In sci.environment \yvind Seland wrote:
In article , Brian Sandle writes:
In sci.environment Thomas Lee Elifritz wrote:
February 2, 2004
Brian Sandle wrote:
What about the earth systems?
[...]
Lots of nice 64 bit processors starting to coming out, write yourself some
software.
My observation is that the physical scientists do not get involved in
statistics in the way social scientists do. I thought to get talking about
correlations to see what understanding could be engendered.
So what do you think of the physical scientists' amount of learning
from partial correlations?
Physical scientists, and in particlular atmospheric scientists
have learned time after time that placing
too much faith in correlations give you nasty surprises. If you just
correlate a big enough number of variables you will always get a few
good correlations just by chance. Just correlating "everything" will get you
so many hits that it is not worthwile to explore this pathway.
You are speaking of putting a whole lot a variables into the Pearson
correlator and looking at what correlations are significant at the 0.05 0.01
level &c. Then I suggest to go to partial correlation.
If you have a hypothesis that there could be a connection, as in your case
between space shuttles and ozone depletion,
I was interested, so I put the two ranks side by side adn thougth there might
be a correlation.
making a raw correlation
make some more sense. Before doing the correlation, and if not then,
certainly straight after, you should think about whether it makes any physical
sense. After all the correlation may have been a fluke,
Yes, with the significance level at 0.05, as I had, that means that if the
thing were done 20 times such a result would be expected once, by chance.
or there
may be a underlying cause to both of your variables.
This is where I understand partial correlation to be useful. The ozone may or
may not be causally affected by several variables: solar UV, shuttle flights
are the two we have looked at so far in this.
So now *your* ordinary correlation table looks like:
ozone flights UV HC
ozone 1 0.4 ? ?
flights 0.4 1 ? ?
UV ? ? 1 ?
HC ? ? ? 1
As for your hypothesis, I think the latter is the case.
The increase in space shuttle flights and use of CFC and other halocarbons have
just come at more or less within the same time frame.
All those things are changing together. One technique of finding out is to do
actual experiments in which two variables are physically held constant and
what happens to the other two is examined. That is quite often possible in
physical sciences, though rather less so in earth sciences on the whole.
So we want to see if partial correlation can tell us anything about that.
The partial correlation calculation takes into account that the variables are
all changing together then looks at the situation if one or more are
artifically held constant mathematically. A lot more figures turn up.
Holding one variable constant would produce another table like the above,
with perhaps the same but more likely different figures. I suggested holding
flights constant. But each of them could be held, giving in total four new
tables.
(Then it is possible to hold two constant at once giving another four tables)
I am not very versed in this, but I understand that if a partial correlation,
mathematically holding a variable constant, is no different from the simple
correlation of the two variables, then it is unlikely that the held variable
is causal.
Now as to your physical explanations.
Part 1: Solid fuel rockets emit chlorine compunds in the stratosphere,
which depletes ozone.
They don't until they have been activated. They emit HCl. Someone said
earlier in the thread that HCl does not react with ozone.
see:
http://ess.geology.ufl.edu/HTMLpages.../lecture4.html
* Polar stratospheric clouds (PSCs) provide surfaces for reactions that
convert chlorine compounds from relativly inert forms to reactive
species that destroy ozone in sunlight.
* PSCs condense in the frigid cold of the stratospheric polar vortices
and provide heterogeneous surfaces for reactions that convert the
reservoir species to more reactive ones.
* The most important reaction is between the two chlorine reserviors:
ClONO2 + HCL - Cl2 + HNO3
* The molecular chlorine produced is in the gas phase and is photolyzed
by even weak sunlight to give chlorine radicals - active chlorine -
that can catalyze ozone destruction.
And there is more trouble related.
True, but the problem with that argument is that this amount
is quite small compared to other emission types.
The rockets also produce a lot of water which may be used to produce the
PSC.
Which brings up argument 2: The emissions are relatively more important
since they happen in situ.
This argument does not hold. The chlorine emitted near the ground
are not different from the chlorine emitted from the solid fuel rockets,
From near the ground comes HCl form of chlorine, as from rockets, too, but
also halocarbons, which do not come from rockets.
The HCl from the sea and volcanoes (near the ground) is emitted (except for
large volcanoes) into the troposphere and washed out by rain. It does not go
across the tropopause.
Yes, both the halocarbons and the rocket HCl end up producing active chlorine
under UV activation, but the question is the level/altitude at which it is
being produced, and the understanding of the whole system mechanism.
An idea is to look at time of launch. Then see what heppens when the rest of
the hours of sunlight have had their effect, maybe with stratospheric cloud
present, added to a night and what happens the next morning.
so whether something is transported across the tropopause or emitted
in situ have no effect on the total ozone depletion.
Excuse me but is that tested or hypothesis?
Thus, since the
amount of chlorine transported from below is much higher than the emissions
from the space shuttle, the contributions from the space shuttles
are small.
From http://www.business.gov/busadv/faq.cfm?catid=113
" In a 1990 report to Congress, NASA found that the chlorine released
annually in the stratosphere (assuming launches of nine Shuttle missions and
six Titan IVs--which also have solid rocket motors--per year) would be
about 0.25 percent of the total amount of halocarbons released
annually worldwide (0.725 kilotons by the Shuttle 300 kilotons from all sources)."
Not zero however and in a sci-fi scenario with tens of launches a day
it would certainly have a large effect. This scenario is however unthinkable
economically.
Maybe a threshhold is being approached an it does not take that much.
Some of the Arianes also have powder strap-ons, and I don't know about
Russian and Chinese rockets.
But what about the water?
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