This post is prompted by recent posts by Steve Goddard on WUWT about the GHE and the lapse rate on Venus. They muddle the effects, in a way that is quite often seen in the blogosphere. The meme is that surface warming is due to the lapse rate and not to the GHE. Often on WUWT this comes down to even more simplified assertions that warming is due to atmospheric pressure.
The fact is that the dry adiabatic lapse rate, and the mechanism that creates it, are an intrinsic part of the greenhouse effect that causes warming at the surface. More below the jump.
The elementary GHE explanation
A brief explanation of the GHE usually notes that the atmosphere is differentially transparent, letting most sunlight through, but obstructing IR. This is described as trapping heat, or blanketting or whatever.
This sometimes is criticised for inexactness. It's true that the heat isn't trapped - it all gets out. And a blanket isn't quite right.
Still, the reasoning does reliably lead to the right answer. With physical situations, where there is a flow driven by siome kind of potential and the flow is impeded in some way. the potential builds up behind the impedance. It has to, in order for the flow to get through. It happens with electrical circuits, river flows, even in economics. But it's true that more can be said about the mechanism.
The dry adiabatic lapse rate
I posted a while ago on the dry adiabat and the atmospheric heat pump which maintains it. Any gas in motion in a gravity field will have a vertical component to the motion. When it moves down or up, it is compressed or rarefied. Respectively, it is heated or cooled. This heat change then diffuses in to the gas at the new level. Both up and down motions have the effect of pumping heat downward, until the dry adiabat lapse rate is achieved. This is a temperature gradient, warming in the downward direction, and the critical level is g/cp, where g is the acceleration due to gravity and cp is the constant pressure specific heat of the gas.
As I said, maintaining such a gradient requires a heat pump, because heat tends to move down the gradient by conduction. The energy for the pump comes from atmospheric motions, which are thus attenuated. But when a large heat flux is passing through the air, as in the solar flux which causes all kinds of differential heating, there is energy to drive and sustain these motions. And because the energy sink is the need to overcome conductive leakages, the drain is small, because, the conductivity is low (although there are other demands too).
Adiabat, solar flux and no GHG
Suppose then we have an atmosphere of nitrogen, which does not absorb or emit IR. Sunlight is converted to heat at the surface, and to maintain energy balance, this heat must be radiated back to space.This goes straight through the N2. The surface will warm to just the temperatrure that is needed, on average, to achieve that outward radiation level.
The surface will not be uniform. Some parts will be hotter than others, and this will set up local convection cells. Bigger cells will take heat from the tropics to colder parts. The N2 will be in motion.
So, I hear you say, shouldn't the energy balance include convection from the surface. Well, there will be some exchanges, but on balance, heat flux to the N2 goes nowhere. For N2 cannot emit it to space. It can only conduct it back to the surface (somewhere).
The important thing to note is that the dry adiabat has nothing to do with IR properties. It only requires gravity and motion. The N2 atmosphere will have that g/cp gradient. But it will start from the surface temperature fixed by the IR balance, and get colder as you go up.
An example often cited is of the Earth without GHG's. To keep in balance with sunlight, after allowing for albedo, the Earth has to emit about 235 W/m2. At a uniform surface, the temperature required to do this is about 255K. This is much less than the temperatures we have, and the difference of about 33K is ascribed to the greenhouse effect.
When there are GHG's that absorb and emit IR, there are many changes. One thing that doesn't change, though, is that the heat absorbed from the Sun must still get out to space. The question is what warming will occur to make that happen.
One thing that does change is what actually emits the heat. GHG's, like surfaces, emit according to their temperature. At many wavelengths, the main GHG's, H2O and CO2, are dense enough to absorb nearly all the IR emitted from the surface. Kirchhoff's Law says that, at each wavelength, emissivity equals absorptivity. So at these absorbing wavelengths, water and CO2 are also the main sources of emission that go out to space. At other wavelengths (the atmospheric window), the emission to space is direct from the ground.
But the absorption and emission happen at different places. Much of the IR is absorbed soon after leaving the ground. At those frequencies, IR is also emitted from the same gases high in the atmosphere. The absorption and emission are not directly connected. The heat absorbed has to somehow reach those upper levels before emission can occur. That is another interesting story.
The adiabat and heat balance
Meanwhile, the adiabat is still there. It is determined by the properties of the non-GHG gases (at least on Earth). And it ensures that the emission from GHG occurs at a much lower temperature than the surface.
For the adiabat, I've used the analogy of a battery, which maintains a voltage difference between its ends of, say, 1.5V, but does not determine the actual voltage there. That depends on how it is "earthed". In the non-GHG case, it is "earthed" at the ground, where the temperature is fixed by radiation balance. Since no "current" (heat flux) flows through the battery at the top, it's voltage is just 1.5V less, passively determined.
When emission occurs from GHG's at TOA, a similar heat balance equation determines the temperature there. That is where the adiabat "battery" is "earthed". The temperature gradient below is still fixed, and hence so is the surface temperature - at a much higher level. That is the complete greenhouse effect.
So for the Earth?
If GHG's had uniform strong effect over IR frequencies, then the whole 235 W/m2 would be emitted from TOA, and this would have to happen at the snowball earth temperature of 255K, on average. TOA for this purpose is probably about 10 km or more, so the Earth's surface would be about 355K. very hot. The actual location of the emission depends on the GHG concentration. If it is lowered, some of the emission will come from lower levels, reducing the average height, and hence the temperature at the surface.
It should be said too, that on Earth the dry adiabat is rarely attained. That is mainly because the air is not dry, and water phase changes reduce the gradient considerably.
In fact, as I've mentioned, the Earth has an atmospheric window, through which some of the IR (about 40 W/m2) is emitted with a spectrum corresponding to the warmer ground surface. That leaves less to be emitted at the top, and so the temperature can be lower. Indeed, the effective emission from GHG's is at about 225K - so instead of a uniform 255K emission temp, we have part at about 288K and part at about 225K. That reduces the ground temp by about 30K.
You can see how this works for the Earth from observed spectra. Below is a plot of IR spectra observed near Barrow, Alaska (source , from Grant Petty's textbook "A First Course in Atmospheric Radiation"). The top plot shows , in effect, outgoing IR. It's only part of the thermal spectrum, but shows a dip between 600 and 800 cm^-1, which is emission from CO2, and to the right of that, the atmospheric window, where IR comes largely unimpeded from the surface. Note that in the dip it tracks the BB curve for about 225K, and in the window, about 268K - the spectrum is taken over a polar ice sheet near Barrow AK. The lower plot shows the IR reaching the surface below. Corresponding to the emission CO2 dip above, there is a peak caused by atmospheric (low level) CO2 emission.