Temperature Reconstructions

Medieval H-E Double Hockey Sticks

Scientists attempt to determine the temperatures of the past using a variety of methods. The results are called temperature reconstructions. The majority of the evidence indicates that modern temperatures are unusual compared to the last thousand years or so, but such an assertion is hotly contested by skeptics.

Medieval Warm Period & Little Ice Age

Much of this debate revolves around how warm it was in the Middle Ages, or specifically the existence of a widespread “medieval warm period” (MWP). To a lesser extent, the debate also involves the “little ice age,” a relatively cold period between the MWP and today.

The line of thought is this: the warmer temperatures of today are just the recovery from the little ice age, and even then, it’s not as warm as it was during the MWP. The MWP was good for civilization, so we should welcome warmer temperatures.

“It was warmer in the Middle Ages, even the IPCC says so!”

The most famous depiction of the MWP is this graph from the IPCC’s First Assessment Report (FAR) from 1990.1

It’s been used by a variety of skeptics, including The Great Global Warming Swindle, and S. Fred Singer’s newest book Unstoppable Global Warming.

There’s just one problem.

Central England is not the world!

The graph was created by climatologist Hubert Lamb to represent temperature of Central England over the last 1100 years.2 The UK Meteorological Office defines Central England as the triangular area enclosed by Bristol (west), London (east), and the county of Lancashire (north).3 Everyone agrees that there was a MWP in England and other areas near or in the North Atlantic.

The graph uses a combination of thermometer readings for the last 350 years, and records of growing seasons before that to infer temperature. It was first published in 1965 and has been updated several times since.

The IPCC used the top graph. Each point represents a 50 year average of yearly temperatures, with the most recent point centered on 1925. The middle and bottom graphs represent summer and winter temperatures. The solid line prior to the year 1400 is Lamb’s opinion, and it is this line that is the basis for the IPCC’s graph.

The IPCC incorrectly described their graph as “global temperature variations of the last 1000 years,” and this error has been repeated by the skeptics to this day. Regardless of the graph’s description, the text of the report talks about regional, not global, warmth. It is unlikely the authors intended for it to take on the significance that many have assigned it.

Lamb updated

The version of Lamb’s graph in the FAR is a crudely drawn schematic with an inconsistent time scale. Because the final years of the 20th Century had not yet occurred, the last portion of the graph (centered in 1975) is an extrapolation. If we return to the original 1965 graph and include the 50 year period beginning in 1950 and ending in 1999, we get the red line. Recent temperatures in Central England have been among the warmest in the thermometer record. The blue line represents the average from 1959 to 2008 and the corresponding 50 year periods going back to the beginning of the thermometer readings.

The Hockey Stick

Over the past decade, considerable effort has gone into the creation of more geographically diverse reconstructions of temperature. The first and most famous of these is the hockey stick. This is a temperature reconstruction of the past 600 years, first published in 1998,4 and later extended to the past 1000 years in 1999.5 The thousand year version was featured prominently in the IPCC’s Third Assessment Report (TAR) from 2001.6

The study was authored by Michael Mann, with co-authors Raymond Bradley and Malcolm Hughes (collectively referred to as MBH). Mann and Bradley are climatologists, and Hughes is a dendrochronologist - an expert on tree-rings. The two papers are referred to as MBH98 and MBH99. The papers combined a wide variety of data from various northern hemisphere locations. These are called proxies, because each contains climate information that can be used to infer temperature and/or precipitation. Such data sources include tree-rings, corals, ice cores, lake and ocean sediment, and bore-holes (in the ground).

The further back in time you go, the more limited the sources of data and this is represented by the grey error bars which increase in magnitude earlier than 1600 AD. The black line represents the estimated northern hemispheric mean temperature anomaly. The red line on this graph represents the northern hemisphere temperature from thermometers.


The hockey stick did not come without its share of caveats

No one had attempted to combine temperature data in such a way and from so many different sources, so the paper was the first of its kind. The uncertainty involved in the reconstruction is reflected in the title of the 1999 paper, “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations.” The TAR assigned a probability of “likely” or “greater than 66% percent” that present day temperatures have exceeded those of the past millennium. The conclusions were not intended to be the last word on the temperatures of the last 1000 years, although many took it that way, ignoring remaining uncertainty and focusing only on the dramatic hockey stick graph.


