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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 76 to 100 out of 271:

  1. And another way to learn about the Medieval Warm Period in North America: "Severe though the six multiyear droughts since the mid nineteenth century have been in terms of environmental and social impacts, as climate events they were dwarfed by a series of megadroughts that struck the West between about 900AD and 1400AD. These droughts were sufficiently long in duration that it actually makes more sense to describe the Medieval climate of the West as not so much afflicted by a sequence of droughts but as simply more arid than in subsequent centuries or now." The Medieval Droughts would make man shudder today, and in fact may happen again. Medieval Drought North America The mega droughts of long term duration indicate that there was a definite world wide climate effect. One does not have a drought centuries long without other areas of the world being affected as well. It is also quite well defined as to time line in North America.
  2. The mega droughts of long term duration indicate that there was a definite world wide climate effect. One does not have a drought centuries long without other areas of the world being affected as well.
    Unsupported assertion of worldwide impact here, Camburn, based on evidence from a very limited percentage of the World's surface (5% for North America). Given the trajectory of drought in the article you link to (showing a pattern not too dissimilar to the more recent Hockey Sticks, and the indication that parts of the USA were simply more arid in the Medieval rather than periodically dry, I would be very concerned about water availability for agriculture or cities in parts of North America over the coming decades. Of course that is exactly what is forecast by climate science as the world warms, so thanks for providing evidence agreeing with that. Never forget as well, Camburn, that a strong MWP = HIGH climate sensitivity and is really bad news for everyone, particularly climate skeptics who for some bizarre reason seem to think the MWP is good news for their arguments.
  3. skywatcher@77: The MWP was certainly present in the Western CONUS. And from this it appears it was present in China and the surrounding area as well. Confirmation of MWP in China As far as a strong MWP= High climate sensitivity, that remains to be determined unless you have papers that show otherwise.
  4. skywatcher@78: I am always concerned about drought in the crop production area of the USA. Just as I am concerned about how the sun affects hydrological cycles in the crop producing areas. Drought and the suns effects in the Upper Great Plains There are numerous studies that show sun cycle and hydrological cycle effects. Temperature also plays a role, but the sun does as well.
  5. This was suppose to be posted with the above link. "A 2100-yr decadal-resolution salinity and aridity proxy record of lacustrine ostracode-shell Mg/Ca ratios from a closed-basin lake in the northern Great Plains shows statistically significant periodicities of ∼400, 200, 130, and 100 yr"
  6. It's pretty simple physics, Camburn @78. Something had to cause the MWP. The hotter the MWP, the larger the climate sensitivity to those causes, and thus the larger sensitivity to CO2 as well. See hockey stick own goal and Do critics of the hockey stick realise what they're arguing for? and the papers referenced therein. Swanson (2009) makes a similar point about large internal variability suggesting high sensitivity. The greatest irony of the climate 'skeptic' movement is the ferver with which they've attacked the hockey stick, when in reality the hockey stick should be their best friend. The flatter past temperatures are, the lower climate sensitivity is (hence the 'own goal' title).
  7. Camburn, you cannot force large global climate changes from relatively limited forcings (increased solar, reduced volcanic) without climate sensitivity being high. Where are the big forcings, comparable in scale to recent CO2 emissions, that would mean we could be happier about the MWP being strong? And your link indicates there was a warm period sometime in the Medieval in China, but not when they occurred, how long, strong or how they synchronised with events elsewhere on the globe. Your link is to an editorial, I'm sure there are actual papers that indicate warm episodes of climate at times during the Medieval in China. Nobody disputes that there are relatively many places in the world, notaby NH, that experienced warmth at times in the Medieval. It's a credible hypothesis that there were periods where quite a lot of the Northern Hemisphere experienced more-or-less synchronous episodes of favourable climate (illustrated in the relatively more 'bent' Hockey Sicks), but that still does not excuse you for assuming global significance and synchrony from a few scattered sites. If the Medieval Warm Period does turn out to be a globally significant event, I will be much more worried about the magnitude of 21st Century global warming. Dana, as ever, has just explained it more eloquently than I!
