Greenhouse gases, are not so much “trapping” heat, as acting by “tapping” heat. They are acting as a vector (tap) enabling the flow of energy between the adiabatic controlled atmosphere and the IR radiation that eventually leaves the atmosphere. And it is because the adiabatic cooling reduces the temperature, that the apparent temperature of earth from space is cooled. This may reconcile the “Dragon slayers” with mainstream skeptic views.
Introduction: why colder means warmer
Talking through my post yesterday with a physicist (The CO2 Greenhouse effect is real (sometimes), it was clear I needed to spend a bit more time explaining what may appear counter intuitive. That is why when the apparent temperature of the earth from space decreases, that this must mean the planet is warmer.
This is easiest to explain using the analogy of a house. To the right is a thermal image of a house, clearly showing the windows and doors are much warmer than the better insulated walls and roof. Why are they warmer? Because the internal heat can more easily penetrate the thin insulation of windows and doors. So, for a constant level of heating (at night**), if all the house were covered in the glass windows with the red (hot) appearance, then more heat would be lost from inside. Conversely, if there were no windows or doors all the outside would be the cold blue and less heat would be lost.
So, if the house is “warmer” to the view, then more heat is being lost – so tending to make it colder inside. Whereas if the outside is “cooler” then less heat is being lost tending to make inside warmer. So warmer is cooler!
** Obviously, in a real house, the windows also allow sunshine in, but for the sake of this analogy we are ignoring sunshine.
Problem with greenhouse gas warming.
Yesterday I introduced a summary of the way greenhouse gases actually work rather than the way we are told they work. Normally we are told CO2 absorbs IR and so:
more CO2 causes more heat to be trapped
But anything that absorbs also emits and so it is just as true to say that because CO2 emits IR:
more CO2 causes more heat to be emitted
Obviously, this makes the “standard” explanation of CO2 warming a complete load of non-science and I have a lot of sympathy with those who dismiss it as such.
Recap of actual greenhouse warming.
To recap my post yesterday “The CO2 Greenhouse effect is real (sometimes)“, if we consider a transparent atmosphere. Being transparent it has no effect on incoming or outgoing radiation. So all the surface radiation reaches space and the radiation power reaching space is purely dependent on the surface temperature (and emissivity).
If however, we interpose a thin layer of gas which is semi-transparent, it interferes with a small fraction of that radiation. Let us suppose this fraction were 1%. That would mean that 99% of the radiation would pass through without being affected. But 1% would be absorbed and 1% emitted.
This fulfils the basic requirement of physics which is normally contradicting by those explaining “greenhouse warming”. If anything is able to absorb IR it must also be able to emit it.
(For the purpose of this post, I am ignoring the fact that CO2 behaves differently at different wavelengths. This is because I do not try to quantify the greenhouse effect. So whether it is 1% or 0.0001% the direction is the same.)
So, when there is a semi-transparent layer with interacts with 1% of radiation, looking from outside (like the house photo above), 99% of what we see is the surface at around 15C, but 1% is this thin layer. And if this thin layer is colder there is less radiative emission (but if it were warming there would be more)
To give an example with the house. Think of an old style house with solid brick walls. On a cold day, the outside might be perhaps 2C above ambient. If however, we add an insulating coat to the house, the effect is to stop heat reaching the outside, so now there is less heat to warm the outer surface of the house and it now only reaches 1C above ambient. So, for a house at a constant temperature,
a house with an outer surface which insulates is cooler and emits less radiation.
turning around, for a house with constant input of heat (equivalent to a planet with constant solar heating):-
A house where the outside surface is cooler is hotter inside
(for the same level of heating)
One of the key arguments against the greenhouse effect I’ve seen put is from those sometimes referred to as “skydragons”. And this argument I think really boils down to this:
the scale of the purported greenhouse effect is very similar in scale to that of the adiabatic effect – so the higher temperature of the earth is due to this.
This gives us two very contrasting views:
- Consensus science: the greenhouse effect can be calculated so it is real therefore the adiabatic effect has nothing to do with the higher temperature of planets like earth.
- Skydragons: The adiabatic effect is real physics, and therefore it is the greenhouse gas theory that must be wrong.
I would now like to propose a solution that I think bridges this gap and means that both are right (at least in significant parts).
Recap: Calculation of total greenhouse effect
As we have been told too many times, the temperature of the earth is determined by the balance between incoming and outgoing radiation.
[Note: this section is only showing how the greenhouse effect is usually calculated. Before jumping to comment please note the end where I say: “the theoretical value when we take a real earth with varying temperatures is considerably different.“]
As the diagram to right shows. Sunlight falls on only one side of the earth (daylight) so if the fraction of reflectance or albedo is A, the solar energy per unit area S, Es is the total solar (shortwave) energy collected by the planet per unit time (in units of W), the amount of sunlight absorbed is:
Es= (1-A) SπR2
Outgoing radiation is emitted from the entire surface. According to the Stefan-Boltzman law the power emitted is proportional to the fourth power of its temperature. So emitted heat energy is given by:
Ep= 4πR2 σT4
Where Ep is the planetary (longwave) radiation (in W) and σ is the Stefan-Boltzman constant.
The temperature increases until a radiative balance is reached when EP=Es so:
4πR2 σT4 = (1-A) S πR2
rearranging and eliminating terms we obtain:
T= ((1-A) S / 4σ)-1/4
|For the earth||S is about 1368 Wm-2|
|σ is 5.6704 10-8|
|A of about 0.3|
And this leads to a value of -18C as the theoretical temperature of a sphere the size of the earth, with the same distance from the sun and at a uniform temperature.
