Discussion > GHG Theory step by step
A good start, Mr Cat. Other points that seem to be missed out, or utterly ignored, is that, in environments where there can be very little water, such as a desert, during the day, the temperatures can get very high, yet at night, the temperature plummets; if there is no air movement, any water on the ground can be frozen, while the air above remains warm – an event I have witnessed (and already mentioned far too many times on here). To me, this basically invalidates any supposed “greenhouse effect” attributable to CO2. Maybe it is just a phrase used in an attempt to explain a complex situation to the simple.
Another point is that there seems to be the general assumption that these “greenhouse gas” molecules are just absorbing all this energy; there seems to be little consideration that they will also be transmitting it, too, in all directions, slightly more than half (dependent upon altitude) of which will be to space.
Radical Rodent
5 / .04 = 125
So here is 125 times more water vapour than CO2.
Assuming similar effectiveness at capturing photons of similar energy levels, one can only assume that CO2 is a very small player in the temperature control system upon earth.
As water is 'special' ie changes phase at useful temperatures and its latent heat properties, it would give valuable cause to the belief that water moderates the temperature on this planet in a well controlled negative feedback system.
Steve Richards
Schrodinger's Cat, thank you for starting this thread. As a non-physicist, I was not going to comment so soon but ....
Jul 17, 2017 at 2:06 PM | Radical Rodent your comment about deserts hot by day, and flipping cold at night, is something that niggled me from 15+ years ago,
golf charlie
Interestingly, a post over at No Tricks Zone talks about CO2 being a very weak GHG.
The point about the desert implies that the ground radiates its heat to space without much interception by GHG. We know that there is hardly any water vapour, being a desert. It seems to back up the idea that CO2 is pretty useless.
It reminds me that to be active in the IR, molecules require a dipole. Water vapour is a good example. CO2 is a linear molecule with no dipole when stretching or twisting. It does have one when bending because the atoms move out of alignment. I'm guessing that this reduced dipole functionality limits its IR absorbance.
We can continue to develop what happens next in our story but I'm hoping that we get some explanations, challenges or disagreements.
Schrodinger's Cat
The current working hypothesis is that escaping radiation, in some frequencies, is absorbed then released then absorbed until it gets to an altitude where the air is thin enough for escape without further interruption. Each release after absorption is omnidirectional. A lapse rate exists in this scenario where it would not in (say) a wholly nitrogen atmosphere. As we know, temperature drops with altitude and pressure in the troposphere where this hypothesis mostly belongs. To calculate how the energy moves through the atmosphere, the The Schwarzschild equation is employed (Google it - v complicated). Basically, you want to know the flux at altitudes. A simple algebraic model for that follows;
A model of simplified radiative atmosphere of 4 stratified and optically thick layers (energy within one layer radiates to both its near neighbours only). At the atmosphere top, the air is thin enough to allow escape to space of all radiation from below. At ground, there is no radiation arriving from below but there is incoming radiation [σTe^4]. Where σ is the S-B constant and Te is from the Planck emission eqn.
Each layer is radiating both up and down while exchanging radiation from below and above (background radiation of space is small and omitted). Each layer is resolved for σTe^4
Escape ● At equilibrium, outgoing = incoming......................● ∴ σT1^4 = σTe^4
Layer 1 ● 2σT1^4 [=2σTe^4] = σT2^4..................................● ∴ σT2^4 = 2σTe^4
Layer 2 ● 2σT2^4 [=4σTe^4] = σT1^4 [=σTe^4] + σT3^4.....● ∴ σT3^4 = 3σTe^4
Layer 3 ● 2σT3^4 [=6σTe^4] = σT2^4 [= 2σTe^4] + σT4^4..● ∴ σT4^4 = 4σTe^4
Layer 4 ● 2σT4^4 [=8σTe^4] = σT3^4 [= 3σTe^4] + σTg^4..● ∴ σTg^4 = 5σTe^4
Ground ● σTg^4 [=5σTe^4] = σT4^4 [= 4σTe^4] + σTe^4....● ∴ σTg^4 = 5σTe^4
(a) With Te^4 constant, Tg^4 would increase with the number of optically thick layers [concentration]. If Co2 is able to add to the atmosphere in this way then it will increase surface temperature.
