I often see the statement "co2 causes warming, its simple physics".
Firstly, I find it hard to believe the physics of the atmosphere are simple, physics is never simple - particularly at a planetary scale!
My understanding of the greenhouse effect is that it in fact ignores several laws of physics.
Firstly "greenhouse" is a misnomer as radiative forcing does not warm a greenhouse (even though glass absorbs infrared like co2), in fact the glass actually slightly cools a greenhouse over say panes of polished salt crystals (that don’t absorb infrared). The question is actually why is it cooler outside of the greenhouse? (greenhouse warm due to a lack of convection)
I understand the concept that Co2 in the troposphere (often incorrectly shown a blanket over the earth) absorbs a few narrow bands of infrared, and as it increases in the atmosphere it becomes more opaque to infrared until all the leaving infrared energy is eventually absorbed causing slightly increased emission (about the point we are at), but within the laws of physics this could not heat ground air, as it would have to flow against the actual heat flow. To enable co2 to warm surface air you have to essentially set thermal conductivity in the atmosphere to zero (an un-physical assumption) and assume local thermal equilibrium, ignoring the lower temperatures of the atmosphere above.
Or am I (or the IPCC) misunderstanding the effect? Common sense would actually say that as Co2 is present and greatest at the surface of the earth, the absorption would occur at or very close to the surface (rather than the troposphere) and would then be drawn upward to space via convection (as is observed in the temperature data available i.e. the surface is warming faster than the troposphere in contradiction to the IPCC models).
Co2 induced warming calculations basically assume the atmosphere is in a constant state of energy balance, but it is in fact in a constant state of flux and highly choatic.
Either way, to ignore physical effects such as radiative convection seems to me to an entirely un-physical representation of the “greenhouse” layer.
So is the “greenhouse” effect really simple physics, or is it physics simple?
2009-01-09T02:40:03Z
Gnet, my understanding is that the atmosphere cools with altitude as the the pressure reduces with altitude
DaveH2009-01-09T14:38:35Z
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I see the point you are making.
The traditional greenhouse explanation says that as the IR is absorbed by the GHG’s then some is radiated to space, and that some is radiated back down. But that can’t be so. Radiation goes from the hotter body to the colder body... never the other way around.
The energy gained in atmospheric CO2 by outgoing IR simply cannot be re-radiated to the (warmer) lower atmosphere and surface. We are being told porky's.
I've tried to avoid questions in this topic, but I'm impressed that there's finally a question asking about the science.
You are quite right about greenhouses, oddly enough, they are mostly warmed by their lack of convection, and not by the greenhouse effect, so in that sense the greenhouse effect is misnamed. However, because the effect is misnamed doesn't mean the science is wrong, and while you always make some simplifications in moving from an actual physical phenomenon to a scientific model, the key is to understand the magnitude of the approximations that you're making, and I think people have a pretty good understanding of the physics.
First you have to understand that there are several methods of energy transfer involved--one is convection of both sensible and latent heat, and the other is radiative transfer. The lowest part of the atmosphere is the troposphere (I think you're a bit confused on the meaning of that), and the troposphere is characterized by both radiation and convection being important, that's why atmospheric scientists will refer to "radiative-convective equilibrium" as setting the lapse rate for the troposphere. These forms of energy transfer take place at vastly different rates, convection occurs at meters per second, or tens of meters per second for the strongest thunderstorms, while radiation occurs at the speed of light. So when you say something "couldn't heat the ground air" because it would counteract convection, well it does so constantly.
Now carbon dioxide is fairly well mixed throughout the troposphere, so it occurs in the upper levels also. Perhaps you think that any absorption of radiation from the ground will be trapped near the surface, but this is false, because just as CO2 absorbs radiation it also radiates it, both upwards and downwards, so some of that radiation gets transferred to the upper levels, and some will still escape into space. It's just that adding more CO2 will mean that a bit more of it will be re-radiated back in the downward direction and shift the equilibrium temperature of the Earth to be slightly higher.
When people say "it's simple physics," I think they do so because it's a very simple calculation to consider the Earth with no atmosphere and calculate an equilibrium temperature, then add an absorbing layer and re-calculate it and find that the equilibrium temperature goes up. Of course, to be a realistic model you need to get a lot more sophisticated, but you can make the layers arbitrarily thin and add more of them and keep re-doing the calculations until they're sufficiently accurate for your purposes. So, it's simple to understand that adding CO2 will increase the equilibrium temperature, but to understand how much requires much more involved modelling.
Also, while as you say the atmosphere is in a constant state of flux, which complicates things, if you were to average over a sufficiently long time period the mean equilibrium temperature of Earth MUST be set by the radiative equilibrium--because there is no convection or conduction to space, only radiative transfer. That's why if you decrease the amount of IR radiated to space, the temperature of the Earth must go up. You can shuffle it between the ocean, atmosphere (and to a smaller extent the ground), but the energy content will go up.
[As an aside, the temperature in the troposphere typically falls with height (although certainly not always) because the upper atmosphere is farther from the heat source (the ground). The sun's radiation is not very effective at heating the atmosphere because air is transparent to visible light. The sun does heat up the ground, which warms the air by conduction, convection and radiation.]
You're right, physics is rarely simple, and atmospheric physics especially so. The "simple physics" explanations of the greenhouse effect are usually quite wrong. And it appears to be the case here that you've taken your understanding of the GE from one of them.
To begin with, no, the GE doesn't operate at all like an actual greenhouse, which acts simply to restrict air movement within its walls. And few people today believe that does, so there's little reason to dwell on it.
