Friday, May 18, 2012

4396.txt

cc: fiona.smith@metoffice.gov.uk, "Tett, Simon" <simon.tettatXYZxyzoffice.gov.uk>
date: Thu May 12 09:35:28 2005
from: Tim Osborn <t.osbornatXYZxyz.ac.uk>
subject: Re: Palaeo reconstructions
to: "Jenkins, Geoff" <geoff.jenkinsatXYZxyzoffice.gov.uk>

Thanks for the opportunity to see this, Geoff. I have a few points to raise:
(1) There were no years, or indeed any x-axis labelling, on the figure.
(2) Perhaps this will be said in a caption, but please make sure that somewhere (caption or
figure) it is clear that this is northern hemisphere mean annual temperature, rather than
global mean annual temperature.
(3) It might also be worth saying that the apparent narrowing of the range in the first 2
centuries isn't because of smaller uncertainties, but rather because of few reconstructions
from which to define the "envelope".
At 16:41 11/05/2005, Jenkins, Geoff wrote:

Are you happy for us to use it
in the booklet of slides we are doing? I will credit you as the source.

(4) Yes, fine with me.

I assume zero is the 61-90 mean for both instrumental and
reconstructions.

(5) Correct.

My only concern is with the very obvious difference between
reconstructions and instrumental over the last few decades. I recall
Keith mentioning this when he gave us a talk, but I don't remember the
reason. If we don't give one then the obvious mismatch will raise
unanswered questions and may cast doubt on the reliability of the
reconstructions. Does IPCC comment?
I suppose another way round would be to simply miss off the last 30 yrs
of the reconstructions. Your advice would be appreciated.

(6) There are some issues with whether reconstructions capture the post-1980 warming. But
in this case, these issues are dominated by effect of there being very few reconstructions
past 1980 and thus (as with the first two centuries - point (3) above) the envelope is
poorly defined. Indeed only one extends to the end of the envelope (hence the convergence
to a point) and this is the borehole record which couldn't resolve any acceleration in
warming on time scales of a decade or two anyway! I would suggest cutting off the
reconstruction envelope in 1980, after which there are far fewer reconstructions available
(and those that are available are based on fewer constituent proxy series). This seems
honest because it will be clear that we are not using the reconstructions to "confirm" the
recent warming, for which we rely on the instrumental temperatures, but taking it through
to 1980 will be far enough to show that while the instrumental series is still just within
the reconstruction envelope, it is not central and thus there are possibilities of
reconstructions bias which deserve investigation even if they aren't disastrous to our
interpretation. I think that's a fair view.
Hope this is useful,
Best wishes
Tim

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