Talk:Greenhouse effect

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/Archive 1 (the Great Edit War; note that this page has an arbcomm decision associated with it: see [1])

[edit] Inconsistant

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] Re-write

I saw a really clear and precise explanation of the greenhouse effect at a website. But I hesitate to rely on it without some checking from the Anome or Axelboldt or anyone else who really knows the science.

of all the sunlight that passes through the atmosphere annually, only 51 % is available at the Earth's surface to do work. This energy is used to heat the Earth's surface and lower atmosphere, melt and evaporate water, and run photosynthesis in plants. Of the other 49 %, 4 % is reflected back to space by the Earth's surface, 26 % is reflected to space by clouds and atmospheric particles, and 19 % is absorbed by atmospheric gases, particles and clouds. [2]

(William M. Connolley 22:07, 8 Sep 2003 (UTC)) Yes. But this isn't a description of the GH effect. Its a picture of how solar radiation is distributed through the atmos. But I've used it in my re-write.

I like the percentage breakdowns [But they are illustrative only: they would vary strongly in space and time. WMC]. The mention of clouds is especially interesting, because one theory is that higher temperatures will evaporate more seawater and increase cloud cover. This would increase the percentage of solar radiation reflected by clouds and decrease the percentage that hits the earth's surface. Could this be a self-regulating effect of the atmosphere? Does the greenhouse effect come with a "thermostat"?

The percentage breakdowns are generally misleading because different numbers (for water vapor for example) are listed from different sources, but these different sources use different definitions -- these differences are almost entirely semantic, and yet the text gives the impression of great uncertainty on how to compute radiative transfer. In fact, there is almost no uncertainty in radiative transfer. I have added a sentence to this effect as a placeholder Climateguru

Maybe it depends on how much warming is causd by trace gases, and how much cooling is caused by increased cloud cover. Has anyone studied that? (How about the IPCC, since they're such highly regarded experts?) --Uncle Ed 17:07, 8 Sep 2003 (UTC)

(William M. Connolley 22:07, 8 Sep 2003 (UTC)) I have done a major re-write of the how-GH-effect works section, using the model I like, from the "bad greenhouse" page. This has now relegated the former explanatory link to a terrible example of how to get it wrong. Sorry. The text is somewhat untidy and perhaps over-pernicity in places, I look forward to its evolution.

The first paragraph currently reads:

The clearest and most direct way to understand the "greenhouse effect" is to see that the surface of the earth is warmed by two sources: the sun, and the atmosphere [1]. It is thus warmer than it would be in the absence of the atmosphere.

I think this description is so simplified that it doesn't explain what the greenhouse effect is. If the atmosphere reflected most light at the sun's peak frequencies, but was mostly transparent to infra-red radiation corresponding to the surface temperature, it the earth would still be warmed by the same two sources. But in this case the surface wouldn't be warmer than it would be in the absence of the atmosphere, would it?

Matthew Woodcraft

(William M. Connolley 13:38, 8 Nov 2003 (UTC)) I wrote that para, in contrast to the wurbley stuff about "insulation" that was there before.

I don't understand your example. If the atmos reflected most solar, then there would be little heat input to the system, and the whole thing would cool. If the atmos was mostly transparent to IR, then it wouldn't emit in the IR, so it wouldn't heat the earth.

So I think the first para is OK, though simplified (it doesn't explain how the atmos comes to be warm, for example).

Well, the first paragraph currently states (by saying thus) that an earth with an atmosphere will necessarily be warmer than one without an atmosphere. And that's just false, isn't it?

Well, it could be, if the atmosphere were entirely transparent to radiation. But in an atmosphere roughly resembling ours (largely transparent to viz (and thus heated from the bottom), somewhat opaque to IR) its true. This is currently explained in para 4 at the moment. Why do you think its so false?

Because by saying The clearest and most direct way to understand the "greenhouse effect" is..., it suggests that it's giving a sufficient explanation. But the effect relies on particular properties of the atmosphere which aren't obvious, and aren't mentioned in the proposed explanation.

It also seems to me that without explaining how the atmosphere gets to be warm, it doesn't really count as a description of the greenhouse effect. Perhaps there isn't room for a description in the first paragraph. How about opening the article by just saying that the greenhouse effect is the name of the phenomenon by which the atmosphere warms the earth, and leave explanation for later? Matthew Woodcraft

You're welcome to try to improve it. The reason the actual physics is up there at the front is that its so common to see dubious non-physical explanations up front. I wouldn't want to see the explanation buried.

