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Cause of Greenhouse effect

  1. Oct 2, 2015 #1
    As I understand and I think correctly, that Green house effect is in essence conversion of lower wavelengths in the Sun's energy to higher wavelength near infrared regions which can heat up the medium more efficiently. So if we have a closed transparent box containing some amount of green house gas under normal pressures and room temperature, and keep in the sun, the temperature of the box goes on increasing. But to pin point the cause of heating up due to green house gas effect only and not due to stoppage of convection currents with the surroundin we must have two such identical containers one with green house gas like CO2 and other filled with say nitrogen, We should then check up whether the container with CO2 gets heated up more. have such experiments been done?
     
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  3. Oct 2, 2015 #2

    DaveC426913

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  4. Oct 2, 2015 #3

    davenn

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    Your idea has some flaws
    in a closed system like that... the temperature is going to rise quickly anyway regardless of what gas is inside
    consider how hot it gets in your closed up car on a sunny day



    Dave
     
  5. Oct 3, 2015 #4
    You would not expect the same temperature rise regardless of what gas was inside .With some gases the temperature should rise more quickly and remain hotter longer depending on there thermal capacity.
    Some double glazing units have gas injected because of the gases thermal capacity but they don't use Co2 it's way down the list.
    Yes sure the temperature will rise quickly regardless of what gas is used but more to the point some will be better at it than others.
     
  6. Oct 3, 2015 #5

    DaveC426913

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    The point is, it's not a valid test.
    A valid test could be designed, but we'd need clarification on exactly what it is we're trying to test here.

    Greenhouse gasses do not work the way the OP suggests they do. They work on solar energy reflecting off the surface of the Earth, preventing it from exiting the atmosphere.
    You'l want to examine this process carefully before
    a] doubting it, and
    b] trying to test it.
     
  7. Oct 3, 2015 #6

    davenn

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    yes exactly .... that was going to be my response
    thanks Dave :smile:
     
  8. Oct 3, 2015 #7

    sophiecentaur

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    You could refine that statement. In a steady state situation, all the incident energy gets radiated and it is not "prevented" from leaving. What in fact happens (basic simple model) is that the effect of the 'greenhouse gases' is to raise the surface temperature of the planet to a value which allows the radiated energy to equal the incident power. They present a 'high resistance path' (electrical analogy) to the IR radiation because they absorb and re radiate down as well as upwards. There is a constant (average) level of received power at the surface, dominated by higher frequency radiation, which is more or less independent of the presence of the greenhouse gases, which is analogous to a constant current electrical source and the higher path resistance causes the voltage to be higher, to enable the current to pass.
    I wouldn't mind betting there is a Water Analogy of that Electrical Analogy too!!! :wink:
     
  9. Oct 3, 2015 #8

    DaveC426913

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    Yes, that was my point: it's much more complex than the OP's setup. Gotta do some readin' before doin' some questionin'...
     
  10. Oct 4, 2015 #9

    sophiecentaur

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    When someone asks a question in the "homework" section, they are expected to show that they have actually tried. Perhaps the same demand should be made for OPs on all forums.
    That could take some of the fun out of things, tho'.
     
  11. Oct 4, 2015 #10
    I agree with sophiecentaur and admit my mistake in presenting the opening thread. You mean to say that whatever experiments we need to do to confirm experimentally the cause of Green House effect have to be done on open systems. I would very much like to know about such experiments. Also my asking this question many others who have this misconception that our closed car kept in sun explodes due to green house effect only will be clarified.
     
  12. Oct 4, 2015 #11

    sophiecentaur

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    That is a 'real' greenhouse effect, in as far as there is a direct equivalence between the car windows and the greenhouse glass. The difference between that and the effect due to the atmosphere is that the heat loss is reduced in a greenhouse by the fact that the glass just stops exchange of the air with the outside air (convection). It has nothing (very little) to do with the absorption and re-radiation of EM by the glass. That mechanism is very sophisticated for non-scientific minds to grasp, I think.
    The greenhouse effect is a handy term to use in describing global heating but it doesn't imply the same mechanism - just the result. Bad journalism, perhaps, as the term is used by people who have no idea about (or choose to ignore) the mechanisms at work in the atmosphere. (The blessed Lord Lawson, for one - along with a tame Scientist he seems to have trawled the World to find!)
     
  13. Oct 4, 2015 #12
    Well could you not simplify it.If the atmosphere was totally glazed in (one big bubble) and more greenhouse gases were present at any given time than another would the temperature remain higher and for longer when the gases were present than when they were not.
    Which is basically what the op is asking.
    I can't see exactly why the negative no can do replies to an honest question.
     
