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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.


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


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Comments 126 to 150 out of 271:

  1. "One of the most often cited arguments of those skeptical of global warming is that the Medieval Warm Period (800-1200 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." This is not fair. Almost to the point of dishonesty. I am a skeptic by nature. No, the Warm Period does not PROVE that the warming is not caused by man, but at the same time all the alarmists models do not prove that it is. You are putting words into half our mouths by claiming that all of us skeptics use this as evidence that definitively proves our point. That is far from the truth. The alarmists do themselves a great disservice to their cause by ridiculing their opponent for not jumping onto their bandwagon. You just make it more difficult for us to agree with you when you do this. You all were the ones that excluded the Warm Period to get more drastic results. You were the ones that used only 12 trees in your tree ring data, including one freak tree to bend the graph favorably in your direction. Why is it that the Medieval Warm Period is excluded as being an anomaly, yet this freak tree is considered good scientific evidence? Don't treat us skeptics all the same. We have different ideas and mindsets. I am not a scientist, so your complicated graphs and explanations mean very little to me. Because even with my limited knowledge of astrophysics, I know that all it takes is 1 little miscalculation to throw everything out of whack. Accurate temp records have only been kept for the last 100 years. So when you point to ice core records that shows "accurate" records over the last million years, forgive me if I don't jump to a conclusion from them. Explain to me the definitive proof that shows there is a cause and effect scenario with CO2 and rising temperature. Because I could just as easily do the same thing as this author did and point out the flaws in the logic of showing the rise in CO2 leading the rise in temperature during the 80's, and saying that is proof of cause and effect. Of course CO2 is going to continue to rise even if the temperature drops if the temperature had been relatively high for the previous 80 years. Yes the warming may have started around the same time as the Industrial Revolution, but this is also when accurate temperature recordings were beginning to be kept, as well as it was the same point in time when we began moving out of a 500 year cold period, which had been the coldest period in the last 10,000 or so years. Like I said, I am not a scientist. But I do understand the scientific method, and what I see with these models, and the definitive conclusions being made from them, goes against everything I been taught about the scientific method.
  2. Eric from Indiana: Suffice it to say, it doesn't strike me as properly skeptical behaviour to make claims of scientific incompetence or even malfeasance with regards to studies of the medieval climate anomaly without substantiation.
  3. Eric, I'm a little unsure on where to start here. You are saying that your "your complicated graphs and explanations mean very little to me" so how much are you prepared discuss this really? Oddly, you readily swallow "that used only 12 trees in your tree ring data, including one freak tree to bend the graph favorably in your direction." How skeptical were you of that claim? What about proxy reconstruction that dont include trees? (and yet give same picture) As has already be pointed out, MWP is a/ different from today in that it occurred in different places at different times, b/ something that climate models reproduce from known forcings. This is not proof, but you cannot have proof in science, only in mathematics. What we do have is observation that are consistent with the predictions of climate theory. Furthermore, note that if it was hotter globally (little evidence to support this) than today, then this is evidence for high sensitivity and you should be more worried. "Explain to me the definitive proof that shows there is a cause and effect scenario with CO2 and rising temperature." Warning - this involves understanding some science but I doubt you would be visiting this site unless you had some interest in what the truth was. Start with The Big Picture. AGW theory is based on physics, not correlations with paleoclimate. If you have a theory of climate that is consistent with known physics, then of course you want to be sure it also explains the past. Paleoclimate studies are fundamental to this but they are for checking theory not the foundation of it. Their value is limited by the difficulties in knowing what both the past climate was globally and by knowing what forcing were present so this is happy-hunting ground for pseudo-skeptics. The important tests of theory are still based in physics; on what is predicted and then validated from measurement today. I fear that you have been misled by some dubious information sources. Please take some time on this site to find out what the science really says.
