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]]]]]]]] CARBON DIOXIDE -- AN ALTERNATIVE VIEW [[[[[[[[[[[[
Sherwood B. Idso (27/11/88)
(From New Scientist, 12 November 1981, pp. 444-446)
[Kindly uploaded by Freweman 10602PANC]
Many experts argue that rising levels of carbon dioxide in the
atmosphere will lead to a climatological catastrophe, but they
base their opinions on computer models. Here we present an
alternative view, based on "natural experiments", suggesting that
the effects of carbon dioxide will not be so drastic.
["Dr Sherwood B. Isdo is a research physicist with the United
States Department of Agriculture's Water Conservation Laboratory
in Phoenix, Arizona." (p. 444:1)]
Carbon dioxide is only a trace constituent of Earth's
atmosphere -- with a concentration of about 0.03 per cent by
volume. Yet CO2 stands in the centre of a raging controversy
about climate. The gas is fairly transparent to solar radiation,
but rather opaque to some wavelengths of thermal radiation, and
so acts as a classic "greenhouse gas": it allows the rays of the
Sun to pass through the atmosphere and heat the Earth, but it
absorbs a significant proportion of the heat radiated by the land
and sea, and radiates some of this energy back to the earth's
surface. Thus the Earth is warmer than it would be if there were
no CO2 in the atmosphere. But how much warmer? That is the
question that now divides the world's meteorologists.
At the end of the last century two Swedish physicists, S.
Arrhenius and T.C. Chamberlin, estimated that if the
concentration of CO2 were doubled, the Earth would be warmer by
8oC. Later, scientists came to agree on a figure of 2-3oC. This
latter figure is also predicted by the most sophisticated general
circulation models (GCMs) of the atmosphere but it is an order of
magnitude greater than several recent experiments imply.
Consequently, the views of scientists have become polarised. At
one extreme many physicists look to the computer models and view
the steadily rising concentration of atmospheric CO2 as a prelude
to climatological catastrophe, while, at the other extreme, a
group is beginning to emerge that concentrates on more empirical
data. This group suggests that increases in atmospheric CO2, far
from being detrimental, is actually beneficial. These scientists
feel that any change in the climate caused by the rising levels
of CO2 will be indistinguishable from natural climatic
fluctuations. But more significantly they foresee that higher
concentrations of CO2 will tend to stimulate photosynthesis, and
so increase the productivity of crops and the efficiency with
which they use water, thus helping to feed the world's
population.
The divergence of opinion is becoming more acute because the
burning of fossil fuels, such as coal and oil, is causing the
concentration of CO2 in the atmosphere to increase at such a rate
that experts estimate that by AD 2025 it may be twice as high as
it was before the Industrial Revolution. This date is close
enough to prompt the scientists who belong to the "CO2
catastrophe camp" to urge the world's governments to curtail the
use of fossil fuels. But if the emerging group of more
empirically-minded scientists is correct, such pressures can only
be counter-productive to our future well-being. Given the
seriousness of the problem (or non-problem, depending on how one
views it), along with its many political ramifications (New
Scientist, vol 90, p 82), we must endeavour to break the impasse.
As an advocate of what is currently the minority viewpoint --
that increasing CO2 in the atmosphere will not lead to imminent
catastrophe -- I would like to set forth the alternative to the
long-unchallenged majority position, to help those in power reach
a decision in an open-minded manner. To this end I will present
the case against CO2 as a significant modulator of the Earth's
climate.
First, I should like to make a philosophical distinction
between the two approaches. Those who work with general
circulation models calculate on theoretical grounds the effect of
a two-fold increase of CO2 on the radiation passing both to and
from the Earth. They then feed that information into a computer,
which calculates the resulting change in global temperatures.
The empirical approach, on the other hand, depends on finding
some natural event -- such as the passage of a dust cloud through
a particular locality -- which temporarily disturbs the heat
balance of the atmosphere. By monitoring the temperature changes
and the flow of radiative heat during such natural events, it is
possible to measure the response of the real world to the
perturbation. I call this response the Earth's "surface air
temperature response function"; we observe the change in
temperature of the lower atmosphere in the face of a measured
change in heat flow and extrapolate from this the expected global
effect of a doubling of CO2.
The first investigators to publish an empirically-derived
value for this response function within this context were R.E.
Newell of the Massachusetts Institute of Technology and T.G.