The hockey stick came under heavy criticism, primarily from two Canadian mathematicians. The first is Stephen McIntyre, who also discovered the “Y2K error” in the United States temperature series (see section 4). He is a former mining executive with a bachelor’s degree in mathematics. He runs the Climate Audit website, primarily dedicated to examining temperature reconstructions and measurements. The other is Ross McKitrick who is an environmental economist and associate professor at the University of Guelph in Ontario. He’s also a senior fellow at the Fraser Institute — a Canadian think tank. They published papers in 2003 and 2005 criticizing the statistical methods of the hockey stick — two in Energy and Environment, and more significantly, one in Geophysical Research Letters.7

“Mann should be in jail!”8

This quote pretty much summarizes how the skeptics feel about Michael Mann. Skeptics find just about everything wrong with the MBH papers, but it mainly comes down to these three points.

  1. Improper use of statistics, namely the use of “principal component analysis” (PCA) which is a method of extracting useful information from noisy data.

  2. Improper use of proxies, namely the overemphasis on North American bristlecone and foxtail pines.

  3. They accuse MBH of improper behavior. That is, withholding data and methods and therefore making it difficult to replicate their findings.

In addition they accuse the scientific establishment of collusion.

Pick your panel

The controversy over the hockey stick eventually made it into the editorial pages of newspapers and then to the attention of skeptical members of Congress. Rep. Joe Barton (R-Texas), then chairman of the House Energy and Commerce Committee, and Ed Whitfield (R-Kentucky), then chairman of the Subcommittee on Oversight and Investigation, sent letters to MBH demanding a response to alleged errors, in addition to their data and methods and a description of their funding and involvement drafting the TAR.9

Rep. Sherwood Boehlert (R-New York), then chairman of the House Science Committee, condemned Barton and Whitfield’s inquiry as “misguided and illegitimate” and the precedent it set as “truly chilling”.10 As an alternative, Boehlert requested that the National Research Council formally review the hockey stick. Barton set up his own panel.

The Wegman Report

Barton’s panel was made up of three statisticians: Edward Wegman (Chair), David Scott, and Yasmin Said, a former grad student of Wegman’s.

Wegman Report

The report was informally circulated to nine reviewers. Two of the reviewers didn’t wish to be named for fear of “potential negative consequences.”11 The panel was only asked to evaluate the statistics of the hockey stick. Neither the panel members nor the reviewers had any knowledge of paleoclimatology.

Temperature Reconstructions Tables

Wegman Report findings12

The Wegman Report found the M&M statistical criticisms to be “valid and compelling.” They recommended:

  • The chapters of the IPCC reports not be written by the same scientists who provide the underlying papers. This makes little sense, as the authors of the reports are leaders in their areas, and by definition publish work relevant to the field.

  • They suggested, “Federally funded work including code should be made available to other researchers upon reasonable request.”

  • Climate research relevant to policy decisions should undergo a mandatory “evaluation phase” by statisticians.

  • “Funding should focus on interdisciplinary teams and avoid narrowly focused discipline research.”

In addition, they performed a “social network analysis” of Mann and his co-authors, not unlike methods the FBI use to track the membership of crime syndicates.

NRC Panel

The NRC assembled a panel of 12 experts on climate, geography, geology and statistics. The panel was chaired by Gerald North.

NRC Report

The NRC report was reviewed by 13 anonymous reviewers. Neither the public nor the panel members knew the identity of the reviewers until the report was released. To make certain that all reviewer comments were properly addressed, the report was also overseen by two anonymous monitors.

NRC Report

General conclusions of the NRC Panel13

Rather than focusing exclusively on the hockey stick, the NRC panel reviewed the current state of temperature reconstructions. They found:

  • It is warmer now than at any time in the past 400 years. This is not a controversial conclusion since this period has the highest quality data includes the time of the “little ice age.”

  • “Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900.” “Less confidence,” in this context is roughly equivalent to the IPCC’s use of “likely” or “2:1 odds.”14 The increase in uncertainty in this period is consistent with the hockey stick’s error bars.

  • Although several areas had unusual warmth during medieval times, including Europe, “the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.” There was no single, geographically diverse “medieval warm period” per se.

  • Data before 900 AD, which was beyond the scope of MBH98 and MBH99, is too sparse to make a firm statement.

Hockey stick findings15

Of the Hockey Stick specifically, the panel found:

  • “The basic conclusion of Mann et al was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence” with proxy indicators that “in many cases appear to be unprecedented during at least the last 2,000 years.”

  • “The committee finds it plausible that the northern hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium.”

  • They did not agree with the statement from Mann et al that the “1990s was the warmest decade, and 1998 the warmest year” in a millennium. The quality of the data is not sufficient to make such a specific claim.

  • Of MBH’s PCA methods, the report says, “In practice, this method, though not recommended, does not appear to unduly influence reconstructions of hemispheric mean temperature; reconstructions performed without using principal component analysis are qualitatively similar to the original curves presented by Mann et al.”

