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Tree Rings & Michael Mann’s Hockey Stick

June 14, 2020

By Paul Homewood


The Spectator reviews a new book about tree rings:



History is only as good as its sources. It is limited largely to what has survived of written records, and in prehistory to random fragments unearthed by archaeologists and paleontologists. Climate history is no different. As the effects of global warming accelerate, it becomes ever more urgent to reassemble what we can of the atmospheric conditions of the past to gather evidence from wherever it may be.

Glacial ice cores are one place, with their frozen snapshots of long-ago air and traces of ash and pollen and greenhouse gases such as carbon dioxide or methane. Other climate proxies include the annual accretion of stalagmites, the growth of corals and the incremental layers of bone in the ears of fish. But in recent decades, tree rings have emerged as one of the most precise and revealing of all paleoclimatic indicators.

Each growing season, a tree adds an outer layer of cambium to its core of dead wood. How much is added is a function of thermal and pluvial conditions. The exact width of the ring and its microscopic structure give a very clear picture of the weather during a period of a few warm months, like a postcard from a distant summer.

Valerie Trouet is a leading tree-ring scientist and her fascinating book recounts, among much else, how she and her fellow dendrochronologists are building a data base of astonishing accuracy and detail. In some areas — where deadwood is preserved in alluvium or peat — the record can go back millennia. The German oak-pine tree-ring series gives an annual record of more than 12,500 years.

Unlike carbon14-dating, which can only offer a temporal range, tree rings pinpoint the conditions for a precise year, even the beginning or end of the season. Cross-referencing with known events has helped add a missing climate element to history. The disappearance, for instance, of the pioneering English colony at Roanoke in North Carolina had long baffled historians; after three years, in 1590, a relief ship arrived to find everyone gone. Tree-ring analysis has confirmed not only the intervening years as ones of drought but as the most extreme dry period on the eastern seaboard in the last eight centuries.

Dendrochronology has also helped explain such curiosities as a dip in Caribbean piracy in the 17th century (a spate of hurricanes), riots in Ptolemaic Egypt (rain failure), the Ottoman crisis of the early 17th century and the rise and fall of the Mayan, Mongolian and Uyghur empires. When, a few years ago, Stradivari’s famous Messiah violin was deemed a copy, it was analysis of the rings in its wood that confirmed its authenticity.

But it is the study of historical climate change that has benefited most from this sylvan corner of the scientific world. Using multiple cores from trees in an area, and multiple areas, a comprehensive picture of wider climatic variation can be assembled. Such analysis provided key evidence for the 1998 Hockey Stick graph, which stirred up a hornet’s nest of opposition from deniers, but has since persuaded all but the lunatic fringe that the spike in global temperatures is something seriously abnormal. Oscillating within a given range for 1,000 years or so, global temperatures suddenly started rising beyond that range in the 20th century, to reach skywards in 1998 — and that year is now only the tenth hottest.


Somehow what starts as a perfectly sensible review morphs into Michael Mann and his discredited hockey stick!

But, as the review itself admits, tree rings tell you more about rainfall than temperature, Indeed, in a much better review in Newsweek, we read how the book reveals in detail the effect that a long period of drought had on the declining Roman Empire in the 4thC.

In fact Mann’s Hockey Stick was hopelessly flawed in many ways. (I would recommend Andrew Montford’s book, “The Hockey Stick Illusion”, for anyone interested.

For a start, the Hockey Stick was based on shonky statistics, which were guaranteed to produce a hockey stick curve regardless of the data fed into it. This was because of the way Mann used Principal Component analysis. In simple terms, Mann’s statistics blew out of all proportion any data which showed a hockey stick effect and ignored all other data.

Secondly, as far as tree rings were concerned, it was heavily dependent on bristlecone pines. It has long been known that the marked increase in bristlecone growth in the 19th and 20thC is due to CO2 fertilization, not temperature. When bristlecones are taken out of Mann’s analysis. the hockey stick disappears.

Thirdly, when tree ring and other proxy data diverged from rising temperature data in the late 20thC, Mann ignored the proxies and spliced the temperature data onto his graph.

