Re: AAPG on Global Climate Change
- From: bobg@xxxxxxxxx (Robert Grumbine)
- Date: Thu, 22 Feb 2007 17:05:38 -0000
In article <12tosh8gtaempca@xxxxxxxxxxxxxxxxxx>,
Jo Schaper <jospamnotschaper34@5socket78dot9net> wrote:
Robert Grumbine wrote:
In article <12tmc5houm0e8ea@xxxxxxxxxxxxxxxxxx>,<snip>
Jo Schaper <jospamnotschaper34@5socket78dot9net> wrote:
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.
I sympathize with the problem. It's one of my complaints with my
colleagues, at least for their public pages. Once you're fairly
adept in the field, the conversion isn't hard. But we're supposed
to be communicating, at least some of the time, with folks who aren't.
One of the interesting things (to me at least) is that if you
follow the distribution of water vapor vs. CO2 through the depth
of the atmosphere, you find that as you go higher up, CO2 becomes
increasingly important, eventually being the dominant greenhouse
gas (or at least dominant w.r.t. H2O, O3 may be more important
than CO2 at those levels). This is offshoot of the fact that CO2
doesn't condense and is chemically stable -- so its concentration
is nearly constant through the depth of the atmosphere. With H2O
dropping rapidly with temperature, it drops under 380 ppm in, iirc,
the lower stratosphere.
(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...
But then we get back to observables. The atmosphere doesn't
reach much supersaturation, so if we observe temperature, we
can easily put an upper bound on the H2O vapor levels. And
temperature is both easy to observe, and a single observation
is generally representative of a large area. The latter
is useful, and itself derived from observation. If your house
is warmer than usual today, then temperatures for a couple of
hundred km radius around you are also likely warmer than usual.
This can be, and is, made rigorous in the field.
But H2O vapor is also fairly easy to observe, at least where
there aren't clouds (which, I grant, is a major limitation) --
H2O vapor has strong lines in the microwave around 22 and 87 GHz,
and satellites with sensors there have been flying since
1978 and ca. 1984, respectively. (Plus radiosondes have been
observing it, at least for the lower atmosphere, where most
of the water is, for a half century.)
These different sources are then used to decide how bad
the models are -- whether the weather models or the climate.
In some cases, the answer is 'surprisingly not-bad'. Sometimes
even 'good'.
--
Robert Grumbine http://www.radix.net/~bobg/ Science faqs and amateur activities notes and links.
Sagredo (Galileo Galilei) "You present these recondite matters with too much
evidence and ease; this great facility makes them less appreciated than they
would be had they been presented in a more abstruse manner." Two New Sciences
.
- References:
- AAPG on Global Climate Change
- From: Peter D. Tillman
- Re: AAPG on Global Climate Change
- From: Jo Schaper
- Re: AAPG on Global Climate Change
- From: Robert Grumbine
- Re: AAPG on Global Climate Change
- From: Jo Schaper
- AAPG on Global Climate Change
- Prev by Date: Re: AAPG on Global Climate Change
- Next by Date: Re: folding, plateaus,. uplift, problems
- Previous by thread: Re: AAPG on Global Climate Change
- Next by thread: Re: AAPG on Global Climate Change
- Index(es):
Relevant Pages
|
