Friday, April 13, 2012

3387.txt

cc: mduvall@bates.edu,k.briffa@uea
date: Tue, 29 Jan 2002 12:02:58 +0000
from: Tim Osborn <t.osbornatXYZxyz.ac.uk>
subject: Re: Arctic synthesis
to: Konrad Hughen <khughenatXYZxyzi.edu>

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At 16:53 23/01/02, you wrote:
>Dear Tim,
>
>Good to hear from you. I feel like I've been underwater with the holidays
>followed by proposals, so there hasn't been any delay up to now with the
>synthesis project.
>
>Your timing is impeccable though, as the NSF Arctic sciences program is
>having an all-hands meeting next month, and I'll be presenting a poster about
>this project - current and, more importantly, future status. I've just
>started working on the poster, and this will let me include future site
>coverage accurately.
>
>If you could email the data and descriptions to me, that would be great.
>Also, if you could, please cc the message to our data manager, Matt Duvall
>(mduvall@bates.edu).
>
>Thanks again. I'll talk to you soon,
>Konrad

Konrad,

I've attached a file containing our calibrated, gridded reconstructions of
warm-season (April-September) temperature anomalies. The top of the file
gives some information, but here's some more points that you should note:

------------------------

(1) Referencing the data. The major reference to the calibrated, gridded
data is still in preparation (Osborn et al., 2002, see below). A second
paper, that is in press (Briffa et al., 2002) describes the climate signals
that are apparent within these tree-ring data and will show many of the
yearly maps of the Osborn et al. (2002) data - so both references should be
given.

Osborn TJ, Briffa KR, Schweingruber FH and Jones PD (2002)
Annually-resolved patterns of summer temperatures over the Northern
Hemisphere since AD 1400 from a tree-ring-density network. In preparation.

Briffa KR, Osborn TJ, Schweingruber FH, Jones PD, Shiyatov SG and Vaganov
EA (2002) Tree-ring width and density data around the Northern Hemisphere:
part 1, local and regional climate signals. The Holocene (in press).

Finally, if you need a reference to something that is actually published,
then our Briffa et al. (1998) paper showed uncalibrated anomalies from the
same data set for a few years

Briffa KR, Jones PD, Schweingruber FH and Osborn TJ (1998) Influence of
volcanic eruptions on Northern Hemisphere summer temperature over the past
600 years. Nature 393, 450-455.

------------------------

(2) The data are already calibrated. If you're going to re-calibrate them,
then you don't need to know the details. If you're going to use them in
their present form, then you should know how we did it. In this version,
we only did a "local" calibration - i.e., we only used grid boxes with
tree-ring chronologies in them, and calibrated them against instrumental
warm-season (Apr-Sep) temperatures from the same grid box (or occasionally
an average of temperatures from immediately adjacent grid boxes was used to
maximise the instrumental data available for calibration). They were
calibrated over the period 1881-1960, or a shorter period if instrumental
data were not available for the full period. The instrumental temperature
data were already expressed as anomalies (degrees C) from the 1961-1990
mean, thus we consider our reconstructions to also be estimates of
temperature anomalies (degrees C) from the 1961-1990 mean. This does *NOT*
mean that our reconstructions have a zero mean over the 1961-1990 period;
the calibration instead forces the reconstruction mean over the 1881-1960
calibration period to match the instrumental mean over 1881-1960. This
latter point often causes confusion, especially where our reconstruction
has a significantly non-zero mean over 1961-1990! This is the case in the
more northern grid boxes (the ones you are most interested in, I guess),
due to a decline (relative to observed temperature) in the tree-ring
density in those regions over the past few decades, leading to low values
in 1961-1990. It is because of this decline that we use 1881-1960 as our
calibration period. The cause of this decline is not clear (see Briffa et
al., 1998); if one assumes that it is anthropogenic in origin then this
allows one to use the reconstruction back to 1400 AD because a similar
underestimation of temperature wouldn't have happened in the past.

Briffa KR, Schweingruber FH, Jones PD, Osborn TJ, Shiyatov SG and Vaganov
EA (1998) Reduced sensitivity of recent tree growth to temperature at high
northern latitudes. Nature 391, 678-682.

------------------------

(3) As with many tree-ring-based reconstructions, the process of removing
the effect of tree-age can also remove real, long timescale variations in
climate. Thus these reconstructions are probably not fully capturing the
century and longer climate variations. You saw, I think, our paper in JGR
in 2001 (Briffa et al.) where we tried an alternative method of removing
the effect of tree-age which didn't lose real, longe timescale
variations. This method has so far been applied only at the regional or
hemispheric scale. Thus, the finer-scale gridded reconstruction being
supplied here have not been based on this method. We are in the process of
"forcing" the enhanced long timescale variability into these gridded
reconstructions, but have not completed this task yet.

Briffa KR, Osborn TJ, Schweingruber FH, Harris IC, Jones PD, Shiyatov SG
and Vaganov EA (2001) Low-frequency temperature variations from a northern
tree-ring-density network. Journal of Geophysical Research 106, 2929-2941.

------------------------

(4) Please don't give these data out for purposes other than the updated
Arctic synthesis. We will make them available ourselves once the papers
are complete.

------------------------

I think that's enough details for now!

Best regards

Tim


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Attachment Converted: "c:\eudora\attach\osborn_briffa.dat"
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Dr Timothy J Osborn | phone: +44 1603 592089
Senior Research Associate | fax: +44 1603 507784
Climatic Research Unit | e-mail: t.osbornatXYZxyz.ac.uk
School of Environmental Sciences | web-site:
University of East Anglia __________| http://www.cru.uea.ac.uk/~timo/
Norwich NR4 7TJ | sunclock:
UK | http://www.cru.uea.ac.uk/~timo/sunclock.htm

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