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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 51 to 75 out of 156:

  1. jzk - yes it would be neat if we had a whole bunch of global proxy data with sufficient resolution (detail), say at the annual (yearly) scale, but AFAIK there's not a great deal - and we know how worked up the fake-skeptics get about the tree-ring data. You just have to work with what you have. The real dagger to the heart of the MWP (to my mind) is the global circulations. Mann's reconstruction agrees very well with how we expect the circulations, and their teleconnections to operate. Not perfectly of course, but a warm MWP wouldn't agree at all. For instance ENSO (La Nina/El Nino) was weaker (lower amplitude) then, and the natural oscillation in ENSO was longer (around 80 years?). This is consistent with a cooler tropical Pacific than today. Additionally, the Amazon rainforest was wet - consistent with a more southward displacement of the ITCZ (inter tropical convergence zone) than exists today. Again suggesting a cooler global climate back in medieval times.
  2. And as if to complicate the situation once again - Last Millennium Climate and Its Variability in CCSM4 - Landrum (2011) suggest something else again.
  3. Rob, I refer only to the two chart chosen by Skeptical Science to contrast todays warming with the MWP. The Mann chart shows a several hundred year period that includes, according to his data, both years of warming and years of cooling. The Skeptical Science chart shows only a decade at a warming peak. It is it any wonder one shows "red" and the other not? It may very well be that the MWP was cooler than today, or not. Let the science show us that.
  4. Rob, Let me put this another way. I could take Mann's data from the MWP and produce that global chart based on 950 to 1250. Then I could produce another chart with the same data, but only 980-990. Then, I could put them next to each other. One would be all red, and the other would be all white. Could I make the claim "The MWP was warmer than the MWP?" How is that a helpful comparison? Again, none of what I say contradicts the premise that the MWP was cooler than today, just the way that the charts are used to illustrate that position.
  5. jzk 54, a comparison between a single decade and several centuries is far from ideal. There is, however, some justification for it in this instance. In particular, very solid evidence suggests that this will be the coolest decade of the next several centuries. (How many depends on just how soon we break the fossil fuel habit.) That being the case, we know that the average global temperature over the next few centuries will be higher than that for the current decade, and hence higher also than the average over the several centuries of the MWP. In other words, the comparison does strongly suggest that the current warming will result in greater overall warmth in this century than at anytime in the MWP. Of course, it is not appropriate to look at a decade to century comparison and conclude that this decade is warmer than any decade in the MWP. However, a large number of studies have been done which suggest exactly that. It is more likely than not, on current evidence that this decade, and even the 1990s was warmer than any decade in the MWP. However, the evidence for that is not so strong that it can be stated categorically.
  6. Tom, "There is, however, some justification for it in this instance." While you may have evidence to suggest that the MWP was not that warm, that warming will continue, that the decade chosen will turn out to be a cool decade, or anything else related to this issue, comparing those two charts is meaningless and bad science. That is my only point. In fact, it detracts from the real case that you are making because it appears to be manipulation.
  7. jzk, given that false claims that 'the MWP was warmer than current temperatures' are made on a regular basis, a graphic comparing temperatures over those two time periods seems a perfectly reasonable response. Yes, in an ideal world we would be able to tell the deniers, 'comparing multiple centuries to a few years is "meaningless and bad science"' and they would shut up and go away. This is not an ideal world. Instead, we present a direct comparison of the exact things they cited. If they want to change their claim to 'some decade in the MWP may have been warmer than the past decade' we can switch to pointing out that all available evidence suggests otherwise. Until then I really don't see the problem in providing direct evidence that the claim they are making is false.
  8. "a graphic comparing temperatures over those two time periods seems a perfectly reasonable response." CB, But that is not what you did. You compared several hundred years that contained both warming and cooling to a peak of warming. That is no comparison at all. Compare the warmest decade of the MWP to the warmest decade now. That would be a fair comparison, and if you used Mann's data, it would still confirm your case which seems to be, "yeah some warming then, but not as much as now." I just want to promote good science.
