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What evidence is there for the hockey stick?

What the science says...

Since the hockey stick paper in 1998, there have been a number of proxy studies analysing a variety of different sources including corals, stalagmites, tree rings, boreholes and ice cores. They all confirm the original hockey stick conclusion: the 20th century is the warmest in the last 1000 years and that warming was most dramatic after 1920.

Climate Myth...

Hockey stick is broken

“In 2003 Professor McKitrick teamed with a Canadian engineer, Steve McIntyre, in attempting to replicate the chart and finally debunked it as statistical nonsense.  They revealed how the chart was derived from "collation errors, unjustified truncation or extrapolation of source data, obsolete data, incorrect principal component calculations, geographical mislocations and other serious defects" -- substantially affecting the temperature index.” (John McLaughlin)

The "hockey stick" describes a reconstruction of past temperature over the past 1000 to 2000 years using tree-rings, ice cores, coral and other records that act as proxies for temperature (Mann et al. 1999). The reconstruction found that global temperature gradually cooled over the last 1000 years with a sharp upturn in the 20th Century. The principal result from the hockey stick is that global temperatures over the last few decades are the warmest in the last 1000 years.

Figure 1: Northern Hemisphere temperature changes estimated from various proxy records shown in blue (Mann et al. 1999). Instrumental data shown in red. Note the large uncertainty (grey area) as you go further back in time.

A critique of the hockey stick was published in 2004 (McIntyre & McKitrick 2005), claiming the hockey stick shape was the inevitable result of the statistical method used (principal components analysis). They also claimed temperatures over the 15th Century were derived from one bristlecone pine proxy record. They concluded that the hockey stick shape was not statistically significant.

An independent assessment of Mann's hockey stick was conducted by the National Center for Atmospheric Research (Wahl & Ammann 2007). They reconstructed temperatures employing a variety of statistical techniques (with and without principal components analysis). Their results found slightly different temperatures in the early 15th Century. However, they confirmed the principal results of the original hockey stick - that the warming trend and temperatures over the last few decades are unprecedented over at least the last 600 years.

Figure 2: Original hockey stick graph (blue - MBH1998) compared to Wahl & Ammann reconstruction (red). Instrumental record in black (Wahl & Ammann 2007).

While many continue to fixate on Mann's early work on proxy records, the science of paleoclimatology has moved on. Since 1999, there have been many independent reconstructions of past temperatures, using a variety of proxy data and a number of different methodologies. All find the same result - that the last few decades are the hottest in the last 500 to 2000 years (depending on how far back the reconstruction goes). What are some of the proxies that are used to determine past temperature?

Changes in surface temperature send thermal waves underground, cooling or warming the subterranean rock.  To track these changes, underground temperature measurements were examined from over 350 bore holes in North America, Europe, Southern Africa and Australia (Huang et al. 2000). Borehole reconstructions aren't able to give short term variation, yielding only century-scale trends. What they find is that the 20th century is the warmest of the past five centuries with the strongest warming trend in 500 years.

Figure 3: Global surface temperature change over the last five centuries from boreholes (thick red line). Shading represents uncertainty. Blue line is a five year running average of HadCRUT global surface air temperature (Huang et al. 2000).

Stalagmites (or speleothems) are formed from groundwater within underground caverns. As they're annually banded, the thickness of the layers can be used as climate proxies. A reconstruction of Northern Hemisphere temperature from stalagmites shows that while the uncertainty range (grey area) is significant, the temperature in the latter 20th Century exceeds the maximum estimate over the past 500 years (Smith et al. 2006).

Figure 4: Northern Hemisphere annual temperature reconstruction from speleothem reconstructions shown with 2 standard error (shaded area) (Smith et al. 2006).

Historical records of glacier length can be used as a proxy for temperature. As the number of monitored glaciers diminishes in the past, the uncertainty grows accordingly. Nevertheless, temperatures in recent decades exceed the uncertainty range over the past 400 years (Oerlemans 2005).

Figure 5: Global mean temperature calculated form glaciers. The red vertical lines indicate uncertainty.

Of course, these examples only go back around 500 years - this doesn't even cover the Medieval Warm Period. When you combine all the various proxies, including ice cores, coral, lake sediments, glaciers, boreholes & stalagmites, it's possible to reconstruct Northern Hemisphere temperatures without tree-ring proxies going back 1,300 years (Mann et al. 2008). The result is that temperatures in recent decades exceed the maximum proxy estimate (including uncertainty range) for the past 1,300 years. When you include tree-ring data, the same result holds for the past 1,700 years.

