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Climate Hustle

How does the Medieval Warm Period compare to current global temperatures?

What the science says...

Select a level... Basic Intermediate

While the Medieval Warm Period saw unusually warm temperatures in some regions, globally the planet was cooler than current conditions.

Climate Myth...

Medieval Warm Period was warmer
The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made.

One of the most often cited arguments of those skeptical of global warming is that the Medieval Warm Period (800-1400 AD) was as warm as or warmer than today. Using this as proof to say that we cannot be causing current warming is a faulty notion based upon rhetoric rather than science. So what are the holes in this line of thinking?

Firstly, evidence suggests that the Medieval Warm Period may have been warmer than today in many parts of the globe such as in the North Atlantic. This warming thereby allowed Vikings to travel further north than had been previously possible because of reductions in sea ice and land ice in the Arctic. However, evidence also suggests that some places were very much cooler than today including the tropical pacific. All in all, when the warm places are averaged out with the cool places, it becomes clear that the overall warmth was likely similar to early to mid 20th century warming.

Since that early century warming, temperatures have risen well-beyond those achieved during the Medieval Warm Period across most of the globe.  The National Academy of Sciences Report on Climate Reconstructions in 2006 found it plausible that current temperatures are hotter than during the Medieval Warm Period.  Further evidence obtained since 2006 suggests that even in the Northern Hemisphere where the Medieval Warm Period was the most visible, temperatures are now beyond those experienced during Medieval times  (Figure 1).  This was also confirmed by a major paper from 78 scientists representing 60 scientific institutions around the world in 2013.

Secondly, the Medieval Warm Period has known causes which explain both the scale of the warmth and the pattern. It has now become clear to scientists that the Medieval Warm Period occurred during a time which had higher than average solar radiation and less volcanic activity (both resulting in warming). New evidence is also suggesting that changes in ocean circulation patterns played a very important role in bringing warmer seawater into the North Atlantic. This explains much of the extraordinary warmth in that region. These causes of warming contrast significantly with today's warming, which we know cannot be caused by the same mechanisms.

Overall, our conclusions are:

a) Globally temperatures are warmer than they have been during the last 2,000 years, and

b) the causes of Medieval warming are not the same as those causing late 20th century warming.

Figure 1: Northern Hemisphere Temperature Reconstruction by Moberg et al. (2005) shown in blue, Instrumental Temperatures from NASA shown in Red.

Basic rebuttal written by dana1981

Update July 2015:

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


Last updated on 7 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page


Many thanks to gp2 who generated the temperature pattern for the last decade based on NOAA data.


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Comments 201 to 238 out of 238:

  1. I agree that the few barley grains found could have been imported.  It is logical that the earliest Vikings would have experimented with cereals.  These experiments might well have failed given the short growing season both then and now.  Henrickssen, however, believes that these grains were locally harvested.  

  2. "My interpretation of D'Anrea's studies is that he believes in present day CO2 induced global warming."

    Meaning what? That therefore his data must be wrong?

  3. What about these studies using senentments ect indicating the MWP was global? Shows a NYT reporter intervieweing 2 scientist who measured sentiment samples around Indonesia.  Look at 11:15

    MWP in China

  4. aristotelian...  Making the rounds at all the climate denial sites, I see.

    What WUWT, CO2Science and Science&PublicPolicy are not going to tell you is that there are many places where no MWP shows up at all. Such as, the Tibetan Plateau.

  5. re: "aristotelian... Making the rounds at all the climate denial sites, I see.

    What WUWT, CO2Science and Science&PublicPolicy are not going to tell you is that there are many places where no MWP shows up at all. Such as, the Tibetan Plateau."  

    Where talking about 1000 and 2000 (Roman Warm Period) years ago.  I don't see how the absence of evidence is evidence of absence in this case.

  6. Also, one of these links is a video of a NYT reporter interview scientists. As a layman, who am I more likely to believe, some random website I found the immiediatly lables someone "a denier" which is a clear reference to the holocost deniers so is a gross ad hominin attack or some scientists interviewed by the NYT?


