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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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 151 to 175 out of 271:

  1. realscience... Your original comment was that the MWP didn't appear in MBH98, which is obvious because the data didn't go back to medieval times. MBH99 added 400 years to MBH98 and took the data back about the peak of the MWP. Mann's 2009 work of course went back even further and showed the cooler period prior to that. You are making an erroneous inference that somehow the MWP magically "came back into view." It didn't come "back" into view, it merely came "into" view.
  2. Of course, if one could somehow find a reconstruction of Arctic Sea Ice levels for, say, the past 1,400 years or so, surely that would help prove the non-regionality of the MCA. After all, Arctic amplification will help magnify any warming present during the MCA, so any reduction in sea ice cover should stand out like a sore, throbbing thumb with respect to the years between then and now. What's that? We do have such a reconstruction? Oh, yes, SkS covered Kinnard et al 2011 here, Arctic Sea Ice Hockey Stick: Melt Unprecedented in Last 1,450 years Pretty bleedin' obvious, I'd say.
  3. Rob Honeycutt @151, how true your words are is revealed by this comparison of MBH 99 and Mann et al 2008 CPS method: As can be seen, MBH declines from a high point in the MWP which is about as high as that found in other more recent proxies. While there is some substance to the claim that MBH 99 does not show the LIA, there is no substance to the claim that it does not show the MWP. That it is the later, not the former claim deniers continuously shows well the substance in their arguments.
  4. Additionally, using the same logic, the warmth of the MCA should also be obvious in the global methane record. From Mitchell et al, 2011: Oh, there it is.
  5. And to add one more to DB's list, 'realscience' continues the erroneous accusation that Mann's group somehow wished the MCA away because they didn't want it. Lets think about that for just a second. In real science, who, exactly, whould benefit from a small, invisible MCA? Why, those arguing for low climate sensitivity of course! If Mann et al had an agenda, as has been insinuated, they would not have produced a 'hockey stick'. They would have produced a graph with very large wiggles for every significant natural climate event. That would be a graph supporting high climate sensitivity, and thus very great concern for the magnitude of our current anthropogenic forcing of climate. Fortunately, Mann and all the dozen or more others that followed him had the integrity not to do that - they followed the data and produced reconstructions that best suited their data. It supported slightly lower climate sensitivity! A large MCA = high climate sensitivity. There is so much irony in the fact that so-called skeptics ought to be lauding Mann, when instead they chosen to maliciously attack him. But then nobody ever said skeptics were actually rational in their arguments about climate! [and for those that don't know, it's "Medieval Climate Anomaly", rather than MWP, to account for the fact that there are precipitation changes in the time period, and regions with cool episodes, not accurately reflected in a 'Warm' name. Something else that has come out of deeper understanding as the science has advanced beyond the thinking of the great H H Lamb.]
  6. Aside from the Holy Hand Grenade, what other sacred relic proxy's are out there? How about Martín-Chivelet et al 2011? Abstract:
    Remarkably, the presented records allow direct comparison of recent warming with former warm intervals such as the Roman or the Medieval periods. That comparison reveals the 20th century as the time with highest surface temperatures of the last 4000 years for the studied area.
    Fig. 7. Synthetic time series of relative δ13C values for the last 4000 years, based on the three stalagmites. The curve is based on the deviation of each δ13C value from the mean calculated in each stalagmite for the 1570–670 yr BP interval (i.e., the longest interval of coeval growth in the three samples). The smoothing curve is based on adjacent averaging (n=10) of the stacked relative δ13C values of the three stalagmites. The temperature scale is based on the linear model correlation obtained from the cross-plot of Fig. 6. According to that model, the error of the temperature estimates is ±0.26 °C.
    [Source] Proxy, proxy, my kingdom for a proxy...
  7. I think it's worth nothing that realscience has agreed here with the essential conclusions of climate science (essential in terms of informing present & future policy) before piling on too much over medieval temperature anomalies.
  8. Daniel, Thank you for the studies. I would completely discount the glacier study. We have a much more direct proxy in ice cores that seem to disagree. Would not glacier extent have to do with precipitation not temperature. That would explain the difference. Perhaps with the ice cores the glacier extent can help with precipitation patterns. The stalagmite is more interesting. I have not sen that proxy before. Certainly this finding disagrees with sst of southern spain, but water and land temperatures often diverge. Thank you for that. I will look into it.
  9. Rob, As tom has corrected me, it was mbh '99 not mbh '98. I read them a long time ago and I'm not great with dates. tom and Skywalker, I am not accusing mann of a secret agenda. Jones and Mann talked about it, and defining the terms. It was discussed with cook. Esper publicly disagreed. We do have private correspondence that goes to reasons, but lets not rehash it. As to high MCA meaning high sensitivity, it does not follow. Let's go to the discussion of what forcings might be involved. First changes in albedo, but those are stronger today as there is less ice. Solar, volcanic activity, and ocean circulation and heating patterns. If these can be modeled better, then sensitivity can be nailed down better. More variability that is not well modeled does give more uncertainty to the range. I would not expect that our best average estimates would strain far from 3 degrees c for a doubling of CO2. I certainly am not saying that a hotter MWP means less forcings from ghg, only that the data seems to be going there. I am also not saying that the MWP was warmer than today, only that it likely was global, and that because of the high uncertainty in these temperature reconstructions we should not rule out that it was hotter than today.
  10. "I would completely discount the glacier study." I take it you refer to Kinnard, the study of Arctic sea ice cover. It is the height of irony for someone proclaiming skepticism to ignore data inconvenient to their position. "Would not glacier extent have to do with precipitation not temperature." Glacial advance and retreat is governed by mass-balance. Mass-balance is the balance between depositions in the glacial accumulation zone and losses in the ablation zone. It is a dynamic measurement and differs for each and every glacier in the world. It is affected by temperature, altitude, season, precipitation and insolation. It is quite possible for some glaciers to advance in a warming world or retreat in a cooling world. Given the volume of glaciers in the world, there is an ample record of advancement & retreat going back multiple millennia. Some support a warm MCA, but only in certain time periods, some the opposite. The overall glacial record suggests regional warmth for the MCA, with a varying temporal placement. It is thus most inconvenient to the fake-skeptics, who wish to ignore it or to wish it away. Between sea ice, stalagmites, glaciers (ice cores and moraines), trees, CO2 and CH4 proxies, there exists a converging and strengthening consensus of record showing the MCA to be a general period of warmth largely centered in the North Atlantic and Europe. Heterogenous, but not homogenous.
