date: Tue, 1 Aug 2006 22:05:40 -0600
from: Jonathan Overpeck <jtoatXYZxyzrizona.edu>
subject: Re: response to your question
to: Keith Briffa <k.briffaatXYZxyz.ac.uk>
Hi Keith - thanks. This makes sense to me. I'll
cc Susan so she understands the issue better, and
also can advise on any strategy we should adopt
to make sure we communicate effectively.
>The TAR was, in my opinion, wrong to say
>anything about the precedence (or lack thereof)
>of the warmth of the individual year 1998.
>The reason is that all reconstructions have very
>wide uncertainty ranges bracketing
>individual-year estimates of part temperature.
>Given this, it is hard to dismiss the
>possibility that individual years in the past
>did exceed the measured 1998 value. These errors
>on the individual years are so wide as to make
>any comparison with the 1998 measured value very
>problematic, especially when you consider that
>most reconstructions do not include it in their
>calibration range (curtailed predictor network
>in recent times) and the usual estimates of
>uncertainty calculated from calibration (or
>verification) residual variances would not
>provide a good estimate of the likely error
>associated with it even if data did exist.
>I suspect that many/most reconstructions of NH
>annual mean temperature have greater fidelity at
>decadal to multidecadal timescales (based on
>examination of the covariance spectrum of the
>actual and estimated data over the calibration
>period. This is the reason many studies
>implicitly (Hegerl et al.,) or explicitly (Esper
>et a;., Cook et al.) choose to calibrate
>directly against decadally-smoothed data.
>The exception is the Briffa et al (tree-ring
>density network based) reconstruction back to ~
>1400. This has probably the best year-to-year
>fidelity � but for summer land only and does not
>go back anyway to the MWP.
>We are on much safer grounds focusing on
>decadal/multi-decadal timescales and so this is
>where we place the emphasis. As for the �warmest
>decade� � this is likely to be the 1990s or the
>last 10 years � but again, the proxies do not
>cover this period, and we do anyway state that
>post 1980 is the warmest period � which I think
>is fair enough.
>Professor Keith Briffa,
>Climatic Research Unit
>University of East Anglia
>Norwich, NR4 7TJ, U.K.
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