Talk:Greenhouse effect

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[edit] Inconsistent

The figure showing scattering losses and absorption is very low resolution and does not show absorptions and windows in the mid-IR. Anyone have a better figure??

[edit] CO2 lifetimes

158.169.131.14 wrote:

NOT correct. The lifetime of CO2 is in the order of FIVE years, not hundreds. If you don't believe me, look at http://www.icsu-scope.org/downloadpubs/scope13/chapter01.html. This gives some figures. The atmospheric reservoir of CO2 is now about 750 Gigatonnes. Photosynthesis uses some 100 Gigatonnes/year (both land and marine plants). Exchange with the ocean comes to another 100 Gigatonnes/year. Lifetime is the reservoir size divided by turnover rate. Work it out yourself)

First of all 158.169.131.14 it woud be a good idea to get yourself an account. Second, to put comments like that in the talk page (here) not the article.

Secondly, your calculation of the lifetime is too simplistic. What is of interest is the lifetime of a CO2 anomalty in the atmosphere, not the individual CO2 molecules. See Greenhouse_gas; http://www.grida.no/climate/ipcc_tar/wg1/016.htm. IPCC tar says the lifetime is "5-200" years (FWIW sar says 50-200) and no single lifetime can be determined. It refers you to ch 3 for the details but they are elusive...

==: Please, IMHO, the lifetime of CO2 or any other atmospheric gas is not relevant to describing the greenhouse effect, nor is the precise quantitative measure of any energy transport method. These issues should be discussed somewhere else ... say for example news on global warming or in Atmosphere Chemistry. We should stick to known science and refrain from speculations in areas that are obviously still basic research. Tanuki

[edit] greenhouse effect

you should add more detail about what it is and a more recent news on the greenhouse effect —The preceding unsigned comment was added by 67.83.135.189 (talk • contribs • WHOIS) .

[edit] 25°C vs. 30°C

I saw some edits, and someone claimed that without green gases, Earth's surface would be "25°C" cooler; while the other claimed that person was speculating or something like that. How is possible to know which one is right and wrong? --Gh87 11:54, 2 November 2006 (UTC)

Remove the atmosphere and it is a simple blackbody calculation. T = ((1-albedo)*solar flux/(5.67*10-8 W/m2/K4)1/4. Solar flux is 342 W/m2 at the top of the atmosphere, so it reduces to 278.6 K * (1-albedo)^(1/4). Where modern average temperature is 287 K. The question comes in, what does one assume for the albedo. Current global albedo, with clouds, is about 0.3. Albedo of the modern surface, neglecting the atmosphere, is about 0.15. However, if you remove the greenhouse effect, the whole globe will quickly freeze solid and the albedo of that snow and ice covering could be 0.6-0.8.
So depending on what you want to assume:
  • albedo = 0.15: 20 degrees C
  • 0.3: 33 degrees C
  • 0.6: 66 degrees C
  • 0.8: 100 degrees C
~30 degrees is what is normally given, but that assumes rather nonsensically that you can remove the entire greenhouse effect while having no effect on cloud cover or any other aspect of the Earth's albedo. So I guess take your pick on what you want to say about what happens when you "turn off" the greenhouse effect. Dragons flight 13:05, 2 November 2006 (UTC)

Is there something missing from this temperature calculation? What about the heat inside the earth which is constantly being transmitted from the hot interior to the surface? What is the magnitude of this geothermal heat transfer? Shouldn't the effect of this be added to the solar flux effect to calculate a non-greenhouse temperature? Penurious (talk) 17:34, 28 April 2008 (UTC)

Conduction of heat from the Earth's interior is well below 0.1 W/m2, and makes a difference of only about 0.01 K in temperature. Raymond Arritt (talk) 17:42, 28 April 2008 (UTC)

This leads to a quatitative argument against a large significance for anthropogenic global warming. Pls debunk the following. Penurious (talk) 15:47, 8 May 2008 (UTC)

 Entire current greenhouse effect of atmosphere: 30 degrees C
 Estimated CO2 fraction of greenhouse effect: 10% or 3.0 degrees
 Estimated increase in CO2 effect for doubling of CO2: 5%
 Resulting temerature increase from maximum possible
   anthropogenic greenhouse effect: 0.05*3.0 == 0.15 degrees.
I really don't have a clue about this, but could it be that they meant 10% + 5% so that would mean 0.15*30 = 4,5 °C, an increase of 1.5 degrees.
— Apis (talk) 16:14, 8 May 2008 (UTC)
Here's a crazy idea: Instead of doing our own back-of-the-envelope calculations, maybe we could cite the results of calculations from the academic literature. Just a thought. Raymond Arritt (talk) 16:36, 8 May 2008 (UTC)
I strongly disagree, it's much more fun to make things up as we go along. ;)
— Apis (talk) 20:41, 8 May 2008 (UTC)
I humbly ask the experts for citations and reasoning to refute this. I'm not against citations.

