Thursday, December 15, 2011


From: Jonathan Overpeck <>
To: Bette Otto-Bleisner <>, Keith Briffa <>, Tim Osborn <>, Eystein Jansen <>,,,
Subject: Urgent - FINAL review/edits of 6.5.8 Sensitivity
Date: Wed, 12 Jan 2005 16:55:36 -0700
Cc:, Jean-Claude Duplessy <>

Hi all on the list above... Some of you have received this already straight from David, but
some other key people have not. Eystein and I would appreciate it very much if you would
please read/comment/and edit the attached section 6.5.8 (Sensitivity) NO LATER THAN
THURSDAY NOON, Eastern time (6PM GMT).

Please send responses to all on the address list ABOVE, plus Peck.

Thanks, Peck

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Date: Wed, 12 Jan 2005 13:29:53 -0500
To: joos <>
From: David Rind <>
Subject: Re: Fwd: Re: Fwd: 6.5.8 Sensitivity
Cc: David Rind <>,
Jonathan Overpeck <>,
Dominique Raynaud <>,
Eystein Jansen <>,,,
Jean-Claude Duplessy <>,,
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Dear Fortunat (and others),

Here is the revised section 6.5.8. I've put in most of your changes (and also most of
those suggested by Stefan, particularly with regards to clarifying the sign of the
radiative forcing). Most importantly, I've removed the table - I agree it seems to imply
a solidity that is really not there. The one thing I have not done is condense it
greatly (of course!). The real reason for going into such detail, rather than just
saying, "well, the forcing and response are uncertain, so we can't conclude anything",
is I think it's important to show that paleoclimate scientists have gone to some effort
to try to deduce climate sensitivity from the paleorecord, the parameter that is
probably of most interest to IPCC. In that respect the details are important, as are the
magnitudes of uncertainty represented in the different studies. Obviously, at any point
in the proceedings the section can be shortened, but I thought it useful to start with
this level of quantification, and show paleoclimate has this similarity with the rest of
IPCC in addition to more qualitative concepts.

I've responded to your individual comments below.

At 6:15 PM +0100 1/11/05, joos wrote:

Dear David,
Here my comments on the updated climate sensitivity section. Please
apologize if I formualate my comments straight away, but I need to leave
very soon. Many of my comments might have to do with presentation.
Your main conclusions in paragraph f are fine.
My view is that it would be ideal to address the issue from a
probabilistic view point. this is of course not always possible.
1) Maunder Minimum section:
Several studies using Monte Carlo approaches show that almost any
climate sensitivity is posssible when taking into account uncertainties
in radiative forcing input data as well as observational records over
the 20 century as constraints. See the Paris report for more
The uncertainty does not only arise from indirect aerosol effect, but
also form the whole range of forcing agents that all have an uncertainty
attached. E.g. Reto Knutti did some evaluation of his results where he
assumed that the aerosol forcing is exactly know (No error) -> even then
climate sensititivity remains unconstraint. Clearly, uncertainty is
growing when going further back in time than the last century as done
here. Then, the numbers provided in the table are useless, as you now
state in the last sentence of the revised text.
2) Other sections:
I think similar concerns also hold for the other sections. For example,
the LGM global cooling is very uncertain. I have just heard yesterday a

talk by Ralph Schneider who showed how different SST reconstructions
(Alkenone, Cd/Ca, MAT, radiolare etc) disagree. global SST cooling might
be anywhere between 0 and 4 K or so. Of course, CLIMAP and the recent
GLAMAP update provide a reasonable estimate. However, the point is that
uncertainies are huge.
The table is a very focused and stand alone thing for the reader. It
gives the impression that climate sensitivity for different period can
be well evaluated. However, this is not the case.
3) My conclusion:
- The table should be dropped. I have quite a strong feeling here, as it
seems to me that the number in the table are very hard to defend and
should not be made prominent.

The table and reference to it has been dropped.

- The whole section should be condensed considerably. Your main
conclusions in paragraph f are fine.

Well, removing the table will shorten this section!

Further comments:
1) section d) 1. para: solar forcing reduction estimate range up to

0.65% for MM e.g. Reid, 97 and Bard et al.

Correction made, and reference added (and I also corrected the numbers as Stefan
suggested, although the upper number is actually larger given the Reid estimate).

2) section d, last para equilibrium
The statement that transient effects are not important is very hard to
2a) The warming and forcing up to today is considered. Certainly, we are
now far from equilibrium ( a lag of 30 years or so).
2b) the volcanic forcing is very pulse like and I do not see how the
equilibrium concept holds here. It can only be evaluated in a transient
3c) The MM is probably not in equilibrium climate, as solar forcing has
likely varied over the MM as indicated by radiocarbon, althoug sunspots
were not present

I've removed the word "transient" but I have justified the equilibrium aspect of the
sentence with a reference (we investigated that issue by running from 1500 through the
Maunder Minimum, and seeing what the prior changes in solar forcing did to the Maunder
Minimum cooling - the effect, as noted in the reference, was small in our model).

3) section b) end of 1. para: How should such a 'general climate
sensitivity' be defined?

For now I've simply suggested what should also be factored in; I don't know that it's
our place to come up with a new definition per se, although if IPCC is interested, we
could try!

4), section c) Somewhat a mix of model and observations. end of 2 para:
It is not clear which forcing was operating in these different models
(at least it is not stated in the text) and hence one can not directly
imply a climate sensitivity in the way done here. For this the forcing
that went into the model simulations must be known.

I looked at each of the references and saw what forcing they actually used - they were
all very similar except for one which used current orbital parameters (not really
important). This comment is now included.

Hope this is useful and looking foreward to further debate the issue.

Thanks for the comments!


ps - Jonathan, the attached Endnote library includes the references we discussed
yesterday, as well as all the ones relevant for this section.




Jonathan T. Overpeck
Director, Institute for the Study of Planet Earth
Professor, Department of Geosciences
Professor, Department of Atmospheric Sciences
Mail and Fedex Address:
Institute for the Study of Planet Earth
715 N. Park Ave. 2nd Floor
University of Arizona
Tucson, AZ 85721
direct tel: +1 520 622-9065
fax: +1 520 792-8795

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