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“The Medieval Warm Period was just as warm”—NOT!

by Donald Prothero, May 16 2012

As I discussed in a previous post (April 11), the people who deny anthropogenic global warming (AGW) have many similarities to creationists. Despite the fact that the reality of AGW is supported by a 95% or greater consensus of qualified climate scientists, the critics (mostly non-scientists, or scientists in fields that do not qualify them to assess climate science) keep on repeating the same false tropes over and over again, no matter how many times they are debunked. This is analogous to the shopworn old arguments of creationists, who invariably trot out fallacious arguments like “evolution contradicts the Second Law of Thermodynamics” even though it has been corrected hundreds of times by scientists. The creationists have such a strong denial filter to resolve their cognitive dissonance that they either don’t realize why their “Second Law” argument is invalid, or they are deliberately and deceptively using it over and over again because it impresses their scientifically illiterate following.

The same is true of the long-debunked example of cherry picking, “It hasn’t warmed since 1998″ (see my April 11 post). Another common false statement is “The planet warmed just as much during the Medieval Warm Period, but eventually it cooled down again.” They argue that if this warming preceded our modern injection of carbon dioxide into the atmosphere, then maybe our current global warming not caused by our burning of fossil fuels. Climate deniers repeat this old saw over and over again as if it’s somehow a devastating blow to the huge body of data about our recent climate changes. They often illustrate it with the anecdotes about how the Vikings could colonize Greenland for a while, then as climate cooled in the late Middle Ages, these colonies failed when Greenland became too cold again. The story about the fate of Viking colonies in Greenland is true—but the rest is not.

The Northern Hemisphere temperature curve of Moberg et al. (2005). Note that the Medieval Warm Period is nowhere near as warm as the modern episode of global warming.

As climate scientists have long understood, there were numerous small fluctuations of warming and cooling over the last 10,000 years of the Holocene. Most are well understood to be consequences of the different orbital variations of the earth around the sun (the Croll-Milankovitch cycles), with some component of solar activity. But in the case of the Medieval Warm Period (about 950-1250 A.D.), the temperatures were only 1°C warmer in the Northern Hemisphere, much less than the temperature changes since the beginning of our current global warming (Fig. 1). The warmest years of the Medieval Warm Period are comparable to the mean annual temperatures recorded about 1960—and the earth has warmed dramatically in the past 50 years.

More importantly, the Medieval Warm Period was also only a local warming in the North Atlantic and northern Europe. If you look at the record of global temperatures over this interval, the mean temperature of the earth did not increase significantly, and actually cooled by more than 1°C. And in contrast to the current episode of global warming, which is caused by burning fossil fuels, the Medieval Warm Period was triggered by several well-known non-human causes: a long-term drop in volcanic activity (which can warm the earth by letting in more solar radiation) and an episode of unusually high solar activity. In addition, there may have been a strong long-term oceanographic effect, the North American Oscillation, that might explain why the warming was local to the North Atlantic and not global.

Likewise, the warmest period of the last 10,000 years prior to 1800 was the Holocene Climatic Optimum (5000-9000 B.C.) when warmer and wetter conditions in Eurasia caused the rise of the first great civilizations in Egypt, Mesopotamia, the Indus Valley, and China. This too was largely a Eurasian phenomenon, with 2-3°C warming in the Arctic and northern Europe. By contrast, there was almost no warming in the tropics, and cooling or no change in the Southern Hemisphere.

To the Eurocentric world, these warming events seemed important, but on a global scale the effect is negligible. In addition, none of these historic and prehistoric warming episodes is related to increasing greenhouse gases. The Holocene Climatic Optimum, in fact, is predicted by the Milankovitch cycles, since at that time the axial tilt of the earth was over 24°, its steepest value, meaning the poles got more solar radiation than normal. By contrast, not only is the warming observed in the last 200 years much greater than during these previous episodes, but it is also global and bipolar, so it is not a purely local effect. The warming that ended the “Little Ice Age” (from the mid-1700s to the late 1800s) was due to increased solar radiation prior to 1940. Since 1940, however, the amount of solar radiation has been dropping, so the only candidate for the post-1940 warming has to be carbon dioxide.