Here are citations supporting the above numbers: the 30 degrees are from this section of talk on the greenhouse_effect article. The 5% increase in CO2 effect comes from http://www.john-daly.com/forcing/moderr.htm in the section 3 on radiative forcing. He mentions an increase of downward radiation of 3.7 W/M**2 out of 74 W/M**2 for CO2 doubling. The 10% CO2 effect I get from the greenhouse_effect article. Penurious (talk) 21:13, 8 May 2008 (UTC)

Daly is (a) dead and (b) incompetent. I'm not going to bother wasting time discussing his fantasies. If you want to, usenet beckons. Don't waste our time here William M. Connolley (talk) 21:29, 8 May 2008 (UTC)
Well, I don't think there's anything wrong with asking questions, but this is the wrong forum for it. :) This page is for discussing how to improve the article not the topic itself.
— Apis (talk) 21:43, 8 May 2008 (UTC)

[edit] First Paragraph

How about: "The greenhouse effect, discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896, is the process in which the emission of infrared radiation by an atmosphere warms a planet's surface." When do I get my brownie points?

Millenddoug

Greenhouse effects is a compilated subject to understand. The whole cycle/process is very confusing. I hope staring at the computer page will help me understand. With confidence, Tino

[edit] Clarification needed re effect vs greenhouse operation

From my reading, there is a lot of confusion out there because actual greenhouses do not in principle or practice actually employ the real greenhouse effect re reradiation of infrared. A one sentence clarification to this effect in the first para would be helpful imho. Otherwise it takes a lot of reading to get this important point. (Assuming I am not myself just as confused) —The preceding unsigned comment was added by 167.191.250.81 (talk) 18:45, August 20, 2007 (UTC)

[edit] Radiation transmitted by atmosphere

The absence of scales, or indeed a clear indication of what property of the atmospheric gases is being presented, renders this diagram confusing. Presumably the most relevant region is around 12 microns, corresponding roughly to the black body temperature of 288K, but 8-13microns is an atmospheric window, which why it is widely used by thermal imagers. The diagram of atmospheric absorption correctly shows that neither CO2 nor water vapour have much effect here, CO2 appears to absorb in the 12-20 micron region, whilst water vapour appears to absorb both above and below this region. Since there are no scales on the vertical axis it is difficult to conclude anything from this diagram. If these are extinction coefficients for the gases, then the diagram would indicate that water vapour would have a much greater effect than CO2, if they represent the proportion of absorption due to typical atmospheric concentrations of the two gases, then the effect of CO2 is spectacularly greater than that of water vapour. Could somebody please clarify this.

As presented, it would appear that as the surface temperature increases, the amount of energy in the CO2 absorption band reduces, whilst that in the water vapout absorption band increases, implying that water vapour becomes the more important greenhouse gas, which I suspect is not the case.Gordon Vigurs 20:48, 7 September 2007 (UTC)

Water vapor is already (by a large margin) the most significant contributor to the natural greenhouse effect. 76.231.189.193 18:05, 8 September 2007 (UTC)
You are correct, the graph is very misleading - as the surface temperature increases, the amount of energy in the CO2 absorption band also increases. But not as much as the energy in other bands.
The reason the Sun has no energy in that band is simply because it is a long way away ... and the graph shows the energy at the top of the atmosphere. Q Science 07:27, 15 September 2007 (UTC)

Mentioning of greenhouse effect on Venus is even more confusing. A surface temperature of about 450K corresponds to peak emission at about 6microns, where CO2 does not appear to have much effect. How then can this effect be attributable to the high CO2 concentration in the Venusian atmosphere?

Also the 'Rayleigh scattering' term. What particulate or droplet size is assumed? Is this some kind of average? If Rayleigh scattering is only significant below 1 micron, how can weather radar, operating typically at millimetric wavelengths, possibly work? Gordon Vigurs 17:55, 9 September 2007 (UTC)

The weather radar article gives a good explanation. Rayleigh scattering depends on the ratio of the wavelength to the particle size, not on the absolute value of either. Raymond Arritt 19:31, 9 September 2007 (UTC)
True, and the reason for my request for information regarding particulate size, but irrelevant to my criticism of this diagram, which is attenuation as a function of absolute wavelength. It shows no attenuation at microwave frequencies.Gordon Vigurs 06:49, 10 September 2007 (UTC)
On another note:
What is the chage in CO2 emission to outer space with respect to CO2 concentration. Why is change in absorbtion by CO2 discussed but change in emission is not? All things being equal, if we add CO2 to the atmosphere there will be a resulting increase in radiation absorption AND there will also be a resulting change in re-emission into outer space which offsets the absorption. Does emission increase or decrease as a function of CO2 concentration? If it increases, is its change less than the change in absorption? If not, ineasing CO2 concentration will tend to cool rather than heat the atmosphere. We know that greenhouse gas emission to outer space sometimes increases as a function of gas concentration: otherwise re-emission to outer space would now be zero. Why is this never discussed? Penurious (talk) 17:27, 28 April 2008 (UTC)
The atmosphere gets heated by the radiation from the earth. So the atmospere heats up and thus will also emit radiation. However, the atmosphere emits it's radiation in all directions, both up into space, and down towards the earth. So (this is the important part) half of the radiation emitted by the atmosphere is going down, back to the earth, again. So in effect the CO2 in the atmosphere works as a insulating blanket. If you double the amount of CO2 you basically add another blanket. This is a pretty crude simplification, but hopefully it can convince you that adding more CO2 would not cool down the earth. :) Apis (talk) 20:13, 28 April 2008 (UTC)

