cc: cddhr@giss.nasa.gov, Eystein Jansen <eystein.jansen@geo.uib.no>, Keith Briffa <k.briffa@uea.ac.uk>, t.osborn@uea.ac.uk, joos <joos@climate.unibe.ch>
date: Fri, 14 Jan 2005 09:31:33 -0700
from: Jonathan Overpeck <jto@u.arizona.edu>
subject: Re: 6.5.8 on climate sensitivity and last millennium
to: Stefan Rahmstorf <rahmstorf@pik-potsdam.de>

   Dear David and Stefan and Co - Thanks Stefan - you raise some good points. The time going
   forward suggestion is good, and I think crafting the prose more carefully in other areas is
   good too. We DO have to be careful about misrepresentation of what we say, and have to
   anticipate what could happen in order to word it so it won't happen.

   David, can you take these into account for what will be the final ZOD 6.5.8? I'm sure there
   will be more later, but not before ZOD unless others get you comments before you're done
   with this revision.

   Thanks, Peck

     Hi friends,
     I'm not working on this topic myself so I'm by no means an expert. But I am still quite
     concerned with the wording in 6.5.8 on the last millennium.
     First, to avoid misunderstandings, I would like to suggest again to describe forcings
     and climate changes going forward in time, rather than going backwards in time. Even
     colleagues here that I discuss it with misunderstand the present version with backwards
     reasoning - it leads to phrases like "deforestation warming" (used by David in his last
     mail), although deforestation caused cooling - backwards in time you can see this as a
     warming, but should you call it "afforestation warming" if you look back in time? I
     suggest to use the physical, forwards, time arrow in the discussion.
     The section states:

     If one takes mid-range estimates of solar and anthropogenic forcings, and assumes that
     volcanic, tropospheric ozone and land albedo changes cancel out, the resulting radiative
     forcing change is ~-2.4 Wm-2.

     I don't think we should give a "mid-range" of the forcing like this; the assumption that
     ozone, land albedo and volcanic changes cancel is hard to justify in any case. For the
     forcing we need to give a range in my opinion, not one number. If we give a range, it
     will become clear that the forcing is too uncertain for drawing conclusions on climate
     sensitivity from this time period.
     The section contains the sentence:

     Relating forcing to response, the sensitivity from the models is then on the order of
     0.6C/ Wm-2 (or higher, depending on the model used); the sensitivity from the
     observations, if taken at face value, would be considerably less.

     Although this was toned down quite a bit, it still sounds like we can say that
     observations suggest a considerably smaller climate sensitivity compared to models.
     Skeptics will trumpet this sentence around the world. And I don't think we can justify
     such a statement.
     I agree that apparently models have not used all forcings. But I think it is not as
     clear-cut as you say that they would come to much larger temperature change as compared
     to the data if they did. I talked to Martin Claussen, co-author of the Bauer et al.
     paper, as he works on this period. Martin strongly disagrees with your statement

     Bauer et al. used a large aerosol effect and still needed a large deforestation warming
     to bring her results in line with the Mann et al. reconstruction (in fact, it was done
     specifically for that reason)

     The GHG, deforestation and orbital forcings are the known forcings, while solar and
     volcanic aerosols rely on uncertain proxy reconstructions. To cover this uncertainty,
     Bauer et al. did a sensitivity study and used different plausible scenarios, e.g., 0.24%
     and 0.32 % for solar change and a couple of scenarios for volcanic forcing, and they
     discuss the results of all those.
     I agree that for 2.4 W/m2 with best guess climate sensitivity you get 1.5 C in
     equilibrium - but much of this forcing is due to a rapid rise in the 20th Century, and
     the climate is not in equilibrium with that - transient runs suggest that only 1/2 to
     2/3 of the equilibrium warming is realised (here the "looking back in time" issue is
     again a possible source of misunderstanding - it is of lesser concern whether the MM is
     in equilibrium, but rather to what extent the present is). That gets you down to below 1
     C from the 1.5 C equilibrium value.

     Then you state the Mann et al. data are 0.5 C below the 1990s in the Maunder Minimum. I
     can see they are 0.4 C below the reference level (I believe this is 1961-1990). The
     mean of the 1990s is 0.3 C above this level (I calculated this from the Jones data) -
     so I find that the Mann data are in fact 0.7 C below the 1990s in the MM. The
     difference between model expectation for 2.4 W/m2 and the actual found in the Mann data
     is almost gone then. Add to that the possibility that the Mann data may somewhat
     understimate the variability, and I do not see any significant discrepancy between
     models and data, which we should mention and which we could defend as real - even for
     "best guess" sensitivity and forcing, let alone considering the uncertainty in those.
     Cheers, Stefan


     --
     Stefan Rahmstorf
     [1]www.ozean-klima.de
     [2]www.realclimate.org

--

   Jonathan T. Overpeck
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   Professor, Department of Geosciences
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