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B Carbon dioxide and global warming

  1. Nov 19, 2016 #1
    Hi

    I have a simple question about global warming. The percentage of carbon dioxide in the earth atmosphere is about 0.04% . My question is how is such a small percentage of this gas able to have such a powerful affect on global warming? Thanks
     
  2. jcsd
  3. Nov 19, 2016 #2

    CWatters

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    I think it's wrong to focus on the percentage...

    If you could somehow travel to another planet and bring back a load of nitrogen and release it into the atmosphere you could reduce the percentage of the atmosphere that is C02, however it wouldn't change the absolute amount of C02 in the atmosphere.

    Just for info C02 isn't the biggest cause of the greenhouse effect..

    https://en.wikipedia.org/wiki/Greenhouse_effect

     
  4. Nov 19, 2016 #3

    mathman

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    The answer is that it is simply a quantitative result. The earth receives a lot of energy from the sun. Most of it gets radiated back. A small average difference is enougth to increase the temperature of the earth.
     
  5. Nov 19, 2016 #4
    So I am guessing that it is 'relativity' easy to calculate the amount of heat that the Earth receives from the Sun and then also the relative amount of infrared radiation that effectively becomes trapped due to individual greenhouse gases. I will check out the wiki article. I guess I am looking for something that goes someway to introducing the mathematics that is used in modelling climate change, not something at post graduate level but something that is most likely at an intermediate graduate level. Thanks
     
  6. Nov 19, 2016 #5
    I understand that methane can have a much greater effect than CO2 but I have also been reading somewhere in some paper recently that at according to some climate modelling, CO2 still has the greatest effect. I think the author of the paper I was reading did experiments (computer modelling) where they removed the CO2 from their model and they found that other gases such as H20 were not sufficient to sustain a greenhouse gas effect and the Earth jus got seriously colder. Thats parphrasing it roughly.
     
  7. Nov 19, 2016 #6

    mfb

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    The density of carbon dioxide in the atmosphere is 1020/m3. Do you see how such a huge number can have a powerful effect?

    It doesn't help to compare the amount of CO2 to the amount of N2 if you want to measure the effect of CO2 on the climate. The amount of CO2 we have is sufficient to absorb a significant fraction of infrared radiation going through the atmosphere, only the absolute concentration matters here.

    Earth would be significantly colder without CO2, but even colder without water vapor.
     
  8. Nov 19, 2016 #7
    Radiative heat from the sun that reaches the earth will travel through the atmosphere, hit the earth, reflect back through the atmosphere and then disappear into space. The amount of heat that actually stays at earth depends on how much is absorbed,reflected and transmitted by the atmosphere and how much is absorbed and reflected by the earth surface. Nitrogen and oxygen are almost perfectly transparent to heat radiation. Visible light is also radiation and this is the reason that you can see very far on a clear day. There is also infrared radiation and ultraviolet radiation. Infrared radiation is heat radiation and ultraviolet radiation is what gives our skin a tan in the summer. You might remember that glass windows transmit visible light and infrared, but not UV radiation, so you cannot get a tan if you are behind glass. Glass and other materials absorb, reflect and transmit radiation differently depending on the wavelength of the radiation. It turns out that carbon dioxide has absolutely no effect on ultraviolet light or visible light, but is an almost perfect absorber of infrared radiation, exactly for the wavelengths of the heat that is being reflected back into space from the earth. So CO2 molecules let all radiation coming from the sun through, allowing it to heat up the earth. The earth absorbs this heat and sends part of the heat back into space, but at a different wavelength. Then, CO2 absorbs part of this heat that is being reflected by the earth. Methane is also doing it in this manner. Water absorbs also part of the incoming radiation from the sun, as well as absorb part of the radiation from the earth. Absorbed heat is then locally heating the atmosphere, but also reflecting heat back to earth and into space.

    I tried to find a decent website and I found that this one was explaining it rather well:
    https://scienceofdoom.com/2009/11/28/co2-an-insignificant-trace-gas-part-one/
     
  9. Nov 20, 2016 #8
    Thanks - yes it is beginning to make more sense. I would really like to see the numbers a bit, as in the basic models used to estimate the amount of Infrared absorbed and then re-emitted by those molecules. I understand the idea that the re-emitted IR is emitted in all different directions so that on average less of what was absorbed is transmitted back out into space, but I would like to see how it is calculated. Thanks
     
  10. Nov 20, 2016 #9

    CWatters

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    The absorption spectra of greenhouse gasses can be measured rather than just calculated.
     
  11. Nov 20, 2016 #10
    It's not plausible to do a precise calculation of atmospheric warming since many parts of Earth are subject to local changes of cloud cover and ocean currents which are not entirely predictable.
    It's more practical to measure what is actually happening over a period of time, then build models based on understood thermodynamics that explain the data.
    From there it's possible to extrapolate trends within a reasonable margin of error.
    You might find this NASA article interesting.
    http://climate.nasa.gov/news/2264/twelve-years-of-satellite-data-help-decode-climate-change/
     
  12. Nov 20, 2016 #11
    Thanks - I will check out that link.
     
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