There are also a whole host of other major flaws in the Hockey Stick, not related to tree rings.


The Spectator review is written by Philip Marsden, who apparently is an English travel writer and novelist. Perhaps next time they might get someone who actually knows about the subject to write their reviews.

Far from Marsden’s brainless assertions about lunatic fringes and unprecedented temperatures, there is a wealth of data which proves that today’s climate is not unusual, and that the world has been warmer than now for most of the last 10,000 years or so.


For more technical detail from Steve McIntyre and Ross McKittrick, see here and here.

  1. Geoff B permalink
    June 14, 2020 11:55 am

    I never understood why this reliance on tree rings for climate data over the last 2000 years, there are written records of crop yields from Roman times, mainly for tax collection, we have our own domesday book from 1086, again for tax collection. Also parish records, with dates the barley was sown, the harvest festival service, and as we all know there is no hockey stick. I would agree that this only works in the civilised areas, basically Europe and middle east and tree rings have a place in places like usa and australia.

    • ianprsy permalink
      June 14, 2020 12:17 pm

      It seems to me it’s a common phenomenon in all sorts of situations – policy-based research. Local example was a dodgy green belt review to justify taking land for sheds.

    • Harry Passfield permalink
      June 14, 2020 4:12 pm

      Geoff, here’s a clue: “Valerie Trouet is a leading tree-ring scientist…”. She is merely protecting her shonky ‘science’.

    • Adam Gallon permalink
      June 14, 2020 9:15 pm

      Mann’s trees, were in the south-west corner of the USA.
      We have various written records, for small parts of the world, the issue is trying to generate something that covers the world.

    • mwhite permalink
      June 15, 2020 8:57 am

      In 2016 I caught a program on BBC radio 4. It was about the summer of 1976 through the tree ring data.

      That year and others in the mid 1970s tree growth rings were very small. Funny I don’t remember it being that cold back in the summer of 76.

    • Mike Jackson permalink
      June 15, 2020 11:13 am

      You can add in Pepys diaries for mid 17th century and written records for just about every civilisation there has ever been. One thing civilised societies breed very fast is tax = tax collectors = bureaucracies = masses and masses of facts because the bureaucratic mindset has existed forever.

      They may not have had reliable thermometers but they knew the difference between hot and cold and if they didn’t their crops would soon tell them!

    • igsy permalink
      June 18, 2020 9:01 am

      You’ve answered your own question!

  2. TonyN permalink
    June 14, 2020 12:15 pm

    Oh Dear!

    “…. but has since persuaded all but the lunatic fringe that the spike in global temperatures is something seriously abnormal.”

    I read the review yesterday, and I have also been concerned about this silly comment more reminiscent of a brain-fart that a considered thought.

    Anyway, to air my own cerebral effluvium, the worry I’ve got about tree rings is that for every live and growing tree that is and ever was, the last few years of growth-rings must contain more moisture and will be fatter than earlier, drier rings. So, could it be true that all tree ring counts start with the past years apparently recording ‘better growth’ and hence could be misused to claim ‘climate change’? . It would be almost worth buying the book to find out if this observation has any credence, but the review has put me right off academic studies that clearly need to ‘bend the knee’ to the latest political fashion in order to get funds.

    To coin a phrase, It isn’t the physics that bother me; it is the metaphysics.

  3. MrGrimNasty permalink
    June 14, 2020 12:17 pm

    NTZ does a great job of assembling all the non-hockeysticks published each year, latest:

  4. June 14, 2020 12:26 pm

    Dendrochronology is useful for dating, in that corings can be overlapped to go back and determine the age of structures, etc. The growth ring data can tell a lot about temperatures and moisture by looking at each year’s growth.

    In recent times, it has been surmised that the incredible tones found in the Stradivarius violins is due to the close growth of the wood during the Little Ice Age. Of course Michael Mann removed that from the time line, so guess that leaves Stradivarius out in the cold (or heat).