But you would never get this straight story that paleoclimatologists have known for years if you examine any of the climate-denier sites. Instead, they are loaded full of misinformation written by people who do not have formal training in, nor do they actively participate in, climate science research. Instead, like the creationists they keep repeating and recycling debunked and outdated ideas, unwilling or unable to read anything that does not get through their strong filter of confirmation bias. And, like the smug fundamentalist that I mentioned in my March 7 post, they are self-righteous and utterly sure that their arguments are sound, yet they never even bother to read or consider anything that might go against their ideological biases (any more than creationists will read or understand anything that undermines their biblical literalism). Such a behavior may be well understood in the world of psychology and neuroscience, especially as an example of confirmation bias—but it is indefensible in the scientific community, where peer review weeds out the false and bad ideas and data (like creationism and climate denialism) and scientists must learn to accept ideas which have become overwhelmingly supported by the evidence, such as climate change and evolution. Such ideas may not tell us what we want to hear, but that’s all the more reason to believe that they are probably real and not the result of wishful thinking.

52 Responses to ““The Medieval Warm Period was just as warm”—NOT!”

  1. Mal Adapted says:

    “a long-term drop in volcanic activity (which can cool the earth by letting in more solar radiation)”

    Did you mean to say “can warm the earth” here?

  2. noen says:

    I agree with Chris Mooney that we will NEVER convince climate denialists or other right-wing delusional beliefs, with facts. What is needed is a coherent narrative that can appeal to people with conservative values and so inch them away from their false belief system.

    So a conservative gay man giving his story about how discrimination on the right hurt him would help combat their false beliefs about gays better than citing research disproving them.

    Or Bruce Bartlet, former Bush official, saying that conservative economic ideas are insane does far more good than citing Paul Krugman ever could. Even if they are saying must the same things.

    So in the climate change arena what is needed is to find someone who is or was respected who has changed his or her position to come out and tell their story of how their mind was changed.

    As the previous thread amply showed, NO amount of evidence will ever convince a die hard climate denier like markx because his mind is closed. The door is shut and will *never* let in evidence contrary to his preconceived bias. Ain’t gonna happen.

    Find that other voice though and we have a chance,.

    • itzac says:

      I think stories of small business owners hurt by changing climate in their area could also prove persuasive.

      • Donald Prothero says:

        There ARE examples of conservative/GOP climate scientists who do good science, like Kerry Emanuel at MIT. But few started out as deniers–they were good climate scientists from the beginning who say the evidence in their data, and only lately has their political party veered into the realm of anti-science

      • Donald Prothero says:

        Sorry: “saw” the evidence in their data…

    • tmac57 says:

      Here is a good candidate of a conservative scientists who started out as skeptical about climate change,and who followed the evidence to arrive at the opposite conclusion:
      ‘Dr. Barry Bickmore – How to Avoid the Truth About Climate Change’

      Barry Bickmore is Associate Professor of Geological Sciences at Brigham Young University. His research specialties are low-temperature geochemistry and geoscience education. In this presentation, he discusses how he moved from being a climate change “skeptic” to being an outspoken advocate of mainstream climate science. He then discusses how it is that people like him can so effectively avoid the truth about climate change.

    • Max says:

      People deny the problem when they don’t like the proposed solutions, namely regulations to cut CO2 emissions. When they see more palatable solutions like nuclear power, they’re less likely to deny the problem.

      • tmac57 says:

        That’s very consistent with motivated reasoning.It makes me wonder how they perceive their change in attitude.
        Two more conservative Republicans and AGW accepting climate scientists(not former skeptics though) are James Hansen,and
        Richard Alley.

    • farmerjulia says:

      T. Boone Pickens comes to mind. A Texas oil man, Bush lover. He said he no longer denies global warming because he can see the effects on the glaciers.

      • noen says:

        Probably also because he can see himself making money with his wind farms. Nothing wrong with that but I bet is *encourages him* to give up his denialism.

  3. Old Rockin' Dave says:

    Another way to get the deniers rethinking is to point out some of those who have bought into AGW and are making plans for coping with it.
    This includes most of the insurance industry, the Defense Department, the CIA, and Canadian Forces. It might also make them a little worried that the Russians are preparing for exploitation of a warming Arctic with new military formations, plans for mineral exploration, and a program to build new nuclear-powered icebreakers; then again they would probably think the solution is for us to close the “icebreaker gap”.

  4. Claudio says:

    Citing former sceptics may work for some ‘mainstream’ conservatives but probably not for the hardcore. They will answer that “So-and-so used to be sceptic until he got government money”

  5. BillG says:

    Denying the problem is not the problem – the solutions and proposals are. Which are complicated, costly, nebulous and possibly foolish.

    I get the vibe from many that drastic measures are needed ASAP, then get skeptical only when severe personal sacrifice is presented. No proposal is guaranteed, nor is a certain outcome if we do nothing.