  4. Whoops - forgot the link. Start with the Big picture
  5. Eric from Indiana @126: 1) The article does not say that all, or even nearly all deniers use this argument. It merely says that it is "one of the most cited arguments". Indeed, when I check the list of 173 denier arguments listed by frequency of citation, I find the closely related argument that that global warming is not anthropogenic because "Climate has changed before" is first on the list. The also related argument that "The Hockey stick is brocken" is 16th, and this argument is 30th. That certainly makes it one of the most frequently cited denier arguments. So, the claim in the article is accurate, and it is you who are being unfair "almost to the point of dishonesty" by putting words into others mouths. That, however, is a side issue for me. What I wonder is, do you argue against these claims when you see your fellow deniers make them? If you think the argument absurd, but sit in silence while fellow deniers make these claims, you show that truth is not what you are interested in. 2) No scientist has set out to expunge the MWP from the record books. Nor have scientists made conclusions based on very limited and dubious data. As an example, consider the following graph: The important line for this discussion is the greenline, which is a temperature reconstruction excluding all tree ring data, and all data from proxies that have been considered dubious by deniers. It still shows a MWP that is cooler than current temperatures. The myth (and it is a myth) that you have been fed saying the temperature reconstructions are based on just one tree in the Yamal series are false. (The easiest clue that it was false was that it was made on a denier site.) 3) If you want proof that increasing CO2 increases temperature, read this post. Having done so, please point out the flaw in the logic. If there is no flaw, than increasing CO2 increases temperature. Note, posting on Skeptical Science is a privilege based on your compliance with the comments policy. You have already violated several comments policy, and continuing to do so may well result in portions, or the entirety of future posts being deleted. Finally, I have referred to you as a "denier". You may object to being classified by a stereotyped and negative description. You may even find it offensive. However, you yourself are using stereotyped negative descriptions ("alarmist") in your post, so I figure, "Sauce for the goose is sauce for the gander".
  6. Let me briefly add to what scaddenp says: if you haven't read the science, and you claim to be a skeptic, then you shouldn't have any opinion on the science. Actual skeptics use evidence-based reasoning to establish event probabilities. With that having been said, my advice is to back away from the foregone conclusions and actually engage the science. Spend time with it. If you took the time to write on this blog, you must feel somewhat strongly about the issue. If you do feel strongly, make no assumptions, start with the basics, and ask questions. There are plenty of posters here willing to answer questions. Also, as scaddenp indicates, proxies are based on testable physics. Proxy data rules out a great deal of uncertainty. When astrophysicists determine the type of newly discovered extra-solar planets, they do not get clear resolution. They get a rough approximation. Yet even a rough approximation tells them that what they are looking at must fall within a certain range of physical phenomena. They're not looking at a dog or a beer can. Same goes with paleo studies. Because proxy studies are cross-referenced and anchored to known physics, the range of physically possible scenarios for a given point in time gets very slim.
  7. Eric,
    I am not a scientist, so your complicated graphs and explanations mean very little to me.
    Then you shouldn't have an opinion on the subject -- or rather, any opinion you hold is by your own admission an entirely uneducated one.
    Explain to me the definitive proof that shows there is a cause and effect scenario with CO2 and rising temperature.
    And how are we to do that, when our "complicated graphs and explanations mean very little to" you? Sort of a Catch-22, isn't it? But everything you need is already here. All of the answers are here, if you're willing to look for them. You should spend less time lecturing, founded on an ignorant opinion, and more time studying. Or you can just go on believing what you'd like to believe, simply because that's in your comfort zone. That's where you're at now, that's where all deniers are (yes, all), and I rather suspect that that is where you're going to stay. Actual facts mean nothing to you. It's much easier to accept the (false) declarations that fall nicely in line with your predetermined beliefs than to work through all of those "complicated graphs and explanations."
