Friday, June 15, 2012


cc:,,,,,,,,,, Maureen Joseph <>,,,,,,,,,
date: Thu, 31 Aug 2000 12:39:30 +0100
from: Barrie Pittock <>
subject: RE: Extreme climate phenomena

Dear Tim et al.,

Here are my comments on table 3-9#5. I will not attempt to make
track-changes on the table as it will be too confusing. Overall the table
now looks good and will accomplish the task it was designed for, that is, to
set out the basis for impacts assessments which refer to possible changes in
these important phenomena. I still wonder if we have given enough
cross-references to the relevant sections in WGI (or to other sources in the
case of hail or regional changes in TC frequency). Originally I wanted
another column to do that, but it is getting a bit unwieldy so maybe some
strong textual or caption statement re where to find this (more)
original/substantial material would be useful.

My main problem is with the footnotes, which as no doubt everyone has said,
are now mixed up. I also want an additional one re regional changes in
frequency of TCs. I also still have worries re the THC, especially in
reconciling what we say here with what is in Chapter 19 and in WGI. Thus:

Footnote 'd' is a bit of a problem. It is probably meant to apply to small
scale phenomena such as hail and lightning, which are certainly not modelled
explicitly in GCMs. But for lightning Price et al have in fact parameterised
it crudely in a GCM, and some TC specialists remain sceptical of any GCM
modelling anything like a TC, even though "TC-like vortices" having many of
the same characteristics have been modelled at relatively fine resolution in
GCMs and RCMs. So 'd' shooed probably be slightly reworded and applied to
hail, lightning and TCs. Maybe say "No detailed modelling in GCMs, but
physically plausible based on broad-scale GCM modelled changes and/or finer
scale models."

Footnote 'e' probably applies to lightning (Dessens), although I forget
exactly what was the basis of Price and Rind (1994)'s parameterisation - I
think it was depth of convective cloud, in which case this footnote is not
the whole story for lightning.

Footnote 'f' applies to hail, and 'g' to heat index, which I think would be
better called "comfort index".

Footnote 'h' applies to drought, and 'i' to TC intensity. Actually I think
some of the literature on TC intensity suggest that not just peak intensity
will change, but also average intensity, and I wonder if it is wise to give
it high confidence, given that some TC specialists say that until it is all
done at very high resolution we can have little confidence. I would put it
at medium confidence so that it has to be considered but is not taken as
sure thing.

Footnote 'j' is a problem in that we really should distinguish between
partial shutdown or slowing of the THC and total shutdown. This seems
critical to the differences that were present between the WGI accounts and
that in WGII Chapter 19. And if total shutdown is contemplated beyond 2100,
then some idea of why it is there with very low confidence is needed. I do
not think we have answered Tim's query on this point in his email of 21
August. It may be more critical for WGI and WGII chapter 19 than for this
table, but it must be made consistent. How likely and serious would a total
collapse be? Would western Europe get colder than now, or only less warm
than if the THC continued? My suggestion is that the entry re THC should be
"Partial of full shutdown", and the footnote might read "Refers to
likelihood during the 21st century only. However, climate changes over that
period may lead inevitably to a (partial or) full shutdown of the THC during
subsequent centuries. This low probability but possibly high impact event
may represent a significant risk."

Re TC changes infrequency. I think the entry should state "No change in
total world frequency", and in a footnote that "However, changes in regional
frequencies (ie., spatial distribution) are quite possible due to changes in
broad-scale atmospheric features such as ENSO."

I would also rank the increased frequency of hail as low to medium
confidence, again because hail is not explicitly modelled and we leave
ourselves open to criticism from hail specialists if we claim too much.

Congratulations Tim on carrying on such a useful multi WG discussion.



NB. Note current email addresses.

A. Barrie Pittock
c/o <> until 7 September 2000,
and then c/o <> until 25 September 2000.
Back c/o <> from 4 October 2000.

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