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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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.

Comments

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Comments 1 to 25 out of 167:

  1. PROXY-BASED RECONSTRUCTIONS...OVER THE PAST TWO MILLENIA ...Using a greatly expanded set of proxy data... Recent warmth appears anomalous for at least the past 1300 years whether or not tree-ring data are used... ...with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels. (above from abstract) ...a multiple proxy...database consisting of a diverse (1209) set of annually (1158) and decadly (51) resolved proxy series. including tree-ring, marine sediment, speleotherm, lacustrine, ice core, coral, and historical documentary series... ...59 (extend) back to AD1000... ......the conclusion that recent Northern Hemisphere warmth likely exceeds that of at least the past 1300 years thus appears reasonably robust... Mann et al, Sep2008 (PNAS, so no pay wall) GLOBAL SIGNATURES AND DYNAMICAL ORIGINS OF THE LITTLE ICE AGE AND MEDIEVAL CLIMATE ANOMALY ...The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally. Science abstract Mann et al, 27Nov2009 Intervals of regional warmth and cold in the past are linked to the El Nino phenomenon and the so-called "North Atlantic Oscillation" in the Northern hemisphere's jet stream... Comparisons between the reconstructed temperature patterns and the results of theoretical climate model simulations suggest an important role for natural drivers of climate such as volcanoes and changes in solar output in explaining the past changes. The warmer conditions of the medieval era were tied to higher solar output and few volcanoeic eruptions, while the cooler conditions of the Little Ice Age resulted from lower solar output and frequent explosive volcanic eruptions. Penn State press release, Mann et al, 27Nov2009
  2. I would consider moving the Climategate to its one argument. I believe this played much bigger in the media than you give it credit for. Maybe someting like.. Climategate clearly proves that AGW is a myth invented by scientists looking to make a quick buck... I wouldn't of thought to look here for information discussing that information.
  3. Why do you comply same cherry-picking as the IPCC? Many of the graphs presented still have Korrajärvi upside down. And the glacier-graph is ridicilous why do you cut it from 1600 the whole data from Greenland example can be seen here: http://www.foresight.org/nanodot/?p=3553 You are presenting a graph which ends just before the MWP and other warm periods. Why? Secondly, what statistical method allows you to do "the trick". Many other proxies than just tree-rings show the same divergence - which happens to be in the cooling phase of the PDO and between solar maximums. Much more likely explanation for the "divergence" is the UHI-effect and the effect of CO2 being weak (cloud feedback). if you use non-tree ring samples and UAH temperature data you get this: http://wattsupwiththat.files.wordpress.com/2009/04/lanser_holocene_figure11.png The IPCC "hockey stick" reconstructions are just pure quackery to hide the flaws in their biased theory. Even the Institute of Physics is aware of this possible scientific malpractise and cherry picking: LINK
    Response: [RH] Embedded link.
  4. protestant, i guess you've just discoved that climate has changed in the past. It has been warmer and colder, what a surprise! Good for you, next step will be understanding that unfortunately it does tell us much about the ongoing warming trend. PDO and solar minimum has nothing to do with the divergence problem and we cannot attribute it to UHI effct. In this post many proxies (other than tree rings) are shown that confirm the recent sharp temperature increase. Please read the comment policy and resrtain from talking about quackeries and the like.
  5. "Good for you, next step will be understanding that unfortunately it does tell us much about the ongoing warming trend." should read "[...] does NOT tell us much [...]" :p
  6. how boreholes can be used as proxy records? Do they measure the temperature at different depths? Is its temperature determined by the date the borehole was made? Thanks.
  7. Is it safe to say "the last few decades are the hottest in the last 500 to 2000 years"? My local skeptic threw "Vostock Ice Core data which clearly shows the Roman Era as Warmer than now" at me ...
  8. when 18,000 year old glaciers like the Chacaltaya melt away entirely after haveing obviously survived the MWP its a no brainer that it is warmer now at least in Bolivia.
  9. John, I would check whether the Holland paper is peer-reviewed even by E&E "standards". poptech - interesting that you fixate on this. (especially in light of Mann 2009 and all the other reconstructions) What do you think it means? Got "demolitions" for all the other proxy papers as well or this is a crusade against Mann?
  10. By a couple of common metrics such as overweening attention paid to what frankly appear as imaginary slights against the author on the part of a host of what he characterizes as conspirators, that "Casper and the Jesus paper" item appears to have been written by a crackpot. Yet the author has been published, patiently refuted and refuted again in a very professional manner by a number of researchers. I'd call that an excellent demonstration of scrupulous inclusiveness of outsiders on the part of the climate research community.
  11. John, another paper for you. Ice cover in Alps in 2003 at 5000yr low
    Response: That URL gives me a cookie error - is that a problem at my end or the publisher's?
  12. Can anyone tell me if there are temperature reconstructions that specificall avoid the use of the misleading bristlecone pine data? Cheers
  13. I smell a rhetorical question. Marty knows enough about temperature reconstructions to speak of "misleading bristlecone pine data" but wants help finding a reconstruction without 'em. Sorry, I'm not biting. Anybody else up for a pointless argument?
  14. doug_bostrom wrote : "Sorry, I'm not biting. Anybody else up for a pointless argument?" I would direct marty to NOAA Paleoclimatology (hope they're not considered biased and part of the big conspiracy), where he can check out data from boreholes, pollen, insects, etc. But, marty, why do you think bristlecone pine data is 'misleading' ?
