It's the new year, 2013, and I will start it with some sort of "hardcore skeptic bashing".
Needless to say, it's not the first time when you can read a text on this blog that could be described in this way. Your humble correspondent is balanced when it comes to the catching of mistakes spread by "alarmists" as well as "skeptics", "believers" as well as "infidels", and so on.
The glass greenhouses resemble the greenhouse effect in the sense that they make it harder for the heat to escape. However, the methods to achieve so are different. The greenhouse mechanically stops convection; the greenhouse effect reduces the escaping thermal radiation by absorption and re-emission of some of it downwards.
Roy Spencer wrote the following text today:
In reality, the greenhouse effect doesn't depend on the sunlight at all. Also, as Spencer says, it would work even if the atmosphere were absorbing the visible (solar) radiation more proactively than the infrared (thermal) radiation from the Earth.
The wrong idea that the sunlight is essential for the greenhouse effect is actually widespread – and some slightly misguided and mixed-up popular presentations of the greenhouse effect (in movies, perhaps on both sides) may be blamed for this situation. The first Google hit I get for greenhouse effect definition is the free dictionary.com and it also says that it's essential for the Earth to allow the incoming light to get through.
Let me reformulate Spencer's six basic "corrections of generally popular misconceptions" in my words:
- Greenhouse effect requires a warm surface but the origin of the heat may be arbitrary, e.g. geothermal, not necessarily solar. This differs from the popular delusion that the sunlight has to be there to drive the essential processes in the greenhouse effect.
- To determine the temperature at a given place, one must consider both incoming and outgoing energy. At thermal equilibrium, these fluxes (in the opposite directions) are equal. This contrasts with the general misconception that only the incoming energy matters.
- Entropy still increases, in agreement with the second law of thermodynamics, despite the preserved increased temperature difference. The usual misconception seems to assume that the thermal exchange has to lead to the equal temperatures everywhere (higher entropy) – and this thermal exchange becomes stronger when we add the infrared absorption etc. However, in reality, significant temperature differences may be created with a tiny flow of energy per second as long as the energy loss is even smaller i.e. as long as you insulate the surface well enough which is what the greenhouse effect does.
- Infrared absorption rate and infrared emission rate are almost always (for each layer of the air) very different from one another. This contrasts with some (mainly) skeptics' incorrect idea of a perfect balance. When you derive the rates, they're different because the emission rate grows with the fourth power of the absolute temperature while the absorption rate doesn't. What the absorption rate may depend upon is the amount of radiation around which may depend on the fourth power of the layer that emitted the heat which is mostly a hotter layer (closer to the surface) – that's really why the absorption mostly wins. Also, you can't derive the equality between the two rates from equilibrium because there are many other terms (convection etc.) that must be added to the budget when you demand that the budget is balanced. The infrared radiation-related terms don't have to cancel and usually don't cancel by themselves.
- Radiation going up and radiation going down from a layer aren't equal, either. This is a similar point as the previous one. The emission of radiation mostly depends on the "last molecules" on the surface and their temperature and the molecules at the top of a thin layer are slightly cooler due to the lapse rate (note that the temperature outside the flying aircraft is chilly) which means that in this discipline, the radiation going downwards slightly wins again. Despite Roy Spencer's explanation of the origin of the asymmetry, the first commenter Docmartyn seems to pretend that he hasn't seen any explanation.
- The existence of the lapse rate requires the greenhouse effect by itself. In the previous points, I suggested the point that the lapse rate is a necessary condition for the asymmetries that allow the greenhouse effect to operate. Once the greenhouse effect operates, it increases the insulation of the surface which means that the lapse rate becomes even larger. But the opposite causal relationship holds, too. There wouldn't be any lapse rate to start with – no cooling with the altitude – if there were no greenhouse gases (mostly water wapor) in the atmosphere. The lapse rate is close to the adiabatic one which arises because the colder air at higher altitudes is heavier (at the same pressure) so it drops down, shrinks (because the pressure is higher at lower attitudes), and heats up (squeezing a gas makes it hotter). But you see that this mechanism only works if the air at higher altitudes is actually colder then the air near the surface – and the greenhouse-effect-induced heating from the surface up is a necessary condition.