Ok, I've had a go. Matthew Woodcraft

(William M. Connolley 12:42, 9 Nov 2003 (UTC)) I quite liked MW's new very simple first para, which on reflection was nice and accurate. I've changed Mav's non-earth-centric stuff somewhat, so that it notes other planets but then only talk of earth. Because: all the rest of the article is about earth; and I don't think you can be completely general anyway (imagine the case of a star so dim it mostly radiated in IR, for example; or a planet whose atmospheric composition was v different). Oh, and lower down, I've sneaked in a change from global warming hypothesis to global warming, because gwh is currently a rather poor page.

The fact that this article concentrates on the Earth is a major disservice to the reader. The greenhouse effect is a general phenomenon and should be discussed as such with examples given from (and comparing/contrasting) the present and past effects on the Earth, Mars, Venus, other astronomical objects (such as moons) and laboratory settings (not to mention the effect in actual greenhouses!). So that is what should be concentrated on in regards to expanding this article. When the article then gets large enough, then the Earth-specific stuff can and should be summarized here and moved to a daughter article. --mav 23:24, 9 Nov 2003 (UTC)

Well, OK. But I know nothing about GH on other planets, so I'll leave that to those who do!

The little I know about the "greenhouse effect" is that having an atmosphere on a planet helps insulate it. So it tends to (a) avoid rapid fluctuation of temperature as it rotates every day and (b) is quite a bit warmer than it would be without atmosphere. Venus and Earth are the main examples, although science fiction writer Robert A. Heinlein extended the concept to Ganymede for Farmer in the Sky.

I don't care how much or little we generalize, although I'm inclined to follow mav's suggestion just because I trust his judgment. The main thing is that the greenhouse effect be based on known facts; and that what is known about this effect not be confused with speculation about the "runaway greenhouse effect" predicted by some pro-Kyoto environmentalists. --Uncle Ed 16:51, 10 Nov 2003 (UTC)

WMC commented in History: "60-70 from ipcc '90 (would be better from TAR)". Good idea, please do insert the water vapor percentage from TAR. (SEWilco 09:57, 13 Dec 2003 (UTC))

[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] New Articles: Global climate change and Climate forcings

Please read the new articles and consider commenting on them and/or moving some material to either one. Note that climate forcings is not specific to global climate forcings, so if it makes sense to create a separate section please do.

I hope this helps get this part of Wikipedia sorted out.

Posted to all discussion pages listed in the "See Also" section of global climate change. --Ben 03:48, 2 Dec 2004 (UTC)

[edit] Lindzen quote

Removed the following:

For a theoretical case if no other greenhouse gases were in the atmosphere, Richard Lindzen estimated 98% (Global warming: the origin and nature of the alleged scientific consensus. Regulation, Spring 1992 issue, 87-98 [3]).

as it is an unsupported quote from a Business & Govt publication. Seems rather an irrelevant statement. -Vsmith 16:24, 10 Apr 2005 (UTC)

For the estimate of what percentage greenhouse effect would remain if carbon dioxide were removed, I've seen numbers of 85%, 88%, 90%, 94%, 95%, and 98%. For the 98%, I think Lindzen may include atmospheric convection feedbacks, but I'm not sure. Is the 98% an extremum? Yeah, definitely. It's on the edge of the numbers I've seen, in the list there. 5.8 degrees of predicted warming is also an extremum outside of the mainstream calculations, but we include that too. The NPOV policy instructs us to include prominent information that can be attributed to prominent groups or individuals, and this certainly qualifies. — Cortonin | Talk 18:56, 10 Apr 2005 (UTC)

I modified the tone slightly, since it's not clear precisely how he calculates this number, and I presented the atmospheric convection feedback alluded to by the surrounding text in the reference. It is a frequently quoted number, so its absence will only warrant its inclusion later. — Cortonin | Talk 19:31, 10 Apr 2005 (UTC)

To obtain this estimate, he may have included various theorized atmospheric feedbacks. and he may have just pulled it out of his hat - seems a bit odd to add our interpretations or suppositions (isn't that akin to orig. research?). I don't think the NPOV policy means that we must include all prominent info no matter how questionable or irrelevant. To me this extremum is both questionable and irrelevant as we don't have direct info on how the number was derived or imagined. Therefore, I say begone with the meaningless tidbit. Vsmith 02:56, 11 Apr 2005 (UTC)

(William M. Connolley 08:59, 11 Apr 2005 (UTC)) I too say begone with it. For one thing, For a theoretical case if no other greenhouse gases were in the atmosphere is my words, not his - and I realised they were wrong. If anyone wants the 98% value in, it needs to be (a) properly sourced (to science, not Cato) (b) correctly described.