  15. I just read an article saying that it was responsible for the hockey stick appearence and wondered if it was true that it makes temperature records have a hockey stick shape. Obviously if there are hockey stick shaped graphs that don't include the tree ring records of bristlecone pine then the article is clearly wrong. The article is on a site called spiked and I was rather hoping that someone knowledgeable about this issue might like to clarify things either here or in a posting to the letters page on spiked. http://www.spiked-online.com/index.php/debates/copenhagen_article/9056/ Since I haven't made up my mind about the science I obvioously flit between sites like spiked and this one. I'll have a look at the NOAA site and if it's not too technical I'll see if I can figure it out myself seeing as no one here can just give me a straight answer to what seemed to me a straight question. cheers
  16. Marty, read the post at the top of this page. See all the graphs of temperature data from sources that do not include bristlecone pines?
  17. marty - an excellent question. Kudos to you for checking additional data sources; many people don't bother. As Tom pointed out, there are a number of different temperature measurements (proxies) listed at the top of the page. I believe the Mann 'hockey stick' is composed of data from ~100 different temperature estimates, including some tree ring data. Mann felt (with some reason) that the later part of the tree ring data set was distorted, possibly due to other influences (drought) on tree growth for those proxies. It doesn't really matter, though, as including/excluding the later period tree ring data doesn't really change the graph by more than a couple percent either way. This is a popular 'skeptic' tactic, marty, which you might see elsewhere - picking a tiny piece of evidence, pointing out issues (correct or not) with it, and using that to claim that a conclusion based on many, independent lines of evidence is therefore invalidated by that tiny piece.
  18. Ok well for the sake of trying the debate I've posted a question about this on the comments box accompanying the article with a link to the article above. unfortunately I couldn't raise KR's claim about the small difference the inclusion/exclusion of tree ring data makes since I hadn't read it at that point. I would like to see what anyone following the link will have to say. I would disagree about the skeptic tactic bit though since it is a 'tactic' used by science in general. A tiny seemingly peripheral detail like the weird backwards looping about of a planet bought the whole glorious earth-centric universe tumbling down. Cheers.
  19. A small inconsistency can be an issue with a larger theory, marty - if the measurement is accurate/repeatable. Retrograde planetary transits certainly fit in that category. Unfortunately, many 'skeptic' arguments involve disagreements on interpretation, completely incorrect data, arguments about data normalization and calibration, etc. - and each (mis)point is used to argue that the Earth isn't warming, or we aren't the cause, or it won't hurt us anyway. An error in measurement (if it exists) is of quite different importance than a solid measurement that contradicts the theoretic predictions. I haven't seen any of those, really; the only recent issues I've seen that required consideration were ocean heating measurements and the tropospheric hot spot - and after the discussions I have to agree that these do NOT invalidate the conclusions of human driven global warming. You might find the list John has put together of the standard arguments quite interesting.
  20. To clarify my previous statement, marty: An invalid measurement (paper, data, whatever) is a lack of evidence for something. It really doesn't say anything about other evidence supporting a particular conclusion, just that the particular measurement has issues. A valid and contradictory measurement, on the other hand, is evidence against something. That's the category that retrograde planetary transits fall into. I really haven't seen any valid contradictory evidence regarding the conclusion of human driven global warming - nothing that held up as valid under examination.
  21. KR says: "I believe the Mann 'hockey stick' is composed of data from ~100 different temperature estimates, including some tree ring data." The Mann hockey stick in MBH98 was mostly created by incorrect standardization (normalization) that overweighted the Bristlecone proxies, a simple error. If you are referring to a newer version of "Mann 'hockey stick'" then please specify what version you are referring to.
  22. Eric - please read the last two paragraphs of this posting for an answer to that question.
  23. John, I don't know what's happening with Tamino's blog, but links are not working anymore. Fortunately internet archive keeps his blog until the end of 2008. Here is the link for the post not alike.
  24. All of the threads from before March or so of this year are gone at Tamino's site. It's a huge loss, as there was a lot of very good information there.
  25. In another thread the so-far unpublished McShane and Wyner paper was pointed out by David Walters. Some (me, for instance) have speculated that McShane and Walters drop some remarks indicating their unfamiliarity with the topic of their statistical analysis, even as they suggest climate researchers in turn are suffering from a lack of statistical expertise. Perhaps this is in fact a problem? 1) Why did McShane and Wyner regress their proxy PCs against the global mean temperature time series rather than against the whole NH temperature field over time, like Mann does? In other words, why throw away all that detailed info and calibrate against aggregated data only? 2) Why did M&W not notice that their calibration using the first PC of the instrumental field, rather than the global mean of instrumental, gives a reconstruction indistinguishable from Mann et al. 2008? 3) …and why did they not use this calibration — also aggregated but apparently better — in their Bayesian run, when it also clearly gives the best fit even by their crippled (because again, taken relative to the instru mean time series, not the whole field) RMSE criterion? "Gavin's Pussycat" at OpenMind Ordinarily I don't think I'd be referring to a blog comment by a cat as opposed to a formal comment or the like but apparently this paper has been pushed into the limelight before benefiting from full review. So, fair game, I guess.

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