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My book says that the ability of a gas to absorb infrared radiation affects its potency as a greenhouse gas, but I don't understand why. So what if a gas is able to absorb a lot of IR? How does that affect its potential as a greenhouse gas?

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    $\begingroup$ Here is a good definition of IR radiation, which is basically heat. Understanding the definition of IR is the first step in really understanding the answer to your question. $\endgroup$ – airhuff Mar 8 '17 at 18:49
  • $\begingroup$ What is occurring in the greenhouse effect? If a gas does not absorb IR does that matter? $\endgroup$ – Jon Custer Mar 8 '17 at 18:56
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There is only one difference between a greenhouse gas and a non-greenhouse gas: a greenhouse gas has a greater ability to absorb radiation in the infrared (IR) region of the electromagnetic spectrum than does a non-greenhouse gas.

Once a greenhouse gas absorbs IR radiation, that energy must be converted to another type of energy such that the the energy is conserved. The only thing that can happen at IR energy levels is that the absorbed IR energy gets transformed into thermal energy, resulting in an increase in heat in the atmosphere.

From there it follows that the more strongly a particular gas absorbs IR radiation, the more heat is retained in the atmosphere, which might have otherwise just radiated off into space as IR radiation. This means a gas absorbing strongly in the IR is a more "potent" greenhouse gas than one that only absorbs weakly or not at all in the IR.

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  • $\begingroup$ I'd add that, even if a gas doesn't absorb the IR and warm up but just reradiates it, it will still warm the earth. This is because the radiation absorbed from the earth's thermal emissions will be re-radiated in random directions so some will head back towards the earth, warming the earth rather than being lost to space. $\endgroup$ – matt_black Mar 18 '17 at 19:14

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