Dopplick of Scott Air Force Base in Illinois. Writing in the
June 1979 issue of the Journal of Applied Meteorology, they
concluded from studies of the temperature of tropical
sea-surfaces, and the way energy is transferred between the
oceans and the atmosphere, that for every extra watt applied to
the surface of the Earth the mean surface temperature would
increase by 0.1oC; that is, that the mean global value of the
Earth's surface temperature sensitivity was about 0.1oC per
watt/sq.m. I applied these values to calculations of the
radiative perturbation -- changes in the passage of energy
radiating to and from the Earth -- caused by doubling the amount
of atmospheric CO2. From this I was able to conclude
independently that the resulting increase in the mean global
surface air temperature if CO2 were doubled would be only about
0.25oC.
In coming to this conclusion, I had drawn upon more than a
dozen years of field experience related to three independent
"natural experiments". The first of these concerns the way in
which dust in the atmosphere above Phoenix, Arizona,
redistributes itself in altitude between summer and winter. In a
series of papers in Science, Nature and elsewhere, I wrote that
dust low in the atmosphere exerts a significant thermal
blanketing effect on the Earth, much like a greenhouse gas, but
that dust at higher levels has a much smaller effect. Thus, from
measurements of radiation and temperature, I could evaluate the
radiative perturbation that the vertical redistribution of dust
produced, and the effect it had on surface air temperature. By
dividing the change in temperature by the radiative perturbation,
I produced a local value for the surface air temperature response
function -- that is, the amount the temperature changed for a
given change in local radiation. It was 0.173oC per watt/sq.m.
The effects of moisture
The second natural experiment I used was the annual arrival of
the summer monsoon season at Phoenix. From equations I had
developed that relate atmospheric thermal radiation with surface
air temperature and vapour pressure, I could calculate the
changes caused by the influx of moisture over the city. Checking
long-term weather records, I found that in going from a surface
vapour pressure of 4 to 20 millibars, the surface air temperature
just before dawn increased by approximately 11.4oC. Dividing
this change in temperature by the change in energy that caused it
yielded a second value, 0.196oC per watt/sq.m, for the local
surface air temperature response function. This was very similar
to the figure calculated from the data on dust storms.
The third natural experiment dealt with the change in surface
air temperature caused by the annual variation in solar radiation
received at the Earth's surface. I made this evaluation at 105
stations scattered across the United States -- and for all
interior locations the mean result for the surface air
temperature response function was 0.185oC per watt/sq.m, the same
as the average of the two "local" results from Phoenix. For 15
stations on the West Coast, however, where the Pacific Ocean
greatly influences the climate, the result was only half as
great. I took this number to represent an upper limit for the
world's seas, and therefore calculated an approximate upper limit
for the whole globe, taking account of the area covered by land
and by sea, of 0.113oC per watt/sq.m.
The good agreement among the separate evaluations of the
surface air temperature response function gave me considerable
confidence, as they involve three different perturbing mechanisms
(variations in altitude of dust, a temporal variation in water
vapour, and the movement of the Earth in its orbit around the
Sun), two different wavelengths of radiation (solar and thermal),
and two different time-scales (days to months). All that was
lacking was a demonstration that this common result also applied
to time-scales of the order of decades to centuries, and that it
incorporated effects due to the thermal inertia of the oceans --
that is, the fact that the temperature of the oceans responds
only sluggishly to changes in energy input.
I approached this task by considering the earth without an
atmosphere. A simple calculation gives the mean equilibrium
temperature of an airless globe as -18.6oC. The current mean
temperature of the globe is about 15oC, so the total greenhouse
effect of the entire atmosphere is to raise the surface air
temperature by about 33.6oC.
The heat the atmosphere radiates back to the Earth's surface
is 348 watts/sq.m. Dividing the temperature change of 33.6oC
(the difference between an airless Earth to the present state) by
this radiative energy yields a mean global surface air
temperature response function of approximately 0.1oC/watt/sq.m;
and this result must have included within it the effects of all
significant feedback processes between the Earth, the oceans and
atmosphere that operate over large time scales.
This number is just slightly less than the value of
0.113oC/watt/sq.m, which I had acknowledged must be an upper
limit. It is identical to the value that Newell and Dopplick
determined, so it seems to be the value of the Earth's surface
air temperature response function that should be used in
evaluating the climatic effects of CO2. And this value gives a
temperature increase of 0.25oC for a doubling of CO2
concentrations in the atmosphere.