  9. jzk @56, your claims are unjustified. To begin with, Mann et al 2009 make exactly the sort of comparison you claim to be unjustified. Specifically, figure one (intermediate article above) is a comparison of temperatures over a three century period in the Middle Ages to a thirty year period in the twentieth century. With a bit more work, John Cook could have compressed his two figures (from Mann et al 2009) into one by showing the MWP compared to a 1999-2008 baseline. Clearly, therefore, there is no inprinciple difference between Cook's comparison and the data presentation in the original graph. If there were any merit to your claim that such comparisons are bad science, then you must reject that chart itself for not (impossibly) choosing a three century baseline in calculating the anomalies. As it stands, I suspect editors and reviewers of Science Magazine know a little about what constitutes good and bad science. They appear to have judged this comparison of a three century interval with a thirty year interval good science for the very simple reason that, without such comparisons the mean temperature cannot be differences cannot be shown at all. That is something you should consider seriously. Without the sort of comparison, which you in a now deleted post called "absolute fraud" the information in the anomaly map could not have been presented at all. So yours is a standard which amounts to censorship of the data. The question with a diagram in science is always only three things: Does the diagram accurately present the information collected by the study; do the inferences drawn from the information in the graphic actually follow from that information; and is the graphic presented in a way that avoids confusion. On these three counts, nothing prevents a comparison of three centuries data to a single decade. What is necessary for such a comparison is that the conclusions be appropriately restricted, and that the data be presented in such a way as to avoid unwarranted conclusions. On the first point, (accuracy of information) there is no question; on the second, as the unwarranted inference you are making is not made by either Mann et al, nor John Cook there has yet to be shown a problem. On the third point, I believe that a caution to the effect that the data presented does not show that all decades within the MWP where cooler than the 2000s. But that is not a question of poor science, but of poor presentation.
  10. jzk @58: "I just want to promote good science" No, you want to apply a cooky cutter standard without any understanding of context or purpose. Such "cooky cutter standards" cannot exist in real life, except as reduced to basic principle: 1) Do not present false information; 2) Do not make unjustified inferences; and 3) Always let readers know what you have done, so that they can check it for themselves. If you want to argue the article represents bad science, you have to engage with the argument in the article and show how it is unjustified based on available evidence. Failing to do that, at best you can show that there are people out there easily confused by a misunderstanding the author failed to consider (possibly because it is so obviously an unjustified inference).
  11. jzk, again... the claim that the several hundred year MWP was warmer than current temperatures comes from skeptics. SkS showing a graphic proving that claim false is thus perfectly reasonable. Rather than complaining here about a graphic disproving the claim, perhaps you should be instructing 'skeptics' on how to make less ridiculous arguments. Also, "...to a peak of warming". Really? On what basis do you claim that current temperatures are a peak? All available evidence indicates that they are just a spot on a continuing upward slope. "Compare the warmest decade of the MWP to the warmest decade now. That would be a fair comparison" It would not be the comparison that 'skeptics' constantly make and it is not possible with current data. We do not have sufficiently detailed geographic and temporal data to create a map of (or even conclusively identify) the warmest decade of the MWP. Though again, I find it odd that you suggest that the peak of the MWP should be compared to the not-peak of the current warming to be 'fair'. Surely, if we were really trying to determine whether modern warming is within the range of natural variability (as claimed) we should compare the peak of the MWP to the eventual future peak of the modern warming, no?
  12. jzk: The historical instrumental temperature record (those graphs of NASA GISS, UAH, HadCRUT, and the like) are compared to an arbitrarily defined 30-year climatological baseline period in order to clarify temperature anomaly information. I am not aware of any serious valid criticism of this practice. As long as the uncertainties and caveats due to proxy data (the differences in practice) are made clear in the applicable literature, is there some appreciable difference, in principle, between this standard practice and what has been done in Mann et al 2009 with regards to the Medieval Climate Anomaly?
  13. Composer99, Mann didn't take his chart and compare it to 1999-2008, skeptical science did.
  14. "Also, "...to a peak of warming". Really? On what basis do you claim that current temperatures are a peak? All available evidence indicates that they are just a spot on a continuing upward slope." CB, Peak so far. If temperatures continue upwards, then it would be fair to redo the graph. Then, there would be a new "warmest" decade to use. Compare the warmest decade currently to the warmest decade in the MWP. Using Mann's data would still yield your desired result that the MWP was cooler than today, but it would be doing it in a fair presentation of the data. The comparison in the charts presented is meaningless. As I stated, I could show the same thing by comparing any time period to a shorter time period contained in itself. What would that show? Nothing.
