How the Greenhouse gases "work"
Elmer Ogyrzlo
some statements to the press by Jack Barrett who is at Imperial College in
London and appears to have recently published some something in
Spectrochemica Acta, ~1994?
To paraphrase his arguements they run 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 10 to the 4 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."
Several letters to the New Scientist suggest that Jack Barrett may be
talking nonsense. The essence of these comments are as follows:
"Jack'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 proportioal to T to the 4th). 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 irrelevant!"
If the above response to Jack Barrett's theory is correct, then the usual
textbook view of the mechanism of energy trapping by greenhouse gases is
essentially incorrect, since it suggests that if there were no drop in
temperature with altitude there would be essentially no radiation trapping
by these gases. Is that true? If, in fact, most of the lower atmosphere
is heated by "weather" i.e. convective and water evaoration(latent heat
effects) from a heated surface, and greenhouse gases keep absorbing and
reradiating their energy in milliseconds, eventually losing it to space
only from the uppermost altitudes, I could almost be convinced that the
planet would be warmer with a pure N2/O2 atmosphere that doesn't radiate.
Help.
Jim Cook
ogryzlo@ Chem.ubc.ca (Elmer Ogyrzlo) says:
>
(snip)
>
> "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 10 to the 4 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."
>
scientist. However, I will quote a relevant paragraph from an
internal Audubon report, James H. Cook and Jan Beyea (1990)
Relative Halocarbon Contributions to Global Climate Change:
Background Information for CO2 Diet for a Greenhouse Planet.
"The global warming calculated for CO2 is not linearly related
to its concentration (Fisher et al., 1989; Lashof and Ahuja, 1990):
dTs(CO2) depends on the atmospheric CO2 concentration. Since the
main CO2 absorption bands are saturated, infrared opacity scales as
the log of the CO2 concentration (Ramanathan et al. 1985). The
global warming due to CO2, being proportional to CO2 opacity, is
thus proportional to the log of the CO2 concentration. This means
that dTs(CO2), the derivative of CO2 warming with respect to CO2
concentration, is inversely proportional to the CO2 concentration.
Since halocarbons absorb at wavelengths outside the main CO2
absorption bands, their dTs values are much less sensitive to the
atmospheric CO2 level (Fisher et al., 1989). They calculate that
the doubling of CO2 (340 to 680 ppm) would reduce dTs(CFC-11) and
dTs(CFC-12) by about 15%, and would have no effect on dTs(CFC-22).
>
(snip)