Wednesday, December 5th 2012, 12:59 PM EST
In my previous article (An Accounting Error by Climate Science). I explained why one needs to consider the entire mass of the atmosphere when accounting for the additional warmth retained by planets with atmospheres.
That in itself removes any need for concern about greenhouse gases (GHGs) in our atmosphere because they form such a tiny fraction of the total mass. In particular, our emissions of CO2 add virtually nothing to total atmospheric mass.
I also explained why the extra warmth at the surface is provided by compression of descending air converting potential energy (PE) to kinetic energy (KE) and not from downward radiation from GHGs in the air warming the ground beneath.
We can now take another step and consider the thermal behaviour of GHGs and compare that with the thermal behaviour of non GHGs which form by far the greater proportion of our atmosphere.
Imagine an atmosphere with GHGs and no other gases.
Radiation comes in and is instantly absorbed and re emitted by those GHGs which reach and maintain the maximum temperature possible at that particular level of irradiation due to the constant arrival of new solar input.
50% is instantly radiated upwards and leaves the system for space.
50% is instantly radiated downward, hits the surface which warms and instantly radiates out from the surface again back to space.
The GHGs cannot absorb any of the upward return of radiation from the ground because they are already at the maximum temperature permitted by the incoming solar radiation.
Being at that temperature those GHGs cannot absorb any more energy and the upward longwave gets a free pass out of the system.
If it were otherwise there would be a positive feedback loop as the molecule would be continually heated both by sunlight and by the ground faster than it would be emitting and the ground would be continually heated by sunlight and heat from the molecules above faster than it would be emitting.
That is simply not possible because the energy available is limited by incoming solar irradiation. Such a positive feedback loop would require unlimited energy input.
The net effect of an atmosphere comprised entirely of GHGs is that radiation should be flowing in and out nearly as fast as on a planet that has no atmosphere at all.
It couldn’t be quite as fast as on a planet with no atmosphere because there would also be non radiative energy exchanges between the GHGs and the ground via conduction and convection.
If, in contrast, an atmosphere were comprised exclusively of non GHGs then hardly any upward or downward radiation would be possible and almost all heat shedding would have to be by radiation from the surface rather than from the atmosphere.
Thus the air circulation would have to carry out the task of carrying energy back to the surface again every time solar heating on the day side had lifted it up into the atmosphere via convective uplift.
That heat shedding should take longer than for a planet with a GHG atmosphere because the process of returning the energy to the surface takes longer than the direct radiation out that GHGs can achieve.so it is the planet with the non GHGs that should have the warmer atmosphere.
It is the non GHGs which participate more in non radiative processes of energy transfer and since those methods of energy transfer are slower than radiation then one would think that the more energy that is tied up in them the warmer the system should become.
So we have the surprising logical conclusion that GHGs should actually facilitate atmospheric cooling by permitting upward radiation which could not otherwise occur.
It would appear that logically it should be the less radiatively active gases that cause the Earth to be warm enough for us to survive on it.
The truth would seem to be exactly the opposite of the conventional wisdom.
The Sting In The Tail
Note that throughout the above I used the qualifying words ‘would’ and ‘should’.
The reason is that neither scenario works out quite like that at all because the radiative characteristics of molecules have NO effect on the temperature that the atmosphere will reach.
The only factors that are relevant are atmospheric mass, the strength of the gravitational field and the power of solar input.
I understand that to be, or once to have been, settled science.
So, how does it really work?
What happens in practice is that for the atmosphere with more GHGs the circulation will have to work less hard to deliver energy back to the surface prior to radiation out to space.
For the non GHG atmosphere the circulation would have to work harder.
That is all there is to it.
GHGs make it possible for an atmospheric circulation to be slower than for a planet with less or no GHGs.
If mass, gravity and solar input are the same then both planets will reach the same temperature irrespective of the radiative characteristics of the constituent molecules.
What About Climate Change ?
Since the speed of air circulation on each planet will be different the permanent climate zones will be distributed differently.
Then, if compositional changes occur, involving changes in the net radiative balance of the entire atmosphere the climate zones will shift as the atmosphere has to work more hard or less hard to maintain top of atmosphere energy balance.
But it will always be successful and will remain at the same temperature.
Causes Of Climate Change.
Changes in climate zone distribution will always be a result of any changes in atmospheric composition that affect the net radiation balance of the entire atmosphere.
Such changes can result from a variety of natural causes or from human intervention but the effect of our CO2 emissions will be not be discernible because we have to consider the entire mass of the atmosphere.
The main natural causes would be as follows:
i) Solar variability as regards the mix of particles and wavelengths appears to have an effect on the composition of the upper atmosphere which can then affect clouds and albedo in the way I have described elsewhere.
ii) The oceans appear to vary in the rate at which they release solar energy back to the air which affects atmospheric composition via humidity, clouds and global albedo.
iii) Volcanic eruptions can affect atmospheric composition temporarily.
iv) Ocean surface temperature changes can affect the balance of CO2 absorption or release so as to alter the amount of CO2 in the atmosphere.
v) Significant biosphere changes could have effects on atmospheric composition.
But sun and oceans have by far the largest effects against which our puny efforts count for nothing.