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I Do greenhouse gases need to be polar?

  1. May 2, 2017 #1

    I have been wondering if greenhouse gases always need to be polar molecules. Can a gas also absorb energy, and heat up, without being polar?
  2. jcsd
  3. May 2, 2017 #2


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    Staff: Mentor

    Obviously no, since CO2 and CH4 are not a polar molecules :smile:

    What is important is that the molecule be active in the infra-red, which corresponds to vibrational excitation. This precludes non-polar diatomic molecules, for which pure vibrational transitions are dipole-forbidden, but for molecules of 3 and more atoms, there are asymmetric vibrational modes that are active in the infra-red.
  4. May 2, 2017 #3
    Ah, thanks, and apologies for my ignorance. I guess what I wanted to know was can you heat up a gas through an absorption line?
  5. May 2, 2017 #4


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    Science Advisor
    Gold Member
    2017 Award

    I don't know the definitive answer to this but I suspect that there could be insufficient power in the limited bandwidth of a well single absorption line (even starting with 1kW/m2 from the Sun) to give a measurable temperature rise from that mechanism.
    Someone may well tell me different.
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