Here is a little more reading. [4] (SEWilco 20:29, 11 Apr 2005 (UTC))

"4. CO2 contribution to the ~33°C natural greenhouse effect
- Lindzen: "Even if all other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 percent of the current greenhouse effect." Cato Review, Spring issue, 87-98, 1992; "If all CO2 were removed from the atmosphere, water vapor and clouds would still provide almost all of the present greenhouse effect." Res. Explor. 9, 191-200, 1993.
- Lacis and Hansen: removing CO2, with water vapor kept fixed, would cool the Earth 5-10°C; removing CO2 and trace gases with water vapor allowed to respond would remove most of the natural greenhouse effect."

(William M. Connolley 21:17, 11 Apr 2005 (UTC)) Thanks. OK, so for Lindzen, thats the same non-sourced thing again (Cato), except it is clear now that he includes clouds rather than just GHGs (Res Explr, whatever that is: who is the author?). So its now quite clear that asserting that WV causes 98% of the GHE is wrong, even from Lindzens numbers. But we don't know what Res Explor actually said or who wrote it.

the icecaps are going to melt and we are going to die in short talk

[edit] Yet more tedious septic nonsense

yet another anon added:

The greenhouse effect due to Carbon Dioxide is in dispute and any claims that atmospheric H₂O contributes less than 95% to the greenhouse effect should be suspect. The actual effect of carbon dioxide is closer to 1-2% otherwise Mars' mean temperature would be substantially warmer due to having 13 times as much Carbon Dioxide by mass than Earth. Mars has an atmosphere containing over 95% CO₂ while Earth has roughly 1/30th of 1% CO₂ (ImYourDragon@aol.com)>>

This is the same old tedious nonsense so I've removed it. Notice the characteristic lack of sources; unlike the figures already in the article... William M. Connolley 22:03, 23 December 2005 (UTC).

05:46, 22 March 2006 (UTC)~An Accurate CO2 Percentage?05:46, 22 March 2006 (UTC)~

Sorry, I am not sure how to fully do this, but:

Water Vapor has an IR absorbtion rate of 1, CO2 is also 1, where are Methane has an absortion rate of 32. According to the IPCC.

Water Vapor makes up about 1% of the atmosphere, where as CO2 makes up .03768% (according to Wikipedia.) While I realize the amount of water varies from place to place, the difference is alot closer to 95% than 60%.

I don't think CO2's contribution of 21% to the greenhouse affect is anywhere close to correct. It should be closer to 3.2%. Have a reference?

(Josh Coray)

[edit] Runaway?

The article says...

The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback.

I've never felt very happy with this. Venus has a large GHE because it has a deep atmos with lots of CO2 in it. runaway doesn't obviously have much meaning. And the last sentence implies change but AFAIK nothing much there is changing. William M. Connolley 21:21, 7 January 2006 (UTC).

Maybe add that Venus also has 90 times the atmosphereic mass, of which 96.5% is CO2, and that it receives about twice as much energy meter as the earth? Just for a reference. That might help put it in perspective.

Josh Coray

OK there were some misconceptions in the original framing of the paragraph. The runaway phase is now pretty much over - most of the CO2 has been outgassed from the crust and the water vapour lost. The runaway greenhouse theory is pretty mainstream as an explanation of the difference between the atmospheres of Earth and Venus - it is not about what is happening but how it got wher eit is. Unfortuately, the distinction between the current situation and the process leading to it is sometimes lost in popular science. I've tried to improve it but I think this could be further improved - another thing to add to my list (although I'll have to find some better references than I could by googling).--NHSavage 21:29, 13 May 2006 (UTC)

I'm dubious about closeness-to-sun mattering much (given the high albedo) William M. Connolley 22:14, 13 May 2006 (UTC)

As I say, I need to do more research on this one but the conventional argument is that this is about the goldilocks effect, (Mars is too cold, Venus too hot and Earth just right...) or the Habitable zone and Venus is just too close to the sun to be able to retain its water vapour. I have some questions about this given that the articles seem to argue that the ozone layer is important to prevent UV dissociating water but we did not have an ozone layer until we had oxygen from plants. Still AFAIK it is really not controversial, although the timescale over which venus lost its water and whether there was liquid water for any length of time on Venus seems to still be the subject of active research. The whole idea probably deserves its own article which I might do sometime....--NHSavage 11:53, 14 May 2006 (UTC)