    [TD] Nearly every claim on Skeptical Science is backed up with references to, and often links to, scientifically peer reviewed publications.  Those are even more reliable than interviews with scientists, because some scientists make claims in interviews,  editorial writings, and web sites that they are unable to make in peer reviewed publications, due to the strict quality controls over peer reviewed publications.

  7. aristotelian, you misinterpreted Rob's reply.  When he wrote "where no MWP shows up at all" he did not mean absence of evidence of the temperature.  He meant there is solid evidence of the temperature, and that temperature was not anomalously warm. That is existence of evidence, not absence of evidence.  Click the Intermediate tab at the top of the original post on this page, to see more evidence.

  8. Thanks, Tom. That makes more senese.

  9. Aristolean,

    This Skeptical Science article describes an article written by 78 climate researchers who combined data from around the world to make a graph of the temperature for the last 2000 years.  The MWP does not show up in analysis that combine all the world data.  The MWP was evident in some areas, but other areas were colder at the same time so the global temperature was not warmer.  By contrast, in the current warming everywhere is warmer.  It is possible to find warm areas somewhere in the World at almost any time in the past you choose, they are just balanced by other cold areas.  Do you believe a careful analysis by 78 scientists or a blog post on WUWT?  (note this confirms Mann's Hockey stick data, again)

    In general, WUWT leaves out most of the data.  You can make anything appear to be true if you leave out enough data.  The IPCC report does not leave out any data and they find no MWP.

  10. aristotelian...  Three of the four references you provided are well known AGW denial sites. I've spent a large amount of time going through the MWP material shown on CO2 Science, in particular. Many of their written synoses of the research actually contradict the actual research they present. And they, curiously, do not (though they are clearly capable) endeavor to sythesize the data into any kind of multiproxy reconstruction.

    The other thing you'll notice at CO2 Science is that, what they call the MWP does not occur at the same time in all the datasets. And this is clearly pointed out in most of the research, but CO2 Science goes to great lengths to hide this fact.

    When you read the actual research papers what you find is that the MWP was "heterogeneous." That is, it's not homogenous. It occurs at different times, at different places. And it doesn't occur at all in still other places.

    When people go to such lengths, as CO2 Science does, to hide this elemental fact, then yes, I call those denial websites.

  11. aristotelian... Whenever you hear someone say (or see it stated somewhere) that the MWP was global, ask one simple question: "Was the MWP heterogeneous?"

    That's all you have to say.

  12. Is the current Global Warming trend hetetrogeneious?  If the artice shelf melting was not a indicator of global warming 1000 yeears ago, why is it considered an indicator, today?

  13. Aristotelian, SkS is not some random website and if you believe so, you have not paid any attention. It won the Eureka Prize of the Australian Musem for advancement of climate knowledge. That's not a popular online vote in which everybody who has no clue can click and bots can click automatically. Every SkS article is backed up by peer-reviewed science papers, readers are encouraged to explore them, so the kind of dissimulation that happens at Idso's site can't happen. Several of the SkS moderators have had publications in high impact science journals in the past 3 years. In comparison, it took years for Watts to publish one paper, and when he did, it failed to confirm the very premise to the existence of his website. It also brought nothing new to previous publications by others that had already done that.

    So really ,the choice is between a website with established expertise demonstrated by winning awards from scientific organizations and a record of publications in the field, against some random interview in the NYT. That is a much better representation of reality in this case. 

  14. Aristotelian...  This is definitely not a 1000 year representation, but it gives you a better idea of the changes in spacial distribution of temperature on the planet for the past 150 years.

  15. aristotelian...  RE: "artice shelf melting"

    It would be helpful if you could provide links to what you're referring to.

  16. And to answer your question, modern warming is homogeneous, whereas the MWP was heterogeneous. Peaks of MWP warming occur during a span of about 600-800 years. Some places it was as early as 1400 years ago, some places as recent as 600 years ago.

    If you go back to the CO2 Science site and look closely at all the graphs they present, you can clearly see this.

  17. MWP has warming occuring in different places at different times and not at all in some places. Warming now is occuring at different rates in different places, but the whole planet is warming. eg see here spatial distribution for warming since 1980. Compare that with spatial distribution during the MWP shown in the intermediate version of this article.