    • tmac57 says:

      BillG-Denying the problem, most definitely IS the problem.You cannot begin to have a rational discussion about solutions with the public,when a large portion of them is claiming that it is based on fraud and part of a conspiracy to take over the world.
      You also cannot have a rational discussion with someone when their attitude is “AGW is not real because I don’t like the solutions that have been proposed!”

      • Phea says:

        Then you have the problem of dealing with people like me. I believe AGW is real. So is over-population, running out of fresh water, and weapons of mass destruction. All are serious problems that will probably eventually cost millions, perhaps billions of human lives. I don’t necessarily consider this a bad thing. As brutal and deadly as the black plague was, thinning out the herd had some extremely positive results.

      • Syd Foster says:

        Except in this case, it’s the field the cows are standing in which is getting thinned out.

      • Somite says:

        Also you are talking about consequences of what we are doing on my children and unrelated innocent. Basic morality does not allow for that when all we have to do are small cheap decisions that are trivial at the end of the day and have to be done any way.

        Things like changing to renewables are achievable and are cheaper than the status quote, only the greediness of a few is preventing us from doing it.

      • Phea says:

        If ending the worlds, or even my own personal dependence on fossil fuels required small, cheap, trivial decisions, I’d do it tomorrow. AGW wouldn’t even enter into it, as just eliminating oil spills would make it worthwhile. So please… educate and inform me.

        My wife and I drive fewer than 10 miles a day, (but we do use gasoline), I have a newer more efficient furnace, (that still burns natgas), and I’ve replaced all my light bulbs with CFL’s, (but our local power plant is coal fired).

        Small, cheap, trivial decisions? What reality are you experiencing that I somehow missed?

        I still stand by my original statement. The earth is getting way too over populated, and it will and should be corrected. It would be nice, and relatively easy if we just stopped breeding like rabbits.

        Even if solving the AGW problem was as painless as say, world wide disarmament, or solving over population, it ain’t going to happen, for one simple reason. Human nature, which, the necessity of greed, is a large part of.

      • Somite says:

        You are conscious about it. The problem is that most people aren’t. Most people do their commute in an inefficient car, don’t care about CFLs (or even better LEDs) and couldn’t tell you how they obtain their energy.

        If most people cared about these things it would make a positive impact in the world.

      • BillG says:

        tmac, if everyone recognized the problem and all parties agree on AGW, we are still back to the REAL problem. That discussion has already started with no clear solution.

      • tmac57 says:

        You have to agree that arriving at a solution to the problem has very much been complicated by a well entrenched,and well funded and politically connected opposition to AGW that have mounted a massive disinformation campaign set on derailing any chance of their being a reasonable dialog about accepting AGW,and what to do about it.
        That side of the argument has shown that they cannot be trusted to represent their side of the discussion with good faith and intellectual honesty.They have poisoned the well so badly that now the general public does not know who to trust.That will be the legacy of the deniers who have set back action on Co2 mitigation by some 20 years,and now they seek to place the blame for it at the feet of the climate scientists who all along just wanted to get people to look at the facts.Pathetic!

  6. Canman says:

    While skeptic sites like WUWT and Climate Audit seem to have a lot of confirmation bias, they do not impress me as being creationists. They seem to be technically savy and nitpick new climate studies that come out (a valuable activity). When I hear climate skeptics being criticized, it is usually with blanket dismisals and derisive labels like “denialist”. Contrast this with creationists, where all their dubious details are gleefully pointed out.

    Im sure there is a lot of good science being done in the climate feild, but it has also become very politically charged. I can’t beleive that they are immune to confirmation bias. At the Climate ect blog (especially in threads that include Willis Eschenbach), I see a lot of what looks like valid criticism of preisthood of climate scientists.

    • Somite says:

      I think you are being deceived by what Chris Mooney calls the “smart idiot”. People that know a lot of fact sand still deny the conclusion. Also many people simply are unable to arrive at the correct conclusion because they may know facts but lack the expertise to arrive at the correct interpretation or “knowing enough to be dangerous”

      There is a third possibility of a malicious denial agenda as well.

      Always realize that there is nothing stopping deniers from gathering their own data and publishing their own result in a peer-reviewed journal. It just doesn’t happen because they lack the expertise or they do publish something that doesn’t contradict AGW in the publication but they say it does to reporters and speaking engagements,

      Deniers should not be taken seriously until they follow the process of science.