  14. Oct 4, 2015 #13

    sophiecentaur

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    It depends on what the questioner really wants to know. I can't see any "no can do" in my reply. I am just pointing out that there is a much better model than a glass envelope to explain the global warming effect. And it's not that difficult to understand as it can be communicated without any of the dreaded Maths - although there's a lot of Maths involved in a proper quantitative approach.
    The word 'greenhouse' is good enough on an elementary level but, if you want to get a fair 'Physics Explanatio'n, it doesn't describe the mechanism although the result is broadly the same. Why would one prefer an inadequate model just on the basis that it is easier to understand?
    The atmosphere is one big bubble. The gases are kept in place by gravity, rather than a membrane. If more greenhouse gases were introduced then the surface temperature would go up - unless there were some other mechanism which the simple model doesn't include. This proposed "big bubble" would need to be specified more than just with a name. What would its characteristics be? Would it absorb or reflect IR wavelengths? Would it have a vacuum outside it or something else? What would its thermal conductivity be? I guess it would be possible to make a model of the Earth with an atmosphere of non-greenhouse gases and surround it with a suitable membrane which would produce the same surface temperature as we get on an ideal model with greenhouse gases. I would take some convincing that it would be a better approach than what's used at present; there are too many loose ends and undefined variables. Assuming you got the membrane to work OK in one particular set of circumstances then could you be sure it would track reality as circumstances changed?
    Otoh, it is not hard to construct experiments to establish the absorption of IR by greenhouse gases. This is what's already been done and it gives fair results (given all the other variables at work in a real atmosphere. Why take a step backwards?
     
  15. Oct 4, 2015 #14
    I doubt that it would be a better approach than what is used at present though a model which is easier to understand does have benefits.
    By simplifying a subject it becomes more likely to be understood and if reality is not tracked properly as circumstances changed then either the glass envelope model is wrong or something else.You learn more sometimes by being wrong than right.
     
  16. Oct 5, 2015 #15

    sophiecentaur

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    You seem to be defending an inferior model. I wonder why. The glass reduces convection losses. The gases reduce radiation losses. Whic has more relevance in space?
    Isn't this about the 'learning' you refer to? Why not move on?
     
  17. Oct 5, 2015 #16

    JDoolin

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    Aren't the mechanisms behind glass, and the mechanism behind greenhouse gasses the same in principle? The glass is transparent to visible light... It lets the visible light penetrate to the ground, which absorbs part of that light as heat. Then the ground re-emits the light (largely) in the infrared spectrum. The glass is opaque to the infrared spectrum so this heat cannot escape.

    Heat can still escape through the glass, via conduction. But it can no longer escape via radiation.

    Conceptually, I think this model is great, and almost perfectly analogous to greenhouse gasses. For greenhouse gasses, they are transparent to the incoming light, and then they are opaque to the outgoing light. Once they absorb 100% of that outgoing light, you're concerned about their heat-conductivity properties instead of their band-pass properties.

    However the reason that the OP's experiment wouldn't work, if I'm right, is that whatever transparent container holds the gas will also interfere with the transparency and the conductivity of the gas. But in principle, if you had a perfectly transparent container material that wouldn't block any light at all in any wavelength... e.g. an "ideal glass" we could at least approach the idea "gedanken"-style... Am I right?
     
  18. Oct 5, 2015 #17

    sophiecentaur

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    Question: If you replaced the greenhoouse glass with a set of louvres, which allowed convection but still intercepted the IR, would the greenhouse stay the same temperature?
    Evidence: As soon as the Sun gets too strong, greenhouse operators open the roof windows ( 20% of the area, max) and the temperature drops considerably. Is that not experimental evidence that the effect on the IR by the glass is much less compared with its action in suppressing convection losses?
    That's my problem with accepting the 'greenhouse' explanation for what goes on in our atmosphere. It's just not thought out enough.
     
  19. Oct 5, 2015 #18
    The experiment you suggest has been performed. See:
    http://www.tufts.edu/~rtobin/Wagoner AJP 2010.pdf
    with result similar to those predicted above.

    It has been known since Professor Wood published his short paper http://scienceblogs.com/stoat/2011/01/07/r-w-wood-note-on-the-theory-of/ that the greenhouse gases do not behave in the same way as a greenhouse, which works by preenting convection. But it is too late now to change their name. Greenhouse gases work by absorption.
     