(b) The surface flux, 5σTe^4 = the near-surface flux, 5σTe^4.
(c) At ground, the flux received from the near-surface, σT4^4 = 4σTe^4, whereas the flux at the surface is 5σTe^4. ∴ greater by the flux derived directly from the insolation.
There are a number of steps in the above to take issue with. Whether valid or not - heh, I'm no physicist. My doubts lie in my conclusions (b) & (c) which don't appear to square with the 'trapped heat' narrative.
ssat
Jul 17, 2017 at 3:58 PM | Unregistered CommenterSchrodinger's Cat
Thank you.
golf charlie
Thanks, ssat. As you say, the energy works its way up in altitude by a process of emission and absorbance. Many simple explanations of the GH effect say that the downwards emissions of photons warm the surface. This seems to have gone out of fashion.
Now it is explained this way: CO2 makes the atmosphere opaque to outwards going IR radiation. (another way of saying what ssat just said). At some point the radiation can escape to space unimpeded because the atmosphere is thin. We can call this altitude the effective radiation altitude. Adding more CO2 raises the height of the opaque atmosphere, so this raises the effective radiation altitude.
Assuming the lapse rate remains the same, (not affected by more CO2) then the argument goes that to release the same energy from a higher altitude means that the earth surface temperature must be hotter than before.
Schrodinger's Cat
Some comments. There are claims that without GHG there would be no lapse rate. I think this is wrong because, as I understand it, the lapse rate is about work done in raising a parcel of air and nothing to do with GHG.
The effect of increasing CO2 on IR absorbance is calculated using the Beer Lambert law. This normally applies to a sample concentration in a cell with known path length. In the atmosphere, the cell is a segment of a sphere.
As stated earlier, do they take account of convection which can transport heated air upwards? Do they ignore that water vapour is also a GHG with a range of active wavelengths and not necessarily saturated which can receive energy by collision?
Schrodinger's Cat
"Many simple explanations of the GH effect say that the downwards emissions of photons warm the surface. This seems to have gone out of fashion."
There is always a net positive wattage radiating from the ground (land+ice+water) in frequencies that interact with Co2. For that to occur, the average ground temperature must be greater than the average near-ground atmosphere temperature.
In the model, you can see that the near-ground is radiating 4σTe^4 downward and the ground is radiating upward [4σTe^4 + σTe^4]. The fluxes cancel other than the outgoing = incoming = σTe^4. This is to be expected as the cancelling fluxes are those from the temperatures attained in equilibrium.
There are 4 possible scenarios that would perturb that dynamic equilibrium;
Insolation increase........● All temperatures rise.....● σTe^4 > σT1^4 ● Increasing atmospheric volume.
Insolation decrease.......● All temperatures fall......● σTe^4 < σT1^4 ● Decreasing atmospheric volume.
Radiative gas increase..● Escape altitude colder...● σTe^4 > σT1^4 ● Increasing atmospheric volume.
Radiative gas decrease.● Escape altitude warmer.● σTe^4 < σT1^4 ● Decreasing atmospheric volume.
Which have only one of two outcomes: a positive or negative radiative imbalance at the escape altitude.
In the specific case of a radiative gas increasing in concentration, the excess of σTe^4 over σT1^4 will increase overall atmospheric energy until equilibrium is regained by temperature of the near-ground rising so σT5^4 = [σTg^4 – σTe^4].
Which means that the bulk of additional energy is in the atmosphere, not in the ground. The atmosphere will expand and its altitude of centre of mass will increase. That is work done [Mass x altitude rise x gravitational constant].
