from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: Fwd: Your review of Asper et al
Date: Mon, 8 Apr 2002 12:31:13 -0600
From: Chick Keller <cfkatXYZxyz.vega.lanl.gov>
Subject: Your review of Asper et al
I read the review you and Tim Osborn wrote in Science of the Asper et al paper.
Your version, plotted in the review "Blowing Hot and Cold", of the Asper et al proxy
temperatures for the past 1200 years differs from their(figure 3) version, which has
significantly greater amplitude. You state why you get a smaller temperature amplitude:
"For the early 17th century, annual temperature estimates from Esper et al. differ by
about 0.7�C from those of Mann et al. [see figure 3 of (1)].
However, when we regressed the record of Esper et al. against nonsmoothed data (see
the figure), this difference was reduced to about 0.4�C.
Recalibrating both curves against year-by-year warm season temperatures (10) reduces
this difference further to about 0.35�C."
Would Asper et al agree that your version is the more correct for intercomparison with
other proxy results? This is important since, as you must know, Asper et al's figure 3
presents a very different picture of climate variation (and thus climate forcing by the
sun, etc.) than previously thought, while on the other hand your version of their
results shows them to essentially agree with the total range of temperature variation
from other reconstructions (although they still differ on the record from 1100-1400).
For example your resulting curve for the Asper et al work appears to differ little from
that in Crowley and Lowry's Ambio paper in terms of total amplitude, and the two agree
very well after 1450 or so. Also, the Asper et al MWP is only about 0.1�C higher around
1000-1100 than that of Crowley and Lowry.
Finally, it is tempting (but probably dangerous) to compare Esper et al's warming from
850 to 1000 with a very similar warming (both in rate and amplitude) 1800-1950. They
look very similar, but differ dramatically (if you switch to the instrumental record in
the 20th century) over the next 50 years--cooling after 1000 , but warming even more
rapidly after 1950. It is thus tempting to suggest that Asper et al (your version) have
shown us the maximum warming in the past 1200 years that can occur naturally which is
significantly less than warming thru the year 2000.
This probably trying to make too much of a single record, but when added to the other
records as you did in your review, there seems to be a picture emerging of what
hemispheric climate variation can be.
A few other questions--Asper et al used tree rings. Do you think their different
results are due to a different method of treating the raw data, or did they simply use
different tree ring data that gave these differences?
And--I note (and so does Fred Singer) that all proxy data in your figure does not show
temperature increases after 1950 in disagreement with the instrumental records. This
has been attributed to complications due to increased levels of CO2 in the atmosphere.
Has there been any other work done recently to understand this better?
Thank you for putting the Esper et al work into the context of that of yourself and
Charles. "Chick" F. Keller,
Visiting Scientist at
IGPP, Instititue of Geophysics and Planetary Physics
Los Alamos National Laboratory
Los Alamos, NM 87544
(505) 667-4820 or 667-0920
FAX (505) 665-3107
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.