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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

"For now, though, it is enough just to see the Medieval WARM Period shown to be global, and warmer than today." (Musings from the Chiefio)

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

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Acknowledgements

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

Comments

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Comments 26 to 50 out of 271:

  1. Re: stefaan (24) Thanks; I'd known that, but didn't want to prejudice you in any way. Makes one wonder, tho... The Yooper
  2. Hi, "the Medieval Warm Period occurred during a time which had higher than average solar radiation and less volcanic activity" I don't see how the first argument there 'a time which had higher than average solar radiation' can stand on it's own. What measurements do we have for solar irradiance 500 years ago? Can you please cite some papers in support. Thx Hengist
  3. hengistmcstone, there are a few studies here, here, and here, which go back to about 11000 years ago.
  4. Re: @stefaan I don't know whether co2science had been selecting which bits of research to choose to include in their database or not but I know at least one instance where they misrepresented the results of the research. I'd felt for a while in their blog that they had been misrepresenting the results of published research, but I had no evidence so finally after I was pointed to another entry on the co2science website I decided to follow up and I read the original papers and contacted the lead author, I then wrote the results up on my own blog. You can read that here: http://oxfordkevin.carbonclimate.org/?p=469 Basically the lead author stated that "Our figure does not lead one to conclude that past sea surface temperatures were warmer than today as is suggested on these websites" My personal conclusion from this experience is that I cannot trust the work that they have done. Kevin
  5. Stefann, Oxford Kevin, Some time ago I went through as many of the co2science papers as I could bear. What I discovered was that: 1) Very many papers described a "medieval warm period", but differed from each other in actual dates by hundreds of years. Some would say 900, some, 1200, some 1400. Often there might be a period of some warmth for a few hundred years, but the period coming close to modern temperatures would be a peak covering a mere 20 span, and the peaks varied by hundreds of years from one study to the next. So the site will quote papers as seeing the MWP from 900 to 1250, and exceeding current temperatures by 1.5˚C, any yet the actual period where temps were that high were a mere couple of decades, and never the same couple decades from one study to the next. This would be the equivalent of taking the warmest annual temperatures from each country in the past three hundred years, and using each such maximum in concert to determine the temperature for the entire globe in that period. You couldn't even justify doing that in a single year, let alone ten, let alone hundreds. 2) I found several studies that were very selectively interpreted. For instance, one was based on a graph from a paper which showed warming in South American lake sediments. If you went to the paper, there were five other lakes in the study, all of which showed substantially cooler temperatures. They picked the one that showed extreme warmth and ignored the others. In another study, it was clearly stated by the authors that they were not studying climate and that their results could not be taken to reflect the climate in the region. But that didn't stop CO2science. In another case, the study only went back 500 years from the present (to 1500). That didn't stop them from including it in the MWP. In many cases, there were very wide error bars on the temperature and/or period range, or both, but these were always interpreted kindly in favor of the preferred conclusion. In other cases, the papers or graphics were so vague as to be worthless... they looked like they were scribbled on napkins. They either came from non-peer-reviewed articles and write ups (not studies), or else the subject of the study was not actual temperatures in the period in question, so any graphs included were worthless in that respect. After too much time wasted, it became quite obvious to me that there was neither rigor nor honesty in the effort. If there were, it would be published as a comprehensive, meaningful study, such as Mann et al (2009) (which reached the opposite conclusion, by combining all available proxies and systematically determining that while there was regional warming, such warming was not global... but it did so out in the open, in a peer reviewed study, not on a web site with no rigor or honesty). It says a lot, I think, when something is designed as a fun to use interactive map on a web site, yet can't make into the realm of peer reviewed science.
