Note: I’m not going to say anything more than this obviously does not refer to a real glass greenhouse and how it works.
I’ve a couple of times heard statements to the effect that the greenhouse effect is not scientific or even that there is no greenhouse effect.
Whilst the CO2 effect is “proven science” in a general sense that rising CO2 should increase global temperature, I have to sympathise with the view that it’s not science as the greenhouse effect is not a universal “law” but instead the way a real atmosphere tends to work in practice. So, I will try to explain how I understand it.
The Blanket Analogy: a Wet Blanket
To those think CO2 warming is “obvious”. You clearly don’t understand it. The atmosphere is far more complex than the simple idea of a “blanket”. And to those who think in simple terms such as “blanket = warming” because when a blanket is put around a hot person it warms them, answer me this:
If you put a blanket on an outdoor concrete surface will it get hotter or colder?
or to put it another way:-
If you painted one piece of outdoor concrete white and one black, which would have the hotter temperature?
Most people would would likely say “the black would be warmer because it absorbs more sun”, just as the blanket is assumed to warm. But those who understand emissivity, will know that things that absorb better also emits better. Likewise a blanket not only keeps heat in, it also keeps heat out!
The black concrete will help lose heat faster at night just as it will help trap the heat in the sub. Before the blanket the surface already loses as much heat as it gains (otherwise it would continually heat or cool), it therefore follows that this question becomes:
Does a black surface/blanket increase heat absorption more than it increases heat emission?
Remember heat emission happens whether or not the sun is out – but heating by the sun only happens when the sun is out. So, there is far more time to lose heat than gain it.
And the correct answer is … it all depends. A change in temperature with a coating will depend on exactly which frequencies are present in the sunlight and IR and which frequencies are enhanced or blocked by the paint/blanket.
Likewise, there are actually two effects of CO2 in the atmosphere:
- It absorbs radiation and heats up
- It emits radiation and cools down
So, more CO2 could actually lower temperatures if the enhanced emissions from CO2 dominated the absorption .
In this article I want to show that the Greenhouse effect is not strictly true in the sense that “any greenhouse gas in any atmosphere will cause warming”. However, I will concede that in our earth’s atmosphere CO2 is likely to cause warming.
First, we all know that the radiation power (p) from a black body is:
P = εAσT4
This means that the radiation from a surface like the earth WITHOUT AN ATMOSPHERE with an average temperature of 15C would be entirely dependent on that surface temperature and the incoming radiation from the sun.
Now add an Atmosphere
Let us suppose that we now add an atmosphere to our planet.
But let’s start with a very special atmosphere which is completely transparent. As anyone with a 20mm air gap between double glazing knows, air is a good insulator and 10s of miles of air is better still. So surely adding this atmosphere will insulate the earth?
Not this one! As per the diagram above right, as the transparent gases of this atmosphere do not emit or absorb IR (or any radiation), then the atmosphere will not stop any radiation entering or leaving the planet. It has no effect. As such no energy can be lost or gained to the atmosphere and all radiation will pass through. Thus a transparent atmosphere does not affect temperature surface temperature and this “blanket” has no effect.
It will be just as a planet without any atmosphere, because all “Space” can “see” is the surface of the earth at 15C.
What I mean by what space “Sees”
I use this unscientific concept of what “space sees” to mean the temperature of an equivalent black body sphere of the same size as the earth which have the same heat loss through IR emissions.
A Semi-Transparent Atmosphere
Now let’s add a very thin layer of atmosphere which is semitransparent (shown to right as green section) .
Because it now interacts radiately, we need to give it a temperature, so let’s assume it decreases from 15C at the surface to -100C toward space. For simplicity, let’s assume that we have a static atmosphere so there is no significant change in temperature and no movement. And for argument’s sake lets assume the thin strip is at 0C.
Now some radiation goes straight through the atmosphere, but a fraction of the radiation interacts with the thin layer at 0C.
To keep it stable, we cannot allow any significant increase or decrease in energy (at constant atmospheric conditions such as pressure or Humidity).
However, if it is even remotely opaque, there will be a layer of air at 0C interposed between earth’s surface and space and space will partly “see” this layer. Therefore because space “sees” the earth at 15C and the layer at 0C, then the average temperature “seen” by space will be lower than the surface. This lower temperature means the net amount of radiation leaving the earth will decrease, so energy is retained and the earth will try to warm.
So, if we allowed the system to change, it will warm. Therefore, in our model, so long as there is any layer of atmosphere that partly obstructs space’s “view” of the earth, the apparent temperature of the earth surface when viewed from space will be lower, less energy escapes and the earth warms.
But what if the layer we are considering is at a higher temperature than the surface?
If this thin layer of atmosphere were to be higher than the atmosphere, then the situation would be the opposite. Space would “see” a higher temperature and the CO2 would cause the earth to warm. So an atmosphere with CO2 added which is warmer than the surface will tend to warm.
So, which is it?
