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Mann, Hayhoe try to erase the Medieval warm period

October 8, 2019

By Paul Homewood


James Taylor writes for CFACT:



Climate alarmists Michael Mann and Katharine Hayhoe have been caught using dubious, revisionist temperature data in their attempt, as one Climategate email author put it, to “deal a mortal blow” to the extensively documented Medieval Warm Period.

Before climate change became a political issue, it was scientifically well-established that a significant global warming event occurred between approximately 900 AD and 1200 AD. For example, the United Nations Intergovernmental Panel on Climate Change (IPCC) First Assessment Report presented a temperature history and visual graph documenting that the Medieval Warm Period existed and that it brought temperatures at least as warm as today (at pg. 7). Multiple peer-reviewed studies provided additional confirmation of the Medieval Warm Period.

The warming climate of the Medieval Warm Period spurred abundant crop production, fewer extreme droughts and floods, growing human population, and improving living standards. The Little Ice Age terminated the Medieval Warm Period and brought devastating weather extremes, widespread crop failures, famines, plagues like the Black Death, and a contracting human population. (For a good summary of the extensive benefits of the Medieval Warm Period and the devastating harms of the Little Ice Age, see the excellent book, “In the Wake of the Plage: The Black Death and the World It Created.”)

The existence of large historical temperature fluctuations, warmer temperatures than today, and many documented benefits of those warmer temperatures presented a powerful obstacle in alarmists’ attempts to brand our current modest warming an unprecedented climate crisis. One of the many embarrassing emails leaked in the Climategate scandal showed how alarmists deliberately set a goal of eliminating the historical existence of the Medieval Warm Period. Alarmist climate scientist Jonathan Overpeck wrote in an email to fellow alarmist Keith Briffa, “I get the sense that I’m not the only one who would like to deal a mortal blow to the misuse of supposed warm period terms and myths in the literature.”

Also, scientist David Deming testified to Congress that a prominent figure working in the field of climate change asserted to him, “We have to get rid of the Medieval Warm Period.”

We have often been told that the science is settled. Apparently, that doesn’t apply to scientific data and evidence invalidating climate alarmism. Mann last month favorably retweeted an assertion that present temperatures are the warmest they have been for at least the past 5,000 years. Hayhoe earlier this year gave a presentation in which she presented a graph (without any scientific citation) asserting temperatures steadily and consistently declined for 4,000 years – without any significant variation – prior to the warming of the past 120 years that finally and mercifully brought an end to the Little Ice Age (at 7:41).

As documented above, the existence of substantial historical climate variations such as the Medieval Warm Period and the Little Ice Age were scientifically well-documented and not in dispute before climate activism politicized the issue. Alarmist scientists were on record searching for justifications to eliminate these inconvenient climate variations that blew gaping holes in their alarmist theories. Now, conveniently, alarmists like Mann and Hayhoe claim the Medieval Warm Period, the Little Ice Age, and other well-documented warm and cold periods simply did not exist.

An old sarcastic saying goes, “When the facts doesn’t fit the theory, change the facts.” Mann and Hayhoe provide perfect real-world examples of such perniciousness. Powerful scientific evidence supported near-universal agreement about the existence of the Medieval Warm Period. Then Mann and Hayhoe, supported by little or no compelling evidence, waved a magic wand and made the Medieval Warm Period conveniently disappear.

Climate realists, however, will stick with the powerful scientific evidence, the long-established scientific “consensus,” the peer-reviewed scientific literature, and the findings of the IPCC. Sorry, Mann and Hayhoe, but you have been caught red-handed.

  1. charles wardrop permalink
    October 8, 2019 11:59 am

    Such practises amount to lies based on scientific corruption which, when proven, should be met by loss of their posts. Has that ever happened to climate catstrophists scared of the truth?