The ground temperature [Tg] derives from the insolation +σTe^4 and rises to drive fluxes to equilibrium at the escape altitude.
ssat
"There are claims that without GHG there would be no lapse rate. I think this is wrong because, as I understand it, the lapse rate is about work done in raising a parcel of air and nothing to do with GHG."
The GHG-free atmosphere would have a temperature of the average ground temperature [288oK - 33oK = 255oK]. It achieves that via conduction at the ground. A non-GHG molecule cannot radiate energy and maintains its temperature as its mass and its velocity are constant and independent of altitude [as in the thermopause].
"As stated earlier, do they take account of convection which can transport heated air upwards?"
Convection does move energy upward, a short-circuiting effect on the absorption/re-radiation path and reducing the lapse rate. Do they ignore that water vapour is also a GHG with a range of active wavelengths and not necessarily saturated which can receive energy by collision? There are many lapse rates in a range of ~ 6.5 to 10 K/km depending on such as water vapour presence.
ssat
Schroedinger's Cat
"There are claims that without GHG there would be no lapse rate. I think this is wrong because, as I understand it, the lapse rate is about work done in raising a parcel of air and nothing to do with GHG."
I would agree with the claims that without GHGs there would be no lapse rate, but not because GHGs drive the lapse rate.
The lapse rate occurs because there is a troposphere with a decline in temperature between a surface heated by insolation and a tropopause at which the heat is dispersed as IR.
Because of GHGs heat transfer between surface lower atmosphere can be by radiation. The rest of the transfer to the tropopause has to be by convection. Convecting air obeys the ideal gas law, expanding and cooling as the pressure drops. Hence the lapse rate.
Reduce the GHGs and the tropopause lowers. With no GHGs at all the tropopause is in contact with the surface and the troposphere disappears.
Surface heat radiates directly to space. With no lower atmosphere heating temperature rises with altitude and convection does not happen. No convection, no lapse rate.
Entropic man
S's Cat
My comments above, if they reveal anything, is that down welling long wave infra red [a concept important enough to have an acronym, DWLWIR] is an effect of atmospheric GHG, not the cause of Greenhouse Effect.
I have no real idea what goes on in the climate models but as the 'science is settled' I assume they all run the same, despite their differing outputs. However, the daftness of DWLWIR, either as an energy source or escape delayer, is part and parcel of all 'consensus' explanations you can find in the public domain.
ssat
EM "I would agree with the claims that without GHGs there would be no lapse rate, but not because GHGs drive the lapse rate. The lapse rate occurs because there is a troposphere with a decline in temperature between a surface heated by insolation and a tropopause at which the heat is dispersed as IR".
Precis: There isn't a lapse rate but because there is a lapse rate, a lapse rate occurs.
?
ssat
Then, of course, there are actually two lapse rates: the adiabatic lapse rate (ALR), which is the rate at which a parcel of air cools as it rises, assuming no loss or gain of energy from the environment; and the environment lapse rate (ELR), which is the rate of cooling to be found in a stationary column of air as altitude increases. The ELR tends to be greater than the ALR, thus the rising air is always in cooler air, thus continues to rise; when the ELR is not as high as the ALR a thermal inversion occurs, and the rising parcel levels out. This often occurs in warm places or at warm times, and can be seen by rising smoke or vapour from cooling towers flattening out; it is prevalent in the Persian Gulf in summer, with the thermal inversion effectively acting as a lid on a pressure cooker, resulting in the Gulf’s notoriously high temperatures and humidity. The presence, or otherwise, of greenhouse gases is irrelevant to lapse rates.
As the parcel of air is cooled below its dew-point temperature, it experiences saturated ALR, which is about half that of the unsaturated lapse rate, as the condensing water releases its latent heat, and potentially drives the thermal engine of a thunderstorm; the only “greenhouse gas” needed, here, is water.
Radical Rodent
Seat
Not quite.
