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| Greenhouse cooling musing |
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| andre |
 Posted: Tue Jul 15, 2008 5:52 am Post subject: Greenhouse cooling musing |
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It is argued that the difference between Earth black body temperature of about 255 kelvin and the average global temperature of about 288 Kelvin is due to the greenhouse effect. see: http://geography.berkeley.edu:16080/ProgramCourses/CoursePagesFA2006/Geog40/L5.pdf
Is this correct? Suppose (null hypothesis) that there were no greenhouse gasses in the atmosphere and that heat exchange was only possible by convection and conduction.
So the sun would heat up the Earth surface, which would heat up the lowermost molecules of the atmosphere. Due to the expansion this air is lighter and would rise up, convection, transporting the heating to higher parts of the atmosphere, as it happens today. Because of the lapse rate/ temperature gradient, the expanding rising air cools but may remain above environment temperature, so that the upward convection may continue for a while, removing the energy away from the Earth surface. All very basic.
When the sun sets, the radiating Earth surface cools down, cooling the lower part of the atmosphere by conduction. But the contracting air is denser than the environment and there is no tendency for convection at all. The air becomes stable, cool below and warmer in the upper layers. This is known as an 'inversion'. Hence this warming of the atmosphere by conduction and convection is basically one way until there is a dynamic equilibrium in the energy exchange between the daytime convection and night conduction cooling of the lowermost layer.
Hence it appears that we don't need greenhouse effect to heat the atmosphere to above the black body temperature, just an atmosphere, consequently not all, if any, of that 33 degrees difference between average atmosphere temperature and black body temperature is caused by greenhouse effect. Actually in a one way direction, the atmosphere should warm until convection is no longer possible and the lapse rate would be equal to that with the surface temperature at the maximum daily temperature, that is still without greenhouse effect.
But with the greenhouse gasses in the atmosphere, it can radiate it's energy to the earth and space and hence it can cool again in that process. This means that greenhouse gasses also contribute to the cooling of the atmosphere, how much is uncertain, but one could just as easily assume: "the more greenhouse gasses, the more cooling". |
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| Bunbury |
| Posted: Tue Jul 15, 2008 8:54 am Post subject: |
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| Quote: |
| Hence it appears that we don't need greenhouse effect to heat the atmosphere to above the black body temperature, just an atmosphere |
You cannot expect anyone to accept this claim based on your qualitative thought experiment. You need to do some calculations. You also need to review some of your assumptions:
- the surface, including the oceans, doesn't cool down as rapidly as you suggest when the sun sets
- air is a very poor conductor of heat; in the absence of convection what is the balance between radiation back to space and conduction to the ground layer? Have you done this calculation?
- when the sun sets in one place it rises in another, causing a phenomenon called "wind" - does this have no effect on your inversion layer?
Sorry, you can't rationalize your beliefs with unsubstantiated speculation. |
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| KALSTER |
| Posted: Tue Jul 15, 2008 9:48 am Post subject: |
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I also seem to remember reading that IR at night from the earth surface as it cools causes more of an upwards convection gradient than during the daytime, as heat is aborbed by the atmosphere during the day from above as well. Is this true?
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| Lynx_Fox |
| Posted: Tue Jul 15, 2008 4:19 pm Post subject: |
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The fundamental problem with the thought experiement is it's viewed as single slab.
Andre run your thought experiment with lets say three layers and consider that the middle layer is radiating heat back to the lower layer and to the surface--this results in the lower layer rising in temperature to remain in equilibrium. |
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| andre |
| Posted: Thu Jul 17, 2008 12:15 pm Post subject: |
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I think we thing of a labaratory experiment using a long cylinder with insolating walls and a bottom of alternating distinct higher or lower temperatures and see what difference there is in temperature change of the top in a inert dry N2/O2 atmosphere inside the cilinder and with moist air with a considerable CO2 concentrration.
Meanwhile check this out:
http://www.informaworld.com/smpp/content~content=a788582859~db=all~tab=content~order=page
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abstract
The writers investigated the effect of CO2 emission on the temperature of atmosphere. Computations based on the adiabatic theory of greenhouse effect show that increasing CO2 concentration in the atmosphere results in cooling rather than warming of the Earth's atmosphere. |
Excerpts from the article
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Traditional anthropogenic theory of currently observed global warming states that release of carbon dioxide into atmosphere (partially as a result of utilization of fossil fuels) leads to an increase in atmospheric temperature because the molecules of CO2 (and other greenhouse gases) absorb the infrared radiation from the Earth's surface. This statement is based on the Arrhenius hypothesis, which was never verified (Arrhenius, 1896).The proponents of this theory take into consideration only one component of heat transfer in atmosphere, i.e., radiation. Yet, in the dense Earth's troposphere with the pressure pa > 0.2 atm, the heat from the Earth's surface is mostly transferred by convection (Sorokhtin, 2001a).