  2. Michael Adams permalink
    October 8, 2019 12:14 pm

    MSM will give Mann and Hayhoe full coverage no doubt and suppress contrary views. Its how we got ourselves in to this Climate Extinction frenzy. If only one side is given a voice then that voice becomes a fact.

  3. October 8, 2019 12:16 pm

    They’re on a loser there, but it gives an idea of the desperation of their position.

  4. October 8, 2019 12:17 pm

    In the 1970’s when working at The Smithsonian Institution in Washington, DC, I had contact with a couple of women who were working on another project. They were always wanting to access some of my data and I felt they were playing fast and loose with how they were using it. One day, I told one of the women that if scientists began to fudge their data and make false claims, when the general public caught on……and they would……..academia would be forever trying to get their reputation back.

    I believe that we reached this point some years ago. When I hear, “scientists say” or “experts say” I roll my eyes and wait for the scam to be revealed. There have always been bad actors, but there was a time in the not too distant past when they would have been severely sanctioned by their own colleagues. It is high time we ALL question the credentials of these so-called “scientists” and “experts” who “say.”

  5. megagriff permalink
    October 8, 2019 12:28 pm

    This is hardly new is it?! Mann’s defamation suit against Mark Steyn started eight years ago (I think) and is still ongoing. Mann recently lost a similar suit against Tim Ball. Just to remind you, both Steyn and Ball called Mann’s hockey stick fraudulent and in Ball’s case, the court agreed. Steyn’s case has been referred to the Supreme Court as it’s seen as a constitutional (First Amendment) issue.
    Still – it can’t do any harm to keep reminding people that Mann is is a fraud and has been totally discredited, can it?

    • October 8, 2019 3:09 pm

      One of Mann’s latest wheezes is insulting Steve McIntyre in terms that would end in court if it were the other way round.

      • October 9, 2019 12:06 pm

        He must be feeling left out. Needs more attention, so create another tantrum.

    • H Davis permalink
      October 8, 2019 3:10 pm

      Just to be factually correct, the court did not rule on the validity of the hockey stick graph. After long delays and a promise to the court to produce specific data Mann did not comply. Ball moved to dismiss the case (which Mann brought) and the judge agreed. The judge also awarded costs to Ball. See the court’s ruling at:

      However, legal scholars have suggested that because Ball used a “truth” defense he was entitled to discovery of all relevant information Mann might have. By not producing such information future litigants may assert that that the hockey stick is false with more confidence that their position will be upheld.

  6. GeoffB permalink
    October 8, 2019 12:50 pm

    Mann and Hayhoe seem to have overlooked the fact that for at least the last 2000
    years, mankind has been able to read and write, and accounts of the weather, crop yields, and natural disasters are well documented. You do not really need bristle cone pines and lake sediments to show what was happening. It is all in the journals. The Domesday book was 1085 and it is a complete record of agriculture in most of England.

    • Ben Vorlich permalink
      October 8, 2019 10:27 pm

      The Egyptians monitored Nile floods every year as it was vital to the nations survival. Too much or too little would damage the nation’s wellbeing. Also many of the cuneiform tablets from mesopotamia are government records, I would imagine any other civilisation capability of writing would have records which could be used to infer climatic events.

  7. James L. Neill permalink
    October 8, 2019 12:58 pm

    I believe that there was a post here about how Mr Mann had lost a libel case in British Columbia, Canada because he refused to produce the infamous hockey stick calculations when requested by the judge. Those calculations formed the basis of the case, interesting?

    • Martin Burlin permalink
      October 8, 2019 1:15 pm

      We should call this denial, “An inconvenient truth”? I recall it has been used before.

  8. Don B permalink
    October 8, 2019 1:03 pm

     Alaska’s Mendenhall Glacier is melting and retreating, exposing the trunks of ancient trees. Carbon dating has revealed that many trees were growing 1,000 years ago, and some more than 2,000 years ago. In other words, the Medieval Warm Period and the Roman Warm Period were both warmer than our Modern Warm Period, and were warm long enough for forests to flourish.