I tried, and seemingly failed, break the process of heat gain and heat possible the troposphere down into steps.
Let's try again
1) GHGs cause the existence of the troposphere because the only way for the atmosphere to lose heat is by radiating IR at the tropopause. Heat enters the troposphere at the surface as visible light is absorbed and becomes heat. For heat to transfer from the surface to the tropopause there must be a temperature gradient, cooler at the tropopause than at the surface(1st law of thermodynamics).
2) Because the GHGs make the troposphere opaque to IR, heat transfer from surface to tropopause must be by convection.
3) Convecting warm gas obeys the Ideal Gas Law PV=nRT . As warm air rises the pressure decreases, the volume increases and the temperature drops. The pressure gradient depends on gravity and the mass/ unit surface area of the atmosphere. The consequent rate at which temperature drops with altitude is the lapse rate, 9C per km.
4) The GHGs do not cause the lapse rate directly, but the conditions in which a lapse rate develops would not occur without them.
Entropic man
Radical rodent
Nice to be in agreement for once.
Entropic man
Which is an excellent demonstration of our differences, and how you may never understand it, Entropic man. I am not interested in agreeing with people, nor am I interested in “winning” arguments. I am interested in FACTS, whether or not I am offended, or otherwise, by them (as, perhaps bizarrely, the truth can be very, very hurtful). Opinions can be interesting, but they are not the be all and end all of an argument. Sadly, you appear to be quite the opposite; for you, the opinions of one you might have never have met seem to hold greater sway than simple FACTS.
Radical Rodent
EM "The GHGs do not cause the lapse rate directly, but the conditions in which a lapse rate develops would not occur without them."
RR "The presence, or otherwise, of greenhouse gases is irrelevant to lapse rates."
EM "Nice to be in agreement for once."
.ssat
Radical rodent, ssat
I thhnk we are arguing past each other.
Perhaps you could critique these two statements.
1) Without GHGs there would be no troposphere.
2) Without a troposphere there would be no lapse rate.
Entropic man
And what is your reasoning behind such conclusions?
What is the evidence to support your reasoning?
Do note that I have had experience of sub-tropical surfaces cooling to below freezing, while the air above was comfortably warm; the logical conclusion to that simple observation is that the predominant heating of the atmosphere is by conduction from heated surfaces to adjacent air, and convection to disperse the heat through the atmosphere. Cooling is just a reversal of that process, with the slight change in that, because a convection cell is unlikely to form (except on slopes, thus causing katabatic winds, which can be quite ferocious, and ferociously cold, too!), air movement is required to stir the cooled air to higher altitudes – hence, on a typical clear night, with no air movement, the surface chills; with slight air movement (i.e. a faint breeze), condensation of water in air close to the ground forms dew or frost; with slightly greater air movement (a gentle breeze) low-lying mist or fog forms, and so on, until the stronger breeze raises the condensation to form low cloud-cover. (This is why hoar frost only occurs in winter, only when the night has been predominantly clear, and only when there is next to no air movement. A slight breeze setting in towards morning might result in the magic of hoar-frosted trees being wreathed in mist.) In this process, the atmosphere has been cooled. The presence of lapse rates is probably irrespective of atmospheric composition, more likely being caused by the gas laws and gravity. With your logic, Entropic man, the lapse rates on Venus and Mars, both of which have atmospheres nearly 100% GHG, must be truly phenomenal! Have these been measured?
Radical Rodent
Have Real Climate (including at that time WMC aka William M Connolley) resolved the quandary about Lapse Rates that they raised in 2005?
http://www.realclimate.org/index.php/archives/2005/08/the-tropical-lapse-rate-quandary/#comments
"The non-warming troposphere has been a thorn in the side of climate detection and attribution efforts to date. Some have used it to question the surface record (though that argument has won few adherents within the climate community), while others have used it to deny an anthropogenic role in surface warming (an illogical argument since the atmosphere should follow no matter what causes the surface to warm). The most favored explanation has been that the “lapse rate,” or decrease in temperature as you go up in the atmosphere, has actually been increasing. This would contradict all of our climate models and would spell trouble for our understanding of the atmosphere, especially in the tropics."
