Steve

Yes, there are some naturally occurring chlorocarbon and bromocarbon
compounds. One of them, methyl bromide, does have a very significant
ozone-depleting potential. In fact, about 1/4 of the atmospheric MeBr
loading does come from natural sources (I don't know the exact mechanism
of production, but I think it is from a decomposition of leaves in
tropical rain forests). The other 3/4 comes from soil and crop
fumigation using man-made MeBr. However, total MeBr represents only a
small fraction of ozone-depletion from CFCs and halons, none of which
occur naturally. It is probable that trace quantities of chloromethanes
may also be produced naturally, although this has not been confirmed.
Only one of these, tetrachloromethane or carbon tetrachloride, is
considered to have an ODP. Because man produces thousands of tonnes of
this substance annually and it has a long residence time, if nature
produces a few hundred grams, it would be undetectable.

The ozone layer is in an equilibrium state: on the one hand, ozone is
constantly being formed by the photolysis of oxygen molecules forming
atomic oxygen which reacts with other oxygen molecules to form ozone,
during daylight hours. On the other hand, ozone molecules are constantly
being destroyed by both natural and man-made phenomena. If man were not
a chemical manufacturer, the level would be essentially what it was a
couple of hundred years ago. However, by a complex series of reactions,
any chlorocarbons or bromocarbons which happen to reach the stratosphere
have their Cl or Br atoms knocked off them by solar radiation. One
chlorine atom will, by a series of catalytic reactions, destroy an
average of 100,000 - 500,000 ozone molecules before it, itself, is
likely to be incorporated into another compound or "falls out of the
ozone layer". Bromine is 30-80 times worse again. It is therefore easy
to see why traces of ozone-depleting gases can cause enormous damage. To
put this in perspective, the ozone layer is within the altitude range of
12 to 50 km which seems enormous, but if you imagine a column of air
above your head and you managed to pull down every molecule of ozone
within that column to sea level, do you know how thick your ozone layer
would be at atmospheric pressure? On an average, just about 3 mm, so, in
reality, there is precious little to destroy but, without it, life on
earth as we know it would be impossible. We have seriously upset this
equilibrium.

If you look at the document I mentioned in my last message, you will see
that the thermometer data is used from the last 140 years, where the
temperature increase is best estimated at 0.6 +/- 0.2 °C. Proxy data
derived from tree rings, coral growth and ice cores, as well as
descriptive historical records going back 1000 years show a very good
correlation with a better than 95% confidence level. I quote "...the
rate and duration of warming of the 20th century has been much greater
than in any of the previous nine centuries...". Ice core analyses show
four substances, all associated with human industry, (CO2, CH4, N2O and
SO4) follow almost identically-shaped curves to the temperature. These
four substances, together, contribute an effective extra radiative
forcing of about 2 W/m2 (see Fig. 3 in ib. cit.). P. 10 of the same
document presents the "new and stronger evidence" (since the previous
report) that the observed sudden (climatologically speaking) temperature
rise is due to human activities. In my opinion, the computer modelling
shown in Fig. 4 is remarkable in that it shows that the observed global
mean surface temperatures do not fit either the modelled natural
variations (solar/volcanic etc.) or the anthropogenic variations over
the past 150 years. However, put the natural and man-made together and
the fit is almost perfect. This gives me confidence that the modelling
is now excellent. Fig 5 shows the 21st century prediction under 7
different scenarii. Ignoring the "extremist" upper and lower limits of
these predictions, because CO2 has a long residence time, values will
continue to rise and may even double (average prediction) over the next
100 years simply be accumulation. The following phenomena are classed as
"very likely" (0.9 to 0.99 confidence level) in this century:
- Higher maximum temperatures and more hot days over nearly all land
areas
- Higher minimum temperatures, fewer cold days and frost days over
nearly all land areas
- Reduced diurnal temperature range over most land areas
- Increase of heat index over most land areas
- More intense precipitation events over many areas
And the following are classed as likely (0.60 to 0.90 confidence level):
- Increased summer continental drying and associated risk of drought
(over mid-latitude interiors)
- Increase in tropical cyclone peak wind intensities over some areas
- Increase in tropical cyclone mean and peak precipitation intensities
over some areas
(data extracted from Table 1 ib. cit.)
I would add to that a personal view that, apart from increased
precipitation, flooding in coastal regions because of tropical cyclones
will increase because the atmospheric pressure will be lowered in the
"eye" because of the higher intensity, causing higher tides. Finally p.
17 lists the enormous amount of work still to be done to obtain a better
and more complete understanding, both in terms of observations and
studies.

In short, we know, today, a lot more than we did 5 years ago, but we do
not know everything that will allow us to be scientifically categorical.
Notwithstanding, the evidence is sufficient to force any thinking person
who looks, even at this brief document, into having a few alarm bells
ringing in his mind and forcing him to say "can we take the risk of
leaving the world in a barely habitable condition for our
great-great-great grandchildren or, if we do, are we not being so
totally selfish and egocentric that we shall be morally condemned for
taking this risk?".

As I write this, visibility out of the window is barely 100 metres due
to a dust storm. A few years ago, these were extremely rare phenomena (1
or 2 per decade) on this little island. This is the third such this
year. Why are we suddenly having the Sahara desert dumped on us? Maybe
it is nature's way of allowing us to bury our head in the sand?

Finally, if I have misled you in thinking that we are trying to predict
3 billion years of weather, I apologise. We cannot predict reasonably
accurately better than 24 hours or perhaps a few days with a reasonable
"likelihood". However, climate is not the same as weather, but weather
is influenced by climate. Do please read the cited document.

Hope this helps.

Brian

Steve Joy wrote:
>
> Brian,
>
> This is very interesting. Are there any naturally occuring sources of
> chlorinated or brominated hydrocarbons? While the mechanisms may be
> rational, who has a credible mass balance?
>
> No matter what the scientists you reference conclude, aren't you a little
> skeptical of trying to use 100+ years of weather data to predict 3 billion
> years of weather?
>
> Steve
>
> ---------------------------------------------------------------------------------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
> Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
> information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
> ---------------------------------------------------------------------------------

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------