A commentary on ozone depletion theories

Atmowbrio

&Ivironmentvol.

lo,

pp.

W-lb

Pergamon Pmns

1976,

Printed

in

Great Britain.

A CO~NTARY ON OZONEDEPLETIONTHEORIES by R S Scorer Imperial College, London (ReoeW3d 8 Deoemker1975) 1

2

Purpose The discussion

surrounding

the theories

The Chemistry The criticism

of the chemistry

is not

of ozone depletion by NO, and Chlorine has taken a turn unusual in science, that con-

that what has been put into the theories is wrong but that it is not complete enough to

clusions

be valid

really special possible

have been drawn before they are There has been a lot of justified.

pleading, of the form that the best theory has been employed, leading In this case the theories to moralisations. point to a danger in the future and the authors urge that public action should now be taken on the assumption that the danger is real unless the conclusions of the theories can be proved wrong. My purpose is to point out that the theories are wrong, but that it is impossible to prove that Equally, it is their conclusions are. impossible to disprove opposite conclusions. It is therefore essential to bring the discussion back to where the science has got us so far and not todabble in conclusions. In my view there are other far more serious dangers immediately ahead for humanity associated with resource depletion and overpopulation that the argument about the alleged risk of ozone depletion is frivolous. That is a separate issue. The need for this paper arises from the failure of some authors of ozone depletion theories to practice the art of criticism oecause of their concern with potential dangers. They have been determined to reach a conclusion, llave used the best theory available [I certainly cannot formulate a better usable one) and regard that as valid even if the theory is wrong: it is not their duty, apparently to formulate and utter criticisms!

for the purpose of calculating the It is not long effect of a disturbance. since NO, was declared to be a danger because of its natural absence from the stratosphere + Then it became the chief agent for the control of ozone concentration. Now naturally occurring chlorine must be (but has not yet been) included in the balance. It is still believed that the N20 produced organically in the ground is the chief source of NO, for the stratosphere, seemingly because the authors of this theory are unable, at present, to think of any other. Indeed the stratosphere is widely assumed not to contain anything much that can be rained out in the troposphere. This is a patent error if only because water vapour is sometimes enough to produce mother of pearl clouds at heights between 18 and 28km, and possibly higher. The wave clouds are not difficult to observe from the ground, and show optical characteristics which have only been explained on the hypothesis, due to J Hallet, that there are layers of plentiful sulphate particles which change from the anhydrous to a hydrated state on passage through the cloud. Layers of sulphate aerosol are not generated I;n situ, and sulphate is easily rained out in the troposphere, Clearly there are many other natural pollutants present in the stratosphere which have not been taken into account, including NO, and methyl chloride, in quantities at present unknown, On a recent I observed cirrus

flight London-Gander return, cloud 5,OOOft above the

178

Commentary

tropopause and discrete layers of haze up to l:,OOOft above the tropopause at least. The flight 3

went no higher.

Tne Dynamics

The basic assumption of the depletion theories is that the vertical displacement of

One of the problems into

the stratosphere

the ascending

air

of upward motion

is to understand is kept warm.

how

Aerosol

haze could play an important role in this. Ozone is also a warming agent which cannot be ignored if there is any significant absorption of W by the ozone in question. If ozone is patchily

depleted,

the depleted

chemical species is adequately represenThis is ted by a diffusion coefficient. Any observation of a clearly wrong. layered structure contradicts it. Every cnemical species otner than oxygen,n~trogen and CO2, which has been measured with sufficient vertical resolution has shown a layering, often with variations of over 50% This is particularly true in lkm or less. of 03 and I$0 which have often been measured. i)tner substances are more difficult to

patches will necessarily become cooler, and will be flushed out of the stratosphere more Thus quickly than other masses of air, because the chemical species in question haze and ozone - affect the motion of the an eddy diffusion coefficient based eddies,

measure but tne unaided sunset from existence is

Aircraft exhaust obviously pollutes only small tubes of air and these could well have not only a different chemical but also dynamical history from the average air mass, Freons are not uniformly distributed in the troposphere and a mass of air entering the stratosphere with a higher than average freon content could have a quite different history from one with a low freon content.

aerosol haze can be seen with eye from aircraft, sometimes at the ground, and their layered indicated by clouds.

The chemistry is completely altered according to how the high and low concentrations of reacting species are correlated Diffusion coefficients are in space. chosen not on the basis of any mechanical model or theory at all, but so that the theories give the verticaliy smoothed values of some species which have been observed in Since then a few places on a few occasions. the concentration of other substances, eg HCF., has been measured to be within a factor of two or so of the value predicted on this But that is not in the least surbasis. prising, and certainly does not validate a prediction of the effect of a disturbance of one component on the concentration of another, because it takes no account of the layering and correlations therein, The layering is a consequence of the shearing into sheets of originally more compact bodies of air which cross the tropopause, and the geography of that crossing can only be guessed at present; numerical models .are not very good at predicting it yet. Tire influence of hurricanes on the mechanism is one of the many effects that has not yet been explored but is of major importance.

on average conditions is almost certain to be wrong for ozone and haze, or for that matter any other species which can significantly affect the radiative equilibrium of the air and its motion as a consequence.

