Because of the focus on potential warming, just about anyone with some schooling in science has an opinion on the atmospheric effect. That means they have to try and explain the inner workings of the atmosphere to some degree to impress upon their friends or family that they have some understanding of what happens. Since there is a lot that actually is happening, the explanations are far short in one area or another. Without a complete knowledge weighing them down, it is much easier to leap to conclusions. Since there are so many small effects, I doubt that anyone can be assured of a complete knowledge of the subject. I believe the best way to approach this issue is with a more complex explanation from a unique and centrist point of view. So I am defining or redefining the atmospheric center of energy.
The Atmospheric Center of Energy (ACE) is the layer of the atmosphere where the heat content of the atmosphere above equals the heat content below. This layer is approximately at the average cloud altitude. This is not an exact level, it would move around and vary with latitude and all that. It is what I imagine is a good transition layer for heat transfer in the atmosphere.
Thermal energy is kinetic energy because it is motion. So thinking of thermal energy as a potential energy is a little different, but not a bad way to look at the energy in the atmosphere if you consider the average temperature/thermal energy. While there are flows and fluxes in all direction, the average temperature can go up or down with any imbalance. In daylight, that layer moves up, at night it moves down. Kinetic energy moves by three basic means, conduction, convection and radiation. The only way the sun transfers significant energy to Earth is through radiation. What! The sun impacts tides so there is also gravity! Quite true. For now assume it is important but does not interfere significantly with the atmospheric effect.
The dominate method of heat flow varies with conditions. Heat moves inside solids almost exclusively by conduction. The almost is because radiation can flow through solids, but only in significant amounts when solid is transparent to electromagnetic radiation or is permeable to high energy particles. For example a solid block of rock salt crystal, transparent to infrared radiation, can radiate infrared energy from its surface and its interior. The distance the radiation can travel is limited by its path length that depends on the density and composition of the solid. For the radiant energy consider radioactive substances which release gamma rays, alpha and beta particles. Gamma rays can penetrate a good distance in less dense substances. Beta particles penetrate less than Gamma rays and more than alpha particles. Alpha particles are stopped by nearly any substance. What limits the particle's travel is their size, the bigger they are the less they penetrate. Gamma rays are in the electromagnetic category with high energy. It is their wavelength and energy that determine how far they penetrate a substance plus the properties of the substance. In liquids, convective heat transfer is added to the mix. Warmer liquid rises, colder liquid falls to replace the volume of the rising warmer liquid. I am sure you have heard of a lava lamp. Radiant heat can flow through liquids, just like solids, with the same restrictions. In gases, all the modes of heat travel are in operation, but conductive flow is more limited, convective flow can be more rapid and the path for radiant flow is easier, but still has the limits imposed in solids and liquids. Gravity, that other weird form of energy, can play a bigger role with the lighter gases, a decent role with the liquids and a smaller role with the solids.
As I mentioned, heat flows from warmer to colder, but then all things are relative. After Fukushima, nearly everyone is aware that gamma rays and beta radiation can flow into your warmer body. Some understand that a beta particle has a mass, even though it is extremely small, that is traveling at a high speed, so it can smack into you. Since the gamma rays are there too, people accept that gamma rays can smack into you and go deeper. Those same people tend to get lost when it comes to a photon of energy in the infrared band of the electromagnetic spectrum smacking into the Earth or you. It is the same thing though, because that photon is a discrete quantum of electromagnetic energy that exhibits properties of both waves and particles. While not perfect, atomic radiation in general is a example of electromagnetic radiation since that is how our sun creates the energy it provides. Different process, fusion versus fission and radioactive decay, but very similar properties for this illustration.
So what does the Atmospheric Center of Energy have to do with this crap? Well, since you asked. Even though the radiant energy transfer in solid and liquids is a little complicated, in a gas with varying density, temperature and composition, it can be more complicated. The center of energy is convenient for describing some of what is going on. While the conditions related to radiant energy change with density, temperature and composition, the density temperature and composition change with altitude due to gravity and heat content. The heat content decreases as gravitational impact decreases which reduces the density of the gas molecules. We can't do much with gravity, but we can find the average heat content or center of energy as I have called it. Energy is flowing in, out, up, down, sideways, through and not, i.e. potential energy, at this point, it is a pretty busy intersection.