Let me try to explain again by going into gory detail. Peter Deitz at http://www.john-daly.com/forcing/moderr.htm demonstrates how in the IPCC TAR the change in global temperature "best guess" for doubleing of CO2 has been calculated using differential form of the Stefan-Boltzman law S = εσT**4, which he states as being:

                    dT/T = 1/4*dS/S
                    S is heat flow in watts/M**2
                    T is temperature in Kelvin
                    σ is a physical constant 5.6704*10e-8
                    ε is emissivity, a value between zero and 1.0

The technique is starts with assuming some dS=4.3w/m**2 (his reduced value) as a result of CO2 doubling and using S=240 w/m**2 and T=255K as typical values at the tropopause level we can come up with a dT of some 1.14K degrees of global warming (less than IPCC's).

The differential form is arrived at by taking the derivitive of S = εσT**4 with respect to T and dividing by the original equation.

                    dS    =    4εσT**3
                    ---        -------  
                    S           εσT**4

However this assumes that emissivity does not change along with temperature when CO2 is doubled. (This is my big issue here). If it does, we have to use the partial differential form of Stefan-Boltzman which is

                    dT/T = 1/4(*dS/S-dε/ε)

From this we see that if emissivity changes when CO2 doubling happens, temperature increase is lessened or even reversed. In fact it would take only a 2.0% change in emissivity to wipe out all the effect of the increased flow of watts into the atmosphere from CO2 doubling. Why would emissivity change? Because there are 2X as many CO2 molecules and each one is a heat transmitter as much as it is a receiver.

So besides capturing heat, greenhouse gases also cause the atmosphere to be able to cool off. Could it be that with the current state of CO2 saturation in the atmosphere, that point has been reached and passed where we are only cooling the atmosphere by adding CO2?Penurious (talk) 04:42, 29 April 2008 (UTC)

Agreed: increasing GHG's increase emission too. And there is a balance between this and more absorption. In broad terms, it is easy to see that going from no GHG to some GHG results in warming at the sfc. What happens in a real atmosphere with increments to GHGs is far more complex and needs a model William M. Connolley (talk) 07:26, 29 April 2008 (UTC)
Im a bit confused =S emissivity might change a bit with temperature, but if they considered it constant, I assume it probably is relatively constant in the temperature ranges involved (is there reason to believe it isn't?). I'm not convinced that emissivity of the gas would change because you double the amount of molecules though? And I would think that the energy emitted from earth to space match the energy of the sunlight absorbed due to equilibrium (i.e. with 30% reflection that would be about 0.7 * 1370 W/m² * πR²). The GHG in the atmosphere would affect the surface temperature but not the amount of energy emitted back into space? Maybe I'm missing the point, I'm certainly not qualified to judge if there is a mistake in one of the IPCC reports =) Maybe someone else can give a better answer, there are probably some expert around. Apis (talk) 15:06, 29 April 2008 (UTC)
Why does emissivity of the gas change when GHG is added? Because when more greenhouse gas is added the absorption of radiation is increased. This is the supposed basis of greenhouse effect. However according to Kirchhoff's law, the emissivity must equal the absorption; so emissivity increases also by the same amount. Isn't this just basic physics? All these discussions seem to ignore this.Penurious (talk) 13:31, 30 April 2008 (UTC)
Yes, ok, emissivity as far as I know is the ratio of absorption/emission compared to an ideal blackbody. So the emissivity for the gas mixture (atmosphere) would change if you add more co2. But the warming caused by the greenhouse gases is because ca 50% of the absorbed radiation (emitted from earth) is being re-emitted down towards earth again, thus the surface will end up at a higher temperature at equilibrium (the other 50% goes out into space). Adding more ghg gases would increase absorption, but 50% of what is absorbed will still be bounced back again, since that is now more radiation than before, the resulting temperature at the surface will be higher than before. The amount of radiation absorbed from the sun and re-emitted into space will be the same as long as there is an equilibrium (otherwise earths temperature would keep rising/falling). This assumes the atmosphere is a single layer, if you model it as many layers you would have to consider the radiation going back and forth between each. --Apis (talk) 14:44, 30 April 2008 (UTC)
Hope this makes sense, at least that's what I have learned. =) Apis (talk) 16:56, 30 April 2008 (UTC)
If your doubts are at that level, then you might find the basic analytically-solvable version helpful. Start with a black body with no atmos illuminated by SW radiation S. Work out its sfc T. Then add in a layer of "atmos" which is transparent to SW but grey, ie of emissivity e (0<=e<=1) in the LW (assume that the planet absorbs all SW and re-emits it as LW proportional to T^4. Then vary e and see what happens. You will find that sfc T depends on e (increases with e) William M. Connolley (talk) 22:15, 30 April 2008 (UTC)
OK I get it: increased GHG causes increase emission of LW radiation from the atmosphere to earth thus rasing the sfc T surface temperature of the earth. In a manner of speaking, the greenhouse effect of the atmosphere is to partially reflect LW radiation back to earth increasing its radiation input. Ulitimately with perfect GHG saturation (100% absorption of LW), the earth would aquire an equal amount of SW (sun) and LW (atmosphere) radiation input. Would you then say that Kirchhoff's law dictates that with respect to an element of atmosphere, the change in GHG alone (without change in radiation input) cannot change the temperature since emissivity always == absorptivity? And likewise would it be true that, considering radiation effects alone, the atmosphere can't heat up unless the sfc T of the earth does first? Penurious (talk) 15:49, 8 May 2008 (UTC)
Not perfectly sure what your question is. If we assume that the atmos is transparent to SW but grey in the LW, and that solar has no LW, then the radiation that warms the atmos comes from the planet. If the planet gets warmer, or the atmos gets more opaque, it warms. I'm not sure what you are saying re K. If e=1 in the simple one-level model, then adding more GHG makes no difference. If e<1, then increasing e warms the atmos if the solar radiation remains constant.
Its easiest to see this with a toy model. Assume we have solar radiation at S1=(1-a)S/4 per sq m (S/4 cos of the area of a sphere vs cross section, "a" is the albedo). Assume the planet radiates LW at rT^4. Assume a layer of atmos of temp U transparent to SW and of emissivity 0<=e<=1. Then the radiaton downwards at the sfc is S1 + erU^4. The radiation up is rT^4; hence rT^4=S1+erU^4. The radiative balance of the layer gives you 2erU^4=erT^4 hence 2U^4=T^4. Hence T^4=S1/(r(1-e/2)) if I haven't got my numbers wrong. Draw a picture, it will make more sense William M. Connolley (talk) 19:26, 1 May 2008 (UTC)