  8. It seems there is strong evidence that the MWP was glogal, not a local event. We do have the wood's hole reconstruction of SST in 2009, that has similar temperatures. Esper in N. Scandanavia Mxd and TRW supports consistant higher temperatures. The new proxies seem to indicate that past temperatures may well have been above current temperatures. The margin for error is higher than Mann 2009 seemed to indicate, and this had much higher temperatures in MWP than mann 1999. Both of these newer proxies indicate the climate models that say today's temperatures are the highest they have been in 1400 years need to be reexamined. This does not mean ghg are not contributing to current warming, but does mean that natural variability is higher than current reconstructions seem to indicate.
  9. "It seems there is strong evidence that the MWP was glogal, not a local event" This is exceptionally vague (the word seems is the dead giveaway); you'll need to provide a link to substantiate what is effectively your personal opinion. While I'm pretty sure of both the source of your opinion, the blog you read it on and why both of you are wrong, the onus (i.e., homework) is on you to provide it for sensible discussion to ensue. And no, Virginia, regional studies doth not global make. Regardless of spelling. (BTW, a warmer MCA implies a greater climate sensitivity than is commonly you understand the ramifications of that greater sensitivity?)
  10. Esper et al. (2012) is getting serious play out in the trenches -- and being seriously misread. It's a strawman party:
  11. Danial and DSL, 3 very geographically diverse sets of data, constructed with good scientific methodolgy that should make anyone take pause and think that mann might be wrong. If the south pacific SST, antarctic, and N Scandinavia show the MWP, perhaps it is the global phenomenon that was theorzed before 1998, and not simply some localized events. That should make anyone skeptical about the mann2009 temperature Reconstruction. Do you have information why these papers should not be given strong weight?
  12. 1. Your lead source has been misrepresented by the fake-skeptics (they studied 1 site in the Antarctic Peninsula), per the lead author (Li) himself. 2. Per your second source, todays temps are quite a bit higher than those during the MCA: 3. Your third source, the Esper study, delineates a long-term pattern of cooling that mankind has interrupted with the massive bolus injection of formerly-sequestered, fossil-fuel-derived CO2 back into the carbon cycle. There is nothing natural about that. Furthermore, given the already-realized warming and that yet in the pipeline (paid for but not yet delivered), there is little possibility remaining on resumption of that long-term cooling. Per Tzedakis et al 2012, “glacial inception would require CO2 concentrations below preindustrial levels of 280 ppmv” (for reference, we are at about 395 right now…and climbing). Earlier, Tyrrell et al 2007 examined this, concluding that we have already skipped the next glacial epoch. Furthermore, Tyrrell concludes that if we continue our present fossil fuel consumption, we will skip the next 5 glacial epochs. So no glacial epochs the next million years…now that's unnatural. We are currently at 395 ppmv CO2 and growing about 2+ ppmv year-over-year. No down elevators for mankind on this ride. Now a real skeptic would take pause and try to figure out why their understanding of the science was so out of whack with that of the mainstream climate science. Hmm, thousands of scientists (the real skeptics) devoting their lifetimes to studying something on one hand or some guy commenting on a blog on the other...tough call.
  13. realscience: As far as I can see, nothing in those documents necessarily shows that the MCA was a monolithic global warm period. They do show that those regions had their own warm spells during the medieval era. I do not think these papers are enough to cast doubt on the conclusions of Mann 2009 and your suggestion that they should is IMO vastly overstating the "take-home message" the three documents present.
  14. For those interested in a blog from Duke on the SST reconstruction mentioned above Virtually all the proxies used to reconstruct temperatures over the past millennium — the proxies that yielded the hockey stick — have come from land-based sites. But what about the ocean? With oceans covering some 70 percent of the Earth’s surface, how can we infer global temperatures without using sea surface temperatures? These were just the questions asked by Delia Oppo of the Woods Hole Oceanographic Institution and colleagues, and they decided to do something about it. They analyzed sediment cores lying beneath the Indonesian Seas in the so-called tropical Indo-Pacific Warm Pool. Using the ratio of magnesium to calcium in the sediments as a proxy for sea surface temperature (SST), they found that “reconstructed SST was … within error of modern values from about AD 1000 to AD 1250, towards the end of the Medieval Warm Period.” In other words, temperatures during the MWP were comparable to today’s temperatures, putting a significant bend in Mann’s hockey stick stick just above the handle. and further comment about this means about ghg it does not follow that the current warming also must be due to natural causes even if MWP temperatures were comparable to today’s. Regardless of the cause of the MWP warming, the preponderance of the evidence is that the current warming cannot be explained by natural causes and is due to greenhouse warming from emissions of carbon dioxide and other heat-trapping gases.