OOps - sorry, just re-read the article and the bit about closeness to the sun is not in the runaway section is it. I think my remarks above are not wrong just off the point. OK - yes I agree that the albedo will also be important. I suppose the question is - if you have a planet with the same orbit as Earth and the same amount of CO2 in its atmosphere as Venus how hot would it be? It would obviously be somewhat cooler than Venus but I don't have the tools to do a calculation as to how hot. I would guess it would be pretty deadly though (apart from the pressure and CO2 themseleves which alone are deadly...)--NHSavage 12:13, 14 May 2006 (UTC)

Closeness to the Sun does matter for Venus, combined with its weaker gravity. This means water vapor rises higher into the upper atmosphere where the Sun's UV radiation breaks it into atomic hydrogen and oxygen. The Hydrogen then escapes into space. This is why Venus no longer has any oceans. This doesn't happen on Earth. (I got this from Scientific American, but I don't know the issue.) -- MiguelMunoz

[edit] too narrow a definition?

My understanding has been that if a car's windows are left closed on a hot sunny summer day and the interior of the car heats up, or if the interior of an actual greenhouse warms up in the same way, those are instances of the greenhouse effect (and indeed, the latter example is why it's called the greenhouse effect). But the first paragraph of this article seems to say it's the greenhouse effect only if it's applied to the planet as a whole. Michael Hardy 01:43, 4 March 2006 (UTC)

See the section Real greenhouses at the end of the article. Also if you like old controversy check out the edit wars of last spring - it was a bit tedious. :-) Vsmith 03:29, 4 March 2006 (UTC)

05:57, 22 March 2006 (UTC)24.17.159.58 05:57, 22 March 2006 (UTC)Just a note05:57, 22 March 2006 (UTC)05:57, 22 March 2006 (UTC)

[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] Image:Spectrum of blue sky.gif

Removed Image:Spectrum of blue sky.gif as it was redundant with image already in the article. Initially mis-read the caption. Vsmith 13:17, 13 May 2006 (UTC)

[edit] links

website at bottom is incorrect

what cryptic message is this, come upon us as from the oracle, to vex our minds with contrary incomprehensions? Gzuckier 19:48, 31 May 2006 (UTC)

[edit] Style change in introduction feels confrontational

I just read the introduction, and I think there is a style change that spoils the flow of the article. The first few sentences come across as you would expect for an encyclopedic article on a scientific subject, being dispassionate and stating or explaining facts. The change comes with: "No-one disputes the former, or its magnitude; the latter is accepted by a large majority of scientists, although there is some dispute as to its magnitude (see scientific opinion on climate change and attribution of recent climate change)." - this sounds like what it is, an attempt to forestall editors who might want to tweak the wording of this article, or turn it into a diatribe on global warming. This was probably done with the best of intentions, but as it stands, the wording sticks out like a sore thumb and significantly detracts from the value of the article. I would like to try and improve that part of the introduction, but wanted to find out if anyone agrees with me about the need to rewrite that sentence. So, does anyone think that sentence needs rewriting? Carcharoth 21:21, 23 June 2006 (UTC)

I strongly disagree that the wording detracts from the article. If you want to try to improve it, feel free, but bear in mind this is a controversial topic and that wording has survived for a fair while. I don't see any need to improve it William M. Connolley 21:30, 24 June 2006 (UTC)

[edit] Cvection

The claim that the greenhouse effect works by blocking convection is absurd. That's like saying a tea kettle works by blocking drainage. It says nothing about what drives the temperature up. (With your windows and doors closed, your home also acts as a barrier to convection, but that doesn't make it a greenhouse.) Consequently, the claim on this page that the term "greenhouse effect" is a misnomer is also incorrect. The description there of how a greenhouse works makes no mention of how the glass blocks the transmission of infrared radiation. This is confusing and misleading. I first encountered these claims on a web site funded by the oil industry that professed to "explain" the greenhouse effect but only clouded the issue (If you'll excuse the expression). (I first raised this point in the Solar greenhouse (technical) discussion, but this seems to be a better place for it.) --MiguelMunoz 18:38, 28 July 2006 (UTC)

