Stephen Wilde: The Myth of ‘Backradiation’

The Myth of ‘Backradiation’.

Stephen Wilde

1) The so called ‘consensus’ theory of the Greenhouse Effect.

At some point over the past twenty years or so it has come to be believed that the physics of radiation is the primary driving process for planetary atmospheric temperatures.

I do not know how or when that happened but it is contrary to the settled science that I grew up with some 40 years ago.

The idea is that certain gases in the atmosphere known as Greenhouse Gases absorb and emit more radiation than the other, majority, gases such as Oxygen and Nitrogen which make up the vast bulk of Earth’s atmosphere.

Consequently it is proposed that such Greenhouse Gases block radiation emanating from the Earth’s surface from escaping to space and re- radiate a proportion of such upward radiation back to the surface which then becomes warmer than it otherwise would have done.

That, in essence is the Greenhouse Theory of radiative gases and that underlies the entire proposition that human emissions of CO2 and other similar gases such as Methane are capable of heating up the Earth so as to change climates and ultimately cause devastation.

2) The original theory of the Greenhouse Effect.

When I was being educated back in the 1960’s the so called Greenhouse Effect was widely regarded as a consequence of atmospheric pressure. That was the ‘consensus’ view in those days.

It was often stated that the high pressure at the surface of Venus was the cause of the heat at the surface.

The atmosphere of Venus is around 90 times the mass of that of Earth and it was accepted that that was the cause of the surface temperature. Nobody suggested in those days that the higher temperature at the surface of Venus was anything to do with the fact that the atmosphere is comprised of 95% of CO2 as against the 0.04% of CO2 in Earth’s atmosphere.

In fact it was pretty obvious that CO2 had nothing to do with it because the atmosphere of Mars is also around 95% CO2 yet the surface temperature is low so it isn’t anything to do with the simple proportions of the atmospheric concentrations of CO2.

It was widely accepted that atmospheric pressure was the critical factor in accordance with the established science of the Gas Laws details of which can be found here:

http://en.wikipedia.org/wiki/Lapse_rate

Thus:

“While most often applied to Earth’s atmosphere the concept can be extended to any gravitationally supported ball of gas”

and:

“this lapse or reduction in temperature is normal with increasing distance from the conductive source”.

So there it is, out in the public domain, the fact that surface temperature for a planet with an atmosphere has a temperature at the surface higher than the temperature at the top of the atmosphere and the lapse rate describes the rate at which that surface temperature declines with height.

The temperature of the Earth at the surface is completely explained by the Gas Laws and the Adiabatic Lapse Rate with no need to consider the radiative characteristics of the individual components of the atmosphere.

Furthermore it has been shown that the temperature of Venus is much the same as that of Earth at the same atmospheric pressure subject only to an adjustment for distance from the sun. CO2 clearly has nothing to do with the surface temperature.

3) Which Greenhouse Theory is correct ?

Huge confusion has been caused by that ‘consensus’ theory because we have an entire generation (possibly two) of scientists, students and members of the public who think that molecules of Greenhouse Gases are drifting around in the atmosphere like little mini radiators directing heat back down to the surface and warming it up potentially catastrophically. Indeed I did read a newspaper report describing the phenomenon in just that way.

Fortunately, it is utter nonsense. There are no little radiators in the sky heating us up until we descend into oblivion by destroying our environment.

The truth is that it is a matter of pressure (induced by the gravitational field of the planet and atmospheric mass) plus the level of solar input that determines the temperature at the surface of ANY planet.

Those factors and those factors alone determine the amount of energy that any given planet can hold on to before any excess is lost to space.

The temperature of the surface of any planet is limited by gravity, atmospheric mass and solar input alone as per the Gas Laws and if any change in the composition of the planet or its atmosphere tries to increase that temperature then the atmospheric circulation simply reconfigures itself to eject the excess faster to space by a variety of radiative and non radiative processes.

Further evidence in support of that is the simple fact that ALL atmospheric gases are at the same ambient temperature at any given height in the atmospheric column. Thus (relatively) non radiative gases such as Oxygen and Nitrogen participate fully in the effect, no doubt from conduction and convection which would have warmed those gases up in the earliest days of the formation of our atmosphere.