    • Chaswarnertoo permalink
      October 8, 2019 1:17 pm

      Damn and blast, what do you believe? Your lying eyes, or the gospel according to St Greta and the gretards? Heretic!

  9. Gerry, England permalink
    October 8, 2019 2:02 pm

    Their other scam is to claim that the MWP was not global and just confined to the northern hemisphere. There have been lots of papers – peer-reviewed too – that show it was global.

    • Harry Passfield permalink
      October 8, 2019 7:41 pm

      Gerry, considering that 2/3 of the global landmass is in the NH I guess the charge that the MWP was only NH could be explained statistically more favourably than Mann’s hockey stick based, as it was, on bristle-cones selected on one mountain in North America.

      • Gerry, England permalink
        October 9, 2019 2:09 pm

        To that you can add that at the time in question there was little anecdotal evidence being recorded in the southern hemisphere. As pointed out above, the Doomsday Book was compiled in 1085. There also the Anglo-Saxon chronicles but there are no records that I am aware of from south of the equator.

  10. Malcolm Chapman permalink
    October 8, 2019 3:33 pm

    I can’t find any link either here or at cfact to any new publication by Mann or Hayhoe relating to this. The troll-struck ping-pong at cfact seems, on first glance, to offer no further clue. Is this just recycling of old news, or is there some new intellectual and scientific atrocity to enjoy? Thanks in advance for any help.

  11. HotScot permalink
    October 8, 2019 4:00 pm

    Surprising really, that the alarmists who both wail about global populations, and climate change causing mass famine and deaths from extreme weather, don’t get behind we sceptics and welcome a warmer planet.

    I don’t think they know what they are talking about.

  12. October 8, 2019 4:24 pm

    Tucked away in the original batch of climategate emails was one which stated that “we have got to get rid of the mediaeval warm period”

  13. Ben Vorlich permalink
    October 8, 2019 4:34 pm

    They will have to create a virtually stable temperature record for the last 9000years. If the global temperature rises x’C for each doubling of CO2 then then their theory cannot account for any of the warm periods like Minoan and Roman nor any of the cold periods including the LIA as it appears the CO2 content didn’t change a great deal until the 19th century.

  14. saparonia permalink
    October 8, 2019 5:09 pm

    Changing history isn’t a new idea, usually it’s done after the effect. It’s quite refreshing that people who are telling lies to Joe Public attempt this at a time when they are already being rumbled by the majority. Desperation tactics.

  15. Harry Passfield permalink
    October 8, 2019 7:52 pm

    The BBC – and, most illiterately, Jeremy Vine (because he has an equally illiterate following) – is giving comfort to XR by asking: ‘Is it possible they (XR) have a point, and that AGW is going to herald the end the world as we know it?’
    They seem not to understand the law of unintended consequences: their promotion of this anarchist/Marxist group will come back to bite, not only them, but the rest of the country.

  16. October 8, 2019 7:55 pm

    Reblogged this on Climate Collections.

  17. Rambling Don Trumpo permalink
    October 8, 2019 9:23 pm

    (Not sure whether this is a spoof.) Today’s Times includes an article headed “Mayas blamed for starting climate change” (between 1,000 and 1,400 years ago). Instead of pretending warmer periods did not occur, why not simply think of some ancient civilisation that can be held responsible?

