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Green house effect?

  1. May 9, 2010 #1
    I don't understand the green house effect. According to wikipedia, the green house effect is the result of non-directional re-emission of thermal radiation after being reflected from the ground from water vapor, methane, carbon dioxide etc., but shouldn't it cancel itself out? Shouldn't just as much thermal radiation be emitted away from earth that was originally headed toward it as thermal radiation emitted toward it that was originally headed away from it? I just read that, and I can barely make sense of it, so let me try again: Shouldn't just as much radiation headed toward the earth be absorbed by green house gases and emitted away from it as radiation headed away from the earth (because it bounced off of the ground), and re-emitted toward it?
  2. jcsd
  3. May 9, 2010 #2


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    The greenhouse effect results from the fact that the incoming radiation is largely visible light while the outgoing radiation is much more infrared (heat). The greenhouse gasses have very little effect on light, but much more on infrared.

    That is how greenhouses work - light goes through glass, but heat does not.
  4. May 9, 2010 #3
    Greenhouses have physical barriers such as glass which retain the heat.
    The atmosphere does not, after a while the heat will be emitted away the atmosphere is not a greenhouse the greenhouse effect is just that an effect.
    There is no physical barrier between the atmosphere and space.Heat will be retained longer but just as much will be radiated away.
  5. May 9, 2010 #4
    The atmosphere is the barrier. You feel it all the time. If it weren't for the atmospheric barrier your skin would freeze at night if you went outside because you would radiate all your heat into space.
  6. May 9, 2010 #5
    A greenhouse does not work in the same way though.If you want to regulate the temperature in a greenhouse you open the window.
    The temperature is modulated by convection in a greenhouse, the temperature is modulated by radiation with the atmosphere, by the near vacuum of space.
    The atmospheric "barrier" bounds space.
  7. May 9, 2010 #6
    Curiously, some people get really pedantic that the mechanism by which greenhouses (and blankets) work is mainly inhibiting convection (unlike the greenhouse effect of the atmosphere, noting that in general the atmosphere facilitates convection). But clearly what greenhouses (and blankets) do is the same as what the atmosphere (its greenhouse effect) does: elevate the temperature passively by impeding net heat loss (at any given temperature).

    A body reaches thermal equilibrium where it receives energy at the same rate that it gets rid of it. A planet gets rid of incident sunlight by a combination of reflection and thermal (Planck) radiation. Thus, by the "Stefan–Boltzmann law" we can figure out how hot it must be (to emit thermal radiation at the necessary rate). If the planet has no atmosphere (no greenhouse effect) then this will give us the temperature of the surface (otherwise, it will be telling us about the temperature of higher atmospheric layers). Considering how far the earth is from the sun, this law suggests an average temperature that happens to be about 30 degrees cooler than what Earth's average surface temperature actually is. (We can verify our work by comparing with average temperatures of the atmosphereless moon.)

    The atmosphere absorbs some IR, so it heats up until it is also emitting the same quantity of IR. Basically, half of this heat is being emitted back downward, and half is directed further upward (heating next higher layers of the atmosphere). If we increase the amount that the atmosphere interacts with IR, the very top layers will cool (since they become able to radiate their heat faster) but the surface would warm (since the lower layers are absorbing and emitting back more heat than before). (It might be roughly analogous to keeping the air composition the same yet making the atmosphere thicker/taller.) As it is, something like a quarter of the energy incident upon the Earth's surface arrives from the atmosphere (augmenting the energy received from the sun more directly). Since we have the spectral data of the various species of gas, it isn't hard to see how one would start to model exactly how various atmospheric compositions would produce effects of various surface temperatures.

    PF now has a policy, by the way, that discussion of the physical mechanisms is acceptable (for example, it is ok to discuss how much the temperature of some planet would change if the composition of its atmosphere was different, hypothetically, like by adding whatever reactants would be obtained by oxidising a given amount of carbon like, say, the quantity of carbon fuel that on Earth we've recorded mining since the industrial revolution) but PF does not allow discussion of global warming (so you would not be allowed to discuss whether or not mankind is causing any warming of Earth). Make sense? :yuck:
    Last edited: May 9, 2010
  8. May 9, 2010 #7


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    I think this is the key point the OP was asking for. The surface of the earth and the oceans receive light, but only reflect part of the sunlight as light, and converts much of the rest into infrared (heat). Glass and greenhouse gasses allow much of the sunlight to travel to the earth surface, but restrict the infrared flow traveling from the earth surface. There is also some conversion of light into heat within the atmosphere itself.
  9. May 10, 2010 #8
    A planet has a significant surface area.
    Significant because it is large enough to possess a large variation in received and emitted radiation over its surface.
    The area of the planet where most radiation is received is a quarter of a circumference away from the area where most radiation is emitted. The important factor being that absorbtion and re-radiation does not take place at the same locations as is so glibly depicted in many simple diagrams of the greenhouse effect.
    Energy is transported between the two.
    We call this weather.
    There is also the day and night variation ie a separation of half a rotation in time between the receiving emission and areas.
  10. May 10, 2010 #9
    That's a cute way of defining weather.
    Last edited: May 10, 2010
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