  • “Strip bark” bristlecone/foxtail pines, which are trees missing parts of their bark, should be avoided when analyzing the last 150 years because their growth is exaggerated by the increasing concentration of CO2 in the atmosphere. MBH99 had attempted to correct for this effect.


Controversy erupted over the panel’s use of the word “plausible” to describe the possibility that the current warmth is unprecedented in the last 1000 years. In the context of the NRC report, “plausible” means “reasonable.” That is, “there is not a convincing argument to refute the assertion.”16 On the contrary, all of the more recent temperature reconstructions that they examined supported the assertion. The NRC panel was reluctant to assign figures because any conclusion would be an expert judgment, and not a statistical calculation.

In contrast, the IPCC authors are directed to avoid ambiguous language and to assign probabilities based on expert judgment if necessary.17

“Wegman and North agree . . .”

With respect to the statistical issues, North testified, “Dr. Wegman’s criticisms of the statistical methodology in the papers by Mann et al were consistent with our findings.”18 But that is where the similarities end. In his testimony, Wegman recognizes that the techniques in the MBH papers do not change the conclusion, but says, “The method wrong plus answer correct is just bad science.”19 North has responded “[...] the science was not ‘bad’. They simply made choices in their analysis which were not precisely the ones we (in hindsight) might have made.”20 Of the use of PCA and the accusation of manipulation of data, Peter Bloomfield, the statistician on the panel, says:21

Mann’s methods were all quite reasonable choices. I think in some cases a lot of work by others in following up on that have showed that some of those choices could have been made better, but they were quite plausible at the time. I would not have been embarrassed by the work at the time, had I been involved in it and I certainly saw nothing that spoke to me of any manipulation or anything other than an honest attempt at constructing a data analysis procedure.

Mann himself would no longer use the same methods –”knowing what I know today, a decade later, I would not do the same.”22

With respect to the Wegman Report’s “social networking” analysis and its recommendations, North testifies, “It is difficult to see how this data has any bearing on the peer-review process,” and he didn’t see “the need to include statisticians on every team that engages in climate research (which in my view is a particularly unrealistic and unnecessary recommendation), or any of the other findings and recommendations in Dr. Wegman’s report.”23

On the topic of improperly withholding data, North has stated, “I have no cause to think that there was anything inappropriate, professionally.”24

“AR4 abandoned the hockey stick”

It is often said that the IPCC’s Fourth Assessment Report (AR4) “abandoned the hockey stick,” but that is not the case.

From the Summary for Policymakers, “Average Northern Hemisphere temperatures during the second half of the 20th century were very likely (>90% probability) higher than during any other 50-year period in the last 500 years and likely (>66% probability) the highest in at least the past 1,300 years.”25 The difference between this and the TAR’s statement was that they removed the specific language about 1998 and the 1990s and extended the time period back an additional 300 years.

In the body of the report, the original hockey stick is shown along with 11 other reconstructions of northern hemisphere temperature.26

One is the thousand year MBH99 paper, and another, MJ2003, is Mann’s update of the hockey stick, going back a total of 2000 years. One of the reconstructions only goes back to about 1500. The remaining 9 reconstructions do go back as far as the MWP, and although they share many of the same proxies, the methods used to combine them are different. None of them show temperatures as high as they are today.

Mann et. al. 2008

In 2008, Mann and his co-authors published another update to the hockey stick. Additional proxies and improved statistical techniques allow for a more accurate and longer look at past temperatures. Gerald North, the NRC panel chairman, was a reviewer of the paper. Below compares MBH99 (blue) with the northern hemisphere version of the new study (red).27

Like many newer analyses, the new study shows more variability than the original, although the long term trends are similar.

“Strip-bark samples should be avoided”

The NRC panel recommended that strip-bark trees be avoided due to contamination from increasing atmospheric CO2 levels. To address this, the new study reconstructs past temperatures by both including and excluding all tree-rings. The red line includes the tree-ring data, while the blue line excludes it.28

“It was warmer . . . sometime!”

Regardless of how warm it is today, or how warm it will be in the near future, it was warmer at some time in the past.

This graph shows an approximation of temperature since the end of the dinosaurs 65 million years ago.29

Each pixel of the graph represents about 100,000 years. It is apparent that the recent time period, averaged over hundreds of thousands of years, is cool relative to the climate of the distant past.

The current climate, or even that of the last few million years, cannot be compared to time periods so long ago, because so much about the earth has changed. The position, size and geography of continents, in addition to long term volcanic activity, and even the type and mix of plant and animal life are all variables that affect the climate.