  11. realscience @ 159... The papers I've read say that the MCA was "heterogeneous." So, while you can find proxies all over the globe that had a MCA you also find proxies that show no MCA, or you find temporal shifts in when the MCA occurred. What often happens, and is even presented on the Idso's CO2now CO2science website, without reading the literature people see regions of a strong MCA at widely distributed points around the globe and come away with the assumption that the MWP was global and warmer than today. They don't know enough to look closely at the time frame during with warming in each proxy occurs and they don't know that there are 100's of other proxies that show no warming or even cooling during medieval times. In that way, people like the Idso's prey on people's ignorance to create a false impression of the MWP. But what you never get is the Idso's or McIntyre ever producing a real multiproxy reconstruction.
    Response: [RH] Correction: The Idso's site is CO2science.org, not CO2now.org.
  12. Realscience, As I recall, Mann used over 2000 proxies in his 2009 analysis. Do you really think that adding 3 more will make a significant difference? Will they overcome the hundreds of other proxies that suggest the MWP was a local anomaly? Your assertion that Mann is somehow in error because he does not publish a reanalysis every time a new proxy is published is simply uninformed. New proxies are published every month. Mann has undoubtedly added the data you like to his data base and looked at the new graph. It is not worth publishing yet. Mann will likely publish a new reconstruction around 5 years after his last paper (earlier or later depending on how interesting it is). He will include all the new proxy data. If for some reason Mann is not able to publish an update, someone else will.
  13. realscience... My biggest objection to your comments is that you've inferred that Dr Mann was somehow attempting to hide the MWP. That's not at all accurate. If you read Mann's recently published book you find out that he was actually skeptical about global warming, like any good scientist. The whole point to his research was to understand what role natural variability played in climate change today. He says that the hockey stick is merely what arose from the data. And in fact, what he was trying to do with his work was to "contain" the MWP, as in, get statistically significant data that reached back to before the MWP. That is what Tom's chart at 153 shows.
  14. realscience - The best global paleo-temperature proxies, the great polar ice sheets and tropical glacial ice disagree. These are all currently melting at an accelerating rate and significantly contributing to sea level rise, yet in the roughly 500 year-long Medieval Period, global sea level never rose at all. In fact, sea level rise stopped around 4-5000 years ago and was static until humans began to burn massive amounts of fossil fuels - releasing planet-warming greenhouse gases. Current sea level rise, and consequently current global warmth, is anomalous within the context of the last 7-8000 years.This refutes the notion of a warm Medieval Period. I note that you dodged this issue when I previously raised it. For a person with such a pseudonym you don't seem particularly interested in real science.
  15. "people like the Idso's prey on people's ignorance to create a false impression of the MWP" Not to mention relying on them believing what Idso tells them the papers say instead of actually reading them.
  16. Would you like a red pill with that...or a blue pill?
  17. realscience, you're demonstrating a fair bit of missing elementary knowledge on your part. Some examples - not understanding glacier mass balance (#158), confusing glaciers with sea ice (#158), not understanding what a feedback or forcing is and their implications (#159), or not having even read or comprehended the studies involved (apparent from e.g. #140, #148, #158). These may or may not be intentional, but in this light, it is hard to take seriously your various hints that Mann et al, and the dozen or so papers that followed his, have not just followed the data. You've suggested, directly or indirectly, that they have either deliberately muted the MCA signal, or that they have failed to take into account the full body of evidence, yet it's apparent that you're unaware of the contents of the full body of evidence. We have some idea of the forcings involved during the MCA - see Figure 6.13 in IPCC AR4. You might note that the forcings all have a magnitude in watts per square metre. That means that if we are to accept your hypothesis (despite the lack of supporting evidence) that the MCA was globally uniform and large, we still need to find a large enough forcing that operated in the MCA but not today. As detailed by many people, not least in some detail by the IPCC in Chapter 6 above (and more recent papers continue to support this), the MCA is now well-known to be spatially and temporally heterogeneous, consistent with smaller solar/volcanic forcings. Esper's recent paper, another regional study, didn't change this (good discussion at RealClimate). At least I'd agree with you that: "I would not expect that our best average estimates [of equilibrium climate sensitivity] would strain far from 3 degrees c for a doubling of CO2." I'm very glad we agree on that. I'll add a further example of a climate response that has been greater recently than during the MCA - Arctic glaciers and ice shelves, a number of which are exposing land or breaking up for the first time in several thousand years (see section 4.4 in Polyak et al).
  18. Will we see a discussion of Ljungqvist, F.C., Krusic, P.J., Brattstrom, G. and Sundqvist, H.S. 2012. Northern Hemisphere temperature patterns in the last 12 centuries. Climate of the Past 8: 227-249. At some point?
  19. I had read a comment that this month that some form of warm air swept over Greenland and the ice melted temporarily from 55% to 97%. http://www.usatoday.com/weather/climate/story/2012-07-25/greenland-ice-sheet-melt-climate-change/56479518/1 (-Snip-)
    Response: [DB] Your initial statement is off-topic for this thread; please follow the sage direction given you by Composer99. Your subsequent statement was snipped due to ideology.
  20. muttkat: Your inquiry is off-topic on this thread; fortunately there is a post here on the subject of ice sheet surface melt.
  21. Just so it's here, not only might there have been MWP-LIA differences in solar insolation and volcanic activity,but in Law Dome CO2 over last 2000 years. Snow-albedo amplification works both ways, and Northern Hemisphere around snow line is where one would expect to see larger effects. As noted elsewhere, one has to be really careful with apples/oranges comparisons among reconstructions. NH "extra-tropics" is not the same as NH, it's either 50% of the NH (30degN-) or 60% (23.5degN-). Spaghetti charts sometimes require reading fine print somewhere else.