The skeptics mentioned in the previous paragraph make the right first step – to question and look for possible symmetry arguments and related arguments that could imply that the overall effect (the strength of the would-be greenhouse effect) is obliged to be zero. Some of them think that they have found such arguments but all such arguments are ultimately wrong even though one could argue that the reasons why these arguments are wrong are somewhat subtle. In some sense, you could say that the necessary asymmetries (pre-requisites) for the greenhouse effect resemble Sakharov's conditions for baryogenesis – and Sakharov had to be pretty smart to realize that they're necessary and (in principle, if you don't care about the magnitude) sufficient.
Now, every effect that can't be proved to be zero (by symmetry arguments etc.) is guaranteed to be nonzero; particle physicists call this rule "Gell-Mann's totalitarian principle" (everything that isn't forbidden is mandatory: I remember pre-1989 Czechoslovakia, too).
Still, I find it a bit frustrating that almost all the comments on Spencer's blog show the commenters' lack of understanding of the basic physical mechanisms here. Some people say that Spencer hasn't explained the origin of the downward/upward radiation asymmetry – he has. Other people claim that the greenhouse gases reduce the insulating ability of the atmosphere – the truth is the opposite. And there are lots of comments that just uncritically repeat variations of some of the "symmetry myths".
Commenter Nullius in Verba claims that the lapse rate could exist even without the greenhouse gases because of the Hadley cells. This claim is still wrong but it is really, really subtle. I think that the relevant comment here is that even the existence of the Hadley cell circulation depends on the assumption that the temperature of the air depends on the altitude – that's why the winds have altitude-dependent velocities (including direction). One needs a pre-existing lapse rate again and the greenhouse gases – heating from the surface – is necessary for that, too. Quite generally, the temperature's dependence on the latitude is thousands of times smaller than the dependence on the altitude so it would be strange to imagine that the latter is "caused" by the former. The vertical gradients are primary and they're caused directly by the (mostly) "vertical" (mostly) radiation effects – solar radiation and the greenhouse effect. The lapse rate already enters as a pre-existing condition to any sensible derivation of the Hadley cells. Another argument: you may ask how high the troposphere (with the lapse rate) is – i.e. where is its upper boundary, the tropopause. You will find out that the tropopause is determined by a balance between the downward (solar) radiation and the upward (mostly thermal) radiation and without the greenhouse effect, the tropopause would drop to the surface, a flaw that no Hadley cells could really "lift".
Finally, I find it sort of necessary to add the disclaimer that the existence of the greenhouse effect – something that has been known to physics since the 1824 arguments by Joseph Fourier – is extremely far from having any evidence that any dangerous climate change is behind the corner. What's important is how strong these effects are. We find out that the water vapor in the atmosphere is responsible for roughly 30 °C of warming near the Earth's surface while all other greenhouse gases may add 3-5 °C or so.
All these gases are contributors to the normal climate we know and love and hate, depending on the weather and our mood. If you want to study the temperature changes due to changes of the greenhouse gases, you find out that the concentration of the main contributor – water vapor – may be changing and these changes may be important for the regional climate change etc. but there's no reason to expect any trend or our contribution to this trend because the average global water vapor concentration in the air tends to quickly return to the equilibrium that is dictated mainly by the temperature. And the changes of the non-water greenhouse gases lead to pretty much negligible changes of the temperature – probably well below one Celsius degree since the beginning of the industrial revolution.
As we often say, the question whether the carbon dioxide may have a substantial effect depends on the existence of large positive feedbacks – that would amplify the "bare effect" 3 times or 5 times or more. The overall feedbacks are almost certainly not positive and large, at least not this large. I have presented tons of arguments over the years. Let me offer you a simple new one. If the greenhouse effect were amplified by a brutal multiplicative coefficient, this amplification would probably apply to the greenhouse effect caused by water, too. But the bare greenhouse effect caused by the actual water vapor in the atmosphere may be shown to be close to 30 °C and we may empirically exclude that the greenhouse effect from water adds 100 or 150 °C. So we may apparently rule out a 3-fold or 5-fold amplification of the CO2 greenhouse effect, too.