The Antarctic temperature record of the last 600,000+ years is contained within ice cores (top graph), and is independently verified using ice volume data derived from certain marine fossils (bottom graph).30

What is apparent is that the warm periods like today, highlighted in grey, are rare in the earth’s recent history. Ice ages are now the norm, not the exception.

Of particular relevance to mankind are the temperatures of the current geological epoch, the Holocene, represented by the right-most grey area. The warmest temperatures of the Holocene before the 20th Century were likely those of the Northern Hemisphere summer from the end of the last ice age about 10,000 years ago to about 5000 years ago, sometimes called the “Holocene thermal optimum.” This was a continuation of the same process that ended the last ice age. (See section 7 for a discussion of Milankovitch cycles).

The last time global temperatures were significantly higher than today was during the last interglacial period over 100,000 years ago, or the second grey area from the right. If AGW continues at the expected rate, we will easily exceed those temperatures by the end of the 21st Century. This will have serious consequences for civilization and most life on earth due to the rate that such a transition will occur (see section 11).


  1. (Houghton, Jenkins, & Ephraums, 1990)

  2. (Lamb, 1965)

  3. (Hadley Centre) Online here

  4. (Mann, Bradley, & Hughes, 1998) Online here

  5. (Mann, Bradley, & Hughes, 1999) Online here

  6. (IPCC, 2001) Online here

  7. (McIntyre & McKitrick, 2003) Online here, (McIntyre & McKitrick, The M&M critique of the MBH98 Northern Hemisphere climate index: Update and implications, 2005) Online here, (McIntyre & McKitrick, 2005) Online here

  8. (Montgomery, 2006) Online here. The article quotes Tim Ball, one of those retired skeptical scientists who has never published a paper in a peer reviewed journal challenging AGW. The quote is, “He threw out all the data that didn’t fit his hypothesis [...] I personally think [Mann] should be in jail!” Ball appeared in The Great Global Warming Swindle and is a frequent TV guest and editorial writer. He received his PhD in 1983, yet has called himself Canada’s “first PhD in climatology”. He’s also said that he was a “professor of climatology at the University of Winnipeg for 32 years” which is mathematically impossible and a distortion of his title (he was professor of geography and a lecturer before that). And he says that he received a “Doctor of Science” from London University, even though that is an honorary degree that he did not receive. After listing such accomplishments, he states, “For some reason (actually for many) the world is not listening.” Ball sued when these facts were pointed out. Then he withdrew his lawsuit.

  9. (Barton & Whitfield, 2005) Online here

  10. (Boehlert, 2005) Online here

  11. (Wegman, 2006) Online here. Contrary to Wegman’s statement, the reviewers of the NRC panel report were anonymous to the authors and the public. Obviously, someone at the NRC knew who they were.

  12. (Wegman, Scott, & Said, 2006) Online here. The official title of the Wegman Report is Ad hoc committee report on the ‘hockey stick’ global climate reconstruction. The ‘98 and ‘99 “hockey sticks” were reconstructions of Northern Hemisphere temperatures.

  13. (Committee on Surface Temperature Reconstructions for the Last 2,000 Years, National Research Council, 2006) Online here

  14. (Bloomfield, 2006) Online here (requires Real Player). Bloomfield’s quote is, “Where we speak of ‘less confidence,’ we’re more into level of sort of 2 to 1 odds, which IPCC, they interpreted ‘likely’ as that level, roughly two to one odds or better.”

  15. (Committee on Surface Temperature Reconstructions for the Last 2,000 Years, National Research Council, 2006) Online here

  16. (North, 2006) Online here

  17. (IPCC, 2005) Online here

  18. (North, 2006) Online here

  19. (Wegman E. , Questions surrounding the ‘hockey stick’ temperature studies: implications for climate change assessments (testimony of Edward Wegman), 2006) Online here

  20. (North, A Scientific Graph Stands Trial, 2006) Online here. Responding to this question, “I am curious what you thought of the primary part of the Wegman Report, that dealing with the statistical issues in Mann, et al. Specifically, the statement (or similar), ‘Incorrect mathematics + correct result = bad science’ [...]” North’s full reply was, “There is a long history of making an inference from data using pretty crude methods and coming up with the right answer. Most of the great discoveries have been made this way. The Mann et al., results were not ‘wrong’ and the science was not ‘bad’. They simply made choices in their analysis which were not precisely the ones we (in hindsight) might have made. It turns out that their choices led them to essentially the right answer (at least as compared with later studies which used perhaps better choices).”