    • TonyN permalink
      June 14, 2020 1:22 pm

      Re Strads,

      I uhderstood that there was no magic about them; it was just that the ones that survive have been selected by time for their least number of bad notes than other violins, even ones made by the company …

  5. June 14, 2020 12:45 pm

    “Somehow what starts as a perfectly sensible review …”

    Not so. It starts:

    History is […] limited largely to what has survived of written records […] Climate history is no different.

    Climate science is completely different, having effaced the historical evidence from the record. Emmanuel Leroy Ladurie wrote a monumental “histoire du climat” based on the written record (monastery archives, dates of grape harvest etc.) which is completely ignored by Mann with his flat millennial temperature. Leroy Ladurie even acknowledged the contribution of Mann and Jones in a later work, whereas those two shysters have never, as far as I know, acknowledged that a written historical temperature record even exists, as if a dripping stalactite trumps a monk with a quill pen every time. You have to be a certain type of person to prefer the word of a strip bark pine to that of a human being.

  6. June 14, 2020 12:55 pm

    she and her fellow dendrochronologists are building a data base of astonishing accuracy and detail. In some areas — where deadwood is preserved in alluvium or peat — the record can go back millennia

    Her claim is astonishing, but the accuracy – not so much. Dendro is just not as reliable as that.

  7. Broadlands permalink
    June 14, 2020 12:56 pm


    “During the past two decades the ponderosa pine forests of eastern Oregon and northeastern California have been seriously depleted by drought and bark beetles.”
    “All tree-ring measurements agree in showing that a very critical subnormal growth period has existed since 1917. This slowing down of the growth rate is undoubtedly the result of deficient precipitation and lowered water tables. As compared with other drought periods, the present one is the most severe and critical that the present forests have experienced in the last 650 years. Several other periods have exceeded the present one in duration of subnormal growth, but none has approached it for severity. Growth in 1931, the poorest year, was 68 percent below normal.“

  8. Tim Hill permalink
    June 14, 2020 1:17 pm

    “The exact width of the ring and its microscopic structure give a very clear picture of the weather during a period of a few warm months…”

    Is this not a red flag to any literate person on the planet?

    • Nancy & John Hultquist permalink
      June 14, 2020 3:14 pm

      The wood analyses cannot tell temperature, that is, act as “treemometers”.
      The work – dendrochronology [tree + time] – is very useful and properly handled, the results are interesting.
      The title of the book is “Tree Story: The History . . .”
      It does not include “the temperature”

      Climates (note plural) are determined by several things. Temperatures to ½ degree is not one of them.

      I’d like to read some of the book. It gets positive reviews from those not inclined toward AGW and Klimate nonsense.

  9. June 14, 2020 1:56 pm

    Michael Mann is no longer pushing the hockey stick argument and he has not mentioned it in any of his climate science presentations recently. He has moved on to the Anthropocene idea in which human caused climate change threatens the destruction of the planet.

  10. Graeme No.3 permalink
    June 14, 2020 1:58 pm

    A.E. Douglass thought the same in the 1950’s but failed to convince many people,

  11. GEORGE LET permalink
    June 14, 2020 2:04 pm

    Mann’s hockey stick was the number one example of accept the conclusion of fossil-fuel CO2 caused climate change and come up with the “evidence”. Additional ones are all of these “peer-reviewed papers”. Mann could not justify the hockey stick in court and his case against Tim Ball was thrown out.

  12. Stonyground permalink
    June 14, 2020 2:45 pm

    “As the effects of global warming accelerate,…”

    This must be a new definition of the word accelerate that I am unaware of. It presumably means to level off and stop for twenty years.

  13. Gamecock permalink
    June 14, 2020 3:19 pm

    ‘Valerie Trouet is a leading tree-ring scientist’

    Citation needed.

    ‘her fascinating book’

    I’ll be the judge of that.

    ‘recounts, among much else, how she and her fellow dendrochronologists are building a data base of astonishing accuracy and detail.’

    Right, oldbrew. Accuracy of prehistoric weather from a single tree ring. “What was your weather in 2017?” can’t be answered. Tree rings tell us the general temperature/water situation for a plant for a year. It is impossible to separate whether temperature or water.