Figure 6: Composite Northern Hemisphere land and land plus ocean temperature reconstructions and estimated 95% confidence intervals. Shown for comparison are published Northern Hemisphere reconstructions (Mann et al. 2008).

Paleoclimatology draws upon a range of proxies and methodologies to calculate past temperatures. This allows independent confirmation of the basic hockey stick result: that the past few decades are the hottest in the past 1,300 years.

Intermediate rebuttal written by John Cook

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Additional video from the MOOC

Interviews with  various experts


Last updated on 12 October 2016 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

The National Academy of Science's summation of the various temperature proxies are available online at Surface Temperature Reconstructions for the Last 2,000 Years.

Tamino has an interesting blog post Not Alike where he compares the Moberg temperature reconstruction (one of the least hockey stick like reconstructions with a distinct Medieval Warm Period and Little Ice Age) to modern temperature trends. He finds modern temperatures are 0.53 deg.C hotter than medieval times and the modern warming rate is 64% greater than the fastest rate in medieval times.

The NOAA Paleoclimatology Reconstructions Network has made available paleo data for download including 92 high-resolution temperature records over the past 2+ millennia.


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Comments 151 to 167 out of 167:

  1. Jim Milks compiled a list of three dozen replications of the hockey stick, and that's only up through 2013.

    Hat tip to Jack Dale via David Appell.

  2. David Appell is updating a list of all the hockey sticks anybody can find. He's asking for additions from anybody. It's over three dozen now.

  3. This comment will start a bit off-topic and then quickly make its way back, I promise.

    As an introduction, I posted a response to Dr. Richard Muller's response to the following question on Quora:

    Why do people say "the science is settled" when it comes to climate change? Isn't the point of science that nothing is "settled?"

    It may interest people to know that Dr. Muller basically rules that forum when it comes to questions about climate change impacts, and IMHO, he's running amok.  I don't think it comes from the usual ideological motivators; rather, I think it's the hubris that physicists tend to get that leads them to distrust the work of any scientists other than physicists.  That and maybe some misunderstanding with regard to philosophy of science.  In any case, here's where I get back on topic.  

    In my response to Dr. Muller, I quoted Wikipedia to him, pointing out that he'd been wrong in his opinion piece about Dr. Mann's Hockey Stick.

    The quote stated that subsequent analyses had refuted McIntyre and McKitrick and upheld Mann's paper; further, that the Hockey Stick has been replicated numerous times using other methods.

    It's a bit lengthy, but I'd like to post his last response to this exchange in full, as I found it very interesting and troubling:

     First, let me say some words about the IPCC report.

    To be considered a scientific conclusion, the rule of thumb amount scientists is that the probability of being wrong should be 5% or less. In particle physics, the standard is even higher, generally a fraction of 1%.

    The IPCC defines something as "likely" if the probability of it being wrong is 33%. That is very far from a scientific standard. Sometimes politicians need to make decisions and they base them on less than scientific evidence, but 33% chance of being wrong would never be accepted as a scientific conclusion in any major scientific journal. When scientists say that their result is statistically consistent to 1 standard deviation (that's about the same as "likely") the conclusion in their paper is stated as follows: "No statistically significant effect was seen." I can show you one of my papers in which, for a 2-standard-deviation effect, that is a "2-sigma" effect, with only a 5%b chance of being wrong, I and my coauthors said that the effect was "statistically insignificant." Those are the standards of science.

    The IPCC is also very clear that their assessments were never intended to be considered a scientific report.

    Your quote about the NAS report, despite the usual reliability of Wikipedia, is mistaken. As I mentioned, I was a named scientific referee on the NAS report, and the report said clearly that there was no evidence that the current temperature is the warmest in 1,000 years.

    Don't get me wrong. Global warming is real, about 1.5C over the past 250 years, and it is caused by humans. But the work of Michael Mann on the hockey stick was incorrect, and the errors were correctly pointed out by Macintyre and McKitrick, and the NAS concluded that the evidence could not be used to conclude on a scientific basis that we are now experiencing the highest temperature in the last 1000 years.