  18. Aristotelian,

    This Wikipedia article describes the break up of the Ward Ice shelf on North Elesmere Island in the Canadian Arctic from 2002-2010.  It was a remnant of the Ellesmere Island Ice Shelf which was 3,000 years old when discovered in 1875 (dated using drift wood on the inner surface of the shelf).  Virtually all of the Arctic ice shelves have melted since their discovery, this is only the first hit of a Google search.  The shelf survived the MWP and has melted with modern AGW.  Since the ice shelf was 3,000 years old, how could the MWP have been warmer, even in the Canadian Arctic where it was presumably the warmest?  

    Please provide a link for your supposed Ice shelf melting during the MWP.  Perhaps your sources of information are feeding you fabricated data.  That is why they are called deniers.





    [JH] The use of all-caps is strictly prohibited by the SkS Comments Policy. Please read the Policy and adhere to it.

  20. Myles, the existence of megadroughts in the past does not disprove the human-causational component of the modern warming period nor does it offer support for the denier meme "the Medieval Warm Period was warmer".

  21. Myles - You've pointed to some data on the American West, which is a region representing less than 2% of the globe. Regional variations are weather, not climate, and if you examine the global data the Medieval Warm Period was just not as warm as the world is today. 

  22. Myles... Just to clarify, the MWP is generally believed to be a heterogeneous event, meaning it didn't happen globally and at the same time. We know that there have been periods of local warmth that exceeded global average temperature today.

    What you don't find is evidence that the global temperature during the middle ages was warmer than global temperature today.

    This is a very common trick pulled by fake skeptics. They will point you to a regional event during the middle ages, call it the MWP, and then compare it to global average temperature today.

  23. Just assume Myles is correct of a moment.  That is, assume the MWP was warmer than it currently is, and that it caused series of one hundred year droughts in the SW of the USA.  It would follow that as the world continues to warm with AGW, we can expect the start of such a series of megadroughts in the SW of the USA.  That should make Myles want to do all he can to avoid ongoing warming.  Is that right Myles?

    In fact, we already see the signs of the in the SW of the USA.  From the site to which Myles linked we have this graph:


    Attentive readers will notice that the drought extent in 2002 extended 80% of the area, far more than the at most 60% seen in the MWP.  I understand there have been worse droughts in the area since then.  Using SW US drought as a proxy for global temperature, we therefore must conclude that it is already warmer than at any time during the MWP.  And expected to get warmer so that we can expect worse droughts still.  

  24. Hi, 

    I encountered the following argument:

    <statement>The earth is now definitely cooler than during the Roman Warm Period.

    There are passes in the Alps that the Romans used, today they are covered in ice.

    The tree line was hundreds of meters higher 3000 years ago, we still find remnants of trees under glaciers.</statement>

    I have no idea if these statements are correct, can anyone point me to the relevant literature ?


  25. bvangerven--

    1. "The earth is now definitely cooler than during the Roman Warm Period"

    It's fairly clear from global Marcott et al. 2013 that the Roman Warming Period was not globally warmer than the present.  

    2. "There are passes in the Alps that the Romans used, today they are covered in ice.  The tree line was hundreds of meters higher 3000 years ago, we still find remnants of trees under glaciers."

    Note regional Marcott.  The Northern hemisphere (north of 30) was likely warmer than present (though Marcott's uncertainty for 20th c. temps is large).  It may be that the Alps were responding to a greater forcing during the RWP.  How about the rest of the globe?  Take a look at Funder et al. 2011 and see if the RWP is noticeable (note that Funder ends in 2000).

    Regardless, what's the point?  Even if the RWP were warmer than present, what would it mean, other than the possibility that climate sensitivty is greater than the current mainstream range?  If the RWP was warmer, does it mean that global warming is going to stop? 

  26. bvangerven @224.

    The talk of Romans and Alpine passes sounds like some of the nonsense that followed on from Kurt Nicolussi's "Green Alp" glacier theory that dates back to 2005. After a lot of noise, the evidence led to some better understanding of the microclimates in the Alps but had a big zero implications for climate on a regional or global scale.

  27. Apparently the myth is "Medieval Warm Period was warmer

    The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made." ...