      (Before people cite Galileo or the ulcer guy considering that their discovery initially went against the grain but they used the conduit of proper science for ultimate validation)

      • Canman says:

        I think there is a lot of truth in Chris Mooney’s “smart idiot affect”, but I think it applies to everyone, not just republicans.
        Starting at 06:40 of the forth video on this page, Ronald Bailey describes a study by the Yale Cultural Cognition Group:

      • Somite says:

        The difference is that it seems to be an important component of right wing denialism, up to the level of presidential candidates.

  7. Trimegistus says:

    The shorter Prothero:

    If you disagree with him about climate, you are wrong, evil, and “right-wing.”

    This proves he is objective and nonpartisan.

    • Somite says:

      No. Just wrong. Likely because you are right wing.

      • Marcus says:

        Surprised Prothero hasn’t taken this opportunity to shill for his book. This doesn’t have to do with evolution so he’ll need to write one fast on climate change and then quote it at least 17 times…

        But if he does write a book on climate change, just imagine all the trees that would be cut down to make it and the chemicals used and disposed…thereby contributing to the problem at hand…What a shame…

      • Beelzebud says:

        No one forces you to come here and read his work…

      • Richard Smith says:

        Y’know, it’s almost – ju-u-u-ust almost – as though he isn’t here to shill his book.

      • Donald Prothero says:

        I DID publish a book on this topic, “Greenhouse of the Dinosaurs”, in 2009. However, this article is based on stuff that was not in my book, so I chose not to shill for it. But it IS in the latest issue of Skeptic magazine and in a flyer Skeptic SOciety is publishing soon–and in my NEXT book…

      • Canman says:

        Not to mention some of the chapters in your book catastrophes.

    • Richard Smith says:

      The shorter Trimegistus:

      LALALALALALA!!! Can’t hear you!

  8. Nyar says:

    I think Brian Dunning makes a good point that Global Warming is a political issue now. It shouldn’t be, but it is. So to get results, we are going to have to come up with a way to solve the problem and satisfy most of the various political interests.

    I personally like Hansen’s proposal to tax carbon, but make certain that the monies collected from the tax go back to the taxpayers rather than the government(s). I think a lot of people believe that Climate Change is just an excuse for the government to impose another tax that most people feel they can’t afford and maybe an approach like this will alleviate some of those concerns.

  9. Phea says:

    In trying to understand what’s going on in the 21st century, I’ve read “World On The Edge”:

    and watched, “Plan B: Mobilizing to Save Civilization”:

    The author, Lester Brown, raises several issues that will become problems that will need to be addressed in the very near future. I’m curious, among the problems he brings up, along with AGW are, over-population, and running out of fresh water. Are there substantial groups of people who also deny that these will soon be major problems for us? If not, perhaps we should concentrate our efforts and resources on solving problems we can all agree on. I’ve found that just establishing common ground and opening a dialogue… getting people to agree, is always the first step for any meaningful, ongoing cooperation.

    While the ongoing pissing contest over AGW makes for interesting books and blogs, and satisfied egos, there are other problems we can move on to, that are just as important to solve, or at least discuss.

      • Max says:

        The main point is that population growth is correlated with LOW child survival. Suppose 50% of children survive. If you have 4 children, there’s only a 68.75% chance that more than one of them will survive. To raise that probability, you have, say, 7 children. Of those, 3.5 survive, and so the population grows.

      • Phea says:

        Interesting Ted Talk, thank you. While I’m not an expert on over-population, I believe I can safely make the assumption that a finite planet cannot indefinitely sustain a growing population. I got the basic point of the video, but key elements that affect Hans’ theory are recent developments. Effective birth control and safe legal abortions to name two. He is right though, back in 1960, when I was 9, I remember Weekly Reader had an issue devoted to the 3 billion population milestone.

        While I agree that infant mortality rates are a part the population equation, to suggest that it is the only, or even a main factor governing population growth is, well perhaps a bit premature, to put it nicely.

        Everything goes through cycles. This might very well be the first cycle of world-wide over-population. There have always been isolated cases of too many people. The results are never pretty, from massive famine and disease, to who gets eaten in the life boat.

        Even if we could sustain a trillion people, we will reach a point, (if we haven’t already), where we are over dependent on technology, in some ways very fragile, delicate technology. Failed technology that destroyed the societies which became dependent on them. An early example of this is the decline of the Sumerian empire when soil salinity became too high to sustain crop growth.

        The difference now is that the entire world is connected to the same machine, and if the machine someday breaks, it will effect everyone, all trillion of us, and it won’t be pretty.

  10. d brown says:

    Most of the BS comes from one big oil company. They are proud of it. Its like they winning something.