I am sorry Miguel, but you are mistaken. The greenhouse effect describes processes by which the heat generated by solar radiation is trapped near the surface of the Earth. For the Earth as a whole, this is accomplished primarily (but not exclusively) by the absorption of infrared radiation by greenhouse gases in the atmosphere. However, for the greenhouses that we build, heat is trapped near the surface primarily (but not exclusively) by the enclosed space which prevents the warmed air from rising to mix with other, cooler portions of the atmosphere. People can build a perfectly functioning greenhouses with materials that are totally transparent to infrared. Talk:Greenhouse effect/Archive 1 and Talk:Solar greenhouse (technical)/Archive 1 have some very long (and argumentative) discussions of this. Dragons flight 19:10, 28 July 2006 (UTC)

On People can build a perfectly functioning greenhouses with materials that are totally transparent to infrared, see http://www.wmconnolley.org.uk/sci/wood_rw.1909.html William M. Connolley 20:21, 28 July 2006 (UTC)

Miguel - I don't understand either of the points you are trying to make.

"The claim that the greenhouse effect works by blocking convection is absurd.". The article is very clear that the greenhouse effect works by the absorption of infra red radiation. It is only the glass structure used in gardening (a glasshouse to give it an old fashioned name) that works by blocking radiation. This is mentioned towards the end of the article after an extensive discussion of radiative transfer.

Secondly I can't see why you claim "It says nothing about what drives the temperature up" when the second sentence of the Real Greenhouses section states "it heats up primarily because the Sun warms the ground inside it". If this was not mentioned your analogy with a home would be valid - however the source of the heat is mentioned as well as the mechanism by which the heat is inhibited from escaping.

I would however add that I think it is a mistake to mention glasshouses in the lead section and some mention of IR there might be useful.--NHSavage 20:57, 29 July 2006 (UTC)

Okay, I also take issue with the claim that the greenhouse effect is so named because of an incorrect analogy with the functioning of a greenhouse. While it may be true that functioning greenhouses can be built without materials that trap re-radiation of infrared, it is nevertheless the case that a significant portion of greenhouses use more standard IR-blocking materials, and that a significant portion of their heat comes from that process. In fact, the article about greenhouses on this very site explains the heating mechanism in terms of trapping re-radiated IR, so to call the analogy false creates a lot of confusion. Perhaps a better statements would be "the greenhouse effect's name come from an analogy to one of the mechanisms that heats up real greenhouses". This sentence doesn't mislead people about the functioning of greenhouses, or about the connection between them and the greenhouse effect.

it is nevertheless the case that a significant portion of greenhouses use more standard IR-blocking materials, and that a significant portion of their heat comes from that process. - who says that a significant portion of their heat comes from that process - you? I'm afraid you;ll need rather better than an unsupported assertion, no matter how "obvious" is may be to you. Solar greenhouse (technical) does *not* say that IR trapping is a major effect - it says, just like this page, that first is the stopping convection and only second is IR William M. Connolley 19:39, 17 November 2006 (UTC)

[edit] Cleanup

This article seriously needs cleanup. --scienceman 17:04, 25 July 2006 (UTC)

[edit] WH nonsense

I deleted http://www.warwickhughes.com/icecore/. Not only is it rubbish, its also misleading: despite purporting to be a statement given in evidence, it wasn't William M. Connolley 15:52, 28 September 2006 (UTC)

[edit] "Real greenhouses"

As far as I can see the section "Real greenhouses" is misleading, incorrect and should be removed. In order for something to be warmer than its surroundings there has to be some phenomenon that makes it warmer. That phenomenon is (roughly speaking) that it lets sunlight through, but not the blackbody radiation from the earth. In order to keep it warmer than its surroundings you have to restrict the transport processes that otherwise will cool it until it reaches the temperature of the surroundings (in this case the glass structure that keeps it from mixing with the surrounding air). That is however not the primary reason for the warmer climate within a greenhouse, it is just a condition. If you leave a house (or any other closed volume with the about the same properties as the surroundings) unheated for a reasonably long period the temperature within the house will not differ from its surroundings.

One of the two links given do actually deal with finding errors in Mr Wood's argument (no. 7 at the moment), while the other one is a rather political page of low quality. Gunnar Larsson 18:32, 16 October 2006 (UTC)

First of all, this has been extensively discussed. Its one of the few points of fact that have an arbcomm ruling in their favour. But we can still talk about it... AFAIK "bad greenhouse" doesn't point to any errors in Wood; be so kind as to quote the piece you have in mind. If the other "low quality" page is mine I fear I disagree with you; I point out the only flaw I know of in Woods paper.

