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  1. #1
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    Why is CO2 a green house gas?

    What happens when a CO2 gas molecule in the atmosphere absorbs a photon of infrared radiation which has been emitted by the Earth’s surface?

    What about photons that are coming in from the Sun?

    As far as I can see, incoming photos would also collide with some of the CO2 molecules.

    So why does more heat come in than goes out? Any assistance will be appreciated.

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  2. #2
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    Re: Why is CO2 a green house gas?

    It is just a matter of absorption efficiency of gases along with their concentration in Atmosphere, in fact all gases absorb energy from EM waves, however CO2 in particular, has a maximum absorption efficiency close to the wavelength of the outgoing wavelength that emanates from the Earth's surface. Note that the energy coming from the Sun upon the Earth has a spectrum that is not the same as that radiated from the Earth. Refer to this graph: https://eesc.columbia.edu/courses/ee...absorption.gif
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    Re: Why is CO2 a green house gas?

    EM radiation from the sun is approximately a black body (cavity radiator) at about 5500 kelvin
    EM radiation from the earth is approximately a black body (cavity radiator) at about 300 kelvin

    CO2, CO, Methane act as green house gases because they allow EM from sun in.
    in andre's graph, the incoming solar radiation peaks in the visible range, and passes through the atmosphere
    while the outgoing earth radiation peaks in the infrared/microwave region where at least some
    of the radiation does not escape.

    Green houses and parked cars work this way. Solar radiation comes in, passing through the glass walls,
    (cavity radiator, 5000 kelvin) warms the plants and whatever is in the greenhouse (car).

    The stuff in the greenhouse (car) re-radiates (cavity radiator about 300 kelvin) but the glass walls do not pass the
    wavelength of the outgoing radiation, so the heat is trapped inside


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  4. #4
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    Re: Why is CO2 a green house gas?

    Another point to consider is that due to the complexity of the structure of the chemical compounds present in the atmosphere, from the simplest (atoms) to the most complex (molecules), there are multiple frequencies at which such absorptions occur at each substance as well as varying along the EM spectrum, so strictly speaking a more accurate value of the % absortion of each substance would be a bit more complex.
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