  11. Jolly says:

    Any excuse that simply allows humans to gallop on as before (read: ignorantly) and require then to do nothing to change their ways will be followed. This planet is fucked. Fucked.

    • Phea says:


      The planet is and will continue to be just fine and dandy. It’s us humans that are fucked. We are, most likely, just another species failure, (the list is quite lengthy). The planet will heal and keep on spinning long after we’re gone.

  12. SEAPvet says:

    There is a very good technical discussion regarding Moberg etal (2005) on RealClimate with an interesting and reasonable sceptical comment here: comment 21:

    That commentator does go on to say “Wisdom transcends simple science. Wisdom dictates that fouling one’s nest is contraindicated. Regardless of established science results, it is my personal opinion that the wise choice is to strenuously work to limit anthropogenic pollution.”

  13. Blaze says:

    I think education and PR could make strides if “weather” could be separated from “climate”. The two terms are synonymous in 99% of minds. A climate scientist tries to present his/her case, predicting the trend in temperatures, winds and whatnot for the next century. All the listener can think of is how their Saturday BBQ was ruined by a completely whackadoodle Friday weather prediction. When the five day forecast has a reliability scarcely better than the newspaper horoscope, it’s hard to take climate predictions seriously.

  14. SEAPvet says:

    I may be looking at this a little too naively, but it worries me that the graphic provided indicates the northern hemisphere average temperature to be between 0.2 and 0.8 degrees warmer over the last 40 years than it was during the so called Medieval Warm Period, and at least equal to the temperature of the MWP for about the 100 years or so prior to that.

    However, places like Greenland are still nowhere near the state they were in during the period the Vikings were there. In 2008, Greenland recorded a population of 49 cattle, up from 5 in the year 2000. Yet from the time of Erik the Red, remains of stables each capable of carrying up to 100 head have been discovered, with ample evidence of feasting on beef and mutton. And the Vikings could not have depended on the annual replanting of ryegrass fields which do not have a long enough growing season to seed, as is the basis of today’s cattle industry.

    The implication is that (according to the graphic) in spite of being warmer for the last 120 years than it was during the so called MWP, Greenland is somehow still not warm enough to replicate the farming culture of that earlier time.

    Likewise, the Vostok Ice cores (Antarctica) do show evidence of a Holocene Climate Optimum and there seem to be some odd contradictions in the Wiki page on the period.

    This statement seems to contradict itself and the paragraph which followed on wiki: “essentially no change in mean temperature is reported at low and mid latitudes. Tropical reefs tend to show temperature increases of less than 1 °C; the tropical ocean surface at the Great Barrier Reef ~5350 years ago was 1 °C warmer and enriched in 18O by 0.5 per mil relative to modern seawater.”

    “In terms of the global average, temperatures were probably colder than present day. While temperatures in the Northern Hemisphere were warmer than average during the summers, the tropics and areas of the Southern Hemisphere were colder than average.”

    But this is perhaps again at least partially contradicted by the final paragraph:
    “While there do not appear to have been significant temperature changes at most low latitude sites, other climate changes have been reported. These include significantly wetter conditions in Africa, Australia and Japan, and desert-like conditions in the Midwestern United States. Areas around the Amazon in South America show temperature increases and drier conditions.”

    And that the Holocene Climate Optimum was evident in Antarctica seems accepted in a number of publications. “All the long Holocene records combined in this work (including recent profiles from coastal and Ross Sea locations) confirm the widespread Antarctic early Holocene optimum (11,500–9000 yr ago).”

    • Somite says:

      Regional changes may not follow the hemispheric or global average. Also, current warming is a relatively new phenomenon and not comparable to pre-Holocene temperatures. It would take great global adjustment and hardship for our current civilization to adjust to pre-Holocene temperatures.

      • SEAPvet says:

        Pre-holocene, as in ice-age conditions?
        I agree that ice-age conditions would create great hardship to our current civilization, but I am afraid the point of your remark in relation to the discussion is unclear to me.

      • tmac57 says:

        In addition to what Somite said,your whole comment is completely beside the point,because no previous age of the earth has experienced a human induced, relatively rapid, increase of greenhouse gases,and even the AGW ‘skeptical’ scientists do not seriously argue that we are not raising the earth’s temperature.Their arguments are about what the earth’s climate sensitivity is to human output of greenhouse gases.The vast consensus currently is that it is approximately 3C.
        We are conducting a very dangerous,uncontrolled experiment with our climate,with no known mechanism for reversing the effects.

  15. Somite says:

    I meant early Holocene. But again you are comparing regional variance to an effect of planetary scale.