  8. Dana1981: I clicked on Swanson 2009 and the link didn't work. I also did a search, and it came up empty. Can you fix the link? As far as "The Hockey Stick", I don't care much about that. The latest proxy data shows more warming than the Hockey Stick shows. I am more interested in accuracy than dogma. Skywatcher: The link I provided about China was full of papers that supported the link. The analysis of those papers as presented in the link showed that the current temps are about on par with the MWP temps in China. During this period, China also experienced prolonged periods of drought, just as the Mid-west and Western Conus did. So now we have Western NA, and Asia. And the findings of F. C. Ljungqvist etal 2012 shows general warmth during the 900-1300 time period. Just as today, there were areas of cooler temps and areas of warmer temps, but overall the proxy data shows the warmth exceeding the coolness.
  9. Swanson 2009. I agree, recent reconstructions (including by Mann and colleagues) show a warmer MWP than the original hockey stick. That's not good news, as it means higher sensitivity.
  10. Camburn, I presume you mean Ljungqvist et al 2012:
    "We conclude that during the 9th to 11th centuries there was widespread NH warmth comparable in both geographic extent and level to that of the 20th century mean."
    ...which is a statement that could have easily come from Mann et al 2008, where peak MWP is comparable to the 20th Century mean! Note L12 is a NH reconstruction, not global. The statement is also quite consistent with what I wrote above. So we have current temperatures, which I'm sure you'll agree, are now rising well above the 20th Century mean, and a forcing which is much larger than that in the MWP (see Dana's other links). The strong MWP = high climate sensitivity argument has still not been refuted by you.
  11. skywatcher@85: I have not studied the MWP in detail as to climate sensativity, so I can neither agree nor disagree with you. I have studied it in detail as to North America and there are more issues dealing with solar effects that drive droughts than temperature. If you think that the MWP proves a high climate sensativty, that is fine. I have not read proof of that and would entertain you providing published documents that confirm your thoughts. dana1981: I am not sure the resolution of the proxy data used to show MWP temperatures has the resolution to either confirm or cast aside the statement that current temperatures were not similiar to the MWP. The latest paper with at least decadal resolution using Greenland Ice Cores shows that the temp over the Greenland Ice Mass was as warm or warmer approx 1,000 years ago. That is only one area tho, and would not represent world wide climate.
  12. Camburn#86: "If you think that the MWP proves a high climate sensativty, that is fine. I have not read proof of that... " The contention, so popular among the so-called skeptics, that the 'MWP' was warmer than today requires high sensitivity. But so-called skeptics insist on low sensitivity; they simply want to have it both ways. To understand the high sensitivity requirement, first note that this post demonstrates that forcing for medieval warming was dominantly solar. Steinhilber et al 2009 reconstruct that forcing (data here). Their max for ~1000 years BP is a short-lived differential of 0.39 W/m^2. On the graph shown here, the 'MWP' represents global warming of ~0.3C. It would take at least another 0.3C to inflate the MWP peak above today's warming. That's more than 0.6C from solar forcing alone, suggesting a sensitivity on the order of 1.5 degree/Wm^2. Looking here, sensitivity is calculated from glacial/interglacial stages as 0.7 degree/Wm^2. You can't have it both ways. A warmer-than-present MWP means higher sensitivity - and that is bad news, both for pseudo-skeptic arguments and for the rest of us.
  13. Muoncounter: The uncertainty of the Mann etal graph being it adds the measaured at the end of the graph requires one to understand that the graph really doesn't prove anything one way or the other. As far as sensativity, I will check the credibility of Steinhilber et al 2009, as there have been several reconstructions of TSI published after 2009.
  14. muoncounter: Steinhilber et al 2009 does not agree with this reconstruction. TSI done in 2010 verses Steinhilber et al 2009. It has to be recognized that the uncertainties of Be10 based reconstructions are quite large, 10%, as written in the 2010 reconstruction.
    Response: [muoncounter] Fixed link
  15. And it has to be recognised that uncertainty could be either way. The fact remains that the actual evidence to date from multiple source supports current climate theory.
  16. Camburn#88: "it adds the measaured at the end of the graph" That adds nothing to the 'uncertainty of the graph,' whatever that means. Have a look at it - the instrumental follows the proxy reconstruction exceedingly well. #89: Your cited TSI reconstruction (data file) appears to be Delaygue et al 2010. The values given are for top of atmosphere, showing on the order of 1365 W/m^2. Score: MWP 1365.24, most recent value (1982) 1365.13; that's a tie within the expected uncertainty. Thus it remains: if you want the MWP to be warmer than today, you must have high climate sensitivity. If you want low climate sensitivity, you must accept a cooler-than-current MWP. This could become known as 'the pseudo-skeptic's dilemma'.
