cc: firstname.lastname@example.org, p.jonesatXYZxyz.ac.uk
date: Mon, 26 Mar 2001 13:56:14 -0600
from: tom crowley <tomatXYZxyzan.tamu.edu>
subject: Briffa et al reconstruction 2001
to: Tim Osborn <t.osbornatXYZxyz.ac.uk>
Dear Tim, Keith, and Phil,
Tim, thanks for the data. attached is a ps file comparing the Briffa et al
2001 time series rescaled to NH mean annual temp using the 1881--1960
interval for fitting (r = 0.81). The UKMO data set refers to the recent
paper submitted to GRL.
Also shown is a stretched version of a modified Crowley-Lowery time series
(termed CL6) - the stretch refers to the rescaling required to agree with a
model best fit to the instrumental part of the record (which in turn
implies a sensitivity of about 3.0 C for a doubling of CO2)
Over the interval 1407-1960 the decadally smoothed correlation between CL6
and Briffa.rescl is 0.76. The correlation from 1000-1993 of the model to
the stretched CL6 record is 0.84 (0.94 over the instrumental record!), the
residuals are better behaved in the TC Science paper, in the sense that
there is now no misfit with the instrumental record. The stretched
reconstructions would agree with the borehole estimates of temperature
change over the interval 1600-1950.
Another point of interest is that the CO2 drop of about 6 ppm from the 16th
to 17th centuries could be intererpreted in terms of a global avg SST
decrease of about 0.6 C - less than the 0.25 predicted by the CL6 record
but comparable to the rescaled Briffa record (one problem with this
constraint is that CO2 also seems to be low in the 18th century, so I need
to think about this more).
Any comments? If there is some agreement as to the legitimacy of this
exercise several shortcomings in the Crowley 2000 study (the instrumental
record disagreement), then some important advances in understanding. That
still leaves the Mann et al record outside the fold but I continue to worry
whether his calibration, which extends to 1980, results in a "flattening"
of the regression line and overestimate of paleotemperatures that you
people discussed in one of your Nature papers.
Any comments? Tom
Attachment Converted: "c:\eudora\attach\Briffa rescl NH ann.ps"
Thomas J. Crowley
Dept. of Oceanography
Texas A&M University
College Station, TX 77843-3146
979-845-6331 (alternate fax)