Global Warming; CO2:-mechanism and relative importance
Andrew Glynn Hughes
I seem to recall an article in a British publication, New
Scientist, some two years ago where a senior academic of
Chemistry from Imperial College, London cast doubt on the role of
CO2 as significant greenhouse gas.
Specifically he said that the CO2 absorbed IR radiation only in two
fairly narrow bands in the IR wavelength spectrum and since solar
IR radiation is broadband then the effect of CO2 in blocking
re-radiation of IR into space would be insignificant.
Would any chemists or physicists out there like to comment? Is this
factually true? I thought that possibly his comment was inaccurate
because the narrow band feature may occur only in pure gaseous CO2
but spreads when CO2 is mixed with atmospheric Oxygen and Nitrogen
in a similar manner to which the valence and conductance bands of
atoms within crystalline structure are smeared in comparison with
isolated atoms. However this doesn't seem likely given the
huge average separation distance between atoms in the atmosphere (on
an atomic scale).
Are other 'Greenhouse Gases' such as methane more significant at
concentrations magnitudes lower than that of CO2?
Andy Hughes
(Just to amuse you transatlantics I'm sure you'll be tickled to
know that as usual our British Summer is in full swing today ie.
drizzle, rain and freezing cold)
Len Evens
I would suggest that you look at the latest IPCC report which
has just been published by Cambridge University Press. I don't
have it handy, but there is an extensive discussion of this issue.
The point is that while the most important bands are close to being
saturated at current levels, there are other bands which do make
a significant contribution as CO_2 levels rise. The result
is that the response is logarthmic rather than linear. The
IPCC report reponds explicitly and quantitatively to the contention
you describe. All of this is of course factored into the calculations
in climate models in which estimates are made of temperature
sensitivity to increased concentrations of greenhouse gases.
That is why the IPCC estimates that doubling CO_2 concentration
would only raise temperatures in the range 1.5 to 4.5 deg C.
Otherwise, presumably the effect would be much more dramatic.
Other greenhouse gases may operate in ranges in which the response
is more pronounced, but the situation is quite complicated.
According to the same IPCC report, for example, the contribution
from methane is quite comparable to that from CO_2 for a 20
year time horizon but relatively small on a 100 to 500 year time
horizon.
If I get a cahnce, I will look up the reference and provide more
detailed information.
Leonard Evens l...@math.nwu.edu 708-491-5537
Dept. of Mathematics, Northwestern Univ., Evanston, IL 60208
Claire Gilbert
Len Evens <l...@schur.math.nwu.edu> wrote:
>That is why the IPCC estimates that doubling CO_2 concentration
>would only raise temperatures in the range 1.5 to 4.5 deg C.
^^^^
ONLY????
>Leonard Evens l...@math.nwu.edu 708-491-5537
Excuse me ... I know I've taken it out of context but the use of
the word ONLY really got MY attention.
Claire Gilbert
Elmer
(Andrew Glynn Hughes) wrote:
> I seem to recall an article in a British publication, New
> Scientist, some two years ago where a senior academic of
> Chemistry from Imperial College, London cast doubt on the role of
> CO2 as significant greenhouse gas.
> Would any chemists or physicists out there like to comment?
In a later issue of the new scientist there was a response which made
sense to me. Here are my notes on the subject:
Jack Barrett who is at Imperial College in London (and was supposed to
publish these ideas in Spectrochemica Acta, ~1994)said something like
this:
- The lowest thirty meters of the troposphere already contains
sufficient CO2 and H2O to absorb all the radiation emitted by the Earth's
surface (except in the "10 micron" {7.5-14 痠} window).
-When CO2 absorbs the emitted radiation it does not re-emit it because
in its radiative lifetime (10盜) it suffers 104 collisions, which are
enough to transfer the energy to N2 and O2 which do not emit IR
radiation. Hence emission of IR from the Earth's surfaces to outer space
is prevented at all wavelengths except in the 10 micron window. Under
such circumstances further additions of CO2 to the atmosphere would be
expected to have little effect on the average global temperature. For
example, the burning of all the fossil fuels on earth would raise the CO2
level to 1400 ppm, from its current level of about 355 ppm. If the above
reasoning is true this could cause vegetation to flourish, as it did
during the Cretaceous period (144-65 million rears ago, when dinosaurs
roamed the Earth) and when it is now thought that the temperatures were
not too different from what they are currently (Nature 370, 453(1994)).