But on to your: That phenomenon is (roughly speaking) that it lets sunlight through, but not the blackbody radiation from the earth. This is simply a bald and unsupported assertion. From a theoretical point of view its a possibility; however, observational evidence is against it. You are correct in your restrict the transport processes that otherwise will cool it - and the transport process that is restricted is movement of heat by movement of air: direct thermal advection. I don't understand your house bit: if you mean, a closed volume with few openings for light, then yes of course: because there is no internal heating from sunlight William M. Connolley 19:15, 16 October 2006 (UTC)

[edit] The relative roles of convection, conduction, evaporation and emission for GH temps

I know this is a sore point on this discussion, but there is a relatively simple experiment that illustrates the importance of emission and selective surfaces for the heating of greenhouses. Take two pieces of blued steel (coated in a fine layer of black iron oxide) and paint one with black paint. Leave both out in the sun with no covering. Carefully touch both with your finger.

The unpainted iron will probably burn your fingers, the painted one will be just warm.

Why is it so? The black iron is a good selective surface, having very little absorbtion (and thus emittance) of thermal IR. Black paint, on the other hand, is close to a black body (usually consisting of carbon black) and radiates lots when it gets hot.

Thus, for collecting heat you want to stop the emission of IR but allow the absorbtion of PAR.

This is not the full story, however. Plants themselves do not thrive when the ground is warm and the leaves are cold, as most of the chemical action takes place in the leaves. Thus, we wish to transfer heat collected from the sun to the air around the leaves. We might try to do this by putting a large selective surface under the plants, but in practice although the collector would get hot, much of the heat would be lost to surrounding air by convection. Thus, we trap the air in a box (and for other reasons too).

Another important source (currently not mentioned in the article) of heat loss in greenhouses is evaporation. Losing half a gram of water a second as vapour is more heat loss than the maximum heating effect on a square metre of direct sunlight. Controlling this is thus very important. Plants naturally try to evaporate water as part of their living process.

Finally, we have conduction. Glazings generally are very poor insulators, being so very thin. More significant is the boundary layer of air, but inside a greenhouse we lose half of this to condensation (as water vapour happily crosses the boundary layer by diffusion). Thus double wall films are quite effective.

So, it is true that IR emission is very important for efficient collection of light energy, and important for reducing losses at night, but convection and water vapour transfer is far more effective at cooling a greenhouse. My double walled, TIR blocking greenhouse is cool in summer simply by opening a 1m² hole in each end and relying on the cooling effect of evaporation, driven by convection. Thus, controlling air movement is the first priority of greenhouse design. Once you have sealed everything up, then you look at TIR for further thermal storage.

We might ask what the best use of a dollar for improving greenhouse heating costs is. Experience is that we should, in order, a) remove all air leaks, b) use IR blocking films, c) reduce conduction with extra layers. Thus, the options on commercial greenhouses are usually a) cheap UV only film, b) UV and IR film, c) double layer designs. Because of the massive effect of air transfer, this is assumed to be controlled first. My greenhouse rep tells me that until the relatively recent development of good IR PE films, polyethylene films with only UV block were the most commonly used. Recent advances in IR coatings have made these now commercially viable for growers.

Finally, there are 90% PAR transmitting woven cloths that block both UV(for longer life and human protection) and TIR(due to the kind of plastic) but allow significant air movement. These are commonly used in Australia for growing certain kinds of Orchid. Although 90% of the energy of the sun is reaching the ground, the large air transfer means that these environments only heat very slightly above ambient, and cool very quickly at night. Their multi-layer woven design means they should be quite good at blocking TIR(there are no visible holes through).

Regarding the real greenhouse effect, clearly convection, conduction and latent heat do not cross space, but do have an effect on global temperatures by moving heat from ground level where the light is absorbed to the upper surface where there is less insulating TIR blocking gas. I do not know anything about the importance and magnitude of that and will leave discussion to experts. -- njh 09:56, 18 October 2006 (UTC)

[edit] {{sprotected}}

Right now, it is semi-protected, so IP users may register and do something so disruptive that it may get heavier than it sounds. Even with "calmer" edits, we can't unprotect it without entering a request in wikipedia:requests for page protection nor can we remove the tag until a season or so. Agree? --Gh87 06:39, 28 October 2006 (UTC)

[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)

[edit] Anon Q

An anon asks (but did it by rubbing out another anon Q, so I've (William M. Connolley 16:34, 9 November 2006 (UTC)) re-added it here):

what about the greenhouse effect, not always associated with the earth's atmosphere?

what about the greenhouse effect which is linked to heat transfer?

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