Because the actual average surface temperature is around 15C, this difference of 33C is taken to be the “greenhouse effect” of the atmosphere.
But As Gerhard Gerlich points out in Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics the theoretical value when we take a real earth with varying temperatures is considerably different.
Anyone who has done science, should be familiar with the concept that compressing a gas causes the temperature to rise. If not, find a bicycle pump, hold your thumb on the end and press down a few times, with sufficient pressure to let out a little air. Very soon it is soon hot to touch. This is compressive heating!
It’s easy to think of this as “more energy in a smaller space”. However the actual reason is that work is being done compressing the gas and this work raises the energy and so temperature.
Likewise, but in the opposite direction, when air rises, it expands, it does work on its surroundings and this loses energy and this time it’s temperature drops.
Adiabatic Lapse Rate
As air in the atmosphere warms and expands, it rises and cools. The declining temperature with altitude is called the ‘lapse rate’. The adiabatic lapse rate (Γ) refers to the change in temperature (dT) of a parcel of atmosphere as it moves up or down (dz) without exchanging heat with its surroundings. It is governed by the planet’s gravitational force (g) and the specific heat (cp) of the atmospheric gases. Earth’s theoretical lapse rate is calculated as:
Γd = – dT/dz = g/cp
This varies, but for the region of atmosphere below the stratosphere where convection dominates as a process, the temperature drop is around 6.5 C km-1
Somewhere I saw a “skydragon” calculation that purports to show that all the adiabatic cooling is the same size as the greenhouse. However, whilst I cannot give a working for them being equal, there are numerous posts talking about the way venus is hotter because of the much more extensive atmosphere and therefore larger adiabatic effect as the higher pressure “heats” the atmosphere
Taming the “skydragons”?
Unfortunately, because “skydragons” have been led to state that the greenhouse warming effect does not exist, this has resulted in a sharp division between the two schools of thought.
However, I now think I have found a way to resolve this dilemma: the two very different calculations, each providing a very big number (of many degrees) neither of which seems to admit the other exists.
I would now like to suggest that the greenhouse effect and the adiabatic effect are in fact two facets of the same thing. To see how this can be possible let’s examine the two big problems.
- The adiabatic effect explains why the surface surface is warming thermodynamically. What it fails to explain is how this warmer atmosphere is connected to the heat sink of space.
- The greenhouse theory explains why the atmosphere is warmer (because it effectively traps heat), but in its normally stated form it fails to explain how “greenhouse” gases, which both emit and absorb IR radiation can act to trap radiation.
Now, using my simple conceptual model we may be able to bridge the two because in my model I show that the greenhouse effect is caused by the cooler atmosphere. So:
it is the temperature of the atmosphere which drives the greenhouse effect.
Greenhouse gases: a tap between adiabatic heating and IR
To explain how CO2 and temperature interact, I would like to use the idea of a fractional distillation column.
To the left is such a column for the separation of oil into various fractions.
The temperature varies from the bottom to the top. Hot gases rise up the column cooling as they go until they condense, and then cooler liquid flows down. This way the most dense and least easily evaporated fractions like bitumen tend to collect at the bottom. Then as we go higher we go through successively lighter fractions (ship oil, diesel and aircraft fuels) until we get to car petrol and finally the bottled gases.
Now let us imagine, that this column has been bubbling away for time enough that all the fractions have separated. But none of the taps out the side are open.
What happens if we then open one tap?
That particular fraction is extracted and if there were only one outlet pipe, for the time that that fraction still has any liquid the whole process would be seen as:
oil → fraction
That is, for the time that fraction’s tap is open only one goes through the process. Tapping the process at one
But now imagine that rather than oil, what is going in is heat. And that unlike oil which can only be in one form, the heat can increase and decrease in temperature as heat flows up and down the column. So, tapping at any point can be a continuous process as new heat flows in.
Now, we have a process whereby opening up one tap at a particular level.
Temperature input → fractional temperature
What greenhouse gases like CO2 are effectively doing is to be the “tap” that connects between the fractional temperature (the adiabatically controlled temperature column of the atmosphere) and IR radiation, They are in effect “tapping” the column and “releasing” it to space.
But, just as within the fractional column, gas is constantly moving up and liquid moving down so that heavier fractions move down and lighter up throughout the column, so in the atmosphere, temperature is both being emitted and absorbed at all levels.
The adiabatic cooling curve controls the temperature of the column and the greenhouses gases control the rate at which this temperature is “tapped” into the IR.
So, if we were to observe the earth from outside, what we would see is a combination of all the “taps” within the atmosphere. But in wavelengths where most of the IR radiation is being absorbed and re-emitted we tend to see the “fractional” temperature from the top of the “adiabatic column”. This is why the adiabatic cooling temperature comes out as the greenhouse temperature.
The greenhouse gas is the sum of the adiabatic temperature of the atmosphere at the levels at which the greenhouse gases operate
So, greenhouse gases, are not so much “trapping” heat, as acting by “tapping” heat. They are acting as a vector (tap) enabling the flow of energy between the adiabatic column and the IR radiation that eventually leaves the atmosphere. And it is because the adiabatic cooling reduces the temperature, that the apparent temperature of earth from space is cooled which in turn (remember a cooler surface causes internal warming) causes the planet to be warmer.
Depending where the “greenhouse tap” has most effect, this tends to cool or heat the atmosphere. So the introduction of “greenhouse gases” tend to increase the flow between radiative and stored heat which tends to mean that the higher taps “mask” the lower ones. This tends to expose higher elements up the adiabatic cooling column, so changing the effective temperature of the earth seen from space.
So, adiabatic cooling and greenhouse cooling are just two facets of the same physical process that warms the earth.