    Actually I think more the concern that many in the general public have is that the Medieval Warm Period was not mentioned by Al Gore in his inconvenient truth and that the Medieval Warm Period is usually not mentioned by those pushing global warming.  I think people see this as selective reasoning or confirmation bias and therfore find it difficult to trust what they are being told. 

  28. MrN9

    "Actually I think more the concern that many in the general public have is that the Medieval Warm Period was not mentioned by Al Gore in his inconvenient truth and that the Medieval Warm Period is usually not mentioned by those pushing global warming"

    Leaving aside that what Al Gore may have said about something or other, which has no relevence, the premise of you argument is that there was a Medieval Warm Period. Given that the topic is Global Warming, the question is - was there a MWP Globally?

    Read the Intermediate rebuttal. Was there actually a MWP? The evidence suggests no.

    So what is your point?

  29. MrN9, it's also going a bit too far to claim that "many in the general public" are concerned about Gore's non-mention of the MWP.  I'd guesstimate that less than 2% of the general adult public knows anything about the MWP/MCA, and most of those people could care less one way or the other.  I ask my first year students (highly selective university) each semester if they can describe how the greenhouse effect works--just the basics.  No more than one or two in 30 gets close.

  30. Glenn Tamblyn and DSL.  What my point is that what is being discussed here is the wrong "myth".  It's not about people thinking: "The Medieval Warm Period was warmer than current conditions. This means recent warming is not unusual and hence must be natural, not man-made." ... People think something more like... "Oh look, people who talk about global warming pick and choose the data which they tell us about, and omit that which does not support their view so as to make their own view sound more convincing".  Exactly how warm or not the MWP may or may not have been is irrelevant.  Most will never understand the complexities of the issues, this is about trust...

  31. I'd say it's less about trust than cherry-picking on the part of the "people."  People for whom critical thinking is not a matter of habit are going to cherry-pick the field of information according to what makes them comfortable.  These people aren't trying to put together an understanding of the situation.  I talk regularly with people who claim, all at the same time, that 1) climate has changed before, 2) it's been much hotter in the past, and 3) climate science is a fraud. (and how do we know about the past?  Climate scientists — the same ones who are telling you that AGW is, in fact, quite real.)  The information is not being put together into a coherent picture, and there's no desire to do so.  The desire is to surround oneself with claims that block responsibility--responsibility for current activity and for future activity.  Trust means going to sources that tell you what you want to hear (in the name of freedom).  

    When people hear that the MWP exists, they have a range of possible responses.  
    1. Those climate scientists are hiding stuff.
    2. How warm was the MWP?
    3. What was the cause of the MWP?
    4. Really?  I think I'll go check it out on Wiki and google scholar.
    5. So? Stop talking to me about stupid crap. I'm trying to get into the game.

    It would be less easy to automatically go with no. 1 if climate science communication wasn't primarily defensive.  Sources that misrepresent the science are not legally bound in the US to represent the science accurately.  Thus, we have some very good, very well-paid rhetoricians who frame the message in the most effective way.  The sources don't simply point to Al Gore not talking about the MWP; instead, they create a narrative of persistent deceit, and they can do that because very few sources put together the actual science into a narrative, the narrative that scientists see.  It's simply too complex for the general public.  There will always be little bits of information that don't get included, bits that can be re-framed and blown out of proportion by experts.  The so-called "climategate" is an excellent example, and it revealed the willingness of the "people" to jump all over anything that suited their interests, even while just a tiny effort toward critical thinking would have revealed the absurdity of the whispered claims.

    The MWP is a good example for another reason.  All due respect to Mike Mann, but his multi-proxy record of the last 1000 years of surface temperature is not fundamental to the theory of anthropogenic global warming.  It's important, but one cannot work backward from it to confirm or refute the basic physical foundation of the theory, regardless of the accuracy of the work.  Yet that is precisely what "denier" opinion shapers claim. At every step, they push the logic that one alleged inaccuracy causes a breakdown of the entire theory.  The details are irrelevant; the logic is the message.  It is the logic of doubt, the logic that says "You're right: you don't need to try to understand this, because everyone has an agenda, and you're never going to get good information."

