Re: Water as a byproduct of combustion...

[Tami Bond <Tami.Bond@noaa.gov>]

Tom,

You would have to draw me out... ;-)

> I have long been puzzled by the emphasis on carbon dioxide as a 
> greenhousegas. Water it the primary greenhouse gas...

The Earth's temperature is higher than one would expect without any 
gases in the atmosphere. It's true that water vapor is the primary 
cause. It's also generally accepted that human activity has little 
effect as the major controlling processes are not releases by people 
but the hydrologic cycle. The water vapor we release by burning coal, 
wood or hydrogen doesn't change the concentration of water vapor in the 
atmosphere by much.

Human activity does have an effect on CO2. The processes controlling 
its concentration are uptake by plants (short time scale), uptake by 
ocean (long time scale), weathering of rock (really long time scale) in 
addition to what we dump in. 

Isn't the water vapor greenhouse effect swamping any comparable effects 
of CO2? Nope. That's because water vapor absorbs at different 
wavelengths than CO2 does. The wavelengths that water vapor affects are 
closer to saturated-- that is, H2O is already absorbing most of the 
radiation at those wavelengths. If you add more H2O, the effect on 
radiation is fairly small, just because there's not a lot more 
radiation to absorb. 

On the other hand, CO2 (and other greenhouse gases) absorbs at 
different wavelengths where there is a nearly open 'window'. A small 
amount can make a large difference. The question of 'climate change' 
involves not what is absorbing the most radiation (water vapor is the 
clear winner) but what is tipping the balance from its previous state 
(other greenhouse gases). 

The H2O-vapor wavelengths aren't completely saturated; hence the 'water 
vapor feedback' effect. That is, if/as things get warmer, there ought 
to be more water vapor in the atmosphere, and that will contribute to 
even more warming-- but again, the warming per molecule of H2O is quite 
a bit smaller than warming per molecule CO2.

Yes, Dan! Soil moisture, evaporation, condensation (or at least 
parameterizations thereof) are in the climate models! Modelers have a 
terrible time with convection processes because they are so small 
compared to the modeling grids, but they're working on it. But these 
models are on a very crude scale, maybe okay (or not??) for predicting 
long-term trends, but certainly not with the resolution required for 
daily weather predictions.

Tami





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