Once one involves ALL atmospheric gases in the warming effect then a little more CO2 from human emissions becomes utterly trivial and not worth consideration especially when one includes our oceans as part of our atmosphere as we should.

4) An example of confusion.

I recently became involved in a blog discussion involving, amongst other things, the fact that where a table is left out in a garden overnight there will be condensation on the top of the table, condensation on the grass all around but little or no condensation on the grass under the table.

There were all sorts of convoluted radiative solutions considered including radiation from the ground to the underside of the table and from the underside of the table to the ground then from the top of the table upwards and from the sky downwards. None of it was satisfactory because the radiation down from the underside of the table would be insufficient to prevent condensation below the table and if radiation were coming down from the sky then the table top being insulated to some degree from the ground should not have been so wet from condensation when it should have been warmed by the sky above. It just couldn’t be made to fit the observations if there is significant downward radiation from the sky as proposed by AGW theory.

However, if one applies the Gas Laws, ignoring radiation downward from the atmosphere the answer becomes clear. The presence of the table top blocks radiation from the ground which remains warmer than it would have been so less condensation occurs on the ground beneath the table. The top of the table radiates freely to space and becomes cold enough to reduce the air above it to below the dew point and so receives condensation.

5) A better concept.

The Gas Laws explain perfectly well why the surface is the warmest place (excluding geothermal activity) for a planet with an atmosphere. Due to gravity there are more molecules of the atmosphere per unit of volume at or just above the surface so when solar irradiation comes along there are more molecules per unit volume for it to react with either directly via radiation or indirectly via conduction from the surface and greater heat is generated at the surface declining with height. That is very simple and very obvious.

It is a mistake to then regard the atmosphere as radiating down to the surface because the atmosphere always radiates upward. Instead we should regard the atmosphere as being heated by energy scattering from the surface upward such that it is the temperature of the atmosphere just at or above the surface that passes energy back to the ground and not the sky as proposed by AGW theory.

Everything makes much more sense on that basis.

6) Conclusion.

An atmosphere of ANY composition will heat the surface below in accordance with the Gas Laws which dictate that the only relevant factors determining surface temperature are surface pressure (from gravity and atmospheric mass) and solar energy input. Those factors, and those factors alone, control surface temperature and indeed energy content for the system as a whole.

A change in composition for any part of the system will only alter the rate at which energy flows through the system so that for example IF more GHGs do indeed slow down the throughput of energy then the system will provide another way of speeding it up again for a zero net effect. All that happens is that the balance changes between one type of energy transfer and another. The system has radiation, evaporation, conduction, convection and the water cycle to freely manipulate as necessary.

As it is, GHGs radiate upwards as much as they radiate downwards and I have seen no evidence that the two directions of emission do not cancel out. Since the vast majority of the atmosphere consists of non GHGs then GHGs actually provide a mechanism for cooling that non GHGs cannot provide so unless demonstrated otherwise I assume that the net effect of more GHGs is zero.

Be that as it may, if it were to be shown that the net effect of more GHGs is to make the atmosphere somewhat warmer then the only effect would be to accelerate other processes such as evaporation, conduction, convection, outward radiation and a faster or larger water cycle. Otherwise, the Gas Laws would be breached.

To achieve that faster throughput of energy the surface air pressure distribution and the position of the climate zones would need to shift a fraction but I have discussed elsewhere how vastly larger such shifts already occur naturally from variations in the behaviour of oceans and sun.

On the basis of the above, the radiative theories behind Anthropogenic Global Warming would appear to be incorrect as regards the addition of any extra energy to the whole Earth system but MAY be correct (if the amount of energy sent downward exceeds the value of energy sent upward) in terms of a miniscule climate effect from a change in the surface pressure distribution and a consequent shift in the climate zones.

Separately, it appears that the proposed positive effect from more water vapour in the air as a result of more GHGs is likely to be incorrect since there has been no sign of increasing global humidity during the late 20^th century period of a slightly warming troposphere.

If anything, the Earth system response to more GHGs would appear likely to be negative as I have proposed.

A positive water vapour response to more GHGs was always implausible because more water vapour would lead to more warming and more water vapour then more warming ad infinitum. In the past there has been much more CO2 and much more warmth without that outcome.

