If you Google the phrase ‘greenhouse effect definition’, you get the following at the top of the returned results:
green•house ef•fectActually, the greenhouse effect would still operate even if the atmosphere absorbed just as much solar as it does infrared. When even Google gets the definition so wrong, how can mere mortals be expected to understand it?
The trapping of the sun’s warmth in a planet’s lower atmosphere due to the greater transparency of the atmosphere to visible radiation from the sun than to infrared radiation emitted from the planet’s surface
The so-called greenhouse effect, which is an infrared effect, is admittedly not as intuitively obvious to us as solar heating. Every layer of the atmosphere becomes both a “source” as well as a “sink” of IR energy, which is a complication not faced with understanding solar heating, with the sun as the source.Luboš Motl (The Reference Frame) agrees with Dr Spencer and adds: (link)
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.Roy Spencer writes of the the issues which people most commonly misunderstand, the issues which represent stumbling blocks to an understanding of how the greenhouse effect operates: and in red
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.
Luboš adds his thoughts:
- The greenhouse effect does not necessarily require solar heating. If the climate system was heated by intense geothermal energy rather than the sun, the greenhouse effect would still operate. This differs from the popular delusion that the sunlight has to
be there to drive the essential processes in the greenhouse effect.
- Temperatures in the climate system are the result of energy fluxes gained versus lost. An equilibrium temperature is reached only after the rate of energy absorbed by a layer (of atmosphere, soil, or water) equals the rate of energy loss. This is contrary to the common misconception that energy input alone determines temperature. At thermal equilibrium, these fluxes (in the opposite directions) are equal. This contrasts with the general misconception that only the incoming energy matters.
- The greenhouse effect does not violate the 2nd Law of Thermodynamics. Just because the greenhouse effect (passively) makes the surface of the Earth warmer than if only (active) solar heating was operating does not violate the 2nd Law, any more than insulating your house more can raise its interior temperature in the winter, given the same energy input for heating. Very high temperatures in a system can be created with relatively small energy fluxes into that system *if* the rate of energy loss can be reduced (see #2, above). Again, energy input into a system does not alone determine what the temperature in the system will be. 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.
- The rate of IR absorption by an atmospheric layer almost never equals the rate of IR emission. IR emission is very dependent upon the temperature of that layer, approximately increasing as the 4th power of the temperature. But IR absorption is much less dependent on the temperature of the layer. So, for example, if you irradiated a very cold layer of air with intense IR radiation, that layer would warm until the rate of IR emission equaled the rate of absorption. But in the real atmosphere, other kinds of energy fluxes are involved, too, and so in general IR emission and absorption for a layer are almost never equal. 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.
- Each layer of the atmosphere does not emit as much IR upward as it does downward. There are people who try to attach some sort of cosmic significance to their claim that the atmosphere supposedly emits as much IR energy upward as it does downward, which is only approximately true for thin atmospheric layers. But the claim is false. Ground-based, upward-viewing IR radiometers measure much stronger levels of downward atmospheric emission than do space-based, downward-viewing radiometers of upward atmospheric emission. The reason is mostly related to the tropospheric temperature lapse rate. If the atmosphere was isothermal (vertically uniform in temperature) then upward and downward emission would be the same. But it’s not. Even if you restrict the analysis to very thin atmospheric layers, the upward emission will be slightly less than the downward emission, because it originates from an average altitude which is slightly higher, and thus colder (except in the stratosphere). (As an interesting aside, many models actually do make the approximation that their individual layers emit as much IR radiation upward as downward, yet they still successfully create an atmospheric temperature profile which is realistic). 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 tropospheric temperature lapse rate would not exist without the greenhouse effect. While it is true that convective overturning of the atmosphere leads to the observed lapse rate, that convection itself would not exist without the greenhouse effect constantly destabilizing the lapse rate through warming the lower atmosphere and cooling the upper atmosphere. Without the destabilization provided by the greenhouse effect, convective overturning would slow and quite possible cease altogether. The atmosphere would eventually become isothermal, as the full depth of the atmosphere would achieve the same temperature as the surface through thermal conduction; without IR emission, the middle and upper troposphere would have no way to cool itself in the face of this heating. This scenario is entirely theoretical, though, and depends upon the atmosphere absorbing/emitting absolutely no IR energy, which does not happen in the real world. 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.