    In this sense, people "trust" Rush Limbaugh, but not blindly.  They trust him until he rubs up against their own expertise and experience.  And if he's just plain dumb where their expertise goes, it doesn't transfer to the stuff they don't know anything about.  As long as his message is about not being responsible for the woes of others, everything is all good.

    Any why was Mann attacked so ferociously?  Because he scored a rhetorical home run with his graphed results.  Easy to understand.  No one puts an attack on Lacis et al. 2012 in front of the general public.  It's a fine publication, and works through the fundamentals in a readable way, but it didn't produce any easy-to-get visuals.  Once you see the 'hockey stick' graph, it can't be unseen, and so denierville's experts have to attack, attack, attack.  Same thing happened to Marcott et al. 2013.  They shape public opinion, and, given the enormous amount of money going into the opinion shaping campaign, it's a credit to the people of the US that perhaps only a third of them have fallen for it hook, line, and sinker.

  32. MrN9 wrote: "Exactly how warm or not the MWP may or may not have been is irrelevant."

    Only to people who place their "trust" with no concern for reality. The only way to show that their 'trust' is misplaced is to provide the facts... i.e. 'exactly how warm or not the MWP' was. If you show that there was no global MWP (as the article above does) and people still respond that you are 'hiding' this thing which does not exist (as you argue they will) then those people are delusional. They aren't engaging with the facts, but operating solely on 'who they trust'... even when those trusted sources are shown to be false.

  33. realiscience wrote (in 2012): "Did you look at Oppo's data points. Some are clearly at this level, meaning that it is within the margin of error that sea surface temperatures could have been higher. ?"


    I read the paper.  Did you see this part? " The modern rate of Pacific OHC change is, however, the highest in the past 10,000 years (Fig. 4 and table S3)."    OHC = ocean heat content

  34. Steven Sullivan @233.

    The 2012 comment by realscience @148 referred to Oppo, DW, Y Rosenthal, BK Linsley, 2000-year-long temperature and hydrology reconstructions from the Indo-Pacific Warm Pool, Nature, 460 1113-1116, doi:10.1038/nature08233, 2009.  You are referring to a more recent paper - Rosenthal, Y., BK Linsley, Oppo, DW, Pacific Ocean Heat Content during the past 10,000 years, Science, 342 617-621, 2013.

    The quote you make from this later paper has been bandied about by contrarians, mainly because they rather enjoyed the paper saying "The inferred similarity in temperature anomalies at both hemispheres is consistent with recent evidence from Antarctica (30), thereby supporting the idea that the Holocene Thermal Maximum, MWP, and LIA were global events," but felt the sentence you quoted needed a bit of trashing as it spoiled their interpretation of what the paper was saying. So some branded your quote "a single sentence of genuflection to CAGW."  Reading this contrarian blather, I think it's fair to say that the contrarians inabilities in understanding 'CAGW' are also persent in their understanding of the relationship between OHC & surface temperatures.

  35. This is a response to Angusmac elsewhere.

    Angusmac, I distinguished between three different meanings of the claim "the MWP was global":

    "Was the GMST durring the MWP warm relative to periods before and after? ... Were there significant climate perturbances across the globe durring the MWP? ... Were temperatures elevated in the MWP across most individual regions across the globe?"

    In response you have not stated a preference to any of the three, and so have not clarrified your usage at all.  In particular, while your citing of the AR5 graphs suggests you accept this first meaning, you then go on to cite the Luning and Varenholt google map app, which suggests you accept also the third, false meaning.  Your question as to whether or not I believe the MWP was global remains ambiguous as a result, suggesting you are attempting to play off agreement on the first definition as tacit acceptance of the third, and false position.

    With regard to the Luning and Varenholt google map app, KR's response is excellent as it stands and covers most of what I would have said.  In particular, as even a brief perusal of Luning and Varenholt's sources shows, the warm periods shown in their sources are not aligned over a set period and include colder spells within their warm periods which may also not align.  Because of the possible failure of alignment, any timeseries constructed from their sources proxies will probably regress to a lower mean - and may not be evidence of a warm MWP at all.  (The continuing failure of 'skeptic' sources such as Soon and Baliunas, CO2Science and now Luning and Varenholt to produce composite reconstructions from their sources in fact suggest that they are aware that doing so will defeat their case - and have taken a rhetorically safer approach.)

