Global Warming Science -


Unprecedented Warming? A Look at CRU Emails and the Medieval Warm Period


[last update: 2009/11/27]



The popular global warming theory: that anthropogenic CO2 emissions have caused unprecedented global warming in the second half of the 20th century, is based strictly on the combination of two features:


  1. The use of computer models – the climate models cannot reproduce the warming from 1970 – 2000 without CO2, and
  2. The requirement that the warming from 1970 through the 1990s is unprecedented.


If it the late 20th century warming can’t be shown to be unprecedented, then there is a major problem with the theory (there has been no warming since the 1990s, which is also becoming a problem for the theory).





The IPCC went to considerable effort to provide evidence that the warming was unprecedented. The “hockey-stick” graph produced by Michael Mann (and used by the IPCC and Al Gore) eliminated the Medieval Warm Period and the Little Ice Age in order to exaggerate the 20th century warming.



IPCC Third Assessment Report (2001)


The adjacent figure shows average northern hemisphere temperature anomalies for the years 1000 - 2000 and is known as the “hockey-stick” graph (due to its shape) as used in the IPCC TAR. The “hockey-stick” graph was eventually debunked by the National Academy of Sciences.






IPCC Fourth Assessment Report (2007)


The adjacent figure shows the replacement for the debunked hockey-stick – the “spaghetti-graph” in the IPCC AR4. The accompanying AR4 text states: “The heterogeneous nature of climate during the ‘Medieval Warm Period’ is illustrated by the wide spread of values exhibited by the individual records”





For more information on paleo-climate as well as the debunking of the hockey-stick,



Many climate scientists disagreed with the Mann / IPCC position that the Medieval Warm Period and Little Ice Age didn’t really exist. But since the IPCC is the global political master of the climate “consensus” the “unprecedented” story became official.


For example, Moberg et al, 2005 []

Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data… According to our reconstruction, high temperatures—similar to those observed in the twentieth century before 1990—occurred around ad 1000 to 1100, and minimum temperatures that are about 0.7 K below the average of 1961–90 occurred around ad 1600. This large natural variability in the past suggests an important role of natural multicentennial variability that is likely to continue.” The following figure is from their paper []




The following two figures compare the Moberg 2005 figure (b) above to the IPCC hockey-stick (left) and spaghetti graph (right), showing what the IPCC is trying to obfuscate.




This IPCC position is at odds with the findings of many other scientists. For example, research at the Harvard-Smithsonian Center for Astrophysics report on a recent paper using proxies, which verifies the occurrence of the MWP: []A review of more than 200 climate studies led by researchers at the Harvard-Smithsonian Center for Astrophysics has determined that the 20th century is neither the warmest century nor the century with the most extreme weather of the past 1000 years. The review also confirmed that the Medieval Warm Period of 800 to 1300 A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide phenomena not limited to the European and North American continents. While 20th century temperatures are much higher than in the Little Ice Age period, many parts of the world show the medieval warmth to be greater than that of the 20th century.


Dr. David Deming (University of Oklahoma, College of Earth and Energy) said in his testimony to congress [] “I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, "We have to get rid of the Medieval Warm Period.” The existence of the MWP had been recognized in the scientific literature for decades. But now it was a major embarrassment to those maintaining that the 20th century warming was truly anomalous.”




CRU Emails


Climategate: On Nov 20, 2009 someone hacked a Hadley Climatic Research Unit (CRU) server and obtained emails between several prominent climate researchers (Mann, Jones, Briffa, etc.) and someone put the emails in a searchable database: []. The following series of emails clearly show that the “unprecedented” warming was far from certain and that the CRU climatologists tried to hide any data that indicated the actual uncertainty (bold emphasis added in emails below).


Email: Keith Briffa to Mann, Jones and others, Sep 22, 1999, (Subject: “IPCC Revisions” [])


I know there is pressure to present a
nice tidy story as regards 'apparent unprecedented warming in a thousand
years or more in the proxy data' but in reality the situation is not quite
so simple.
We don't have a lot of proxies that come right up to date and
those that do (at least a significant number of tree proxies ) some
unexpected changes in response that do not match the recent warming. I do
not think it wise that this issue be ignored in the chapter.

 For the record, I do believe that the proxy data do show unusually
warm conditions in recent decades. I am not sure that this unusual warming
is so clear in the summer responsive data. I believe that the recent warmth
was probably matched about 1000 years ago. I do not believe that global
mean annual temperatures have simply cooled progressively over thousands of
years as Mike appears to and I contend that that there is strong evidence
for major changes in climate over the Holocene
(not Milankovich) that
require explanation and that could represent part of the current or future
background variability of our climate.