  20. Oct 5, 2015 #19

    JDoolin

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    Here's a graph from http://www.skepticalscience.com/saturated-co2-effect-advanced.htm
    http://ishmael.altervista.org/fig1.jpg

    The part of the curve marked 300 Kelvin represents the transparent part of the gas. This is light from the surface of the planet at 300 Kelvin. The part of the curve marked 220 Kelvin represents the opaque part of the gas. This is light from the top part of the gas, well above the surface, which follows the blackbody curve of a 220 Kelvin surface.

    If you increase the depth of that layer, it would also increase the distance over which convection operates, and hence lower the amount of escaping light in the 600-700 /cm bandwidth.

    There seem to be two different concerns about increased Carbon Dioxide in the atmosphere. One is that the bandwidth of infrared opacity of the atmosphere will widen (in the graph above). But also that the Carbon-Dioxide layer of the atmosphere will thicken, which would increase the insulative properties and make the temperature of the outer layer drop (in the graph above), even if no additional light bandwidth of infrared was absorbed.

    Now, according to the article which I linked above, the bandwidth increases according to the absorption coefficient of the atmosphere in a "very crude model". So I think that's where I am... If I saw either the argument, or the experiments which showed quantitatively and qualitatively how this bandwidth expanded (and/or how the depth of CO2 layer increased), then I would no longer have to tell my students "Most climate scientists believe that CO2 induced global warming is happening." I could say "Here, let me show you."
     
  21. Oct 5, 2015 #20
    Greenhouses primary reason for heat trapping is that they stop convection circulation. Ordinarily, hot air rises and is replaced by cooler air. The glass enclosure stops that. So it is a bit wrong to say the mechanism is based on transmission limits in the infrared by the glass.

    That is almost right. The sunlight that hits the earth is in a broad spectrum. The earth re-radiates energy as black-body radiation. That black body radiation is SCATTERED and ABSORBED and RE-RADIATED. Those mechanisms trap some of the infrared energy. The black-body temperature of the earth must be at a steady state, exactly re-radiating the Sun's incident energy back into space. If the earth had no atmospheric blanket, the black-body temperature that balances the Sun's "solar constant" would be about 0-degrees F (IIRC). The earths average surface temperature is about 60-degrees-F. Changes to the earths atmospheric blanket will change the amount of energy that gets retained. That moves the black-body temperature higher, which moves the radiation profile (this is a physics board, someone probably even knows the formula off the top of their head, I use google). A new steady state is arrived at with a slightly higher temperature.

    The experiment you describe is simply absorbance spectroscopy. Different gases have different absorption spectra. You could also fill two bottles, one with black paint and one with white paint, and put them in the same sun. One would have a higher black-body temperature (if I am using the words correctly) ... it would be the black one. It is somewhat trivial to deduce that absorption spectrum changes the steady state temperature. But just as you could mix one drop of the black paint into the white paint and see a slight change in temperature, if you mix a gas that has absorption in the spectrum passing thru that space into a gas that has less, there is a slight temperature increase.

    I'm not sure if that precise experiment has been done. I considered something similar as a simple bit of work to propose to a group of kids as a science fair project.

    But the step from absorption spectrum to energy re-distribution should be one which is strong theoretically.

    The sun radiates as a body at a very high temperature. That is the spectrum hitting the earth surface. The earth radiates as a body at a much lower temperature. That is the spectrum leaving the earth surface. In between, there is an atmosphere.

    That bit of science is remarkably simple. There are a LOT of other things that matter though. The earth's albedo ... how much sunlight is reflected directly back into space. Heat capacity and phase changes matter in the energy flow. The Sun's solar constant matters. Heat energy from the earths core matters. Some people choose to regard the changes to the earths atmospheric blanket as trivial, when compared to other factors. I think that while the system may be complex, the atmospheric factor is large and important. There are feedback loops that are both positive and negative. In particular the heat capacity if the oceans, ocean currents and temperature mixing, the phase changes of water, and the changes in net albedo ... those all can create a lot of noise in the data.

    I am always a bit confused by the use of averages in these systems. If you look at the moon, which has no atmosphere, and is generally (barring eclipses) sunlit on one side and dark on the other, the sunlit side is WAY warmer than the dark side. Is it truly accurate to speak of the AVERAGE temperature as the black body radiation profile of re-emitted radiation? Is it equivalent energy outflo? EG, if right now it is 70-degrees-F at noon for me, and on the other side of the earth, at midnight, it might be 50-degrees-F. Is the energy flow from those two points the same as if they were two points at the AVERAGE temperature of 60-degrees-F?

    And I know ... I could do the math myself. But I'm being lazy.
     
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