(The original working title for this post was “Supposed Seasoned Veteran Makes Rookie Mistake”.)
I needed educate myself on how people retrieve water vapor column from satellite-based spectral measurements. I found what I was looking for – see, e.g., The MODIS Near-IR Water Vapor Algorithm and more recent versions of that document. I also encountered this, IR Expert Speaks Out After 40 Years Of Silence : “IT’S THE WATER VAPOR STUPID and not the CO2″. If you’re not intimately familiar with the details of the science being discussed the “It’s the water vapor, stupid!” argument sounds very informed, very sensible. But if you do understand the science… not so much.
It’s clear from the post that the author doesn’t understand radiative transfer climate feedbacks. His understanding of radiative transfer is limited as well. He’s very wrong but it’s a nuanced kind of wrong. (Read the post. Part of his problem I think is that hasn’t run the full RT calc. He considers atmospheric transmission – which is okay as far as it goes – but that’s only part of the RT calculation. He makes some off-the-cuff observations and then screws up on tying them together. If he’s actually done the full calc he might have caught his mistakes.) It’s probably an honest mistake* but his bottom line is very wrong. You want the correct explanation of what’s going on? See Ray Pierrehumbert’s article in the January 2011 issue of Physics Today, Infrared Radiation and Planetary Temperature. In particular, see Figure 3a and his section on Energy Balance and Surface Temperature. He and Spencer Weart provide a complementary discussion of the physics at Real Climate, A Saturated Gassy Argument.
* NB: It’s not that water vapor isn’t a powerful greenhouse gas. It is. The problem with his argument is that water vapor feedback is significant because there’s CO2-induced warming. Water vapor an amplifier of warming not a driver in and of itself.
A little more on water vapor feedback from a 2008 NASA press release:
“Everyone agrees that if you add carbon dioxide to the atmosphere, then warming will result,” Dessler said. “So the real question is, how much warming?”…
The answer can be found by estimating the magnitude of water vapor feedback. Increasing water vapor leads to warmer temperatures, which causes more water vapor to be absorbed into the air. Warming and water absorption increase in a spiraling cycle.
Water vapor feedback can also amplify the warming effect of other greenhouse gases, such that the warming brought about by increased carbon dioxide allows more water vapor to enter the atmosphere.
“The difference in an atmosphere with a strong water vapor feedback and one with a weak feedback is enormous,” Dessler said.
“This new data set shows that as surface temperature increases, so does atmospheric humidity,” Dessler said. “Dumping greenhouse gases into the atmosphere makes the atmosphere more humid. And since water vapor is itself a greenhouse gas, the increase in humidity amplifies the warming from carbon dioxide…. We now think the water vapor feedback is extraordinarily strong, capable of doubling the warming due to carbon dioxide alone.”
See here for a shorter explanation.
And here’s a nice picture illustrating the effect of an increase in CO2 concentration on outgoing flux:
All other things being equal, doubling CO2 concentration from 300 ppmv to 600 ppmv would reduce outgoing flux by about 1.3%. Water vapor will have a comparable effect in the region beyond about 1250 cm-1. Rather than speculate on the details of water vapor and ozone impacts relative to CO2 I’ll do my own MODTRAN runs when time permits and share the results in new post. Or, if you don’t want to wait to see my numbers, you can do some runs yourself.