    ‘but has since persuaded all but the lunatic fringe’

    I finally made it in!

    ‘that the spike in global temperatures is something seriously abnormal. Oscillating within a given range for 1,000 years or so, global temperatures suddenly started rising beyond that range in the 20th century, to reach skywards in 1998 — and that year is now only the tenth hottest.’

    We had no way to measure global temperatures until 1979.

    ‘global temperatures suddenly started rising beyond that range in the 20th century, to reach skywards in 1998’

    From 287.15K to 287.55K.

    Oh noes!

  14. June 14, 2020 4:18 pm

    Yay! At last the hockey stick vindicated and the hidden decline forgotten. This is how history is made nowadays.

  15. Jerry permalink
    June 14, 2020 5:03 pm

    “has persuaded all but the lunatic fringe that the spike in global temperatures is something seriously abnormal.”

    Oh sure.. In real measurements, there has been NO consistent change of global temperature.

  16. Gerry, England permalink
    June 14, 2020 6:24 pm

    The IPCC dropped the hookey stick having had it front and centre when it first appeared given that it had been discredited by 2 US Senate committees although it is true that the summary reports don’t clearly state that. Dr Wegman was quite scathing at one of the committees saying that given this is all about statistics would you not expect the likes of Mann to work with the statistics community. Obviously not if you are going to fake something. I recall the two Mcs had also shot down all the attempts by the hookey team to prop up the initial paper which included lots of ‘pal review’. I think Steve Mc made the interesting point that nobody had been back to take any further ring samples for over a decade. But given the ‘Mike’s Nature trick’ was needed to fake out the end of the graph you can probably guess.

    The main function was to get rid of any earlier and what would seem greater warm periods such as the Medieval, Roman and Minoan, of which the first causes the greatest problem. Since then there have been around 200 papers to show that the Medieval Warm Period was global which counters the warmists plan B to claim it was Northern Hemisphere only.

  17. dearieme permalink
    June 14, 2020 7:52 pm

    Online I once asked a tree ring enthusiast how a tree could simultaneously act as a thermometer and a rain gauge. All I got in reply was empty bluster.

  18. June 14, 2020 10:37 pm

    “Mann’s statistics blew out of all proportion any data which showed a hockey stick effect and ignored all other data …”.

    I think this is a good general summary of what Mann et al. did and apparently is a common practice in climate proxy studies (from Yale Law School journal on unrelated matters):

    “‘Selecting on the dependent variable’ refers to the practice of restricting one’s set of observations to cases in which some phenomenon of interest has been observed and excluding from the set cases in which the phenomenon was not observed …” viz. approximate correlation with the twentieth century temperature trend.
    “Necessarily, any inferences one draws about the causes of such a phenomenon will then be invalid because in ignoring cases in which the phenomenon didn’t occur one has omitted from one’s sample instances in which the putative cause might have been present but didn’t generate the phenomenon of interest — an outcome that would falsify the conclusion”.

    • June 15, 2020 5:02 am

      This is the original ‘hockey stick’ graph from Mann, Bradley & Hughes 1999 (MBH99):

      What is remarkable a first sight is the complete absence of a range of results for the temperature reconstruction after 1900, obscured by the supposed thermometer trend (red).
      That is because the tree-core samples were screened to heavily favour the samples that approximated the supposed twentieth century T trend, supposedly in response to increasing CO2 — they called that the “calibration period”.
      For the previous 900 years the samples appear to vary up to ~1C (yellow) at any one time and the black line is the 40-year smoothed average that in effect represents a trend only.
      As in so much climate (change) science the process is circular: they selected cases on the basis of meeting a criteria (increasing T in 20th century in response to increasing CO2) and then used those cases as evidence for the criteria.

  19. June 14, 2020 10:38 pm

    The tree ring record for the Andes region in the SH where the regulating oceans are, show remarkably little climate change, the natural variability is repeated over 5Ka and is cyclic, up and down, as seen in most of the proxy records.

