Re: AAPG on Global Climate Change

Robert Grumbine wrote:
In article <12tmc5houm0e8ea@xxxxxxxxxxxxxxxxxx>,
Jo Schaper <jospamnotschaper34@5socket78dot9net> wrote:
<snip>
My guess where the 95% figure came from was it was not pulled entirely out of a hat, but resulted when someone tried to compare the percentages of H20 (g) to other known greenhouse gases on a ppm basis in a 'typical' part' of the atmosphere. This difficulty with this is, of course, that H20 is constantly variable-- with a little manipulation, you can come up with any large number you like, because it is true that H20 is more greatly abundant than even CO2 or other trace greenhouse gases. For example:
take .0038 (CO2) and divide by .02 (midpoint of 0 to .04 often given for % of atmosphere which is water vapor) This gives .19 or 2% CO2 vs 98% water vapor. I could see how someone could get such a number, then use Kentucky windage, and get from there to water vapor being 95% of all greenhouse gases.

An interesting calculation, which I'll keep in mind next time that
William and I go looking for the source of the 95% figure.

It's also incorrect. Water vapor approaching 4% of the atmosphere (40 g/kg mixing ratio) is suitable for near-saturation in extremely
warm air, say 35 C (I'm being fast and loose with water vapor saturation
curve; you can research the details if you're so inclined. The gist
of the argument is unchanged). For ballpark purposes, the atmosphere is near
saturation (70% relative humidity), so you're ok there. Where this
goes wrong is that the atmosphere isn't near 35 C for much of its
volume and the saturation pressure drops rapidly with temperature
(approximately halving for each 10 C cooling). Temperature drops
rapidly with elevation (ballpark 6.5 C per km). Upshot being that
that 2% is itself a very high number for water vapor (saturation at,
say, 25 C -- a temperature above global average surface temperature
of 15 C at any rate).

The outcome of the 70% RH over the globe, through the depth of the atmosphere is a global average column of water vapor is equivalent to 2 cm liquid H2O at the surface (it is the 2 cm which is a hard figure, not the above saturation curve numbers). About 20 kg/m^2. Surface pressure being 10^5 Pa gives an atmospheric mass of 10,000 kg/m^2 (roundly). So water is not 2%, but 0.2%. CO2, at 380 ppm, is 0.038%. If we take these as the only gases of interest (by number counts they're the most common greenhouse gases, but as they're both saturated in their band centers, number counts aren't the best way to look for their climate effects), then H2O is 84% by number.
[snip]


Thanks for the clarification of my math error. The difficulty was in sourcing down some place where both the CO2 and H2O were available in the same units. Most of the places I found CO2 in ppm, but the H20 in percent, not in ppm, because those authors had excluded H20 as irrelevant to the discussion, not being an anthropogenic gas for the most part.

(Decimal points and unit conversions--the main reasons I gladly defer to others in math calculations!)

Looking at the whole water vapor pressure/temperature variability problem over the whole planet gives me the willies over how that can be modeled with any accuracy, even with a computer. Talk about a moving target...

best
Jo

.

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