Friday, June 1, 2012


date: Wed, 28 Feb 2001 12:28:29 -0500
from: "Michael E. Mann" <>
subject: letter to Science

Dear Colleagues,
Below is a draft of a short letter to Science that Tom Crowley and I have put together,
after discussing w/ Phil, Ray, and Malcolm. We feel that a reply to Broecker's recent
"Perspectives" piece is warranted to correct several misconceptions that Wally
unfortunately chose to perpetuate (attached as an html file FYI). We have been given
encouragement to submit this by Julia Uppenbrink at Science.
We are working under a very tight timeline owing to Tom's travel schedule (leaves on an
extended travel on friday) so we would greatly appreciate it if you could respond ASAP w/
comments, suggestions, etc. Please note that we are currently near the length limitations
(and probably shouldn't include more than 15 references) so we're looking to sharpen and
hone, but not lengthen the piece at this point.
Thanks in advance for your feedback,
Medieval Warming Redux
In a recent "Perspectives" opinion piece, W. Broecker suggests that the
"hockey stick" reconstruction of climate change over the past 1000 years -
with extreme warming only in the late 20th century - is incorrect, and that
the so-called "Medieval Warm Period" was at least as warm as the 20th
century and due to oscillations in the thermohaline circulation. To reach
this conclusion, Dr. Broecker rejects traditional empirical "proxy" climate
indicators of past climate (e.g. tree ring, ice core, coral, and long
historical documentary records) that are the foundation of a number of
hemispheric reconstructions, as well as our current best physical
understanding of the factors controlling climate at century-to-millennial
timescales. We disagree with Broecker on several major points:
(1) It cannot reasonably be argued that the Middle Ages were as warm as the
20th century at global or hemispheric scales. Although regional warmth
during the Middle Ages may have 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
less than mid-20th century warmth. This is because it has been known for a
quarter of a century that the timing of warmth during the Middle Ages was
significantly different in different regions (Lamb, Dansgaard, Hughes).
Failure to take this observation into account can lead to serious errors in
the inference of hemispheric temperature trends. Although one analysis of
heat flow measurements suggests warmer temperatures than the surface
proxies during the Middle Ages (Huang and Pollack, GRL. 1997), the
considerable sensitivity of the resulting trends to a priori statistical
assumptions has lead borehole researchers to restrict their attention to
the more reliably interpretable temperature fluctuations during the past
five centuries (Huang and Pollack, Nature). Our conclusion is also
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 (e.g., Cook "segment curse" paper) and
are almost always taken into account in framing interpretations of
long-term trends. For example, Mann et al (1999) verified that a
significant subset of multiple-millennial length tree ring and ice core
proxy climate indicators used to reconstruct the trend over the past
millennium passed rigorous statistical tests for fidelity at the millennial
timescale, and that the basic attributes of the hemispheric reconstruction
using more recent non-tree ring proxies available over the past few
centuries yielded essentially the same result as that based on both tree
ring and non-tree ring based information (Mann et al, Earth Interactions,
2000). 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 changes within the so-called "Little Ice Age" of the
15th-19th centuries are largely in agreement. Furthermore these centennial
changes have been shown to be in "agreement" , rather than "in opposition"
(as argued by Broecker) with evidence from alpine glacial advances (Raper
(3) Physical considerations show that external forcing, not internal
variability, played the dominant role in the transition from the "Medieval
Warm Period" to "Little Ice Age" (these terms are used loosely and are, in
fact, ill advised in the context of hemispheric or global temperature
changes -see e.g. Bradley and Jones, 1993; Hughes and Diaz, 1994). One of
the major points of Broecker's argument is that changes in the
thermohaline circulation are a primary driver of climate change on this
time scale. These results do not consider recent modeling studies (Free,
Crowley) that demonstrate at a high significance level (>99%) that about
50% of the pre-anthropogenic (pre-1850) variance can be explained by
changes in volcanism and low frequency solar irradiance. Although the
latter term is still not well constrained from observational studies, there
are a number of independent lines of evidence suggesting such changes
(Hoyt, Lean, Lockwood).
(4) It is not justifiable to argue that changes in the thermohaline
circulation cause significant hemispheric or global changes in temperature.
Although changes in the conveyor play a major role in the Atlantic Basin,
to a first approximation changes in ocean circulation simply redistribute
heat on the planet without significantly raising global temperature, or
even hemispheric temperature. This conclusion is born out by very low
correlations between warmth in the Greenland sector and the hemispheric
indices over the last 1000 years (Crowley footnote ref.), a low correlation
that is shared by coupled model experiments (Delworth citation)? In fact,
sediment core data from the subtropical North Atlantic often cited as
indicative of a distinct "Medieval Warm Period" and "Little Ice Age"
(Keigwin Sargasso Sea), has recently been shown to be more consistent with
changes in the North Atlantic Oscillation (Keigwin and Pickart), implying a
zero sum pattern of regionally alternating warm and cold superimposed on
far more modest hemispheric variations over the past 1000 years. This
pattern itself may be forced, rather than internal in nature, and would
explain the limited evidence for more dramatic cold and warm periods in
regions such as Europe (see Mann, Sci Perspective, 2000).
The above arguments lead us to conclude that, although the conveyor may be
changing, radiative forcing perturbations 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

Professor Michael E. Mann
Department of Environmental Sciences, Clark Hall
University of Virginia
Charlottesville, VA 22903
e-mail: Phone: (804) 924-7770 FAX: (804) 982-2137
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