Friday, December 30, 2011

1254345329.txt

From: Tim Osborn <t.osborn@uea.ac.uk>
To: Michael Mann <mann@meteo.psu.edu>, Phil Jones <p.jones@uea.ac.uk>
Subject: Re: attacks against Keith
Date: Wed Sep 30 17:15:29 2009
Cc: Gavin Schmidt <gschmidt@giss.nasa.gov>

At 16:06 30/09/2009, Michael Mann wrote:

And Osborn and Briffa '06 is also immune to this issue, as it eliminated any combination
of up to 3 of the proxies and showed the result was essentially the same (fair to say
this Tim?).

Mike,
yes, you're right: figs S4-S6 in our supplementary information do indeed show results
leaving out individual, groups of two, and groups of three proxies, respectively. It's
attached.
I wouldn't say we were immune to the issue -- results are similar for these leave 1, 2 or 3
out cases, but they certainly are not as strong as the case with all 14 proxies. Certainly
in figure S6, there are some cases with 3 omitted (i.e. some sets of 11) where modern
results are comparable with intermittent periods between 800 and 1100.
Plus there is the additional uncertainty, discussed on the final page of the supplementary
information, associated with linking the proxy records to real temperatures (remember we
have no formal calibration, we're just counting proxies -- I'm still amazed that Science
agreed to publish something where the main analysis only involves counting from 1 to 14!
:-)).
But this is fine, since the IPCC AR4 and other assessments are not saying the evidence is
100% conclusive (or even 90% conclusive) but just "likely" that modern is warmer than MWP.
So, yes, it should be possible to find some subsets of data where MWP and Modern are
comparable and similarly for some seasons and regions. And as you've pointed out before,
if any season/region is comparable (or even has MWP>Modern) then it will probably be the
northern high latitudes in summer time (I think you published on this, suggesting that
combination of orbital forcing, land-use change and sulphate aerosols could cause this for
that season/region, is that right?).
So, this Yamal thing doesn't damage Osborn & Briffa (2006), but important to note that O&B
(2006) and others support the "likely" statement rather than being conclusive.
Cheers
Tim

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