To a reasonable first approximation, radiative heating/cooling at a given height in the atmosphere depends on the rate of change of emissivity with height and the rate of change of temperature with height. I'm not aware of any really easy way to explain this, but you can get some insight by drawing a cartoon of a simplified atmosphere with only two layers. Raymond Arritt (talk) 15:36, 29 April 2008 (UTC)

[edit] Pure radiative and convective-radiative model

I don't understand the relationship between the pure radiative model (RM) and the radiative convective model (RCM). In the RCM the greenhouse warming effect seems to be dependent on the temperature decreasing with altitude. Let's assume the temperature was constant throughout the atmosphere. In that case while I can see why the altitude of IR emmisions to space would increase with more greenhouse gases it would seem that there would be no need for the temperature to increase to maintain the balance with incoming solar radiation.
On the other hand in the RM it seems to me that the altitude at which IR emmisions reach space is also going to increase with increasing greenhouse gases. Surely the IR opaqueness is a purely radiative property? And if this altitude increases then the surface temperature won't need to increase to balance incoming solar radiation.
I suppose my point is that in the article the RCM differs from the RM in two respects: (1) decreasing temperature with altitude and (2) IR opacity leading to higher altitude for IR emmissions to space. I assume (1) is the "convective" part but (2) seems like it should be part of the RM. But in that case the RM doesn't explain why an increase of GHGs should lead to warming. I'm sure I'm missing something here but I'd love to know what it is! Rickpoc (talk) 20:45, 10 May 2008 (UTC)

Not sure I understand the problem here. In a pure radiative model, the temperature isn't constant with height, it decreases (assuming a grey atmosphere). Its just a generalisation of the one-layer model. Its not very accurate, at least in part because it can end up with a temperature profile that is unstable: ie, would convect. All a "convective" model adds to a radiative model is to overturn and mix layers that would be unstable. As to why GHGs warms the atmos... does [1] help? William M. Connolley (talk) 19:44, 11 May 2008 (UTC)
I understand the individual explanations of each of the models as to why the GHGs warm the surface. It was the relation beween the models and the role of convection that was confusing me. If I understand you correctly the fact that temperature decreases with height does not depend on convection and would occur in the pure radiative model. Do increased GHGs increase the lapse rate in the pure radiative model? --Rickpoc (talk) 22:12, 17 May 2008 (UTC)
Consider the blanket. how does a blanket make you warmer? by increasing the temperature of the air contacting your skin. change "your" to "earth's", "skin" to "surface", and "blanket" to "GHG". Gzuckier (talk) 15:40, 12 May 2008 (UTC)
Accoding to this link (from the article) the blanket is a bad analogy since it warms by preventing convection. --Rickpoc (talk) 22:12, 17 May 2008 (UTC)

[edit] Real greenhouse

There seems to be at least 3 opinions (one is mine) arguing that the Real Greenhous chapter should be removed. Actually, the only source supporting the claim is one research by Woods at the beginning of 20th century, where is the source criticism in Wikipedia? Has anyone ever reproduced the research? If there is no other sources supporting the claim that real greenhous is not based on greenhouse effect, I strongly agree that the chapter should be deleted. Wikipedia should not show insecure speculations as truth.

If no new references is found in a week, I will delete the chapter. Ok?