It is at this intersection where I feel that those explaining and those attempting to learn atmospheric physics lose it. This is where that dreaded branch of mathematics despised by every self respecting empirical science buff comes into play - STATISTICS! Or as I prefer, probability. Because up from this ACE point is less dense and contains less energy, the probability of energy flowing out to space is greater than the probability of energy flowing into the surface. Remember the down part always contains the Earth which is warmer than space and the up part only has the sun to deal with half the time. Energy is being transferred around by the big three, conduction, convection and radiation. Up is less friendly for conduction and convection and down is less friendly for radiation. Convection is really the odd energy transfer mechanism out. Convection cannot happen without conduction or radiation heating something that can expand and move or rise against gravity. Conduction only requires a difference in potential energy and a path. Radiation only requires potential energy and a path. The parallels between conduction and radiation may help better explain things
Going down, the path for conduction gets wider because there are more molecules to bang into which are its path. Up radiation has a wider path because its best path is no molecules at all. Since there are still molecules at this intersection, the probability of banging into a molecule increases downward which increases conductive efficiency and decreases the efficiency of radiant heat transfer. If the Earth had no atmosphere, there would be no convection because there are no molecules to rise, no conduction because the are no molecules to create the path and plenty of radiant energy because the path would be perfect. So the analogy of resistance to heat flow is very good for describing the atmospheric effect if you think of a variable resistor for conduction and some kind of super duper statistical radiation variable resistance for radiation. There is only one big issue with the resistor analogy, that is the relatively clear path or radiation window from the surface to space.
This image from Wikipedia shows the radiative spectrum of the window. At no point is this window 100% clear and you can see that there are several points where zero energy is transmitted due to water vapor and CO2 mainly. There are some chunks for ozone (O3) and a chunk for Oxygen (O2)near the zero wave length. This would be what it looks like if you are on the surface looking up or in a space ship looking down. At the Atmospheric Center of Energy, it looks the same if we look down and a little different as we look up. The zero percent areas due to H2O now have a little, say a few percent passing through. If we move up in the atmosphere a bit further, those H2O window grow more open. So I think it may be easy to see why water vapor that is warmed at the surface, rises with convection and condenses at or above this level can transfer its heat to space. The higher it condenses the faster it transfers heat to space. The reason that the window from the surface to space is not 100% clear, is mainly due to water and water vapor in the atmosphere. Liquid water has a little different radiation spectrum than water vapor in this range of wave lengths, so above the cloud level, the window become very close to 100%. Since CO2 is fairly well mixed, its parts of the window begin getting clearer higher as the density of the atmosphere decreases. If you figure out the area of the clear part of that image, you would have the amount of energy not transmitted by radiation from the surface to space or from the surface to the ACE.
"Now wait a minute! Radiation from the atmosphere is what slows down the rate of cooling which causes the surface to be warmer than it would if there was no atmosphere! Hurrumph! Hurrumph! Hurrumph!", you stubbornly interject. "Well, just hang on there a second bucko! From the ACE the window is clearer up so it is just as clear from up to the ACE, ain't it! So back radiation or down welling flux or whatever you what to call it, can happen more significantly the higher you go in the atmosphere. It is not my fault that from the center of energy down, it is easier for conduction to do the work." I respond with a chuckle.
"Well, heat cannot flow from cold to hot!" You exclaim with spit flying. "Great!", I say cleaning my glasses, "Then gamma rays can't cause cancer, so let's build more nukes. A photon of energy, whether from a gamma ray, cosmic ray or CO2 molecule goes where ever it goes. For enough of those photons to cause what you call heat flow, the source would have to be warmer, because the photons are also going up and to the sides. The down photons are just increasing the resistance to outgoing flow. If you think about the conductance from the center of energy to the surface, it is like you increased the voltage of the center of energy. That reduces energy flow to the center from the surface and increases the voltage at the surface. Kinda like charging a battery."
"Since you seem to have calmed down, think about that battery thing. A battery is potential energy waiting for something to do. The higher the voltage and the bigger the battery the more it can do. Since the atmosphere is storing more energy, the center of energy rises a little." Now grinning, I say.
The changes at the ACE are gradual, but imagine the radiation window above only has CO2, O2 and Ozone blocking part of the window. So let's look at the sunlight coming in. This is a link that shows a little better the goings on when the sun is shining. Unlike most descriptions, this shows what is happening with that 16 percent absorbed by the atmosphere. Two percent is absorbed by ozone, eight percent by oxygen and six percent by water vapor. A total of seven percent is scatter to the surface and 3 percent scattered to space. That seven percent scattered to the surface is 96 Watts/M^2 on a clear sky day, at high noon at the equator or 24 Watts/m^2 average for the day/night over the whole surface from scattering by oxygen. So on average, 24 Watts/m^2 of the controversial down welling radiation is diffused sunlight. The rest is not scattered back to space is solar energy, delayed on its way to the surface. Since most of the water vapor is absorbed below the ACE, most of it makes it to the surface as reduced conduction. That is also included in the controversial down welling radiation. Statistically, the vast majority of the down welling energy actually increases the potential energy below the ACE which reduces the rate of conduction from the surface. Statistically, most of the impact of CO2 on the down welling is radiation above the ACE downward, increasing the potential of the ACE which reduces the rate of conduction from the surface.
I will try and clean this up, but the perspective of the ACE, may help more people understand what is going on in the atmosphere, so they can move forward to what may change with more CO2, more or less clouds, more or less solar intensity, more of less aerosols and more or less water vapor.
How you understand the atmospheric effect doesn't really matter to most folks. To me though, without a more realistic understanding, you will probably miss some of the picture. How the radiation changes at the surface is more than just some big number getting a little bigger, it is a variety of several smaller changes interacting.
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