  17. Look at slide 7 of Dr. Svalgaards presentation. It shows TSI variability verses temperature reconstructions of the period includeing the MWP. Note that the correlation between TSI reconstruction and temperature in the proxy data is very very poor. Slide 7 on TSI verses temperature in proxy data
  18. So as shown in my post 92 it is very obvious that TSI was not a factor of the temps of the MWP. Hence, I have no clue what the sensativity is/was as I have not seen a good demonstration of either high nor low. So, there is no condrum as far as being skeptical concerning the MWP. I would however, really like to understand why it was warm during that time verses the years prior and after.
  19. What about Door#3, as shown in the Intermediate version of this rebuttal? Perhaps the MWP wasn't really a global warm period after all. That solves the problem of a less-than-stunning TSI increase (from your Svalgaard slides), without upsetting the cherished 'climate sensitivity is low' meme. All you need to do is accept that what some call a global 'MWP' should really be a very local 'MCA.' But that would mean 'it's not about the hockey stick.' Maybe this is why many pseudo-skeptics don't like it when their arguments are held to the standard of logical consistency.
  20. Camburn, as stated by many before, with copious references, you can't have it both ways. Climate responds to forcings. A strong MWP either requires a large forcing comparable to today (not apparent from the Sun in the Steinhilber et al 2009 graph that Svalgaard uses, or in Delaygue et al 2010) or it needs high climate sensitivity to a period where solar activity was respectable for an extended period of time and volcanic activity was relatively low. If you don't see this extremely simple physical logic, there's little reason in discussing it with you. Do you have a hitherto unseen large forcing? If you want low climate sensitivity you need one, or you need to forget the MWP being a large global event. Add to that all the other geological and historical evidence for climate sensitivity somewhere around 3C per doubling (Hegerl et al 2006; Knutti and Hegerl 2008), and the argument becomes ridiculous. It is indeed, as muoncounter eloquently puts it, "the pseudoskeptic's dilemma"
  21. Camburn and others, I refer you to this discussion of the issue by John Cook. It should be noted that solar forcing is not the only change of forcing during the MWP. In particular, unusually low volcanic eruptions contributed, particularly in the early twelth century. Elevated CO2 levels also contributed about 0.2 W/m^2, which is significant relative to other changes at the time. The argument is revisited in this blog post by dana1981. Based on Crowly 2010, net radiative forcing in the MWP was < 1 W/m^2 greater than during the LIA. As 1750 had significantly higher radiative forcing than was typical of the LIA, this means that MWP radiative forcings are significantly less than current radiative forcings relative to the LIA. It follows that if the MWP was warmer than at present globally, climate sensitivity is greater than currently believed, and we have significantly more warming in the pipeline than currently expected.
  22. TomC: "unusually low volcanic eruptions contributed, particularly in the early twelth century." A 12c volcanic lull wouldn't explain the prior Norse settlements in Greenland. You seem to suggest that 'neutral' conditions (0 net warming), requires some level of volcanic activity. Why would that be?
  23. muoncounter @97, there is nearly always some volcanic activity going on somewhere on the Earth. Given that, it is reasonable to assume as neutral conditions an average level of background volcanic activity. However, whether you treat this as a zero level so that reduced volcanic activity is treated as a positive forcing, or whether you treat zero AOD as the zero level, so that all volcanic activity is represented as a negative forcing is only a matter of baselining, and makes not difference to the final calculations. For the IPCC, volcanic forcing is (or should be) benchmarked at the presumed level of volcanic forcing in 1750 to bring it inline with the other forcing measurements.
  24. muoncounter, Greenland settlement wasn't necessarily climate-limited - Medieval warmth is not a prerequisite to have Norse Greenlanders. Inuit are, of course a great demonstration that habitation of that part of the world, whether your technology be limited or advanced, is not strictly climate-controlled. And the Norse were also able to trade commodities with Europe, such as walrus ivory, providing a good incentive for settlement. Their survival well beyond the beginning of the Little Ice Age, on a diet that became much more marine (Arneborg et al, 1999), also shows climate not as the ultimate limiting factor.
  25. TomC#98: 'Some level of volcanic activity' does not necessarily result in a measurable change in forcing. Robock 2002 is an excellent summary: what is needed is explosivity, a tropical location and the right geochemistry. Solomon et al 2011 make the case that there is 'persistent variation' in stratospheric aerosols even without volcanic input.

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