212.50.144.26 (talk) 14:24, 15 May 2008 (UTC)

No. This has been discussed many times before and even is supported by an arbitration committee opinion. Raymond Arritt (talk) 15:00, 15 May 2008 (UTC)
(ec) Hmm, I don't think this is controversial (i.e. it's more or less commonly accepted) I'll add another source though.
– Apis (talk) 15:10, 15 May 2008 (UTC)

ok.. as you wish. However, I consider this not proven (show me the facts!) and so it should not be expressed as a known fact. As long as there can be found no links to real research, it is a belief of some people. I assume real greenhouse warms partly from preventing convection, but I would not say preventing radiation is meaningless. Soil at 27 degrees radiates at 460 W/m2. I'm truly disappointed at wikipedia unless 1) the claim that thermal radiation in real greenhouse is meaningless is proven or 2) claim is removed or told as 'some people think radiation is meaningless while some think it is not'

btw, if it has been discussed many times, doesn't that prove it IS controversial, at least as long as someone actually proves it on either way? —Preceding unsigned comment added by 82.128.226.51 (talk) 19:44, 26 May 2008 (UTC)

Oh, one thing: please links that are actual research, not a reference to another reference etc. like this: http://www.pubhort.org/actahort/books/456/456_45.htm —Preceding unsigned comment added by 82.128.226.51 (talk) 19:52, 26 May 2008 (UTC)

The real greenhouse has to be based on that sun warms up the soil and greenhouse slows the loss of heat. It slows the loss down by ALL of the following: 1. prevent convection (and change forced covection to unforced if it is blowing) 2. prevent conduction 3. prevent radiation. The question is if one or more of these are major factors compared to the others. I have not seen any calculations proving it. —Preceding unsigned comment added by 82.128.226.51 (talk) 20:09, 26 May 2008 (UTC)

"This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably". Yes, also my home temperature will drop considerably if I open a window. That doesn't mean I'm warming my home only by preventing convection. I still need to buy electricity or burn some wood. —Preceding unsigned comment added by 82.128.226.51 (talk) 20:15, 26 May 2008 (UTC)

If you open a window, your house would cool by more than the amount you'd expect if preventing IR radiation were the primary function of the window. Ditto GH's. The alternative explanation, physically blocking the motion of air, is correct William M. Connolley (talk) 22:02, 26 May 2008 (UTC)

I missed all this talk. Perhaps the reason that the Woods work hasn't been re-done is that its correct? That the work is old is quite irrelevant William M. Connolley (talk) 22:02, 26 May 2008 (UTC)

Consider a thermos: if you remove the lid the contents will cool much faster than with the lid on. Consider a thermos with only hot air inside, what would happen if you remove the lid?
Anyway, the experiment mentioned in the reference in the article [2] is pretty straightforward, it could be fun (and affordable) to repeat yourself!
As for warming your house: preventing convection doesn't warm your house at all? it only prevents it from cooling off quickly.
— Apis (talk) 01:18, 27 May 2008 (UTC)

Still, I consider this not proven. See http://en.wikipedia.org/wiki/Reproducibility. See also http://en.wikipedia.org/wiki/Pseudoscience. It is clear that the greenhouse actually warms up by sunshine. Why it is warmer than surroundings? It simply keeps the heat inside. And it does this by slowing ALL mentioned heat transfer mechanics. If ANY of the heat transfer mechanics is not blocked, the greenhouse would be almost as cold as the surrounding. Correct in the article should be "Only part of real greenhouse warming is based on greenhouse effect: preventing other heat transfer mechanics (convection, coduction) plays also significant role." So basicly greenhouse is warmed by SUN, keeping the heat inside prevents it from COOLING, just as Apis mentioned. But just like in a house; actual heat origin is something else than insulation.

Also cold fusion was thought to be correct until it was re-done: http://en.wikipedia.org/wiki/Cold_fusion Saying something is correct does not make it the truth. Prove it. —Preceding unsigned comment added by 212.50.144.26 (talk) 07:27, 27 May 2008 (UTC)

There are many reliable sources saying the primary effect at work in a greenhouse is preventing convection. I don't see the article saying that the greenhouse effect has no impact on real greenhouses at all, only (indirectly) that it's insignificant compared to preventing convection. Maybe the wording could be improved in some places though.
—Apis (talk) 20:05, 28 May 2008 (UTC)

Hmm.. actually reference nr 23 emphasizes the effect on radiation properties of the greenhouse, read ahead from page 7. But what is the significance of the different thermal losses? I have not yet seen any reliable calculations or research (other than mr Wood, but that is a single research, not replicated AFAIK) anywhere that would prove any of them is more significant than the others. As long as that kind of proof has not been found, subject should be considered unknown. There are a lot of urban legends that are thought to be true just because they have been repeated enought. —Preceding unsigned comment added by 82.128.226.51 (talk) 21:03, 28 May 2008 (UTC) One more thing; preventing convection is important in any structure that is warmed up (or cooled down) in any way. Would it be of any use to warm a house without any walls? No, but that does not mean that warming a house is based on preventing convection. —Preceding unsigned comment added by 82.128.226.51 (talk) 21:13, 28 May 2008 (UTC)

