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Re: Human vs. natural influences on the environment
Harold Brashears wrote:
>
> Leonard Evens <len@math.nwu.edu> wrote for all to see:
>
> >snark@swcp.com wrote:
> >>
> >> In article <32244d4d.176361891@nntp.st.usm.edu>,
> >> Harold Brashears <brshears@whale.st.usm.edu> wrote:
>
> [edited]
>
> > >In any case, if I remember correctly, the report forcasts an average
> >> >global 0.5 C change in the next hundred or so years. I still do not
> >> >see that, even if all scientists agreed, as any type of emergency.
>
> [edited]
>
> >The actual range given is from 1 deg C at the low end to 3.5 deg C at
> >the high end by the year 2100. This is about 1/3 less than estimates
> >in earlier IPCC Reports. As indicated in the above quote, the main
> >reason is inclusion of cooling by aerosols, which are somewhat better
> >understood than they were earlier, but about which there is still great
> >uncertainty. It is important to be aware of the full plausible range
> >since as Michael Tobis has often pointed out, the worst case scenario
> >which has some reasonable degree of plausibility, could lead to much
> >greater economic and social disruption than the most benigh or even the
> >best guess.
>
> I think I would place less plausibility on the higher end, based on my
> own observations of the global warming reports. Every time a new or
> revised model is generated, I have observed a downward trend in the
> forcast. This will probably not continue forever, but I am not yet
> convincewd we are through, either.
>
> I will place much more confidence in our ability to forcast the
> climate in one hundred years when I observe that we can take data from
> 1920 to 1960 and predict correctly the climate in 1990. This is a
> mere 30 years.
>
I am a bit mystefied by this comment. Why use only the data from
1920 to 1960 and then try for 1990? As Michael Tobis and others have
pointed out, you can prove anything you want by carefully selecting
which dates in the record you start and finish at. A more reasonable
thing to do is to look at the entire observational record from some
early base point and see how accurately the models can track it. The
more data you try to match, the harder it is to get agreement by
tweaking parameters. The point made by Santer and his colleagues in the
detection chapter (8) of Climate Change, 1995 is that this is now at the
level where we can say with confidence that human activities have
affected climate but the uncertainties are still too large to quantify
the human contribution. They also add that this does not mean that the
human contribution is likely to be small. It could be small but it
could just as well be large. It should also be added that Santer and
his co-athors look not only at the record of average global temperatures
but also at things like regional effects, vertical profiles of important
variables, etc.
Mr. Brashears certainly has the right to use whatever criteria he wants
to judge the significance of the problem, but the rest of us have no
reason to share his beliefs. If we have to choose between him and
Benjamin Santer, I think there is little doubt whom to pay more
attention to. He is of course right to assume the probability of the
higher end estimate in the IPCC Report is less than that of the best
estimate. After all, when you give a range, the probability density
reaches its highest point somewhere in between and is lower at either
end. The mode in this case is about 1 deg C higher than the lowest end
and 1.5 deg lower than the highest end. If indeed we could make
precise estimates of the probabilities of each outcome and of the cost
of each outcome, the contribution from the lower probability event at
the high end when multiplied by its cost would contribute appropriately
to the sum. Unless you can make a cponvincing argument that its
probability is close to zero, which I don't think is supported by the
state of current knowledge, then its contribution to the expected loss
would be large enough, I would think, that it should not be ignored.
For emphasis, let me add again that since its probability is less,
it is already discounted to some extent in the calculation. Of course,
as I have said before, we can't really make these calculations at
present, but we still can do thought experiments to help guide our
understanding. That is, we should hedge our bets to some extent to
account for an event of lower probability but much higher cost.
I am not suggesting that Mr. Bashears has done this here, but some
commentators are guilty of the truncating the probability curve at the
middle or lower end of the range of probabilities. I guess that is
the belief that bad things just don't happen and one shouldn't worry
about them until they hit you in the face. According to decision
theory, you shouldn't become obseessed with such eventualities but you
should take them into account in some rational manner.
>
> Regards, Harold
> ----
> "Monster one minute. Food the next."
> Kiakshuk, Inuit Hunter
--
Leonard Evens len@math.nwu.edu 491-5537
Department of Mathematics, Norwthwestern University
Evanston Illinois
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