    I'm curious what y'all's take on this is.   It strikes me as, well, quite odd.  I feel like, about the question of the use of the IPCC's uncertainty terminology, there's a deep misunderstanding here.  Without having read much, I'm quite sure that climate research uses the same Frequentist standards that Dr. Muller is used to and that, if the IPCC is assessing likelihood based on a large number of such pieces of research, all of which purport to be showing statistically significant results, then in fact the IPCC is being even more conservative with its use of such terminology and not less.  

  4. I'm surpised he refers to M&M. Perhaps it is a different paper, but the one in which they intended to demonstrate that Mann's statistical methods generated hockey sticks was a fraud. They designed their algorithm to sort curves in such a way that the hockey stick shaped ones would come first, and then they showed only the top sample, which obviously happened to be hockey sticks. I believe there are links in this thread from a couple of years ago.

  5. dvaytw @153.

    Just reading Muller's comment @153, I feel Muller is being insincere. The NAS report (assuming this is the report in question) does echo IPCC AR5 Chapter 5 when it says:-

    In terms of the average surface temperature of Earth, these indirect estimates show that 1983 to 2012 was probably the warmest 30-year period in more than 800 years.

    The only issue here is that Mann et al (1998) provided a 1,000 year proxy record not an 800 year record. To ignore this 800 year finding shows somebody playing with Ockkhams broom. But he is less than adept at sweeping things under carpets with such a broom because when he says "there was no evidence that the current temperature is the warmest in 1,000 years," he is plain wrong. There is evidence but it is not strong enough. The relevant part of IPCC AR5 is which says:-

    Based on multiple lines of evidence (...), published reconstructions and their uncertainty estimates indicate, with high confidence, that the mean NH temperature of the last 30 or 50 years very likely exceeded any previous 30- or 50-year mean during the past 800 years (...). The timing of warm and cold periods is mostly consistent across reconstructions (in some cases this is because they use similar proxy compilations) but the magnitude of the changes is clearly sensitive to the statistical method and to the target domain (land or land and sea; the full hemisphere or only the extra-tropics;). Even accounting for these uncertainties, almost all reconstructions agree that each 30-year (50-year) period from 1200 to 1899 was very likely colder in the NH than the 1983–2012 (1963–2012) instrumental temperature.

    NH reconstructions covering part or all of the first millennium suggest that some earlier 50-year periods might have been as warm as the 1963–2012 mean instrumental temperature, but the higher temperature of the last 30 years appear to be at least likely the warmest 30-year period in all reconstructions (...). However, the confidence in this finding is lower prior to 1200(AD), because the evidence is less reliable and there are fewer independent lines of evidence. There are fewer proxy records, thus yielding less independence among the reconstructions while making them more susceptible to errors in individual proxy records. The published uncertainty ranges do not include all sources of error (...), and some proxy records and uncertainty estimates do not fully represent variations on time scales as short as the 30 years considered ... . Considering these caveats, there is medium confidence that the last 30 years were likely the warmest 30-year period of the last 1400 years. (My bold.)

    Thus all Muller's blather about 33% applies to hockey sticks longer than 800 years. He conveniently forgets to mention the "very likely" status of the 800 year hockey stick which in IPCC-speak means a greater than 90% liklihood but less than 99% (which would be classed "virtually certain" ). For longer 1,000 year hockey sticks there is evidence, it does point to recent temperatures being warmer but the evidence is not reliable enough to strongly support it.

  6. MA Roger, I realize it's off-topic, but can you comment about my statement above that,

    "I feel like, about the question of the use of the IPCC's uncertainty terminology, there's a deep misunderstanding here. Without having read much of it , I'm quite sure that climate research uses the same Frequentist standards that Dr. Muller is used to and that, if the IPCC is assessing likelihood based on a large number of such pieces of research, all of which purport to be showing statistically significant results, then in fact the IPCC is being even more conservative with its use of such terminology and not less."

    With his big diatribe about statistical significance, Muller was ripping on my comment that if he's going to point out weak evidence for climate impacts on tornadoes and hurricanes, he should also point out the IPCC describes impacts on extreme drought, extreme precipitation and coastal flooding as "likely" and heat waves as "very likely".  He equates the percentages for those terms as basically equalling "no evidence at all".

  7. PS MA Rodger: the NAS report in question was actually this one:

  8. dvaytw - Dr. Muller seens to have the view that if he has not personally done the work, personally checked the evidence, then it is in doubt. And he will thus blithely dismiss solid work, take as gospel tripe like M&M, etc. So I would take his pronouncements with large blocks of salt. 

