Tuesday, May 22, 2012


cc: mann@virginia.edu,Malcolm Hughes <mhughesatXYZxyzr.arizona.edu>, rbradley@geo.umass.edu,k.briffa@uea.ac.uk,td@gfdl.gov
date: Fri, 02 Mar 2001 10:29:37 +0000
from: Phil Jones <p.jonesatXYZxyz.ac.uk>
subject: Re: Version 1.4
to: "Michael E. Mann" <mannatXYZxyztiproxy.evsc.virginia.edu>, tom crowley <tomatXYZxyzan.tamu.edu>


Mike, Tom et al.,
I'm happy to sign up subject to a few small alterations/additions.
This version reads
a lot better than the first and is shorter and more succinct.
First I agree with Mike's first point re the MWP - the so-called etc.
The distinction
between the early/mid 20th century and the late 20th century is good.
Happy with the
second point, but it would be good to run this by Tom Delworth.
What does REDUX mean by the way ? It's not in our dictionaries. Will
like me understand it. I presume it means something like revisited ?

Now to my few points :

1. In the first paragraph add 'against instrumental records' after
'calibrated and verified' .
This is needed.

2. (1) Few minor wording changes :

Although regional warmth during the Middle Ages MAY HAVE sometimes
significantly warmer than present .....

-- all of them showing warmth in the Middle Ages that is cooler than
late-20th century

(By the way what period do historians mean when they say Middle Ages ?)

Such HETEROGENEOUS patterns of change also occur during the two warming
periods (~1920-45 and since 1975) in the 20th century (Jones et al., 1999
Rev. Geophys),
but the magnitude of THEIR OVERALL warming is sufficiently large to yield
significant hemispheric- and global-scale increases.

3. (2) Remove INDEPENDENT. They aren't in the strictest definition of
the word.


Even the centennial-scale trends within the so-called 'Little Ice
Age' of the 16th-19th
centuries are largely in agreement among the surface proxies. Also,
contrary to
Broecker, evidence from alpine glacier movements agrees with local tree-based
summer temperature reconstructions (Raper et al Ref). Only a few such
comparisons have
been undertaken.

Could also add Luckman, BH, Briffa, KR , Jones, PD and Schweingruber, FH
The Holocene 7 375-389 (1997) to Raper et al.

Finally, send a copy of what is submitted. I'll be able to pick
something up here on
Monday early before going to Japan.

Keith says he'll respond later today.