According to our estimates, convection accounts for 67%, water vapor condensation in troposphere accounts for 25%, and radiation accounts for about 8% of the total heat transfer from the Earth's surface to troposphere. Thus, convection is the dominant process of heat transfer in troposphere, and all the theories of Earth's atmospheric heating (or cooling) first of all must consider this process of heat (energy)-mass redistribution in atmosphere (Sorokhtin, 2001a, 2001b; Khilyuk and Chilingar, 2003, 2004).
When the temperature of a given mass of air increases, it expands, becomes lighter, and rises. In turn, the denser cooler air of upper layers of troposphere descends and replaces the warmer air of lower layers. This physical system (multiple cells of air convection) acts in the Earth's troposphere like a continuous surface cooler. The cooling effect by air convection can surpass considerably the warming effect of radiation.
The most important conclusion from this observation is that the temperature distribution in the troposphere has to be close to adiabatic because the air mass expands and cools while rising and compresses and heats while dropping. This does not necessarily imply that at any particular instant distribution of temperature has to be adiabatic. One should consider some averaged distribution over the time intervals of an order of months. [...]
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| Lynx_Fox |
| Posted: Thu Jul 17, 2008 6:17 pm Post subject: |
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The troposphere isn't on average at either the dry or moist adiabatic lapse rates.
You tend to see dry adiabatic lapse rates only in the boundary layer near the surface during sunny afternoons. You tend to see temperature profile near the moist adiabatic lapse rate during convection in the tropics and during small scale weather events (mesoscale) at mid-latitudes.
Does convection have an effect? Certainly. This is why models predict, and thus far there is far less observed warming in the the tropics where there's more convection, than in the high latitudes where vertical convective heat transport is small.
Convection is already accounted for. |
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| Bunbury |
| Posted: Thu Jul 17, 2008 8:10 pm Post subject: |
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| Quote: |
| The proponents of this theory take into consideration only one component of heat transfer in atmosphere, i.e., radiation. |
This is simply wrong. |
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| KALSTER |
| Posted: Fri Jul 18, 2008 1:04 am Post subject: |
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Sorry for squeezing in between you knowledgeable people here, but I have some questions if you don't mind. Heat absorbed by the atmosphere from radiation: This happens both during the day from above and below, and at night from below only. Heat is lost from the atmosphere only through radiation into space. The more CO2 (and other greenhouse gasses) in the atmosphere, the more heat is absorbed through radiation before it can escape into space. So irrespective of how much of a factor convection is locally, globally absorption of IR is the only thing that warms the earth. Ergo (all things being equal) -> more greenhouse gas = atmospheric temperature rise.
How far off base am I? 
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| Bunbury |
| Posted: Fri Jul 18, 2008 4:37 pm Post subject: |
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| Quote: |
| How far off base am I? |
I'm definitely not that knowledgeable, but I don't think you're off base at all. The only heat loss to space is by radiation (except for a few stray fast molecules).
But the claim in the posted excerpt that climate models ignore convection is silly. The Navier-Stokes equation is in every current climate model (afaik) and it has nothing to do with radiation and everything to do with convection. It's nonsense like this that attempts, deliberately one must assume, to confuse.
(Of course CO2 does make the stratosphere cooler, but that's the problem, since radiation is reduced at cooler temperatures and radiation loss to space occurs from the stratosphere, not the troposphere.) |
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| KALSTER |
| Posted: Fri Jul 18, 2008 6:08 pm Post subject: |
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| Andre wrote: |
| But the contracting air is denser than the environment and there is no tendency for convection at all. The air becomes stable, cool below and warmer in the upper layers. |
As the earth spins there would always be a gradient of solar radiation hitting the surface, so as cooling happens when solar radiation decreases (reducing the amount of IR radiated from the surface) that cold air would always be pulled towards the pressure gradient caused by the upwards convection at the moving sunlit front. So convection would never behave as predictably as in your model as turbulence would cause the direction of convection to behave chaotically, attributed to by geographical variation and the varying IR radiation levels of the different albedos of vegetation, water, rock types, etc. As Bunbury said, turbulence is obviously taken account of in atmospheric models, which include Navier-Stokes viscous fluid models. The supercomputer weather modellers can achieve a certain level of accurate prediction, which would not be possible without taking into account such a large contributor to weather patterns as convection.
So back to my earlier point. Turbulence would always have an overall distributory effect on convected hot air and would have little or no increase in heat radiation into space IMHO. So the relation, greenhouse gas increase -> average atmospheric temperature increase, is still valid IMHO.
Still on base now? 