Email: Phil Jones to Ray Bradley, Nov 16, 1999, (Subject: “Diagram for WMO Statement”).



I've just completed Mike's Nature trick of adding in the real temps
to each series for the last 20 years
(ie from 1981 onwards) amd from
1961 for Keith's to hide the decline.


Email: Raymond Bradley to Frank Oldfield, Jul 10, 2000, (Subject: “IPCC Revisions”



the very strong trend in the 20th century calibration
period accounts for much of the success of our calibration and makes it
unlikely that we would be able be able to reconstruct such an extraordinary
period as the 1990s with much success

Furthermore, it may be that Mann et al simply don't have the
long-term trend right, due to underestimation of low frequency info
. …

Whether we have the 1000
year trend right is far less certain (& one reason why I hedge my bets on
whether there were any periods in Medieval times that might have been
"warm", to the irritation of my co-authors!). So, possibly if you crank up
the trend over 1000 years, you find that the envelope of uncertainty is
comparable with at least some of the future scenarios, which of course begs
the question as to what the likely forcing was 1000 years ago
. (My money is
firmly on an increase in solar irradiance, based on the 10-Be data..).
Another issue is whether we have estimated the totality of uncertainty in
the long-term data set used -- maybe the envelope is really much larger,
due to inherent characteristics of the proxy data themselves....again this
would cause the past and future envelopes to overlap


Email: Chick Keller to Mann, Jones, Briffa, and others, Mar 2, 2001, (Subject: “Some thoughts on climate change proxy temperatures in the last 1,000 yrs” []).


Anyone looking at the records gets the impression that the temperature
amplitude for many individual records/sites over the past 1000 years or
so is often larger than 1°C
. They thus recognize that natural
variability is unlikely to generate such large changes unless the sun
is having more effect than direct forcing, or there is some fortuitous
but detectable combination of forcings. And they see this as evidence
that the 0.8°C or so temperature rise in the 20th century is not all
that special.


The community, however, in making ensemble averages gets a much smaller
amplitude ~0.5°C.
which they say shows that reasonable combinations of
solar direct plus volcanos and internal variability with the help of
THC can indeed explain this AND the 20th century warming is unique
. …


We must address the question: what forcings
can generate large amplitude temperature variations over hundreds of
years, regional though they may be (and, could these occur at different
times in different regions due to shifting heat inertia patterns)? If
we can't do this, then there might be something wrong with our
rationale that the average is low amplitude even though many regions
see high amplitude.
This may be the nubbin of the disagreement, and
until we answer it, many careful scientists will decide the issue is
still unsettled and that indeed climate in the past may well have
varied as much or more than in the last hundred years.


One way would be to note that the temperature amplitude (1000 - 1950)
for each [proxy record] is ~1.5°C. Thus you could conclude that hemispheric/global
climate varied ay over a degree Celcius (although with regional differences)
Another way would be to average the records. The resulting temperature
amplitude would be smaller because extremes would cancel since
variability is large and each region's extremes occur at different


Thus, if people simply looked at several records they would get the
impression that temperature variations were large, ~1.5°C. Imagine
their surprise when they see that the ensemble averages you publish
have much smaller amplitude


(Also, I note that most proxy temperature records claim timing errors
of +-50 years or so. What is the possibility that records are
cancelling each other out on variations in the hundred year frame due
simply to timing errors? as in hitting or missing C&L's triple warming
peak 1000-1200 AD)


Email exchange between: Michael Mann, Edward Cook and Tom Crowley, May 2, 2001, (Subject: “Hockey Stick” []).


Cook to Mann: “My statement that the MWP appeared to be comparable to the
20th century does not imply, nor was it meant to imply, that somehow the
20th century temperature is not truly anomalous and being driven by
greenhouse gases. To quote from my email, "I would not claim (and nor would
Jan) that it exceeded the warmth of the late 20th century. We simply do not
have the precision or the proxy replication to say that yet." Note the use
of the word "precision". This clearly relates to the issue of error
variance and confidence intervals, a point that you clearly emphasize in
describing your series. Also note the emphasis on "late 20th century". I
think that most researchers in global change research would agree that the
emergence of a clear greenhouse forcing signal has really only occurred
since after 1970
. I am not debating this point, although I do think that
there still exists a signficant uncertainty as to the relative
contributions of natural and greenhouse forcing to warming during the past
20-30 years at least