Alright, I checked the Real greenhouses section and nothing in it is contradicted by [23]. (Of course radiation properties are important for greenhouse coverings!)
The urban legend here is that the greenhouse effect is what's keeping greenhouses warm.
Again, there are many sources confirming this. We don't do any research ourself here, only edit an encyclopedia, so we don't need more proof than that most scientists say so. If you want to dispute this then you will have to come up with something credible that says otherwise.
And again, the warming is not done by preventing convection, in a greenhouse the warming is done by the sun (although some also have additional means of heating). The same is true for a house, the warming is not done by preventing convection but typically by radiators. Preventing convection is what keeps the structure from cooling down.
If you build a greenhouse out of an IR-black material and one of a IR-transparent the heat loss difference would be insignificant compared to not preventing convection. (And [23] actually says that radiation loss increase with increased emissivity.)
If you doubt this then I really do think it would be a fun experiment to test yourself (seriously). Don't know how to get hold of a sheet of rock salt though.
—Apis (talk) 05:44, 29 May 2008 (UTC)

Well, I would say there is 2 urban legends here: One saying that greenhouse is based on greenhouse effect and the other saying greenhouse is based on preventing convection ;-). I would not say that radiation prevention is meaningless; I leave in north and here radiation loss is easily seen for example when temperature is near freezing point; all horizontal levels that are not under roof (protected from heat radiating to space) are often frozen in the morning. I think preventing radiation is more important on dry environments (less atmosphere greenhouse effect) and in night. About the Wood experiment: Replicating the experiment is difficult, because the description is inadequate. For example, there are no physical dimensions mentioned on the experiment. What is the volume of his greenhouse? Also, if I understood correctly, in his greenhouse there was only one wall (?) of glass/rock salt. Real greenhouse size is metres and there is 5 walls radiating the heat away (one up to space). He did his experiment only on sunlight; in real life half of the time is night when sun is not shining. How much heat reserving mass was on his greenhouse? What was around the enclosures, open space or was it in a house?

Calculating the heat transfer by radiation is quite simple using Stefan-Bolzmann law: Approximate heat loss by radiation is at 27 degrees about 460 W/m2. Of cource, the environment radiates back, so the net loss depends on temperature difference. Quick calculations tells that loss is about 6W/m2/K. So if the greenhouse blocks the radiation and the temperature difference is 1 degree, it saves heat (virtually heating) at 6W/m2. With temperature difference of 10 degrees, saving is 60W/m2. But what is the back radiation temperature from atmoshpere to ground? The temperature changes from -60 degrees in 10 km to ~20 degrees in ground? I would say that without blocking radiation, the greenhouse will be about the same temperature than the environment in the morning. So can we say greenhouse effect is insignificant on greenhouse?

Calculating convection savings is quite difficult; formula is simple, but finding the correct h (convection heat-transfer coefficient) seems to be difficult. But of course, also convection prevention is essential. However, using walls to prevent convection is obvious in any heated space! —Preceding unsigned comment added by 82.128.226.51 (talk) 10:24, 29 May 2008 (UTC)

What you say about his experiment is true, there are some uncertainties, that's partly why it would be fun to try I think. I would presume the size of the rock salt sheet one can obtain will determine the size of the boxes. :)
Yes, preventing radiation loss is important in a greenhouse, especially during night, but what has that to do with the greenhouse effect in this case? In a typical greenhouse the ir-black surface inside the greenhouse (the ground and plants) usually have smaller surface area than the greenhouse itself, so if the greenhouse is ir-black that would mean larger radiation loss to the surroundings? How much the surroundings radiate back is kind of hard to say without knowing the exact geometry and radiative properties of it.
—Apis (talk) 11:10, 30 May 2008 (UTC)

[edit] Automatic archiving

Is it OK to add automatic archiving to this page? there seems to be a discussions from 2006 and such lying around, not being useful...
– Apis (talk) 16:25, 15 May 2008 (UTC)

Automatic archiving added.
— Apis (talk) 00:59, 18 May 2008 (UTC)

I am not at all happy about the automatic archiving. I might accept it if the archives were directly accessible. What is no longer visible is vital to the subject and its absence might give the impression of a nonexistent consensus. If the archivist would be so kind as to undo the change directly I should be happy but in the meanwhile I am trying to find out how to do this. --Damorbel (talk) 07:32, 22 May 2008 (UTC)

I have replied to this at Wikipedia:New contributors' help page#Automatic Archiving: "Greenhouse Effect". PrimeHunter (talk) 12:21, 22 May 2008 (UTC)

[edit] Intro sentence

Raymond; are you seriously contending 'the atmosphere emits IR that warms the surface'? Later, sure. But not first. If anything, it re-warms the surface by re-emitting IR. You're probably refering to "Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the 'greenhouse effect'." on the greenhouse gas page.

That's wrong, it's both, as the caption says: "The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect."