  18. Mervyn Hobden permalink
    October 8, 2019 9:47 pm

    My late friend and colleague, Dr. A.E. (Ted) Brain pointed out some 20 years ago that good quality white wines were produced in Britain as far north as Newcastle, during the MWP. We have a defunct vineyard at the Bishop’s Palace here in Lincoln where a few years ago an attempt was made to reintroduce vines without success. Ted pointed out that good quality white wines depend on the ‘noble mould’, botrytis cinerea and this needs the right climatic conditions to flourish. Viniculture has been reintroduced to Sussex and Kent with some success, but this far North not a chance. The documentary evidence supporting the conditions in the MWP is voluminous, ‘Greenland’ was colonised and ships sailed in ice free waters all around the Northern shores of Russia to the Pacific. By the 15th C commentators complained that this was no longer possible.Well, I suppose it is possible that Medieval scholars decided to document ‘fake news’ to confuse future generations but I think a more cogent explanation is that debunking recorded history is an attempt to conceal the result of badly written software and using mathematics not applicable to the real physical world. For 1500 years we believed that the motions of the heavens could be accurately predicted by a linear mathematical model based on eipicyles and equants – until the observational evidence proved that it was in serious error and lacked any real physical connection. I have a delightful paper written by an American educational academic, showing that if you put the modern orbital constants into the Ptolemaic model and make some small alterations, it predicts planetary positions as accurately as the Newtonian gravitational physical model. this is because the Ptolemaic model is simply a simulacrum where mathematical factors superpose to give an identical answer – but not containing any real physics. It is perfectly possible that the use of largely linear algebra has led to the same mistaken belief in climate modelling. The only thing that you can really prove using that form of human derived software we call ‘linear algebra’ are its own self consistent theorems. Accurately modelling a non-linear system such as climate by such means is bound to lead to an erroneous long term result, as the results must diverge with time from observed reality – which seems to be occurring.

    • bobn permalink
      October 8, 2019 11:29 pm

      There are commercial vineyards in yorkshire today – though limited in what they can do. Botrytis is not required for quality white wines – it relates to sweet dessert wines. There are over 600 vineyards in the UK south of Birmingham. Wine production in England doubled last year and another large high quality harvest is being gathered at the moment. English wines win international accolades and are exported globally. The last country to wake up to English wine is England. But then england is the least patriotic country in the world – anything but English is the English motto. Just go down the high street – Italian, Indian, Chinese, lebanese – but where is the proudly proclaiming English Restaurant? And how many lame english restaurants dare put their local English wines on their lists? Wake up England!

      • Graeme No.3 permalink
        October 9, 2019 1:26 am

        Sweetness in wine would have been appealing to medieval drinkers, so botrytis would have helped sales.
        I haven’t been able to find the reference by Chaucer about a (walled) Monastery vineyard near Edinburgh, but his father was a successful wine merchant, so don’t discount it.
        Also there were 2 attempts by importers of (French) wine, which was taxed, to get a tax placed on the wine produced by the Bishop of Durham’s estate, which sold at 3 times the rate of French wine.
        When I was in Hereford 3 years ago the hotel restaurent featured several local wines.

      • Ivan permalink
        October 9, 2019 4:42 pm

        Indeed Botrytis cinerea, or grey rot as it is better known, is mostly highly detrimental to wine production. Grey rot is all too common, especially in cool damp British conditions. It is only in very specific weather conditions that it makes the grapes “raisin”, or “nobly rotten”, rather than horribly rotten. I have occasionally seen the effect on a few of my own grapes when we get dry conditions in autumn.
        The main issue for English vineyards is competitiveness, what is the cost of producing a wine to a particular quality. Sparkling wines, we see, can be competitively produced. Others, less so.

  19. Susan Fraser permalink
    October 8, 2019 10:27 pm

    Does anyone know the reference for the MWP graph fig 7.1 on page 7 of IPCC First Assessment Report?
    I’ve never been able to find it

    Thanks all

    • John of Cloverdale, WA, Australia permalink
      October 9, 2019 3:56 pm

      Susan, try this link for the full report.

      Click to access ipcc_far_wg_I_full_report.pdf

    • Susan Fraser permalink
      October 9, 2019 11:32 pm

      Changed my strategy and searched “where did IPCC Graph 7 come from”….thanks to Steve McIntyre over at Climate Audit for answering that exact question in 2008. Its origins are Lamb 1965 for anyone else who wondered.