  22. Estimates of global temperatures during the Me devil Warm Period are somewhat speculative because we do not have direct measurements of global temperatures dating back that far.  There is proxy data that shows that for a specific location, temperatures were warmer during the Medieval Warm Period then they are today and man's contribution of CO2 to the atmosphere could  not have played  a part.

  23. William Haas...  No one claims that man-made CO2 had any effect during Medieval times.  There are, though, other known factors at play during that period, including increased solar activity and low volcanic activity.  But, overall, it is estmated that current global temperature is likely as high or higher than the MWP.  This in spite of the fact that the planet has been on a 5000 year orbitally forced trend toward generally cooler conditions.

    Currently there is no other rational explanation for the temperature trend since modern industrialization other than increased levels of man-made greenhouse gases.

  24. William Haas, read the Intermediate tab of this posting, and take special note of the description of the "seminal paper on this subject."  Click that phrase on that page to get to the details.  It is irrelevant that as you wrote "for a specific location" temperatures were warmer than today.  "Global" warming means more than one specific location, and there was no synchronized global Medieval Warm Period.  So your vague contrarian speculation is irrelevant; you are speculating about a non-event.

  25. WIlliam Haas: "There is proxy data that shows that for a specific location, temperatures were warmer during the Medieval Warm Period then they are today and man's contribution of CO2 to the atmosphere could not have played a part."

    I have direct, anecdotal evidence that it was warmer in my back yard the week before last than all the global warming scenarios ever created predict for the next 100 years. And I'm sure CO2 was not the cause of my back yard being warm.

    On the other hand, I don't think that the specific location of my back yard being warm for one short period of time has much to do with global warming due to increased atmospheric greenhouse gases resulting from human burning of fossil fuels over a period of decades.

    Perhaps you would actually be willing to provide us with an argument as to why your anecdote is more relevant than mine?

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