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| Chris |
| Posted: Fri Jul 18, 2008 9:49 pm Post subject: |
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| KALSTER wrote: |
Sorry for squeezing in between you knowledgeable people here, but I have some questions if you don't mind. Heat absorbed by the atmosphere from radiation: This happens both during the day from above and below, and at night from below only. Heat is lost from the atmosphere only through radiation into space. The more CO2 (and other greenhouse gasses) in the atmosphere, the more heat is absorbed through radiation before it can escape into space. So irrespective of how much of a factor convection is locally, globally absorption of IR is the only thing that warms the earth. Ergo (all things being equal) -> more greenhouse gas = atmospheric temperature rise.
How far off base am I? |
KALSTER, for one who doesn't think he is knowledgable, you already have a better command of basic atmospheric physics than andre does. If you're trying internet forum learning on this subject, you should dismiss anyone who beleives the basics of the greenhouse effect ar wrong, or who questions the idea that it makes the global mean temperature warmer than the effective temperature.
The planet itself is in a vacuum and so it only loses heat via radiation to space. Convection and conduction matter for the *surface* energy balance, however, adding CO2 mainly effects the top-of-atmosphere energy balance where the planet is not longer in radiative equilibrium. Actually it's the latter which matters a bit more, though the surface budget is mainly what regulates the surface-atmosphere temperature gradient and is more or less "dragged along" by perturbations of the top-of-atmosphere balance. So to change the global mean temperature, you're right, you need to effect the outgoing radiation (or the incoming sunlight, or the reflected sunlight which is the albedo factor). When you add greenhouse gases to the atmosphere, you make the planet itself a less efficient radiator, and widen the spectrum where absorption occurs. In wavelengths where CO2 absorbs, you also increase the downward infrared radiation, and the cooling due to evaporation, etc is how the surface comes back to equilibrium after being perturbed by the increased radiative heating; that is automatically accounted for in models, and is necessary for them to reach equilibrium, which is a prerequisite for an estimate of the "equilibrium climate sensitivity" to a doubling of CO2 that is always cited from the IPCC reports. |
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| andre |
| Posted: Sat Jul 19, 2008 11:23 pm Post subject: |
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Chris, Better start thinking about cutting the groupthink ad hominem nonense. It will backfire hard and that's a promise. The exposure of the global warming scam is neigh and your character murder attemps are definitely not helping to uniting the camps for cleaning out the mess.
About turbulence, of course mixing of air also allows exchange of heat but turbulence is not as common as convection. As the atmosphere becomes stable at night due to the cooling of the lower layers, it also suppresses turbulence. |
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| Chris |
| Posted: Sun Jul 20, 2008 3:22 pm Post subject: |
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| andre wrote: |
Chris, Better start thinking about cutting the groupthink ad hominem nonense. It will backfire hard and that's a promise. The exposure of the global warming scam is neigh and your character murder attemps are definitely not helping to uniting the camps for cleaning out the mess.
About turbulence, of course mixing of air also allows exchange of heat but turbulence is not as common as convection. As the atmosphere becomes stable at night due to the cooling of the lower layers, it also suppresses turbulence. |
Andre, better start thinking about not trying to confuse the less learned readers, for say, the past two years. You're a liar, and it's such a shame that anyone would take your posts seriously. |
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| andre |
| Posted: Sun Jul 20, 2008 11:01 pm Post subject: |
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| KALSTER |
| Posted: Mon Jul 21, 2008 12:01 am Post subject: |
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Wow, you guys seem to have been going at it for quite some time now. I do not have any position to defend or advance though so I'll just continue in trying to figure this out.
| Quote: |
| About turbulence, of course mixing of air also allows exchange of heat but turbulence is not as common as convection. As the atmosphere becomes stable at night due to the cooling of the lower layers, it also suppresses turbulence. |
It is calmer at night, granted. But there is always differential pressures and temperatures that can cause and uneven movement of air, encouraging mixing. Also, how sure are you that it is really all that quiet at night? Is it only from your mind experiment, or do you have any reliably obtained data to back it up? I am talking about higher up than you can judge by pointing a wet finger at the sky. 
The fact remains that when the warmer air rises, the air it displaces on top of it has to go somewhere and the space it leaves behind will also be filled with air from somewhere. This implies a much more dynamic system than simply warm air rising in a linearly upwards direction and cooling as it does so. The energy the sun provided during the daytime can only escape the planet by radiating back out into space. The warmer gas expanding as it rises does not get rid of the energy, but merely transfers it into the air it causes to move around, I suspect. Since the sun has been shining for some time now, the energy needed to get an average convection/wind system going has already been provided. All the remaining energy has to either be stored chemically (life, etc), add to the energy of the atmosphere or be radiated back out. The equilibrium point has to shift (all thing being equal) as solar radiation remains constant, but atmospheric absorption increases, no?
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