Global Warming and radiation question

[JGrey]

Hi, I'm new to the forum but I have three questions regarding greenhouse gases.

 

I know that N2 and H2 cannot become greenhouse gases so they don't do radiating, but I was wondering how they might influence atmospheric radiation, like with collisions, etc??

 

I learned in chemistry that CO2 has various "bands" at which it absorbs radiation, but only in certain regions that matters (like at wavenumbers of 667 cm-1 ). What determines which regions matter?

 

I've seen an argument that water vapor is also a powerful greenhouse gas, and my teacher said she didn't know much about this, but the textbook said that water vapor has not increased much, where CO2 has. Any insight on that?

[Chris C]

You are right, it is the greenhouse gases that are doing the radiatiing. You do not need the other air molecules. As a suggestion, see "a saturated gassy argument" at realclimate.org

 

By the way, N2 and H2 can actually become greenhouse gases in very dense atmospheres, such as on Titan, or H2 on gas planets. Not factors on Earth however.

 

The 15 micron region (or wavenumber- 667 cm) is of primary importance on Earth, and the other ones are measurable but not especially meaningful in Earthlike conditions. Though, the band around 4 microns can become important at much higher temperatures, such as on Venus.

 

Water Vapor concentration has increased a bit, but its concentration is set by temperature and circulation, whereas CO2 is set by sources and sinks. Because of this, water vapor responds to temperature change, and goes up about 7% per 1 C increase (and in fact will amplify temperatures in a warmer climate), but please note the crucial distinction between "climate forcing" and "climate feedback."

 

C

[iNow]

Also, IIRC, water vapour remains in the atmosphere for only a few days (recall it's a feedback not a forcing though), and CO₂ remains in the atmosphere for decades or even over a century. In other words, one of the many reasons that CO₂ is so much worse is because, once it's put into the atmosphere, it stays there for so very long.