Day 011 - 12 Jul 94 - Page 34


     
     1        paragraph 5,
 
     2        "While ozone loss is expected to increase with increasing
              chlorine abundance in the atmosphere, the relationship
     3        between stratospheric chlorine and ozone is not simple, as
              indicated by the rapid deepening of the ozone 'hole' over
     4        Antarctica during the 1980s.  Stratospheric chlorine is
              expected to continue to increase to levels higher than
     5        previously experienced. There is a risk that these might
              trigger disproportionately larger ozone losses.
     6
              "6. There is a time lag of several years before
     7        tropospheric chlorine reaches the stratosphere, and takes
              part in ozone destruction. The concentration of chlorine
     8        in the troposphere (3.6 parts per billion by volume) ----
              Is that what that stands for?
     9        A.   Yes.
 
    10   Q.  " .... At present exceeds that in the stratosphere (about 3
              ppbv); ozone depletion can therefore be expected to
    11        increase in the short term, even with a cessation of
              emissions of halocarbons.
    12
              "7.  The Review Group has used chlorine loading ....." And
    13        I want to come back to that .,.. "(the total concentration
              of chlorine held in halocarbons in the troposphere) rather
    14        than ozone depletion potential (ODP) to compare the impact
              of releases of different halocarbons on the atmosphere.
    15        The use of ODP is limited by the failure of models to
              predict ozone depletion correctly and by the fact that it
    16        refers to steady state conditions which precludes
              predictions of changes over the next few decades."
    17
         MR. JUSTICE BELL:  That is a change in attitude really, is it
    18        not?
              A.   Yes.
    19
         Q.   Which is the most helpful measurement.
    20
         MR. RAMPTON:  I will pause there, my Lord, because this might
    21        be the appropriate moment for Professor Duxbury to explain
              the difference between the two standards (or whatever you
    22        call them)- chlorine loading and ozone depletion. Can I
              ask you to do that?  Remember that we are all laymen,
    23        Professor Duxbury.
              A.   Chlorine loading of the stratosphere would be
    24        basically the total number of substances in the
              stratosphere which can liberate chlorine, and we are not
    25        making any assumption about the rate of reaction.  The
              idea is that once substances get to the stratosphere, 
    26        sooner or later the chlorine in them will be liberated. 
  
    27   Q.   Is that a markedly different approach from the use of ODP
              as the measure?
    28        A.   It is different in that the ODP effectively was based
              upon the idea of destruction of the substances which can
    29        occur in the lower atmosphere, the troposphere, whereas
              the chlorine loading more or less says that once things
    30        have got to the stratosphere they effectively all have the
              potential to damage the stratosphere.  The current model

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