At 17:32 01/03/01 -0500, Michael E. Mann wrote:
>Dear All,
>I agree w/ most of Tom's latest changes. However, I feel that the
>following two further changes need to made:
>1. The first sentence should read:
>In a recent "Perspectives" piece, W. Broecker suggests that 20th century
>global warming did not exceed the warmth of the so-called "Medieval Warm
>Period", and was due to oscillations in the thermohaline circulation.
>[two important changes here from Tom's wording: Malcolm, Phil, Ray, and I
>have all challenged, with good reason, the imprecise use of the term
>"Medieval Warm Period", and I see no reason to back down from that here.
>"So-called" is a key qualifier, secondly the essential distinction between
>mean 20th century or early/mid 20th century warmth and *late* 20th century
>warmth must be made clear from the outset. Broecker blurs this in his
>choice of wording. I see no reason for us to be similarly sloppy. Best to
>nip that one in the bud at the outset!]
>2. Point 4 should be again modified. As a co-author of the Delworth and
>Mann paper in question, I assert my privilege to summarize the key result
>therein, which regard *century-scale* (multidecadal) changes in the
>thermohaline circulation. In this paper we did show (see the spatial
>snapshot plots!!!) that the signal in question has a small projection
>onto mean hemispheric temperature in both model and observations. This is
>independent of discussion Tom and Tom D had of other additional
>experiments that Tom D has performed which are not yet published, which
>address the issue of projection of thermohaline circulation changes onto
>hemispheric temperature, in a different manner. However, the Delworth and
>Mann result stands on its own, and supports the statement made, in my
>opinion. I'm cc'ing this to Tom D just to make sure that Tom D doesn't
>disagree w/ my statement regarding the Delworth and Mann signal and its
>projection onto northern hemisphere mean temperature.
>Revised Point #4:
>(4) The burden of proof is on the author if he is to conclude that changes
>in the thermohaline circulation cause hemispheric or global increases in
>temperature. Even in the North Atlantic there is a complex regional
>response to century-scale changes in the thermohaline circulation which
>projects weakly onto hemispheric mean temperature in both models and
>observations (Delworth and Mann),
>and changes in the southern hemisphere can sometimes be of the opposite
>sign to changes in the northern hemisphere (Crowley 1992).
>At 03:32 PM 3/1/01 -0600, tom crowley wrote:
>>Hi all, Mike asked me to incorporate my suggestions into a modified text -
>>here it is. I made a few other minor changes for clarity. Please responsd
>>to this one, Tom
>>ps moved the borehole discussion to pt 1, where it seemed more appropriate
>>Medieval Warming Redux
>>In a recent "Perspectives" piece, W. Broecker suggests that the "Medieval
>>Warm Period" (MWP) was at least as warm as the 20th century and that
>>fluctuations in the North Atlantic thermohaline circulation were
>>responsible for the MWP/"Little Ice Age" transition. This conclusion
>>disagrees with "surface proxy" reconstructions (e.g. tree rings, ice
>>cores), which are calibrated and validated using standard methods employed
>>in paleoclimatology, and with recent modeling results. Broecker rejects
>>the surface proxy data sources because he argues they are unable to
>>register lower frequency trends in the climate system. Broecker's
>>conclusions can be disputed on several grounds:
>>(1) The available evidence does not support the conclusion that the Middle
>>Ages were as
>>warm as the late 20th century at global or hemispheric scales (in this
>>regard we interpret we interpret Broecker's statement to mean "late 20th
>>century global warmth", which is substantially greater than early-to-mid
>>20th century warmth). Although regional warmth during the Middle Ages was
>>sometimes been significantly greater than present, four different
>>hemispheric-scale reconstructions (Jones, Mann, Briffa, Crowley) have been
>>completed for the last 1000 years -- all of them showing warmth in the
>>Middle Ages that is either no warmer or significantly cooler than mid-20th
>>century warmth. These reconstructions also agree that a subsequent cooling
>>then occurred, which was interrupted by several centennial-scale warm
>>oscillations, before the onset of 20th century warming. The conclusion
>>about heterogenous patterns of warmth in the Middle Ages has been known for
>>a quarter of a century (Lamb, Dansgaard, Hughes). Such patterns also occur
>>during the two largest warming periods (~1920-45 and since 1975) in the
>>20th century significant (Jones et al., 1999 Rev Geophys.), but the spatial
>>scale and magnitude of this warming is sufficiently large to yield a
>>significant hemispheric and global increase. As discussed by Broecker, some
>>borehole heat flow data suggest a more prominent warming in the Middle
>>Ages (ref.). These conclusions, however, are not as robust as those based
>>on high-resolution borehole temperature measruements; within the
>>sensitivity of the estimates, the borehole data are not inconsistent with
>>the conclusion of Mann et al and others that the medieval temperature peak
>>was at most comparable to mid 20th century, but not late 20th century
>>temperatures. This conclusion is supported by measurements from tropical
>>glaciers indicating an unprecedented level of recent warming with respect
>>to the last 1,000-2,000 years (Thompson).
>>(2) High-resolution proxy climate records, which form the foundation of
>>recent hemispheric temperature reconstructions, are far more reliable
>>indicators of century-to-millennial scale climate variability than is
>>implied by Broecker. The potential limitations in interpreting long-term
>>climate change from proxy indicators such as tree rings, have been long
>>recognized by dendroclimatologists. However, several independent
>>reconstructions (Jones et al and Crowley and Lowery ), using a wide variety
>>of proxy climate indicators and different statistical approaches, yield
>>similar hemispheric temperature trends. Even the centennial-scale trends
>>within the so-called "Little Ice Age" of the 15th-19th centuries are
>>largely in agreement among the surface proxies and, contrary to Broecker,
>>with evidence from alpine glacial advances (Raper reference).
>>(3) Recent studies show that external forcing, not internal variability,
>>played the dominant role in the transition from the moderately warm
>>interval from about 1000-1300 to the subsequent cool intervals. Before
>>invoking internal variability as an explanation it is necessary to consider
>>radiative forcing changes due to volcanism and low frequency changes in
>>solar irradiance (although the latter is still not well constrained, there
>>are a number of independent lines of evidence suggesting such changes
>>(Lean, Lockwood). Two recent studies using independently derived estimates
>>of volcanism and solar irradiance (Free, Crowley) demonstrate at a high
>>significance level (>99%) that about 50% of the pre-anthropogenic
>>(pre-1850) decadal-scale variance can be explained by changes in volcanism
>>and low frequency solar irradiance. The transition from mild warmth in the
>>Middle Ages to a cooler interval is caused by a small decrease in solar
>>irradiance and a large increase in the intensity of and frequency of
>>volcanic eruptions. These hemispheric changes may have been accompanied by
>>larger regional overprints associated with trends in regional patterns such
>>as the North Atlantic Oscillation (Keigwin and Pickart), explaining why
>>enhanced warm and cold anomalies may have occurred in regions such as
>>(4) The burden of proof is on the author if he is to conclude that changes
>>in the thermohaline circulation cause hemispheric or global increases in
>>temperature. Even in the North Atlantic there is a complex regional
>>response to changes in the thermohaline circulation (Delworth and Mann),
>>and changes in the southern hemisphere can sometimes be of the opposite
>>sign to changes in the northern hemisphere (Crowley 1992).
>>The above arguments lead us to conclude that, although the conveyor may be
>>changing, radiative forcing perturbations from volcanism and solar
>>variability were primarily responsible for centennial-millennial changes in
>>the last 1000 years, with attendant implications for interpretation of
>>earlier Holocene oscillations (e.g, Denton and Karlen). Furthermore, the
>>weight of evidence indicates that the late 20th century hemispheric warming
>>is significantly greater than the Middle Ages.
>>Michael E. Mann
>>Thomas J. Crowley
>>Malcolm (Pending approval of final version)
>>Phil (Pending approval of final version, I think?)
>>Ray (no word recently!)
>>Keith (no word!!)
>>Thomas J. Crowley
>>Dept. of Oceanography
>>Texas A&M University
>>College Station, TX 77843-3146
>>979-847-8879 (fax)
>>979-845-6331 (alternate fax)
> Professor Michael E. Mann
> Department of Environmental Sciences, Clark Hall
> University of Virginia
> Charlottesville, VA 22903
>e-mail: mannatXYZxyzginia.edu Phone: (804) 924-7770 FAX: (804) 982-2137
> http://www.evsc.virginia.edu/faculty/people/mann.shtml

Prof. Phil Jones
Climatic Research Unit Telephone +44 (0) 1603 592090
School of Environmental Sciences Fax +44 (0) 1603 507784
University of East Anglia
Norwich Email p.jonesatXYZxyz.ac.uk


No comments:

Post a Comment