Cook to Crowley: “These chronologies are not good at
preserving high-frquency climate information because of the scattering of
sites and the mix of different species, but the low-frequency patterns are
probably reflecting the same long-term changes in temperature. Jan than
averaged the 2 RCS chronologies together to produce a single chronology
extending back to AD 800. It has a very well defined Medieval Warm
Period - Little Ice Age - 20th Century Warming pattern
, punctuated by strong
decadal fluctuations of inferred cold that correspond well with known histories of
neo-glacial advance in some parts of the NH …

the Esper series shows a very strong, even canonical, Medieval Warm Period - Little
Ice Age - 20th Century Warming pattern, which is largely missing from the
hockey stick
. …

I would not claim (and nor would Jan) that it
exceeded the warmth of the late 20th century. We simply do not have the
precision or the proxy replication to say that yet. This being said, I do
find the dismissal of the Medieval Warm Period as a meaningful global
event to be grossly premature and probably wrong.


Email exchange between: Michael Mann and John Christy, May 23, 2001, (Subject: “IPCC” [] Mann expressed disappointment that Christy went on John Stossel’s TV show).


Mann to Christy: “I'll be very disturbed
if you turn out to have played into this in a way that is unfair to your
co-authors on chapter 2 [of the IPCC TAR], and your colleagues in general. This wouldn't
have surprised me coming from certain individuals, but I honestly expected
more from you


Christy to Mann: “In one of the pre-interviews they asked about the "Hockey Stick". I
told them of my doubts about the intercentury precision of the record,
especially the early part, and that other records suggested the period
1000 years ago was warmer
. …

I've been very disappointed with what has gone
on even with respect to some of the IPCC elders and their pronouncements
for forthcoming disasters
. …

the dose of climate change disasters that have been
dumped on the average citizen is designed to be overly alarmist and
could lead us to make some bad policy decisions. (I've got a good story
about the writers of the TIME cover piece a couple of months ago that
proves they were not out to discuss the issue but to ignore science and
influence government

Regarding the IPCC. The IPCC TAR is good, but it is not perfect nor

sacred and is open to criticism as any document should be. …
Some of the story lines used to generate
high temperature changes are simply ridiculous


Email from Michael Mann to Tim Osborn, Keith Briffa and others, Apr 15, 2002, (Subject: “Your Letter to Science” []).


We can maintain an honest difference about how well those points were
conveyed in the Science piece (for example, you can imagine how the statement in your piece
"This record has a smaller amplitude of century-to-century variability, and is consistently
at or near the upper limit of alternate records produced by other researchers" might indeed
have been interpreted as setting MBH99 apart as, in your words, an "outlier"


Email from Michael Mann to Phil Jones and others, Jun 4, 2003, (Subject: “Prospective Eos Piece?” []).


it would be nice to try to "contain" the putative "MWP", even if we don't yet
have a hemispheric mean reconstruction available that far back


Email exchange between Keith Briffa and Edward Cook, Apr 12, 2005, (Subject: “Review” []).


Cook to Briffa: “as one is honest and open about evaluating the evidence (I have my doubts
about the MBH camp).

I just don't want to get into an open critique
of the Esper data because it would just add fuel to the MBH attack squad. They tend to
work in their own somewhat agenda-filled ways


Briffa to Cook: “Bradley still regards the MWP as "mysterious" and "very incoherent" (his latest
pronouncement to me) based on the available data. Of course he and other members of the
MBH camp have a fundamental dislike for the very concept of the MWP, so I tend to view
their evaluations as starting out from a somewhat biased perspective


Email between Phil Jones and John Christy, Jul 5, 2005, (Subject: “This and that”



Jones to Christy: “I would like to see the climate change happen,
so the science could be proved right, regardless of the consequences. This
isn't being political, it is being selfish.


Jones appended to the email, text from Joe Barton (Chairman of the US House of Representatives) to Rajendra Pachauri (Chairman of the IPCC): “in recent peer-reviewed articles in Science, Geophysical Research Letters, Energy & Environment, among others, researchers question the results of this work. As these researchers find, based on the available information, the conclusions concerning temperature
histories - and hence whether warming in the 20th century is actually unprecedented -
cannot be supported by the Mann et. al. studies
. In addition, we understand from the February 14
Journal and these other reports that researchers have failed to replicate the findings of these
studies, in part because of problems with the underlying data and the calculations used to
reach the conclusions.