Here is the response that came later:
Barrett's analysis assumes that global warming is driven by changes in
the radiation balance at the Earth's surface. This is not so. What
happens is something like this:
It is true that the CO2 molecule suffers many collisions between the
time that it absorbs radiation from the solid Earth, and re-emits it in
all directions. This means that it is in thermal, and radiative
equilibrium with its surroundings at each altitude. As we go up in the
troposphere, the temperature of that atmosphere drops, and hence the
temperature of the CO2 at greater elevations also drops. At these lower
temperatures found at the top of the atmosphere, the energy is radiated
into space because there is so little CO2 above it that the atmosphere is
essentially transparent at these emitting wavelengths. However, at that
altitude the intensity of the emitted radiation is decreases (recall the
Steffan-Boltzmann law says that: I is proportional to T4). Thus the loss
of radiative loss of energy to space from this altitude drops, because of
the presence of the CO2 in the atmosphere. If now more CO2 is added to
the atmosphere then the level from which the emission occurs rises. Since
the temperature of the emitting CO2 is even lower, radiation leaving the
Earth is reduced. The climate then warms until once again the input of
solar radiation just balances the radiative loss to space. The fact that
near sea level the CO2 concentration is sufficiently high to absorb all
the radiation in the main CO2 band is therefore irrelevant!
Dave Halliwell
>I seem to recall an article in a British publication, New
>Scientist, some two years ago where a senior academic of
>Chemistry from Imperial College, London cast doubt on the role of
>CO2 as significant greenhouse gas.
>Specifically he said that the CO2 absorbed IR radiation only in two
>fairly narrow bands in the IR wavelength spectrum and since solar
>IR radiation is broadband then the effect of CO2 in blocking
>re-radiation of IR into space would be insignificant.
Something seems to be getting garbled along the lines, here. Solar IR
is not particularly important in the greenhouse effect. The role of CO2
is in its absorption and reemission of IR radiation of *terrestrial*
origin - that emitted by the earth and atmosphere. You might want to
check the article again and provide a more elaborate description if the
argument presented.
>Would any chemists or physicists out there like to comment? Is this
>factually true? I thought that possibly his comment was inaccurate
>because the narrow band feature may occur only in pure gaseous CO2
>but spreads when CO2 is mixed with atmospheric Oxygen and Nitrogen
>in a similar manner to which the valence and conductance bands of
>atoms within crystalline structure are smeared in comparison with
>isolated atoms. However this doesn't seem likely given the
>huge average separation distance between atoms in the atmosphere (on
>an atomic scale).
>Are other 'Greenhouse Gases' such as methane more significant at
>concentrations magnitudes lower than that of CO2?
Molecule-per-molecule, other gases such as CFCs absorb much more
strongly than CO2. I can't remember offhand the relative placing of
methane. CO2 is the "winner" because of the relative large amount of it
that we are adding to the atmosphere. Sort of a "quantity" versus
"quality" fight.
--
Dave Halliwell I don't speak for my employers, and you
Edmonton, Alberta shouldn't expect them to speak for me.
raymond thomas pierrehumbert
I concur with Elmer's descriptionof CO2 radiative saturation and
the relative role of surface and TOA budgets. For an optically thick
atmosphere, it is the TOA budget plus the lapse rate that
dominantly control the surface temperature. The surface
budget is relatively unimportant. Another way of looking
at it is that the atmosphere is so opaque to IR that the
radiation to space is determined by just the first one
optical depth from the top, which, loosely speaking,
reaches into the mid trop.
If Barrett's argument were correct, it would be difficult
to account for the very high surface temperature of venus.
Just one clarification, though:
>during the Cretaceous period (144-65 million rears ago, when dinosaurs
>roamed the Earth) and when it is now thought that the temperatures were
>not too different from what they are currently (Nature 370, 453(1994))
It's only the tropical temperatures which may (repeat MAY) have
been not much more than the present values. The poles, of course,
were ice free and perhaps 20C warmer than the present annual
mean.