    In addition to this fundamental problem, two further issues arise.  The first is that Luning and Varenholt do not clarrify what them mean by "warm" on their map legend.  One of their examples helps clarrify, however.  This is the graph of a temperature reconstruction from Tasmanian tree rings by Cook et al (2000) that appears on Luning and Varenholt's map:

    (Expanded version)

    They claim that it shows a "Warm phase 950-1500 AD, followed by Little Ice age cold phase."  Looking at that warm phase, it is evident that only two peaks within that "warm phase" rise to approximately match the 1961-1990 mean, with most of the warm phase being significanly cooler.  It follows that, if they are not being dishonest, by "warm phase" they do not mean as warm as the mid to late twentieth century, but only warmer than the Little Ice Age.  That is, they have set a very low bar for something to be considered warm.  So much so that their google map app is useless for determing if the MWP had widespread warmth relative to late 20th century values or not.

    As an aside, Cook et al stated "There is little indication for a ''Little Ice Age'' period of unusual cold in the post-1500 period. Rather, the AD 1500-1900 period is mainly characterized by reduced multi-decadal variability."  Evidently they would not agree with Luning and Varenholt's summary of the temperature history shown in that graph over the last one thousand years.

    The second point is that it amounts to special pleading for you to accept the IPCC global temperature reconstruction that you showed, which is actually that of Mann et al (2008), but to not then also accept the reconstruction of spatial variation on MWP warmth from Mann (2009) which uses the same data as Mann (2008):

    Apparently the data counts as good when it appears to support a position you agree with, but as bad when it does not.  If you wish to reject Mann (2009), you need also to reject the reconstuction in 2008 and conclude that we have not reliable global temperature reconstruction for the MWP (unless you want to use the PAGES 2000 data).  If, on the other hand, you accept Mann (2008), end your special pleading and accept Mann (2009) as our best current indication of the spatial variation of MWP warmth.

  36. Answering here, as requested by the moderators, rather than the other thread that Tom points to (Angusmac elsewhere).

    Angusmac's argument that the MWP was global seems to be akin to using past southern hemisphere summer temperatures as a direct comparison to current global mean temperatures, under the argument that "summer happens everywhere, so it's global"

    ...all while ignoring that it's pretty hard to find a time when both the southern and northern hemisphere had summer at exactly the same time.

  37. You"re kidding with the heat maps, right? Way to cherry pick a nine-year period and compare it with a 300 year period as evidence of increased global temperatures. The irony is, you can make the same argument at any time in history: Some locales are warmer, and some locales colder, just like today. While the western and southern U.S. can enjoy higher than normal temperatures, the northeast can be much colder and snowier. More than anything, it's a matter of time and location. Next thing you will tell me is, CO2 levels were not higher more than 30,000 years ago.

  38. scarletrogue @237 refers to this map from the intermediate post:

    He claims the period to have been cherry picked, even though it merely shows the most recent decade at the time of publication of the paper discussed above.  Moreover, it shows a period that excludes the El Nino of 1997/98 and that of 2010, but includes the La Nina of 1999/2000, and that of 2008, thereby being biased cold relative to trend in that period.  The claim that it was cherry picked is therefore just empty rhetoric.

    Of course, 1999-2008 is no longer the last decade, so here is 2006-2015 (again chosen because it is the last decade):

    scarletrogue goes on to say, "Some locales are warmer, and some locales colder, just like today."  Well yes.  Some places are warmer in the MWP than other places in the MWP; and some places today are warmer than other places today.  But I think scarlet rogue is trying to imply that some places in the MWP are warmer than the same places today.  Comparing to either 1999-2008 or 2006-2015, I can find no such place.  If scarletrogue wishes to claim otherwise, let them specify approximate latitude and longitude of the location so we can compare.  

  39. Northern Hemisphere vs 2016:

    2000 yr NH reconstructions vs 2016


    [PS] While your image is pretty self-explanatory, please note that image only/link only posts can be confusing. An extra sentence to explain the point you are making is good.

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