I don't claim to understand the Physics, but I understand lack of certainty before and after reaching a conclusion, which I believe is why Schrodinger's Cat started this thread.
golf charlie
EM, regarding your two questions, I think Roy Spencer covers both very well. From his site;
While it is true that convective overturning of the atmosphere leads to the observed lapse rate, that convection itself would not exist without the greenhouse effect constantly destabilizing the lapse rate through warming the lower atmosphere and cooling the upper atmosphere. Without the destabilization provided by the greenhouse effect, convective overturning would slow and quite possible cease altogether. The atmosphere would eventually become isothermal, as the full depth of the atmosphere would achieve the same temperature as the surface through thermal conduction; without IR emission, the middle and upper troposphere would have no way to cool itself in the face of this heating. This scenario is entirely theoretical, though, and depends upon the atmosphere absorbing/emitting absolutely no IR energy, which does not happen in the real world.
and supporting that isothermal conclusion, from the kinetic theory of gases is;
The average kinetic energy of a gas particle is directly proportional to the temperature. An increase in temperature increases the speed in which the gas molecules move. All gases at a given temperature have the same average kinetic energy.
or putting it another way, unless/until the gas molecule loses energy, its temperature is constant. So don't conflate the troposphere pressure and temperature changes with altitude: both occur for different reasons.
ssat
Radical rodent
There is planetary atmosphere data here .
Using the formula (surface temperature - tropopause temperature)/ tropopause altitude I got the following lapse rates
Venus (750-300)/55 = 8.2C/km
Earth (280-220)/10 = 6.0C/km
Mars (220-150)/45 = 2.2C/km
Gravity(ms^2) Surface pressure(bar). Greenhouse gases(bar)
Venus 8.9. 90. 85 CO2
Earth. 9.8. 1. 0.0004 CO2, 0.02 water vapour
Mars. 3.7. 0.006. 0.006
What pattern do you see?
Entropic man
Curses. How do you persuade this word processor to do tables without a tab function?
Entropic man
We all know how the greenhouse theory works, don’t we? (I didn’t say we totally agree with every step or that we all understand every bit of it. I don’t, for one.)
I’m suggesting a thread that builds up the process, step by step. If a step is uncontroversial we move to the next. If there is controversy, we discuss the step until either agreement is reached or we agree to differ and move on. It is also an opportunity for interested newcomers (if they realise the site is still active) to learn from others.
It is possible, or even likely that the debate will fall apart at some stage.
I shall start and see if anyone agrees or disagrees or asks a question.
Green House Effect
Solar radiation (insolation) bathes that part of the earth facing the sun in visible light. A portion is reflected (albedo) the rest is absorbed by the surface.
The heated surface radiates long wave radiation (IR) towards space.
The heated surface also warms adjacent atmospheric gases by conduction.
Oxygen and nitrogen are transparent to the IR but carbon dioxide molecules, if they intercept the outgoing radiation, can absorb IR photons and become excited. Water vapour can do the same.
The excited carbon dioxide molecules collide with other molecules (nitrogen, oxygen and water vapour) transferring energy to produce kinetic energy. This raises the temperature of these gases and convection is triggered. Water vapour can do the same.
Collisions are very much more likely than photon emissions. I shall not keep mentioning water vapour but it is present at up to 5% compared with 0.04% of CO2.
Some excited carbon dioxide molecules emit a photon in a random direction, losing potential energy.
Meanwhile, the warmed air becomes less dense than surrounding air and rises due to convection.
COMMENTS: Much is happening and the process has just begun. This seems like a good place to pause. When I hear about GHG theory, most of the above seems to get missed out. How do climate scientists decide attribution to the various paths?