Or better yet, dictionary definitions:

"an atmospheric heating phenomenon, caused by short-wave solar radiation being readily transmitted inward through the earth's atmosphere but longer-wavelength heat radiation less readily transmitted outward, owing to its absorption by atmospheric carbon dioxide, water vapor, methane, and other gases"

"The phenomenon whereby the earth's atmosphere traps solar radiation, caused by the presence in the atmosphere of gases such as carbon dioxide, water vapor, and methane that allow incoming sunlight to pass through but absorb heat radiated back from the earth's surface. "

"warming that results when solar radiation is trapped by the atmosphere; caused by atmospheric gases that allow sunshine to pass through but absorb heat that is radiated back from the warmed surface of the earth"

Sln3412 (talk) 18:00, 20 May 2008 (UTC)

The core problem is that we're tripping over ourselves by trying to explain a complex process in a short and pithy sentence. To begin with, we shouldn't be treating absorption and emission as separate processes (as implied for example by Kirchhoff's law). It doesn't help that the dictionary definitions make this specious distinction. I'll post some more thoughts on this later. Raymond Arritt (talk) 18:16, 20 May 2008 (UTC)
Fair enough, but that is what happens, shortwave in warms ground, longwave out warms air, that radiation in the air goes up and down. Just saying the air warms the ground seems at least incomplete. If the GHG weren't there to catch the thermal and move it around, the atmosphere wouldn't heat from it in the first place.
I suppose it depends what you're trying to say, if you're at a macro level or a micro level, perhaps.
Also, why is the analogy "incorrect"? They both stay warmer, that's the "effect". It is an analogy after all. The mechanism being different doesn't make it wrong. But perhaps that's just quibbling.
Sln3412 (talk) 18:36, 20 May 2008 (UTC)
If the GHG weren't there to catch the thermal and move it around, the atmosphere wouldn't heat from it in the first place. That's a common misconception. A substantial part of the atmospheric heating comes from sensible and latent heat transfer from the surface. The problem isn't as simple as the atmosphere absorbing LW radiant energy emitted from the surface and radiating the same energy back to the surface. Raymond Arritt (talk) 19:07, 20 May 2008 (UTC)
I think that "the GHE makes the planet warmer because the planet is heated both by the sun and radiation from the atmos" is by far the clearest, simplest and easiest to understand description. It throws away all the unhelpful "blanket" analogies William M. Connolley (talk) 19:48, 20 May 2008 (UTC)
I meant if the GHG weren't there, the heating from their interaction with thermal IR wouldn't be there. Not that it's the entirety of what's happening.
Right now, "trying to explain a complex process in a short and pithy sentence" there are at least 3 places with it differently: "the process in which the emission of infrared radiation by the atmosphere warms a planet's surface" "The ability of the atmosphere to capture and recycle energy emitted by the Earth surface", and the effect from "the gases present in the atmosphere which reduce the loss of heat into space". All that's being done is some of the 'heat' trying to escape is being sent back down. How hard is it to explain that at a 50,000 foot level.
Why not just use something basically how the IPCC explains it "Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect. The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth’s greenhouse effect warms the surface of the planet."
Sln3412 (talk) 20:36, 20 May 2008 (UTC)
Just as another data point, the AMS Glossary of Meteorology speaks thusly: "The heating effect exerted by the atmosphere upon the earth because certain trace gases in the atmosphere (water vapor, carbon dioxide, etc.) absorb and reemit infrared radiation." William's explanation is essentially the same but is cleaner and goes straight to the practical effect. Raymond Arritt (talk) 21:37, 20 May 2008 (UTC)

(un-indent) It would be good I think for the intro to state simply the GHE itself is the atmosphere sending radition from the surface back down, reducing radiation loss to space, which is loosely analogous in effect to a real greenhouse blocking convection. The emission of IR down that the GHG got from absorbing it coming up keeping things warmer. The main point seems to be the reduction of radiation loss to space. Sln3412 (talk) 00:00, 21 May 2008 (UTC)

Hmm, Correct me if I'm wrong, but that's not entirely accurate either I think, radiation loss to space is equal to what is absorbed from the sun. (When the temperature has reached an equilibrium at least.) The GHE effect changes the equilibrium temperature at the surface. What William said is probably the most compact way to say it. It feels like it's leaving out many important parts of the puzzle though, but thats probably inevitable if one is to summarize it into a short lead section. :S
— Apis (talk) 01:08, 21 May 2008 (UTC)
Yes, it changes the equilibrium, although I'd say level rather than temperature, although both are involved. The point being that shortwave IR in from the sun heats the ground, longwave IR from the ground is absorbed by the GHG, and what is emitted back downwards by the GHG keeps the atmosphere warmer than it would be. This downward emission is the GHE.Sln3412 (talk) 01:51, 21 May 2008 (UTC)

I am a bit perplexed why the introduction doesn't make it clear this IR from the atmosphere that re-warms the ground is the longwave IR that comes from the ground in the first place. I think it is very confusing for the first time reader to just read it's from the atmosphere, it makes it appear as if that was the original source. The start should be a simplified overview of the process/concept as in the graphic even if it's it's not entirely correct except maybe on an abstract level. The details are in the body of the article anyway. Sln3412 (talk) 22:41, 21 May 2008 (UTC)