As the temperature profile of the atmosphere to the right shows, almost all the atmosphere except the very top “ionosphere” is colder than the surface. The ionosphere is at a very very low pressue and so it will have very very weak interaction. So almost all the effect will be with CO2 lower in the atmosphere and therefore lower that the earth’s surface and so lead to warming.
Effect of increasing CO2
What happens if we now increase the concentration of CO2 in our model? The effect is simple, because our model is static and doesn’t change in temperature, an increase CO2 will mean space “sees” more of the atmosphere at a lower temperature than the surface and therefore the apparent temperature will go down resulting in less heat escaping and a warming world.
However, if the atmosphere is not static, obviously if the atmosphere warms not only will the CO2 cause space to “see” more of the colder atmosphere, but that atmosphere will also be warming warm. So we have two opposing effects on what is “seen” from space due to a change in CO2:
|CO2 effect on apparent temperature seen from space||=||↓↓↓ reducing ↓↓↓
↓↓↓as it “sees”↓↓↓
↓↓↓ layer ↓↓↓
↑↑↑as 0C layer↑↑↑
This introduce a new aspect. Whilst rising CO2 tends to create a greenhouse effect, if this also causes the temperature of the atmosphere “seen” by space to increase, this offsets the first effect and then it could cause more cooling.
When CO2 Can Cause Cooling
For obvious reasons, it will be impossible to find anything about any cooling effect of CO2. So, what follows is entirely my own.
What I’m about to show is that in some circumstances it would be possible for CO2 to raise the effective heat seen from space leading to cooling. This happens when heat is “dumped” at high altitudes above the clouds and most of the rest of the atmosphere.
So, how could such circumstances occur where rising CO2 can cause global cooling?
To demonstrate this see the diagram to left. Here there is a thick layer of cloud which blocks of the surface from the layer at 0C (the top surface of the cloud). So rather than space “seeing” the average temperature of the earth as its surface, it actually sees the temperature of the clouds (or that part of the sky covered by clouds) as the average temperature.
But how could CO2 warm this layer and not the rest?
First, we must understand that because pressure drops, the bulk of the atmosphere is in the lower atmosphere.
We then assume that rising temperature “due to CO2 warming”, causes increasing air temperature in this lower layer.
Then we take account of the ideal gas law that says that
PV = nRT
P is in N m-2 (J m-3)
V is in m3
n is in moles
R is 8.31441 J K-1 mol-1
T is Kelvin
Looking at units we find that PV & nRT are in units of energy or Joules.
Radiation increases energy
If we tale a small volume of air in the atmosphere, the pressure is largely determined by the surrounding air pressure. If that volume of gas absorbs radiation it is given energy so the value PV to increases. But as the pressure is largely determined by the surrounding environmental pressure, P is constant and so V increases. As density is M/V so as V increases, density decreases. This causes the volume of gas to be forced us and rise.
This rising air column takes the additional heat upward. But no matter how far up it goes, this volume of air will always have an excess value PV than surrounding air. It needs to lose this energy. It can do so either by conduction, by radiation, or when some other mechanism comes into play.
With moist air, not only does the water in the air contain huge amounts of energy, but when it condenses the value of n in nRT drops as water becomes liquid. So cloud formation takes energy out of the air column. So the main effect of the radiative heating, is cause the air to rise until it rid itself of excess heat by condensing it out as water droplet. Thus the main effect of the CO2 heating is to dump the heat into the cloud layer. The change in the bulk of air may be small, but as it is all concentrated into this thin layer, there is a large transfer of heat from the bulk atmosphere to this cloud layer. But because the cloud is largely opaque:
Space “sees” this energy as rising temperature at cloud level.
So rising CO2 leads a higher apparent temperature of the earth seen from space and so cooling.
So, now, rather than the main effect of CO2 being to cause the colder atmosphere to become more opaque and “mask” the surface making the earth appear colder from space (and less emissions), instead, because of the way the heat is circulated, the energy absorbed by CO2 is dumped into the cloud layer, raising the temperature of this layer and because the cloud obscures the earth, the apparent temperature of the earth as “seen” from space space is higher. And a higher apparent temperature means greater emissions with the result that more energy is lost.
Unfortunately, if the result is for the total quantity of cloud to increase, this results in a cooler layer over the earth – thus lowering the temperature as seen from space and causing warming. But that only lasts until it rains.
- The blanket analogy is wrong as an insulating layer of gas which is transparent has no effect on how much radiation is lost or gained.
- So called “greenhouse” gases can heat by absorbing radiation but also cool because they increase emissions.
- Going from a transparent to semi-transparent atmosphere will cause cooling if the atmosphere is lower in temperature than the surface.
- Going from a transparent to semi-transparent atmosphere will cause warming if the atmosphere is higher in temperature than the surface.
- Any rise in temperature of the atmosphere tends to increase cooling.
- Even if CO2 causes air to rise, that rising air concentrates heat in the cloud layer. This heats a layer which is blocked from the surface. So, this effect can increase emissions to space. However it only works so long as the total coverage of cloud does not increase.
So, basically, CO2 can be a cooling gas, but only if rain keeps cloud coverage the same.