date: Tue Jul 13 14:46:52 2004
from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: RE: Overpeck calibrated data
to: mmaccracatXYZxyzcast.net, Tim Osborn <t.osbornatXYZxyz.ac.uk>
Dear Mike et al.
Sorry for my tardy response, but it is important that you define the purpose of the
Overpeck Figure as regards deciding on the "best" version of calibration.
The question of what is the "correct" amplitude of variation in the reconstruction must
take into account the rationale underlying the choice of method of calibration (in this
case the way the data are scaled with respect to some modern mean). In our short piece for
Science (perspective on Esper et al reconstruction), that addressed the uncertainty in
regression of paleorecords on climate series, Tim and I showed the effect of scaling based
on either annual mean or summer mean temperatures (see attached powerpoint slides) .
Differences will also result from calibrating either inter-annual or decadally-smoothed
data . If yearly data exist for both proxy and instrumental data , the resulting amplitude
of longer time scale variation expressed in a reconstruction produced by regression
involving smoothed instrumental and proxy data, will not match that of the appropriately
smoothed reconstruction based on regression with annually resolved data. Similarly,
differences will arise when the same predictor series is scaled against different
area-average data (i.e. NH mean, NH north of 20 degrees North; Arctic temperatures). Hence
a direct comparison of the Overpeck ; earlier Crowley; and original Bradley and Jones
reconstructions (none of which is expressed with inter-annual resolution) with those of
Mann et al.; Briffa et al , Jones et al etc.are confounded by the ambiguous definition of
the appropriate regression model (in fact a couple of reconstructions do not use regression
but only scale the proxy series by adjusting the overall mean and variance to match those
of the instrumental series -without considering any least squares fitting) , and
definition of the seasonal and regional focus.
These aspects need considerably more attention as regards simple statements as to which
reconstructions agree or disagree with each other , or with variously-defined model
The bottom line , is that it might be best to stick with published results for your
purposes (ie the curve in Tim's piece (which he will send separately) - even though these
relate to a larger region than the Arctic . If this is not what you want we would still be
willing to discuss a more appropriate recalibration , taking into consideration the best
area, season etc.
At 13:45 05/07/2004, mmaccracatXYZxyzcast.net wrote:
Dear Tim--Thanks very muchfor the offer. The idea of calibrating with the data for north
of 60 N sounds very interesting, and I am sure everyone would like to see. I guess the
issue with it would be that we would need to in this document carefully explain what was
done as it is not published in that way. Or we could basically say that it was done
similarly to your article, but simply for stations north of 60 N.
Or if we want to stick with published info could use the best of the two you mention. It
would be interesting to know how different the two records would be.
Perhaps John Walsh and Gordon McBean (and others) can weigh in.
Best regards, Mike
> Dear Mike,
> Keith asked me to write a quick reply to your email about a calibrated
> version of the Overpeck et al. Arctic temperature history. Both Keith and
> I are away for the rest of the week, but we can reply more fully when we're
> The series, as published by Overpeck et al. was simply left in "normalised"
> units -- i.e., standard deviations. We calibrated it by simple linear
> regression against the observed temperature averaged over all land north of
> 20N -- trying both annual means and an extended summer (April-September)
> temperature. I can tell you next week which performed best, though I think
> it was summer. You are welcome to use either of these series -- I can send
> them next week.
> But, for this particular record, a calibration against temperature from
> north of 60N might be more appropriate for your purposes. We have the
> observed temperature series for this region, and we're happy to perform the
> simple calculation of calibrating the Overpeck series against this if you'd
> prefer. Just let us know.
> Best regards
> Dr Timothy J Osborn
> Climatic Research Unit
> School of Environmental Sciences, University of East Anglia
> Norwich NR4 7TJ, UK
> e-mail: t.osbornatXYZxyz.ac.uk
> phone: +44 1603 592089
> fax: +44 1603 507784
> web: http://www.cru.uea.ac.uk/~timo/
> sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.