How about something like this?
The greenhouse effect is the process in which the emission of infrared radiation by an atmosphere that comes from a planet's surface rewarms that surface rather than releasing it to space. The name comes from a loose analogy with the temperature difference due to the capture of air inside a greenhouse compared to the mixed air in the atmosphere outside the greenhouse. The greenhouse effect was discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896.[1]
--Sln3412 (talk) 20:58, 27 May 2008 (UTC)
Its not a loose analogy, its an incorrect analogy. And "atmosphere that comes from a planet's surface" sounds odd William M. Connolley (talk) 22:06, 27 May 2008 (UTC)
The IPCC explains it as an analogy. So. The analogy can only be incorrect if the two are not similar in any way. Although that's probably just a matter of interpreting what the word means, since we know both keep things warmer than they otherwise would be -- it would be a false analogy to claim they operate in the same way, yes. But this is not an inductive argument fallacy! It's an analogy to what they do, not how they do it.
Analogy: Similarity in some respects between things that are otherwise dissimilar. A comparison based on such similarity. See synonyms at likeness.
An agreement or similarity in some particulars between things otherwise different; sleep and death, for example, are analogous in that they both share a lack of animation and a recumbent posture.
But yes, that wording about the IR is rather clunky, that's why I said "something like". It's IR from a planet's surface heating the air which then (among other things) reheats the surface. The IR, that went up from the ground, comes back down. In a manner of speaking.
--Sln3412 (talk) 17:58, 28 May 2008 (UTC)
As Raymond said above, it's not as simple as IR from the surface heats the atmosphere, a lot of that heating is not from radiation but is caused by convection for example or as Raymond said "A substantial part of the atmospheric heating comes from sensible and latent heat transfer from the surface". What you are trying to describe is a very simplified radiative model. Such a model is described in the article, but I think it's unnecessary (and difficult to incorporate) in the lead section, and it could be misleading. I agree that the current bit about clouds heating the surface feels rather unsatisfying, but it appears to be correct at least. I don't see why the suggested change would be better? (And it would at least have to be something like "The atmospere absorbs heat transfered from the surface" (I think that's right at least?)).
—Apis (talk) 18:56, 28 May 2008 (UTC)
Since the greenhouse effect is IR (thermal radiation) being emitted from the atmosphere downward re-warming the surface (slowing down the thermal IR transfer from the surface or however you want to phrase it), the other heat transfer mechanisms of the surface seem immaterial. That's why I'm ignoring them, the greenhouse effect is the IR part only, isn't that correct? If not, then it's even more confusing to say "The greenhouse effect is the process in which the emission of infrared radiation by the atmosphere warms a planet's surface."--Sln3412 (talk) 23:06, 28 May 2008 (UTC)

(dedent)

  1. "the other heat transfer mechanisms of the surface seem immaterial" – No.
  2. "the greenhouse effect is the IR part only, isn't that correct?" – No.
  3. "If not, then it's even more confusing to say ..." – No, I'm guessing the only heat transfer downwards is done by radiation.


—Apis (talk) 05:59, 29 May 2008 (UTC)

FWIW the IPCC says radiative and by greenhouse gases and thermal IR.
As far as the intro. Surely it can't be that difficult to explain by overview that sunlight comes down and heats the air and surface, the surface releases this and it heats the air, and the air re-heats the ground, impeding radiation loss, like a greenhouse impedes convective cooling. That is the analogy; they both trap heat so you get the same effect. The fact that real greenhouses trap heat by impeding convection rather than radiation seems immaterial to the point.
I suggest using the AR4 WGI glossary (page 946, Annex 1 at http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Annexes.pdf) or a paraphrasing.

Greenhouse gases effectively absorb thermal infrared radiation, emitted by the Earth’s surface, by the atmosphere itself due to the same gases, and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system. This is called the greenhouse effect.

--Sln3412 (talk) 15:30, 5 June 2008 (UTC)

[edit] Wien's displacement law in Basic Mechanism

It seems the Basic Mechanism section could benefit from a reference to Wien's displacement law, which plays a key role in atmospheric "one-way transparency" of solar radiation. By Wien's law, incoming solar radiation peaks in the visible part of the spectrum around 500 nanometers, corresponding with the sun's surface temperature of 5780 K. In contrast, the earth re-radiates energy in the far infrared part of the spectrum around 10 micrometers, corresponding to the planet's surface temperature of 287 K. The atmosphere, including greenhouse gasses, allow the relatively short-wave incoming radiation to pass through efficiently, while co-called greenhouse gasses interfere with the re-radiation of long-wave infrared waves back into space.

A good definition of the greenhouse effect that emphasizes this concept is given by Lueddecke, Pinter, and McManus in the Journal of Geoscience Education, v.49, n.3, May, 2001, p. 274-279, as, "asymmetry of the atmosphere, in which carbon dioxide, water vapor, and other gases are more transparent to in-coming solar radiation than they are to out-going radiation of heat, thereby keeping the Earth warmer than it would otherwise be."

A source that covers the roles of both Wien's law and the Stefan-Boltzmann law in greenhouse warming is at http://www.downbound.com/Greenhouse_Effect_s/322.htm.

Pradtke (talk) 20:48, 31 May 2008 (UTC)

[edit] Should be marked as "Has not been proofed"

78.129.96.207 (talk) 21:28, 2 June 2008 (UTC) Nobody has so far shown any substantial experiment to proofe this effect of CO2. Hence, I would mark this article Fiction.

I'm sorry to tell you that the basics of the greenhouse effect (and CO2's effect), can be shown with a simple highschool experiment. So no, it shouldn't be marked that way. --Kim D. Petersen (talk) 21:32, 2 June 2008 (UTC)