    The 2006 NAS report states in its conclusions:

    • It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. This statement is justified by the consistency of the evidence from a wide variety of geographically diverse proxies.
    • 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. The uncertainties associated with reconstructing hemispheric mean or global mean temperatures from these data increase substantially backward in time through this period and are not yet fully quantified.
    • Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900 because of sparse data coverage and because the uncertainties associated with proxy data and the methods used to analyze and combine them are larger than during more recent time periods.

    So the work he signed off on indicates high confidence in the last 400 years, less confidence in the previous 600, and reasonable uncertainty about 1000 years and greater ago, based on the evidence available at that time. 

    In the intervening decade additional proxies have been located, producing work up to and including Marcott et al 2013, which concludes that recent temperatures represent a reversal of a cooling trend that started 5000 years ago, with current temps warmer than the mean temperatures over 82% of the Holocene (going back 11,500 years).

    Muller's statements regarding paleotemperature reconstructions were reasonable a decade ago, but are now sadly out of date. And his assertions about MBH/M&M simply indicate that he hasn't looked into the M&M work - it's nonsense, multiply debunked, most notably by Wahl and Ammann 2007. M&M's failure to apply PCA selection rules alone invalidates the work, let alone their many other errors and misstatements. Muller is (once more) talking through his hat. 

  9. Also => List of large-scale temperature reconstructions of the last 2,000 years

  10. I posted in a forum about the purpose of the "Hockey Stick" study and the fact that numerous proxy studies have been performed confirming the original hockey stick conclusion: that the 20th century is the warmest in the last 1000 years and that warming was most dramatic after 1920.
    A denier swooped in and made this snarky comment and expects me to make a statement answering their question.

    Which is immaterial and irrelevant.

    State with specificity what the exact average global temperature should be now and the peer-reviewed science to support your conclusion.

    I have no idea how to answer as I don't know what the current global temperature "should" currently be? 

  11. TVC15, the current global mean temperature for the Earth "should be" one dictated by the sum balance of forcings and feedbacks that drive temperatures. As those are not in balance due to human activities, the global mean temperature is increasing as a result. Global mean temperatures will continue to increase as long as temperatures are not in balance with forcings, which they are not, as long as the burning of fossil fuels continues as they are currently used.

    Scientists have evaluated all natural forcings and factors capable of driving the Earth's climate to change and it is only when the anthropogenic forcing is included that the observed warming can be explained.

    Natural vs Anthropogenic Climate Forcings, per the NCA4, Volume 2:


    Scientists have quantified the warming caused by human activities since preindustrial times and compared that to natural temperature forcings

    Changes in the sun's output falling on the Earth from 1750-2011 are about 0.05 Watts/meter squared.

    By comparison, human activities from 1750-2011 warm the Earth by about 2.83 Watts/meter squared (AR5, WG1, Chapter 8, section 8.3.2, p. 676).

    What this means is that the warming driven by the GHGs coming from the human burning of fossil fuels since 1750 is over 50 times greater than the slight extra warming coming from the Sun itself over that same time interval.


  12. @ 161 Daniel Bailey

    Thank you so much for that great response and all the data!

  13. TVC15 @160. Denier's statement: "State with specificity what the exact average global temperature should be now".

    This is a common technique and it's a bit of a strawman argument. Scientists don't make any claims about what the "ideal" temperature "should be". There is no "perfect" temperature for the earth.

    But, there are ideal temperatures for sustaining a global human civilization. We need stable temps to grow our food, we need stable sea levels to insure that our major cities are not inundated, etc, etc.

  14. According to denialists Researchers Question Validity Of Some say it is meaningless to talk about a global temperature for Earth. The globe consists of a huge number of components which one cannot just add up and average.

    Researchers Question Validity Of A 'Global Temperature'

  15. Joris Geelen, their contention is ridiculous. Look up the definition of "average." To expand your knowledge, look up "central tendency" and how it complements "spread." Learn about additional statistics that can be used to further characterize a population. In short, read an elementary statistics textbook. For a specific rebuttal of that particular claim, see the Climate Feedback article. And if you really (?!) need more explanation, see the Rabbett Run post that has links to multiple detailed explanations.

  16. Just joined and wanted to read background on hockey stick. I tried Mcintrye 2004, but it just goes to AGU home page. Is there a correct link?



  17. alisonjane @166,

    The paper you found had the broken link in the OP is McIntyre & McKitrick (2005) 'Hockey sticks, principal components, and spurious significance'

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