    Nothing unusual is happening, just more of the same. Paper linked below.. The volume of data confirmin the natural cycles of change in the last decade must make it clear the nonsense theorised before is wholly delusional and supported ONLY by theoretical models paid to prove CO2 was the cause of climate change. The science proves f that ant effect from CO2 is tiny and has no detectable effect on natural change. The GHE does not occur as guessed but never proven, in fact its is simply the insulation provided by an atmosphere held on to planet under perssure by gravity. Just the gaseous lagging which works pretty much the same on most solar system planets, with its thermal gradient reversng direction at 0.1 Bar, whatever the gas mix. Even NASA agree this is the case.

    • dearieme permalink
      June 15, 2020 12:12 pm

      All very well but how do the authors of your first link know that their tree rings are thermometers rather than rain gauges?

  20. Brian Jones permalink
    June 14, 2020 11:45 pm

    The most interesting thing in all this is the effect of CO2 on growth rings. I would expect growth rings to be bigger during high periods of CO2. If that is the case what use are tree rings by themselves as they only tell part of the storey.

    • June 15, 2020 5:43 pm

      Quite so. Sunlight, CO2 and water are all significant components required for growth, up to a limit. Less is less.

    • dearieme permalink
      June 15, 2020 11:02 pm

      Good point. Tree rings are not only thermometers and rain gauges, they are CO2 meters into the bargain.

      Do they also respond to disease, pests, nutrients other than CO2, and so forth?

  21. Phoenix44 permalink
    June 15, 2020 8:21 am

    The only thing you need to know is that Mann had to “hide the decline”.

    The tree rings and the temperature as measured with instruments went in different directions. The Spectator should be reminded of that one simple but devastating fact, admitted by Mann.

    Either tree rings are not a good proxy at all or the measured temperatures are wrong. If you want tree rings to be right, then temperatures are declining.

  22. Pancho Plail permalink
    June 15, 2020 9:27 am

    OT but perhaps now would be a good time to highlight the state of windpower over the last couple of days.
    Gridwatch is currently showing wind contributing 0% to the total (0.11GW) and it was pretty much the same yesterday.

    • Pancho Plail permalink
      June 16, 2020 8:39 am

      And add to that a further two days of effectively zero wind generated power.

  23. A Norwich Tory permalink
    June 15, 2020 10:22 am

    The Spectator likes punchy little letters … why not give it a try 🙂

  24. A Norwich Tory permalink
    June 15, 2020 11:00 am

    Drax today in The Times. Who knew?

    • Ben Vorlich permalink
      June 15, 2020 12:00 pm

      That’s the second article like that in the last couple of weeks. Not holding my breathe for one by Harrabin any time soon.

      • Neil Wilkinson permalink
        June 15, 2020 12:39 pm

        I wonder who benefits financially from this…..

  25. MrGrimNasty permalink
    June 15, 2020 12:25 pm

    Update on private motor cars and effect on air pollution.

    According to Apple mobility data, with the lockdown on 23rd March, UK car use plunged then sat around -65% for quite a few weeks. It has gradually recovered and is now less than 20% down on ‘normal’.

    WTHG air pollution monitoring station is urban roadside, and the road/junction is one of the busiest and most gridlocked/congested places in Sussex.

    You would expect the effect on air pollution to be dramatic. There is still no apparent trend/link and certainly no significant link.

  26. Coeur de Lion permalink
    June 15, 2020 6:58 pm

    I will write to the Spectator as I’m a subscriber – no – I can’t be bothered. It would have to be too long to get in.

  27. The Man at the Back permalink
    June 15, 2020 9:37 pm

    Surely the news of the week.

    I always say that AGW crowd are 10 years out of date. It is a lag in the system like so much in weather and climate.

    Who would have thought it wasn’t green and wonderful. Oh all of us, thanks to Paul covering this years ago.

  28. M E permalink
    June 15, 2020 11:03 pm

    My memories of geomorphology lectures are that the width of tree rings are mainly due to water availability. It the water source is only rain it will record the precipitation but if there has been a rivulet ,brook or swamp near the tree and this has been changed in time since the tree flourished and the drainage changed naturally, it will not tell the observer anything about the climate and preciptation. A tree near a desert waterhole will flourish until the spring of water gives out.for instance. A swamp or lagoon opens to a river or large body of water after a storm and flood and the land is dry enough for farming. The tree alters or dies.