There are also problems with the surface temperature measuring system as a result of compromised sensor locations and urban heat island effects plus numerous ‘adjustments’ to past data which seem not to have been fully recorded or justified.

How it must work in practice.

We need to get away from the idea that no work is being done where a gas which is receiving solar energy is situated within a gravitational field. Roy Spencer has made a big mistake in saying that there is a once and for all compression with no work done thereafter. Lots of other highly qualified scientists say just that.

They seem to forget that molecules of gas that are receiving solar energy vibrate, moving in ALL directions, by virtue of their kinetic energy which is why they are in gaseous form and not a solid.

Adding solar energy is what makes them zip about to form a gas in the first place and the more solar input the more they zip about.

The power source is the solar input. Switch it off and gravity succeeds in pulling down all the molecules so that the gas forms a solid on the surface once more.

The sun adds the necessary power, work is done as the molecules vibrate and infrared heat energy becomes highest where gravity has placed most molecules available to participate in the process, at the surface. The weight of the whole atmosphere places greatest pressure and greatest molecular density at the surface.

So, at that point we have solar input, the surface pressure and the surface temperature all in balance. Change any one of them and the point of equilibrium changes as per the Gas Laws.

However, on a planet with an open sky there is also the volume of the atmosphere to consider. That volume will be related directly to the total system energy content.

As soon as one changes system energy content by however small an amount the volume of the atmosphere will change instantly to keep the other numbers the same UNLESS one also alters the pressure at the surface AND/OR the level of solar input. Those two factors will change the equilibrium temperature because they increase the total amount of interaction going on between air molecules and insolation at the surface.

GHGs alter system energy content a fraction so the volume of the atmosphere changes by a fraction but because there is no increase in surface pressure and no increase in solar input then the change in atmospheric volume instantly cancels the effect of the GHGs on surface temperature by shifting the entire atmosphere upward leaving less molecules near the surface to participate in the interaction with solar energy.

The difference is that GHGs only affect the timing and location of the throughput of energy and NOT the total amount of interaction between air molecules and solar energy.

GHGs slow down energy loss to space but the increase in atmospheric volume simply dissipates it instantly and in the process the global circulation of the air shifts a miniscule fraction as a result of the increased volume of the atmosphere.

The same thing happens in response to ANY change other than increased surface pressure or increased solar input.

Hence the Venus/Earth observations and the curve fit shown by N & Z.

Summary.

Neither the Earth nor the Earth’s atmosphere can be treated as black bodies:

http://en.wikipedia.org/wiki/Black_body

To give black body status to Earth you have to take a point beyond the atmosphere as the ‘surface’ and only then apply the relevant equations for ascertaining surface temperatures:

http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

Furthermore, treating Earth and its atmosphere as two black bodies separated by a vacuum is wholly inappropriate because the Earth and its atmosphere are a single unit interacting primarily via non radiative processes which is where the Gas Laws come in.

Due to the governing factors of surface pressure and solar input the only relevant transfer of energy to and from atmosphere and surface involves the non radiative process of conduction plus a tiny amount of radiation from the GHGs just at or above the surface.

Conduction from the solar irradiated surface supplies the energy for convection and evaporation which together facilitate the transfer of energy from surface to space, in the process being well able to negate any radiative effects from changes in atmospheric composition.

Additional support is supplied by horizontal air mass movements such as the development and equatorward migration of features such as the ‘Mobile Polar Highs’ described by Marcel Leroux. Atmospheric events such as those have a substantial impact on sea surface / surface air temperatures.

The radiative characteristics of GHGs are neutralized within the system by:

i) The ability of GHGs to radiate directly out to space and

ii) The ability of the atmosphere to reconfigure itself by altering the surface pressure distribution thereby adjusting the rate of energy flow from surface to space.

If the radiative characteristics of GHGs were not neutralised then the Gas Laws would not apply and the concept of the Standard Atmosphere would not work. I suggest that readers Google the Standard Atmosphere and acquaint themselves with its characteristics. Aviation, in particular, relies on it being valid.

So, for bodies separated by a vacuum, apply the Stefan Bolzmann equations but only at a point outside any atmospheres where radiative processes do indeed dominate exclusively.

For bodies not separated by a vacuum, such as a planet and its atmosphere, apply the Gas Laws because non radiative processes dominate by holding the balance of the net energy flow within their control.