  20. Coeur de Lion permalink
    October 9, 2019 7:27 am

    And the Roman and Minoan?

  21. Phoenxi44 permalink
    October 9, 2019 9:30 am

    The trouble they have is that the MWP is well documented in other sciences and areas of study. It appears in geography, painting, history, literature,geography, agriculture and so on. As is the LIA as well.

    Just looking at a tiny sample of proxies and claiming all those other disciplines are wrong is beyond absurd.

  22. October 9, 2019 4:08 pm

    Other books on the LIA. Geoffrey Parker’s magnum opus “Global Crisis: War, Climate Change & Catastrophe in the Seventeenth Century” and Brian Fagan’s “The Little Ice Age”.

  23. October 9, 2019 4:09 pm

    Worth noting that Hayhoe thinks reducing emission to none will “stabilise” the climate.

    That’s a special kind of insane.

  24. John of Cloverdale, WA, Australia permalink
    October 9, 2019 4:17 pm

    “The late tenth to early thirteenth centuries (about AD 950-1250) appear to have been
    exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987, Lamb, 1988) This period is known as the Medieval Climatic Optimum. China was, however, cold at this time (mainly in winter) but South Japan was warm (Yoshino, 1978) This period of
    widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases.
    Cooler episodes have been associated with glacial advances in alpine regions of the world, such neo-glacial’ episodes have been increasingly common in the last few thousand years Of particular interest is the most recent cold event, the Little Ice Age , which resulted in extensive glacial advances in almost all alpine regions of the world between 150 and 450 years ago (Grove, 1988) so that glaciers were more extensive 100-200 years ago than now nearly everywhere (Figure 7 2) Although not a period of continuously cold climate, the Little Ice Age was probably the coolest and most globally extensive cool period since the Younger Dryas In a few regions, alpine glaciers advanced down-valley even further than during the last glaciation (tor example, Miller, 1976) Some have argued that an increase in explosive volcanism was responsible for the coolness (for example Hammer, 1977, Porter, 1986),
    others claim a connection between glacier advances and reductions in solar activity (Wigley and Kelly, 1989) such as the Maunder and Sporer solar activity minima (Eddy,
    1976), but see also Pittock (1983) At present, there is no agreed explanation for these recurrent cooler episodes The Little Ice Age came to an end only in the nineteenth
    century Thus some of the global warming since 1850 could be a recovery from the Little Ice Age rather than a direct result of human activities So it is important to recognise that natural variations of climate are appreciable and will modulate any future changes induced by man.”
    7.2 Palaeo-Climatic Variations and Change
    7.2.7 Climate Of The Past 5,000,000 Years
    pp 201-202,
    Full Report here:

    Click to access ipcc_far_wg_I_full_report.pdf

  25. Mervyn Hobden permalink
    October 9, 2019 7:59 pm

    Botrytis cinerea does indeed prefer dryer, warmer conditions to flourish and so would cease to be effective in the wetter, colder conditions at the end of the MWP. But this again is indicative of the true climatic conditions during the MWP. The reason is probably the same that caused the following Little Ice Age, a change in the bulk output from the sun. As astronomers noted, although sunspots had neen noted in the early 1600s when projection via the newly invented telescope was applied, by the middle of the century there were very few if any sunspots. Some even claimed that the previous observations were down to faulty instruments, even though the Chinese had noted the existance of sunspots and their variable nature many centuries before. Mann and others claim that the sun’s output is too regular to account for climate change, but is this another example of ‘Cookin’tha books?’

  26. October 9, 2019 11:11 pm

    The MWP must have been a lot warmer than today as it allowed birches and barley to grow normally on Greenland. Try that today. They undid the barley and the birches in Greenland today. This is very simple logic and should convince everyone who sees it – except if you are part of a religious cult called Climatology. Then facts don’t matter to you. Mann and Hayhow must be tried for fraud and should do jail time.

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