    However tree rings can be used in cross dating timber framed structures by matching patterns of rings in the same quality of timber in use.

  29. Mack permalink
    June 15, 2020 11:14 pm

    O/T Paul, have you seen the EMBER report on biomass derived energy. Got a mention in The Tines today. Apparently, burning biomass and troughing huge subsidies for the privilege is neither green nor cost efficient for the taxpayer. Who knew? Nice to see the greenies beginning to eat their own.

  30. tom0mason permalink
    June 16, 2020 12:52 am

    One of the best takedowns of Man’s paper was done by the internationally renowned statistician William Briggs …

    Because an assumed probability model said some sequence was rare, it does not mean the sequence was therefore caused by whatever mechanism that takes one’s fancy. You still have to do the hard work of proving the mechanism was the cause, and that it will be a cause into the future. That is shown by making good predictions. We are not there yet. And why, if you did know cause, would you employ some cheap and known-to-be-false probability model to argue an observed sequence had low probability—conditional on assuming this probability model is true?

    Lastly, please don’t forget that everything that happened in Mann’s calculations, and in my examples after the First Error, are wrong because we do not know with certainty the values of the actual temperature/anomaly series. The probabilities we calculate for this series to take certain values can take the uncertainty we have in these past values into account, but it becomes complicated. That many don’t know how to do it is one reason the First Error is ubiquitous.

  31. Coeur de Lion permalink
    June 16, 2020 11:17 am

    My letter to the Spectator
    As a member of the lunatic fringe of climate deniers (Philip Marsden – book review) may I gently remind the alarmists that they harm their cause by name-calling. The shocking Climategate scandal revealed, inter alia, that tree rings are a bad proxy for temperature. Forty-one years of satellite measurement show a continuing emergence from the Little Ice Age at a beneficial 1.3degsC a century. Today we are 0.53degsC above the 30-year mean, a figure that will decline under the influence of a La Niña at the turn of the year and possibly the rather ominous Solar Cycles 24 and 25. No need to panic. Plenty of modern science out there questioning CO2 as the prime climate driver. After 30 years of overheating, nobody now believes the IPCC’s scary ‘projections’. Read Donna Laframboise on IPCC corruption.
    Inflicting misery on millions, this industrial crash clamoured for by Greta Thunberg and the Extinction Rebellion mobsters has resulted in an uptick in atmospheric CO2. So it’s back to the drawing board for the climate alarmist cult!


    • Broadlands permalink
      June 16, 2020 6:54 pm

      “…a figure that will decline under the influence of a La Niña at the turn of the year.”

      The ENSO, of which La Nina is but one-third part, influences Pacific equatorial sea surface temperatures, not air temperatures. The current ENSO 3.4 is still in neutral territory and not headed toward the La Nina threshold… yet.

      • Coeur de Lion permalink
        June 18, 2020 6:50 am

        1998 and 2016 El Niños affected air temperature worldwide.

  32. George Lawson permalink
    June 16, 2020 12:33 pm

    Philip Marsden, a writer on travel, novels and fiction! Yet this man, who obviously has never studied science, or entered into the subject of dendrochronology refers to thousands of experts in the field as “the lunatic fringe”, simply because they hold an opposing view to the likes of Valerie Trouet. His attitude sums up the reason why it is impossible to have a serious point of view discussion with these people.

  33. MrGrimNasty permalink
    June 16, 2020 3:32 pm

    Not sure if we’ve seen this from last year before – didn’t find it on here with search.

    Where to start! One thing I didn’t know, carbon capture could make gas turbine generation too impractical/costly to load balance, Jeeez.

    Mean while China has 1040GW coal generation, enough in construction/development to almost meet the UK’s entire electricity demand twice over.

    And we are into the 4th consecutive day of very low/minimal metered wind power, 0.25GW currently.

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