Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How the Greenhouse Gas Effect works

  1. Jun 22, 2006 #1
    How the Greenhouse Gas Effect Works

    We all know by now that the greenhouse gas effect depends upon absorption by greenhouse gas molecules of infrared energy radiated from the Earth’s surface, and which is then re-radiated by these molecules in all directions. Effectively, half the energy is emitted to outer space, and half is emitted to the Earth’s surface where it is absorbed again. Together with the additional energy continually being received from the Sun, this warms the surface more than would be the case without the GHG, and so causes increased radiation from the surface.

    Consider an area of 1 square metre of the surface, and let us take a value of 235 Watts for the power incident upon this area. Assume an emissivity of 1.0 for the atmosphere. Follow the process through second by second as in the table below.

    Sun’s radiation to____Surface radiation____Back radiation____Radiation to
    Earth’s surface(W)___to atmosphere(W)___to surface(W)_____space(W)


    We see that the radiation to space converges towards 235 Watts, which in fact is the limit set by the power received from the Sun since, in equilibrium, the power emitted must equal the power received.

    Also, the power radiated to the atmosphere converges towards 470 Watts, which is twice the power emitted from the atmosphere to space. Now, the power radiated from a body is proportional to the fourth power of the absolute temperature, by the Stefan-Boltzmann Law.

    Let Ts be the absolute temperature of the Earth’s surface.
    Let Ta be the absolute temperature of the top of atmosphere.

    Therefore, we have
    ( Ts )^4 = 2 × ( Ta )^4

    and Ts = (2)^0.25 × ( Ta)

    Finally, Ts = 1.189 × Ta

    That is, the surface temperature of the Earth is 18.9% greater than the temperature of the GHG emitting to space. This will be true provided that there is at least sufficient greenhouse gas present to be able to cope with the amount of energy being received from the Sun.

    Note that this analysis can be applied to other planets which have an atmosphere containing at least sufficient greenhouse gases, like Venus. It also means that there cannot be a runaway greenhouse gas effect, because the ToA temperature is governed by the emission which must be equal to the energy received from the Sun, and the surface temperature is limited to twice the ToA temperature. This reasoning, of course, assumes that no energy is entering the atmosphere from other sources, such as geothermal energy, and it assumes an emissivity of 1.0 for the atmosphere.

    Aubrey E Banner
    Sale, Cheshire, UK
  2. jcsd
  3. Jun 22, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You're missing the point entirely, AE. You're correct that if the volume of greenhouse gas is constant, there will be no runaway greenhouse effect. Let me be the first to say: DUH! That's not what a runaway greenhouse effect is!

    If the concentration of greenhouse gases is constant, then there is no feedback path, and the earth will simply reach equilibrium. The actual temperature of that equilibrium will depend on the amount of greenhouse gas present.

    However, if there's a positive feedback path involved -- say, higher temperatures lead to more dissociation of water in the upper atmosphere, leading to less dissolved greeenhouse gas, leading to higher temperatures, leading to more dissociation of water, etc. -- then you have a runaway greenhouse effect.

    Your naive "analysis" totally neglects the feedback path that makes a runaway greenhouse effect a runaway in the first place.

    - Warren
  4. Jun 22, 2006 #3

    Please refer to my post "Greenhouse Gas Effect and Carbon Dioxide".

  5. Jun 22, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't need to refer to any other posts to show that your statements here are wrong.

    - Warren
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook