Wednesday, May 16, 2012


cc: "Pittock,Barrie" <>, "Hennessy, Kevin" <>, "Jones, Roger" <>, "Whetton, Peter" <>
date: Wed, 28 Jul 1999 17:52:35 +1000
from: "Pittock,Barrie" <>
subject: scenario leaflets
to: "Hulme Mike (E-mail)" <>

Hello Mike,

I am giving a preliminary response to your suggestion that Peter Whetton
comment on your scenario material in case there is some urgency. Peter
did write an email last Friday night before going on a week's holiday,
but unfortunately the email system failed and it probably did not go and
has been lost. He asked Roger Jones to respond on behalf of the group
but Roger is snowed under at present.

Peter and I did discuss it on Friday. Our main concern (although there
are other more detailed ones) is your use of the 95% confidence limits
of natural climatic variability as some sort of threshold for change.
This is a reasonable thing to do if you are addressing the question of
whether climatic change will be detectable at a "scientific level" of
confidence, but that is certainly not the question I would expect WWF to
want answered, nor is it the one most relevant to giving policy advice.
The relevant question is "What is the best estimate of climate change,
given the assumption that increasing GH gases will cause change?". The
contrast between these questions, the statistical criteria they require,
and thus the answers, is what I was driving at in my comment on your
paper in Nature. It is a very serious difference with serious
consequences for how people will interpret your advice. The results as
you present them suggest that many areas will have precipitation changes
(particularly) which are small compared to natural variability, and
therefore it does not matter. But if the change in mean is some
appreciable fraction of natural variability, say, 50%, that is a very
serious matter which ought to concern policy makers, because it will
have cumulative impacts, especially in regard to large changes in the
frequency and magnitude of extremes (floods and droughts). Surely you
understand that! - refer to the standard diagrams of the impact on
extremes of shifting a normal distribution by one standard deviation.

What you are doing is using a strict Type I error criterion when others
(WWF?) might think a Type II error criterion is more suitable (the
Precautionary Principle), and reasonable people (like me of course!?)
think a criterion in between which measures risk of serious impacts is
what is needed for policymakers. The reference I gave in my comment in
Nature may not be the best - but look at my argument in QJRMS, 109,
pp.46-48 (1983) for a clearer exposition on this point.

The other related matter is that your scenarios for WWF, and for that
matter for IPCC WG2, do not discuss the importance of changes in
extremes, which are arguably the most important changes, however poorly
understood they may be at present. This and the other caveats you are
intending to include in the IPCC material, re scaling, sulfate aerosol
effects, longer timescales, and change after stabilisation of
concentrations, should be in the WWF material also, even if they
complicate things a bit (I have not checked whether some of that is in
your WWF stuff as yet).

I would be very concerned if the material comes out under WWF auspices
in a way that can be interpreted as saying that "even a
greenie group like WWF" thinks large areas of the world will have
negligible climate change. But that is where your 95% confidence limit

Sorry to be critical, but better now than later!

Best regards,


Dr A. Barrie Pittock
Post-Retirement Fellow*, Climate Impact Group
CSIRO Atmospheric Research, PMB 1, Aspendale 3195, Australia
Tel: +61 3 9239 4527, Fax: +61 3 9239 4688, email:

* As from 1 March 1999 I have become a CSIRO Post-Retirement Fellow.
This means I do not have administrative responsibilities, and am
working part-time, primarily on writing for the Intergovernmental Panel
on Climate Change. Please refer any administrative matters or contract
negotiations for the CIG to Dr. Peter Whetton, the new Group Leader, at
<>, tel. +61 3 9239 4535.

"Far better an approximate answer to the right question which is often
vague, than an exact answer to the wrong question which can always be
made precise." J.W. Tukey as cited by R. Lewin, Science 221,636-639.

No comments:

Post a Comment