  6. Thanks to JMurphy who has provided me with some interesting reading which I accept. But I still see a problem with this. On the one hand SkS is saying the causes of the MWP are (largely) solar irradiance - the sun's output. On the other hand it is argued that the MWP may well have been a regional effect not global. How can these two arguments exist side by side? Greater solar irradiance would have an effect across the planet. In essence the intermediate answer to this Skeptic argument "To claim the Medieval Warm Period was warmer than today is to narrowly focus on a few regions... Globally, temperatures during the Medieval Period were less than today." negates the need for (and it could be argued unpicks) the basic argument
  7. hengistmcstone, you may be interested in reading another article on here : Was there a Medieval Warm Period? There, you will find a link to : Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly There is also a link to the press release of the above paper, which may be worthing looking at before you look at the actual paper. More reading, I'm afraid !
  8. Interesting new paper in Climate Change by Koch and Clague. Abstract: The Medieval Warm Period is an interval of purportedly warm climate during the early part of the past millennium. The duration, areal extent, and even existence of the Medieval Warm Period have been debated; in some areas the climate of this interval appears to have been affected more by changes in precipitation than in temperature. Here, we provide new evidence showing that several glaciers in western North America advanced during Medieval time and that some glaciers achieved extents similar to those at the peak of the Little Ice Age, many hundred years later. The advances cannot be reconciled with a climate similar to that of the twentieth century, which has been argued to be an analog, and likely were the result of increased winter precipitation due to prolonged La Niña-like conditions that, in turn, may be linked to elevated solar activity. Changes in solar output may initiate a response in the tropical Pacific that directly impacts the El Niño/Southern Oscillation and associated North Pacific teleconnections.
  9. Apologies in advance if I've failed to find it, sometimes I have trouble navigating this site: I keep hearing from the skeptic corner that the MWP isn't reflected by any of the IPCC's models and therefore the models are completely useless. Is there a page that examines this issue?
  10. Tristan - I doubt facts would have much influence skeptics pushing this argument but you can see these at the paleoclimate chapter in the IPCC report, specifically this section for temperature reconstructions and model simulations of this period. However, more recent work has shown the MWP (better called MCA) to be rather more complex than simple global event (see this paper for instance) so appears a dynamical component involved as well.
  11. #34 Tristan: The difficulty skeptics have with the MWP (MCA) is that they've heard all this anecdotal evidence of NW Europe being warmer, or Greenland being particularly pleasant for the Vikings (less unpleasant would be more appropriate), and they conflate that with the idea that therefore the whole world must have been uniformly much warmer during the MCA. To our best knowledge (see scaddenp's link), warming wasn't uniform, either in space or in time, and so globally it was relatively modest. Some places were warmer, others cooler or not much different. IIRC, Greenland's peak warmth occurs a couple of hundred years before Britain or Central Europe experienced peak warmth. At the same time, other areas were cooler and so global temperature just wasn't as much warmer as some skeptics would like to believe, see Mann et al 2009. The Little Ice Age experienced similar variations, in that it's most pronounced over NW Europe and the N Atlantic region, driven most likely by some increased volcanic activity and reduced solar activity. Where do most of our familiar cultural / historical records of the Western world emanate? NW Europe, hence at least part of the entrenchment of the MWP/LIA ideas. Today's warming is (relatively) much more globally uniform by comparison, and so there's much more of a signal in global mean temperature. Our best idea of climate forcings over these periods (solar/volcanic activity, then the recent CO2 spike) reflect these global patterns. The overall pattern globally is one of relatively subdued changes over the MWP/LIA, then a large rise through the present, reflecting the forcings. Skeptics complain about the 'missing MWP' because they want it to be there, large and proud, in global temperature records. The evidence suggests that it's only large and proud in some parts of the world, some of the time, disappointing the skeptics... again. Oh, and if that palaeo evidence were to be overturned and the MWP was globally really strong, what is the main implication? Very high climate sensitivity, not what skeptics want to hear!
  12. Thanks a lot folks, I love learning about all this stuff
    Response:

    [DB] In addition to using the omnipresent Search function in the upper left corner of every page here, one can also examine skeptic arguments by Taxonomy.  However, much information is contained in blog post form here which may not be found in the rebuttals, so the search function is best overall.

    For example, a search of the term "Medieval Warm Period" returns the following:

    Search Results

    Skeptic arguments matching the search 'Medieval Warm Period':

    Blog posts matching the search 'Medieval Warm Period':

  13. Tristan. One easy thing to keep in mind when mentally testing 'skeptic' posturing about the MCA/MWP. These same people will try to argue in the next breath/ paragraph/ thread that there's no problem with CO2 release because the climate has little or no sensitivity to forcing by GHGs. They fail to notice that an argument in favour of a global, higher-than-now temperature for the medieval period is also an argument in favour of (very) high sensitivity to any forcing. You might not want to advance this point while you're still feeling your way through the science. But it's a handy reality check when you read such things.
  14. I've just read an argument claiming that the maps you display above comparing the MWP to the last decade anomaly isn't repersentative as you're comparing 300 years to 10 so it's bound to appear colder. Perhaps a map of the 10 years at the height of the MWP would be more repersentative, but then I guess you have the problem of which years to pick as you'll be accused of "cherry picking" which ever you do.
  15. Fitz, see Daniel Bailey @ post 20 of this thread. You're looking for a graph, and Daniel has a few for you.
  16. Fitz1309 The argument that a 300 year average is bound to appear colder than a 10 year average is bogus. A 10 year average will have greater variability than a 300 year average because there is less data, however that would be equally likely to make it colder than the "true" temperature as warmer. Picking the warmest 10 years in the MWP would indeed be cherry picking, if someone used such evidence to claim that the MWP was warmer than now that would be bogus. However it would be a good way to show that current temperatures are warmer than the MWP, if it were the case even though you cherry pick the data that minimises the evidence in favour of the hypothesis. Having said which, if you look at the magniture of the difference, I rather doubt cherry picking a ten year period in the MWP would make much of a difference anyway.
  17. Fitz1309, like many 'skeptic' arguments this boils down to arguing that climate science has failed to do the impossible; 'They have not compared 300 years of AGW warming to the 300 years of the MWP'... obviously AGW hasn't been going on for 300 years and thus the data to perform such a comparison does not exist. 'They have not compared the 10 hottest years of AGW (thus far) to the 10 hottest years of the MWP'... proxy data resolution is not detailed enough to identify what the 10 hottest years (globally) of the MWP were with any confidence, let alone to generate a map of global variations for such a small time period. In the realm of things which might be possible; A comparison of the last 50 years to ~1026-1075 would show less pronounced differences than the maps above, but the overall conclusion would be the same. A few small parts of the world were slightly warmer back then, but the vast majority of the globe is much warmer now.
  18. The 10 hottest years in which region? The peak warmth occurred in different times at different places.
  19. I've read the Mann et al 2009 paper linked above and I've a question. The authors compare the overall pattern of change between Medieval Climate Anomaly and Little Ice Age, as reconstructed from proxies and simulated by models. And they conclude : « The observed patterns of change, even when averaged over multicentury intervals, are unlikely to be entirely forced in nature, as there is also a potentially important role for purely internal, natural variability. » Reference to the unforced natural variability is unclear for me. I have often read that such an intrinsic variability cannot create any long term trend. This is a typical Tamino argument : energy is redistributed, not created, by (unforced) natural variability so you do not expect from it any decadal-to-century trend in surface temperature, just a season-to-year signal, up-and-down noise. So, what are Mann et al alluding to when they mention such a « purely internal » variability which could explain part of the averaged multicentury pattern ?
  20. Perhaps because of the global patterns apparent, oceanic effects like discussed in Swanson and Tsonis 2009, also discussed here at Realclimate (guest post by Swanson, MAY have been a factor.
  21. skept.fr @44, earlier in the paper Mann et al wrote:
    "Both simulations give very similar estimates of the global mean MCA-LIA temperature difference (0.16° and 0.24°C for NCAR and GISS, respectively; the latter is identical to the proxy reconstructed mean surface temperature difference of 0.24°C). The spatial patterns of response for the two models (Fig. 3), however, are quite different, as discussed further below"
    The sentence you quoted comes from that discussion. Therefore the observed patterns are most likely the variations in spatial patterns of warming and cooling found in the models, rather than temporal variations in mean global temperature. It should be noted that changes in spatial patterns in temperature can make a small difference in TOA outgoing radiation, and hence there may also be some temporal variation in GMST, but it would be very small relative to the difference in temperature between MWP and LIA, ie, less than 0.06 degrees C (one quarter of the difference between MWP and LIA).
  22. "Therefore the observed patterns are most likely the variations in spatial patterns of warming and cooling found in the models, rather than temporal variations in mean global temperature." I find that a better way to understand it indeed. A while ago, I took a look at the litterature on the SHALDRIL cores, of which a chief researcher was Milliken. He finds that "There is no compelling evidence for a Little Ice Age readvance in Maxwell Bay" and also asserts that "the current warming and associated glacial response in the northern Antarctic Peninsula appears to be unprecedented in its synchroneity and widespread impact." So indeed LIA and MWP seem to have been more likely regional than global in nature.
  23. If the MWP is thought to be regional then I assume the data must just be specific to certain regions and therefore not truly representative of global temperature. Is this the case only with The MWP or does this apply throughout the temperature record as regards the proxy data? Just wondering.
  24. peacetracker - proxies that are records of local temperature have to be treated as just that. Getting a worldwide temperature has to involve multiple proxies rather like trying to construct GISS from an extremely limited no. of stations. There are some global indicators however - eg oxygen isotopes from seawater organisms reflect the amount of global ice. However time resolution isnt much good for small wiggles like MCA,LIA. However, there are non-quantitative proxies (like glacial advance or retreat) that help indicate whether a proxy is likely to valid for a large region. The importance of paleoclimate is to constrain and test models. If the models are correctly reflecting real physics then estimated forcings acting at the time (which often have to be determined from another kind of proxy) should be compatiable with temperature proxies at the same time.
  25. Comparing 1999 - 2008 to 950 to 1250 is simply not a fair comparison. According to the Mann data, 1100 - 1250 was colder than average. (See his data set A for NH mean). How is that a relevant comparison? No such comparison occurs in Mann's paper.

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