Your first misunderstanding is that adding more CO2 to the atmosphere will further close the infrared "window." It's all ready closed! Almost all of the infrared radiation emitted by Earth's surface is absorbed by the atmosphere within several meters of the surface. Adding more CO2 doesn't affect that. What it does affect is where the radiation is able to escape to space.
If we were to look at Earth from afar we'd see that it is radiating energy like an object that is about 255 K. However, if we measure the temperature at the surface we see that it's actually around 288 K. This is because the level that Earth is radiating energy to space isn't at the surface, but high up in the atmosphere, where temperatures are much colder. This in turn is because the atmosphere absorbs and reemits terrestrial radiation in all directions, until the radiation finally reaches a layer of the atmosphere high enough and thin enough to escape to space.
This is where the greenhouse effect comes in. If we add more CO2 (or any other GE) to the air, it makes the lower layers of the atmosphere more opaque, and so moves the altitude that radiation is able to move to space to a higher layer. These higher layers are much colder, and so they do not radiate heat so well. Thus the rate that radiation escapes to space is lower, and the planet will take in more than it radiates. As the higher levels emit some of the excess downwards, the lower levels will warm all the way down to the surface.
This explanation also refutes your second misunderstanding, which is that all the important absorption occurs close to the surface. That is partly true, but irrelevant since increasing CO2 matters high up in the atmosphere, where CO2's absorption bands aren't saturated.
I'd have to say that the basic concept of greenhouse warming is pretty simple to understand. Of course, if you don't understand what a wavelength (wave) is or don't know the difference between infrared and visible light, then you'll have trouble understanding.
You are correct in that a real greenhouse (the building) prevents the air inside and outside from mixing (this is know as convection), and this is why a greenhouse is warmer that the air outside. This is not how greenhouse warming works.
The rest of your question deals with the "saturation gas argument." The argument argues that the atmosphere is already saturated with CO2, and therefore adding more CO2 won't change a thing. It would take too much time to summarize why this argument is irrelevant; but, if interest here is some further reading: http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/
The “simple” physics goes something like this:
Solar heat reaches Earth in the form of visible and UV light. Some of this light is reflected back into space by clouds and light-scattering particles before it reaches Earth’s surface. Most of the light does reach Earth's surface, providing warmth for sunbathers and energy for photosynthesis in plants.
Once this energy warms the planet, it is then reflected off of Earth and back towards space in the form of longwave energy, or infrared light. Some of this infrared energy escapes into outer space, and some will be absorbed by molecules in the atmosphere. Most molecules in the atmosphere, such as nitrogen and oxygen, can not absorb this infrared energy. Greenhouse gases (CO2, H2O, and CH4) are "tuned" to absorb energy at infrared wavelengths. Absorbing energy "excites" these greenhouse molecules.
Energy can neither be created nor destroyed. The energy is released from "excited" greenhouse molecules primarily through collisions with other molecules. This transforms the energy to heat; some of the heat is lost to space, some is directed downwards and warms earth's surface even more. [Thanks Beren... Explanation corrected.]
It is very rare that you will find someone that completely disagrees with modern physics properties such as the fact that CO2 and other gases act as greenhouse gases (capturing escaping infrared energy to then release as heat.) Earth has a natural greenhouse gas effect. It is true that if the atmosphere completely lacked greenhouse gases, then the global temperate would then be about 30°C (55° Fahrenheit) cooler. Water would be locked away as ice, and life would probably not be possible.
Common sense would actually say that as Co2 is present and greatest at the surface of the earth, the absorption would occur at or very close to the surface (rather than the troposphere) and would then be drawn upward to space via convection........
This would be in true in most cases, and I agree with you in the statement "Either way, to ignore physical effects such as radiative convection seems to me to an entirely un-physical representation of the “greenhouse” layer."
This "energy" so to speak that is absorbed by the CO2 layer, is the same as the energy received by photosynthetic organisms, and this is in turn translated into chemical energy, but what does this energy absorbed by the CO2 layer translate in to? If it is in fact thermal energy, then why doesn't this diffuse in turn to the upwards of our atmosphere?
In fact, the atmosphere above is cooler, and I don't remember why this is, because in sense it stands that heat rises, and heat reflected off the surface warms surface air and in turn this "heat" should warm the upper atmosphere as well. But the farther from the surface you get, the less radiance there is, therefor the less "heat" in the upper atmosphere. Also to assume that as pressure goes up, so does temperature, then the pressure of the upper atmosphere is greater than at the surface. However, the atmospheric composition changes through levels of the atmosphere. More Nitrogen composition comprises the upper atmosphere, keeping it much cooler? If this is true, should we be more worried about the amount of N in the upper atmosphere?
Ok I understand that. I was assuming pressure is GREATER with altitude, but now I do remember that pressure drops. But I am still wondering about the nitrogen composition also. While I understand a good bit of conceptual physics, I am not very knowledgeable in our atmosphere. I have studied the so called "ozone" layer some in the distant past. Global warming interests me when speaking in terms of facts, rather than just some news reporter showing a video of a once frozen tundra.
However, going by the fact that pressure is greater at the surface than to space, it makes sense that more "heat" would be present closer to the surface, and that thermal conductivity approaches a limit in relation to altitude. If you find anything else, email it to me if you be so kind. In the near future, I will try to find time on my own to research this and possibly get in touch with some old instructors for insight.
"Science is man's feeble attempt to explain the world around him"