far, the reasoning here is correct. They then construct an example dataset with 5 fictitious data points and show that while the mean temperature is 15C, the radiative average is 15.5C. They then conclude "Thus there is no longer any room for a natural greenhouse effect, both mathematically and physically." However, already the abstract states that the greenhouse effect is supposedly responsible for a 33C difference. How the 0.5C difference in their fictitious example removes the need to explain the 33C difference for the actual Earth is not addressed. They end with "In the preceding sections mathematical and physical arguments have been presented that the notion of a global temperature is meaningless." This is obviously incorrect -- they themselves have pointed out that care needs to be taken to define the average appropriately for the question being posed -- and it's another claim that has no obvious connection to whether, physically, the greenhouse effect exists or not. Finally, what about the second law? It's a bold claim, but one that is even less clearly motivated. Instead of laying out a physical reasoning, section 3.9.3 simply quotes the German climate researcher Rahmstorf as saying:
"Some 'sceptics' state that the greenhouse effect cannot work since (according to the second law of thermodynamics) no radiative energy can be transferred from a colder body (the atmosphere) to a warmer one (the surface). However, the second law is not violated by the greenhouse effect, of course, since, during the radiative exchange, in both directions the net energy flows from the warmth to the cold."The authors then retort:
"Rahmstorf's reference to the second law of thermodynamics is plainly wrong. The second law is a statement about heat, not about energy..."I don't even know how to address this. Perhaps the authors are not familiar with the fact that heat is energy? They go on:
"Furthermore [Rahmstorf] introduces an obscure notion of "net energy flow". The relevant quantity is the "net heat flow", which, of course, is the sum of the upward and the downward heat flow within a fixed system, here the atmospheric system. It is inadmissible to apply the second law for the upward and downward heat separately redefining the thermodynamic system on the fly."I simply can't understand what they are trying to accomplish. Admittedly, the quote from Rahmstorf is not the clearest as it talks about energy flowing from warm to cold in both directions, but this is another example of the tactic commonly used by the authors where they appear to believe that they are in a courtroom. Attempts to explain phenomena are parsed in a way that makes the statement appear to not make sense, apparently in a belief that this serves to also make the underlying physical principle invalid. Nature, of course, is not a courtroom, and while this tactic may serve to make people look inarticulate, it has no bearing on the state of reality. That is the extent of the "falsification" of the greenhouse effect by showing that it violates the 2nd law. There is not even an attempt to lay out a coherent argument. The truth, of course, is that there is no problem with the 2nd law, because the atmosphere does not "heat" the Earth, it merely slows its cooling. A couple of examples will show that this is obvious: Consider two blackbodies in empty space. They are both held at fixed (different) temperatures by an energy input that balances their radiation loss into space when they are far away from each other. Now what happens if they are brought near each other? Some fraction of the radiation from the hotter body will hit the cooler body, and it will start to warm up. But the cooler body is also radiating, and some fraction of that radiation will hit the warmer body, making it warmer, too. There is no violation of the second law, because the net heat flow is from the hotter body to the cooler one. But to say that no heat can flow from the cooler body to the hotter one is equivalent to saying that, somehow, photons are now prohibited from leaving the cooler body in the direction of the hotter one, which is of course preposterous. What has happened is that, as a system, the two bodies now have a smaller surface area radiating into space than they did when they were independent. With a smaller area but the same energy input, the temperature of the system must go up, and both bodies get warmer. It's just that the cooler body gets warmer faster. As another example, let's think about how insulation works. Without insulation, a house will lose heat like crazy and the interior walls will be cold. If you add insulation, the walls will warm up. The insulation nearest the interior wall will be nearly the temperature of the wall, and as you go outwards, it will get cooler and cooler. Is the insulation "heating up" the walls? Of course not, the interior heating system is heating up the walls. The insulation makes them warmer because it lowers the thermal conductivity to the outside, and the heat flux is still in the outwards direction -- the second law is safe. In the case of the actual greenhouse effect, the situation is slightly more subtle, since the source and sink of heat are both in the same direction -- towards space. If there were no materials with wavelength-dependent opacities, it would in fact not work, as trying to insulate the Earth from outgoing radiation would also block the incoming radiation to exactly the same degree. As it's difficult to seriously argue that greenhouse gases don't act as an insulator -- most people should be familiar with the fact that cloudy nights generally are warmer than clear ones -- it's this fact, never pointed out by the authors, which is the most subtle argument for the existence of the greenhouse effect. It is curious that the authors both derive the equation of radiative transfer and even grant it physical soundness, but don't attempt to infer its implications for the problem at hand. Instead they say:
"It cannot be overemphasized, that differential equations only allow the calculation of changes on the basis of known parameters. The initial values (or boundary conditions) cannot be derived from the differential equations to be solved. In particular, this even holds for this simple integral."But differential equations of course work perfectly well also for calculating the stationary states, regardless of whether the initial values are known or not. And if you calculate the stationary state with a greenhouse gas, you will find that the temperature is higher. The authors say that "the atmospheric greenhouse effect does not appear in any fundamental work of thermodynamics ... physical kinetics [or] radiation theory". Yet they could easily have confirmed it with the RTE, but neglected to do so. I'm unsure what to think of the authors. While they appear to know a lot at least about physical formulae and language, they also appear unable to apply this knowledge in any form of coherent analysis. The scathing tone they assume when dismissing climate research in general and explanations by others in particular, by referring to semantics and without a coherent argument (i.e. "Disproof: The concept of the Earth's mean temperature is ill-defined. Therefore the concept of a rise of a mean temperature is ill-defined as well.", "Disproof: The concept of an (sic) radiation budget is physically wrong.") indicate to me that they are not interested in actually learning something. But I do know whether I think this paper has anything to contribute to our knowledge of climate change and the greenhouse effect: Absolutely not.