  21. (Bloomfield, 2006) Online here (requires Real Player). In addition, panel member Kurt Cuffey notes, “It was really the first analysis of its type, the first time that anyone had tried to do a continual time reconstruction of a large scale average temperature of that sort, so it’s not surprising that they could have probably done some detailed aspects of it better, but it was a really remarkable contribution and basically gave birth to a debate that’s ongoing that’s really teaching us a lot about how climate has changed.”

  22. (Center for Science, Technology, and Congress at the American Association for the Advancement of Science , 2006) Online here

  23. (North, Questions surrounding the ‘hockey stick’ temperature studies: implications for climate change assessments (testimony of Gerald North), 2006) Online here

  24. (North, Surface Temperature Reconstructions for the Last 2,000 Years (Press Conference webcast), 2006) Online here (requires Real Player).

  25. (IPCC, 2007) Online here

  26. (Jansen, et al., 2007) Online here. Figure 6.10 (cropped)

  27. (Mann, Bradley, & Hughes, 1999) Online here. Data here. (Mann et. al. 2008) Online here. Data here.

  28. (Mann et. al. 2008) Online here. Data here.

  29. (Jansen, et al., 2007) Online here. Figure 6.1 (cropped)

  30. Ibid. Figure 6.3 (cropped)

Sources cited in Temperature Reconstructions

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Bloomfield, P. (2006, June 22). Surface Temperature Reconstructions for the Last 2,000 Years (Press Conference webcast). Retrieved June 11, 2008, from The National Academies Press: http://www.nap.edu/webcast/webcast_detail.php?webcast_id=327

Boehlert, S. (2005, July 14). house_committee_barton_letter.pdf. Retrieved June 11, 2008, from Global Warming Research: House Energy and Commerce Committee Investigation: http://branch.ltrr.arizona.edu/house_committee_barton_letter.pdf

Center for Science, Technology, and Congress at the American Association for the Advancement of Science . (2006, Fall). House challenge to climate change research fizzles. Retrieved June 11, 2008, from Issues in Science and Technology: http://www.issues.org/23.1/hill.html

Committee on Surface Temperature Reconstructions for the Last 2,000 Years, National Research Council. (2006). Surface Temperature Reconstructions for the Last 2000 years. Washington D.C.: National Academies Press.

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IPCC. (2005, July). Guidance notes for lead authors of the IPCC Fourth Assessment Report on addressing uncertainties. Retrieved June 11, 2008, from Intergovernmental Panel on Climate Change: Working Group I: The Physical Science Basis of Climate Change: http://ipcc-wg1.ucar.edu/wg1/Report/AR4_UncertaintyGuidanceNote.pdf

IPCC. (2007). Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. (S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, et al., Eds.) Cambridge, UK and New York, NY, USA.

Janeen, E., Overpeck, K., Briffa, J., Duplessy, C., Joos, F., Masson-Delmotte, V., et al. (2007). Paleoclimate. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. (S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, et al., Eds.) Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press.

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Mann, M. E., Bradley, R. S., & Hughes, M. K. (1999). Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations. Geophysical Research Letters , 26 (6), 759-762.

McIntyre, S., & McKitrick, R. (2003). Corrections to the Mann et. al. (1998) proxy data base and northern hemispheric average temperature series. Energy & Environment , 14 (6), 751-771.

McIntyre, S., & McKitrick, R. (2005). Hockey sticks, principal components, and spurious significance. Geophysical Research Letters , 32.

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Montgomery, C. (2006, August 12). Mr. Cool: Nurturing doubt about climate change is big business. Globe and Mail .

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North, G. (2006, June 22). Surface Temperature Reconstructions for the Last 2,000 Years (Press Conference webcast). Retrieved June 11, 2008, from National Academies Press: http://www.nap.edu/webcast/webcast_detail.php?webcast_id=327

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Wegman, E. J., Scott, D. W., & Said, Y. H. (2006). Ad hoc committee report on the 'hockey stick' global climate reconstruction. Retrieved June 11, 2008, from Climate Audit: http://www.climateaudit.org/pdf/others/07142006_Wegman_Report.pdf

Wegman, E. (2006, July 19th and 27th). Questions surrounding the 'hockey stick' temperature studies: implications for climate change assessments (testimony of Edward Wegman). Retrieved June 11, 2008, from US Government Printing Office: http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=109_house_hearings&docid=f:31362.wais

Wegman, E. (2006). Response of Dr. Edward Wegman to Questions Posed by the Honorable Mr. Bart Stupak in Connection with Testimony to the Subcommittee on Oversight and Investigations. Retrieved June 11, 2008, from Ross McKitrick: http://www.uoguelph.ca/~rmckitri/research/StupakResponse.pdf