  15. jzk - "I could take Mann's data from the MWP and produce that global chart based on 950 to 1250. Then I could produce another chart with the same data, but only 980-990. Then, I could put them next to each other. One would be all red, and the other would be all white" This is the point I made earlier, although not clearly enough, you cannot do this because the proxy data with sufficient global coverage and resolution (detail at the annual level) does not exist. For the instrumental record we have millions of measurements made all over the world for the last 130-odd years. These have great precision because they were made by thermomemeters, on (mostly) a daily basis. Even then great care has to be taken to ensure spurious signals, and therefore bias, doesn't creep into the record. In the MWP thermometers had not yet been invented so we have to rely on signatures of global temperature embedded in ice cores, tree-rings, coral growth rings, sediment deposits, pollen, cave stalagmites/stalagtites (speleotherms) etc. Not all of these signatures (proxy data) have resolution at the annual scale, most are only indicators of local temperature, they are affected by other factors which have to be zeroed out, and virtually all of them are not complete. They only cover a small interval over the MWP. The purpose of the statistiscal analysis by Mike Mann and others is to combine all this data so as to give an idea of what global temperature was back then. That means weighting the data and putting them into a common reference frame relative to each other. That way you can allow for the fact that the data do not overlap, and have different time resolution, and are only indicators of local conditions. So what you ask cannot be done.
  16. Yes I understand the proxy data, and I am not expecting to plot temperatures from May, 975. But certainly a more narrowed view can be taken. If not a decade, how about half a century? One can see from Mann's data that half of that period was warmer and the second half cooler. And besides, it is not Mann that is comparing a decade of our warmest weather so far to any of his data. It is this site. How about this. Take Mann's chart and compare it to one of the last 300 years (1711-2011). Would the chart be different than the decade one? You bet it would.
  17. jzk -"How about this. Take Mann's chart and compare it to one of the last 300 years (1711-2011). Would the chart be different than the decade one? You bet it would" Of course, but then the Industrial Revolution had not even begun in 1711. Fake-skeptics make the false claim that the MWP was warmer than today, not 1711. So some comparison is necessary in order to demonstrate that that is wrong. It would be nice if an apples-to-apples comparison could be made, but that isn't possible.
  18. I think I understand what jzk is trying to say. If you took the temperature reconstruction and randomly generated noise for it (in line with the expected distribution) to show examples of what the year to year temp might have looked like, some examples would have years (possibly multiple consecutives) as high as the decade just passed.
  19. "Of course, but then the Industrial Revolution had not even begun in 1711." Yes, of course that is the point. How can you compare centuries of data that include both a warm and cool period to a decade of warm data? By using 1711, I would be unfairly averaging in cooler years to the recent warmer ones to bring down the significance of the recent warming. That is my exact point for not doing it in the MWP either. Otherwise the comparison has no meaning.
  20. "How can you compare centuries of data that include both a warm and cool period to a decade of warm data? By using 1711, I would be unfairly averaging in cooler years to the recent warmer ones to bring down the significance of the recent warming" No by including 1711 you would be including a period where global warming had not even begun. The question is: how does present warming relate to the MWP? Present warming is the period affected by human-added greenhouse gases. So how does that compare to a period without large human perturbation? You know, the natural background state. If, as is claimed, the MWP was warmer why does a compilation of the ENSO data indicate that the tropical Pacific was much cooler than present? As I'm sure you are aware ,the tropical Pacific has a huge effect on the global climate through the sea surface-atmosphere heat exchange. And once again, why is the ITCZ anchored further south during the MWP? That indicates an Earth cooler than present too. Maybe it was globally warmer during the MWP, but the evidence thus far indicates otherwise. "That is my exact point for not doing it in the MWP either. Otherwise the comparison has no meaning." And for consistency you'll be telling the fake-skeptic blogs that a comparison is utterly meaningless too. Right?
  21. jzk: The purpose of this post is to refute the contrarian claim that the Medieval Climate Anomaly was globally warmer than the present day, where 'the present day' is, effectively, any given year in which someone asserts this claim. The SkS-created graphic to which you object (thank you for the correction, by the way) does exactly this by showing both the MCA global temperature reconstruction (to the extent that such a thing is possible given the proxy data used) and a (not quite) present day global temperature anomaly, both calibrated to the same climatological baseline. Indeed, the present day data is already a few years out of date. Since the contrarian claim is about the present day, it is IMO sufficient to show that the present day is much warmer than the MWP/MCA. If you want a more complete picture, there are always the Moberg et al 2005 reconstruction with instrumental data in the 'Basic' version of this article and the various other graphs provided by Daniel Bailey upthread (comment #20). Given the inclusion of these graphs in this post (to say nothing of other posts with additional information) I think SkS has satisfactorily minded the ps and qs.
    Response:

    [DB] The Basic tab article in the OP above shows the SkS version of the graphic from Moberg et al 2005.  The original is here:

    Click to enlarge

    With a zoomed-in version of the pertinent section here:

    Click to enlarge

    It is left for the viewer to demonstrate a substantive, non-pedantic difference between the SkS version and the original graphic from Moberg.

  22. Composer99 @71, it is true that some fake skeptics of climate science argue explicitly or implicitly that it was warmer throughout the MWP than it has been over the last two decades. Such a claim is implicit, for example, in any claim about the vikings "growing vinyards in Greenland". Leaving aside the fact that that claim is false, and conflates two true claims (there where viking settlements in Greenland, and wine was grown in the MWP in England); farmer and particularly medieval farmers did not switch cropping practices on an annual or even decadal basis. They where very conservative because a failed innovation literally meant starvation. Therefore, for the vikings to have grown crops in Greenland, Greenland must have been warmer than current temperatures not just for a decade or two, but for centuries. In fact, the comparison above (intermediate level) does show that that Greenland was as warm as the current decade for centuries in the MWP. Of course, some of that time, it would have been cooler, but some of that time it would be warmer. But the implicit claim that the MWP was globally warmer than the last two decades for the entire period of the MWP is refuted by the same comparison. Never-the-less, the more informed fake skeptics (and some uninformed or misinformed genuine skeptics) are arguing that because decades within the MWP where warmer than the last two decades, there is a prima facie assumption that the current warming is natural rather than anthropogenic. The conclusion is a non-sequitor, but the premise is not refuted by the comparison above for exactly the reasons jzk gives. We need to recognize that and not make the erroneous argument that the comparison between three centuries of data and one to three decades could show that no decade in the three century period was warmer than any decade in the two to three decades. That argument is not made above, although it has been in comments and it is an invalid argument. Because the argument has been made in comments, it is clearly a misunderstanding people are prone too, and the intermediate article should be revised to explicitly caution against it.
  23. jzk @69, I was about to ask you to pay closer attention to the actual argument made above, which focuses on comparing the regional distribution of the MWP warmth, compared to the global distribution of the warmth in recent decades. That is, I believe, a valid comparison, and a valid point to make from that comparison (with caution). However, on rereading the article I noticed the concluding sentence, which while correct, is not supported by the figures shown. So not only does the article not caution against the invalid conclusion (from the evidence presented) that you caution against, it appears to implicitly draw that conclusion, and needs to be revised. Having said that, the final sentence is well supported by other evidence, notably by Mann et al 2008, which shows these figures in the supplementary data: You should notice that in all three series, the temperature in 1998 (the highest shown in the instrumental series) is greater than the upper confidence limit for the highest decade in the MWP. Therefore, the concluding sentence above (intermediate) level, while not supported by the figures in the article is well supported by scientific evidence.
  24. Tom @73, I have no problem drawing that conclusion from the Mann data, and of course I have noticed that. That is the kind of presentation that SkS should have. But picking a decade of our recent warmest data (+- 3 years) and comparing it to 300 years of previous data just so that one chart will be full of white and the other red does nothing other than to mislead people that don't actually look to see what is behind the chart. I use the word "mislead" as a description of the effect of the comparison, not the intent. I have no idea what the intent was. If the data are on your side of the initial premise anyway, why not just present it in the most fair way possible?
  25. Here's another way to learn about the Medieval Warm Period “The scientists found the years from 800 to 1300, known as the Medieval Warm Period, had the most frequent fires in the 3,000 years studied. Other research has found that the period from 800 to 1300 was warm and dry,” the university said. “What’s not so well known about the Medieval Warm Period is how warm it was in the western U.S.,” Swetnam said. “This is one line of evidence that it was very fiery on the western slopes of the Sierra Nevada–and there’s a very strong relationship between drought and fire.” http://newswatch.nationalgeographic.com/2010/03/18/sequoias_endured_500_years_fire_and_drought/ Chris Shaker

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