The diffusion theory contains the assumption that every upward progressing species has the chance and the necessity to react with every downward progressing specie when they pass through the same altitude. This is utterly in error. The present 211 and 3D dynamical theories make some adjustment for meridional circulations but make no allowance for the mechanisms discussed here. It is sometimes alleged that because “different” theories agree in their conclusions, the conclusion is more credible, In fact the theories do not differ in essential and are all equally subject to the criticisms. If they did differ essentially in their models their agreement would merely be a fluke or indicate that the answer did not depend on the differences. If they differ in essentials all but one must be, an all may be wrong, and it is not real support for a conclusion that several wrong theories lead to it.

179

Co~entary In

view

of the

slow progress

made over

the last three decades with dynamical weather forecasting in spite of the enormous with model making and computing, progre and the large amount of chemical monitoring that would be needed to handle a numerical model of the stratosphere and tes.t it, it is most unlikely that a quantitative prediction of ozone incidence, let alone depletion, will be possible in any detail in this In my view it will not be possible century. except in a future profitably imagine

with features today.

we cannot

Emerging from this discussion is the theme that the factors which will determine the effect of pollutants which react with ozone lie in the complexity, both chemical The and dynamical, of the atmosphere, history of meteorological thinking has on shorn that catastrophic effects predicted the basis of a simple model have always The atmsophere is not in the been wrong. least in a state of delicate equilibrium: it is a very inefficient machine with an enormous turnover and many complex interacting mechanisms which already produce far greater ozone variations than any human There is no prospect of influence could, indicating whether interference with ozone might produce climatic effects which are advantageous or disadvantageous to mankind. In my view there is unlikely to be any perceptible effect either way because of the large tropospheric and geographical influences which are dominant. 4

Biological

Effects

Even accepting the most extreme predictions of ozone depletion, which I do not, there is almost no basis for predicting any biological consequences, let alone harmful ones. It is a non sequitur to say that if two quantities are correlated (in this case only very crudely) the same linear correlation will hold if one is altered. Al though W is obviously related to skin effects, these are very dependent on temperature, chemical pollutants in the air, and more particularly the pattern of duration of the radiation. Thus the Swedes receive a large

amount of W in the summer but recover any ill

effect

in the winter

whereas

from some

white

skinned people in Queensland suffer a The consequences of cumulative effect, exposure are dependent very much on way of life, and harm is not difficult to avoid completely. The correlation

with

latitude

is not

good, and because of the enormous variations due to local causes the W intensity alone In particular W is a minor influence. incidence

is well

correlated

with

altitude

but skin effects are not. The use of the emotive word cancer for a disease which is avoidable and curable is mischievous in with this context, expecially when associated numerical prodictions of incidence which are without valid foundation. 5

Observations

Ozone depletion by pollution has never been observed. Observations, such as a very crude profile of the concentration of HCL, which it is sometimes claimed have confirmed the depletion theories, have not confirmed anything that the critics of the theories had expected to be otherwise. No observations critical to the theories have been made, and we are in the process of colleotipo fragmentary measurements in a few places which may or may not turn out to be representative of other places. Until extensive measurements have been made which enable correlations in space of the small scale variations of the different chemical species to be calculated, no theory of ozone depletion will be fully valid, and there is at present no plan to obtain such correlations, evidently because their importance is not appreciated, so extravagant have been the claims made on the basis of the diffusion theories. The radiative characteristics of the airparcels with different composition must also be considered in some detail. 6

Conclusion

In the meantime, which is likely several decades, before a calculation proper basis is possible there is no

to be on a

Commentary

100 justification of

for

any kind will

magnitude variations

predictions produce

is on the scale in layers

model,

similar

least,

its

in complexity

assuming

that

sort

of

misleading

depletion

and the biological

on the assumption

are farfetched

as to the nature, of

by

certainly that

and quite causes,

and

the symptoms in

question.

observed, References

can be explained

of way but only

of

suggested

controllability

by

the mechanisms used to explain

An appreciation of all

they are right

climatology

ozone

are almost

wrong and exaggerated, effects

model at

of

theories

a

would be necessary.

the variations

them have an effect

plexity

to that

that predictions diffusion

over-

to the present

an ozone

similar

in a qualitative

the present

tropospheric

variations,

At present

of

whose

For confidence

which produces

statistically with

pollution

and in the total

head amount of ozone. most complicated

that

a depletion

the right the actual

the mechanisms

magnitude, com-

suggests

Jones,

A (1975)

Ozone depletion

New Scientist, Scorer,

R S (1975)

i\iature, 259,

68,

140 969,

and cancer, 14

Haze in the stratosphere, irlo 5531

Hallet, J (1975) PehloniL Commnication.