Soren wrote on Oct 17
th, 2011 at 8:41pm:
What you are suggesting here is that CO2 works on water vapour, rather than directly on the temperature. This is the 'forcing' idea, yes?
Mars has no water vapout so even a large amount of CO2 has nothing to force. That is, CO2 in itself is less effective than in an atmophere that has water in it as well as one that is omposed largely or air -something also not present on other planets.
(noting that we haven't even touched on the contribution of air itself).
Soren,
Not quite. It's
both direct and indirect. You haven't been paying attention, have you? (Feedback is the word you want, not forcing)
This is not some new concept that I'm introducing. On Earth, water and CO2 are important Greenhouse gases by virtue of their atmospheric proportions.
Carbon dioxide works
directly as a result of its properties to absorb Infrared radiation around 15 microns. That in itself causes some warming (forcing). The consequence of that warming in a world that has lots of water, is to evaporate just a bit more water which causes more warming. That's the
indirect or feedback aspect.
Now strangely enough, the Martian atmosphere is close to saturated with water, whereas the Earth's is not (aha! you say) . I thought I'd bring up that little chestnut before you did. However as I said before, there is
very little free water in the Martian atmosphere, compared to that of the Earth which is hardly surprising given its extremely low average temperature.
So the question is, what are the relative warming effects of CO2 and water vapour (feedback)? Well that's a term called the climate sensitivity. Most research puts that figure around 3. In other words for every doubling of CO2 (plus methane, nitrous oxide, HFC's etc expressed as CO2 - equivalent) concentration, you get about 3 degrees of global temperature rise due to forcing plus feedbacks, including water vapour. (for no extra price). The direct forcing component is a bit less than one degree.
Please tell me that you follow that.
As I've said before, the major components of the air - Nitrogen and oxygen are transparent to
longwave IR radiation. They absorb elsewhere of course, but that has no relevance. (Do you want to see their spectra?) They are not greenhouse gases. End of story.