The computer modeling and empirical approaches both make the
same assumptions about radiative changes that would be caused by
an increase in atmospheric CO2, so I find no recourse but to
reject as erroneous the great temperature increases predicted by
the GCMs, as generally run. However, when these models are run
with the constraints on sea-surface temperature that Newell and
Dopplick have noted, they too give an increase in temperature of
0.3oC for a doubling in the CO2 concentration, as has recently
been demonstrated at Oregon State University.
So where do we go from here? The case for the empirical
evaluation of the surface air temperature response function seem
complete; and it cannot be significantly in error. Let me use it
to make further extrapolation concerning the climate.
At present the atmosphere reflects about 90 per cent of the
energy from the Earth back to the ground: that is, its emissivity
is 90 per cent. Suppose the atmosphere were to send even more
radiation back. I have shown that in going from a value of zero
(assuming the Earth had no atmosphere) to the current value of
about 90 per cent the surface air temperature response function
has averaged the same as its current value -- that is about
0.1oC/watt/sq.m. Thus, there is good reason to believe that it
would not change significantly if the atmosphere's emissivity
were to increase by 10 percent, to unity -- that is, if the
atmosphere reflected all radiation from the Earth back to the
ground. This being the case, it is easy to show that the maximum
temperature increase that this change would cause is just
slightly over 4oC. With a response function an order of
magnitude greater, however, the GCMs predict a temperature
increase on the order of 40oC. Which estimate is more realistic?
One way to approach this question is to look at past climates.
We do not know for sure how the composition of the atmosphere has
changed in the distant past, but there is reason to believe that
the proportions of certain greenhouse gases may have varied
considerably; and it is possible that at some time over the past
several million years the atmospheric emissivity may have been
significantly greater (or smaller) than it is now.
But how has the temperature varied? At the meeting on past
climates at the "Carbon dioxide and climate research program
conference" held in Washington DC in April 1980, W. Broecker
noted that over the past few million years the Earth may never
have been more than a couple of degrees warmer than at present;
and this seemed to be the consensus of most of the people at the
meeting. Also, R.K. Matthews and R.Z. Poore, writing in Geology
(vol 8, p 501), have suggested from data on changes in
concentration of one isotope of oxygen, O18, that ocean surface
temperatures have over that entire period remained relatively
constant at about 28oC. Thus, whereas the GCMs lead one possibly
to expect significant temperature excursions from the present
mean global value, the available empirical evidence indicated
that this is not the case for at least the past 50 million years,
and possibly the past 200 million years (Geological Society of
America Bulletin, vol 88, p 390). Consequently, the GCM
predictions that temperature would increase by 2-3oC due to a
mere doubling of the atmospheric concentration of CO2 seem highly
suspect.
But let us consider again the temperature rise that could be
caused by an increase in the atmosphere's emissivity. How big is
the effect on emissivity of doubling CO2 in the atmosphere? In
reality, very little. If the atmospheric CO2 were to increase by
a factor of 10, it would still fill in only about 20 per cent of
the atmosphere's "window" to electromagnetic radiation. Thus,
even an order of magnitude increase in the concentration of
atmospheric CO2 would increase the mean global air temperature by
only about 0.8oC; and such an increase in concentration is
considerably greater than any current predictions.
There is one more point we can learn from past climates.
Recent studies of air trapped in ice cores show that the amount
of CO2 in the atmosphere 18 000 years ago was about half what it
is now (Nature, vol 284, p 155). And other work has shown that
incoming solar radiation at that time was not much different from
now (Meteorological Monographs, No 34, American Meteorological
Society). The GCMs predict a drop in temperature for that time
of more than 2oC (Nature, vol 209, p 9), but data obtained from
three cores in the subtropical gyre -- a current of the Atlantic
at 20oN -- show essentially no temperature difference at all
between than and now (Geological Society of America Memoranda, no
145, p 43). This again indicates that the results of the models
are at odds with reality.
Considering all the available evidence, then, there seems no
reason to suppose that carbon dioxide gas, present in the
atmosphere only as a "trace", has any more than a "trace effect"
on Earth's surface air temperature. Thus, we should not regard
potential increases in concentration due to continued utilisation
of fossil fuels as bad. Quite to the contrary, such an
enrichment of CO2 in the atmosphere is desirable -- because of
its helpful effects on the growth of plants.
* * *
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