    [DB] Please note that the graphic given to you above is from Oppo's own study, showing that today's SST's are unparalleled in the reconstructions.

    The point is not there there wasn't a MCA (the appropriate term that the science uses for your MWP). It's that today's temps are global, are well above those of the MCA and are driven by mankind. And that a warmer and more synchronously warm MCA means a much higher climate sensitivity than is the current understanding.

    And that spells disaster.

  15. Composer99, The removal of the MWP seemed key to mbh '98, but they had serious doubts about the proxies. Some excuses were made that it was really local. There was some bad math. Mann 2009 corrected the bad math, and used some better proxies. Magically the MWP reappears, but following the old line of defense it is now called MCA and down played as being local. These new studies seem to have more reliable proxies, and cast doubt on some of the proxies used. Certainly not using SST, when it is a major part of warming is a problem. The work needs to be redone with the new data. Certainly any good scientist would incorporate new data into the model, would they not. These data points disagree with those in mann 2009 reconstruction, so it needs to be redone.
  16. "These data points disagree with those in mann 2009 reconstruction, so it needs to be redone." Feel free. We're still waiting on McI...
  17. Daniel, Reconstructions are being done, by those more skilled than I as we speak. Peer review takes time. I hope you anticipate better temperature reconstructions and follow the data, not just the personalities.
  18. The global sea level trend is another approach to constraining global temperatures during the Medieval Period: This reconstruction isn't quite accurate, the trend was actually much flatter until the 20th Century, but it does give a good indication that the Greenland & Antarctic icesheets were stable during the Medieval Period, whereas melt is rapidly accelerating today.
  19. Now you stray away from the science into trolling. The point was that you wish an audit of Mann. And that the peer-reviewed science has been waiting on McI to much the same for years now. The Muir-Russell commission did so in essentially 2 days, pronouncing it a "not difficult" task. All of this is off-topic anyway. Please return the discussion to your supposed evidence regarding the MCA. Or lack thereof, as we are finding from the peer-reviewed literature.
  20. realscience @ 140... "The removal of the MWP seemed key to mbh '98..." How could that possibly be when MBH98 only went back 600 years?
  21. realscience @ 140... "Magically the MWP reappears, but following the old line of defense it is now called MCA and down played as being local." And here we have another erroneous assumption that barely skirts the commenting policy here at SkS.
  22. realscience @ 142... You know, McIntyre has had over a decade to produce a multiproxy reconstruction that shows something different than Mann's work. Nearly a dozen other multiproxy reconstructions have been produced in the interim, all confirming the conclusions of MBH98/99. I just don't believe that the peer review process works any slower for McIntyre or anyone else (the Idso's also seem to have taken a long term interest in the MWP) than it does for the other scientists doing the same work.
  23. "[DB] Please note that the graphic given to you above is from Oppo's own study, showing that today's SST's are unparalleled in the reconstructions. The point is not there there wasn't a MCA (the appropriate term that the science uses for your MWP). It's that today's temps are global, are well above those of the MCA and are driven by mankind. And that a warmer and more synchronously warm MCA means a much higher climate sensitivity than is the current understanding. And that spells disaster." 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. That is far from the certainty of unparrelleld. Have you read the text of the paper? Since we have many places hot during the MWP or MCA, and some colder, how is this different from today's climate anomaly, where some places are warmer and some are colder? There certainly is reasonable doubt that today is significantly warmer than the MWP. As to your later statement that this would mean ghg forcing would even be higher, that does not follow. What follows would be a more refined model based on better historical data. Higher variability does not at all require a stronger forcing. When looking at new evidence isn't it important to incorporate it instead of rejecting it out of hand. Is not the science more important than the politics? Certainly the existence of a MWP, does nothing to say today's temperatures are natural. There is strong evidence that they are not, that there is a forcing from ghg and feedback. But really, this thread is about the MWP, or MCA, as some that would like to erase the concept from history would call it. I am merely presenting the current evidence that the MCA was global in effect. That is unless you think Lapland, England, Indonesia, and Antarctica responded to the same local only warming. A MWP says nothing about the climate change we are going through now. There is strong evidence that ghg are a major component of that. But having good scientific, non-political, historical temperature reconstructions is important to accurately model future change.
  24. Rob, Mann himself in 2009 produced that multi-proxy work, that had the MWP come back into view. It is in the front page of this discussion. There has been new proxies since then that look like they raise the temperature of the MWP. Daniel, I did not say I wanted an audit of mann, I said that like the earlier work, 2009 needs to be redone. It certainly does not need to be done by mann. That later 2009 work was actually put up for real peer review in a timely manner, and did not have the math errors of previous work. No trolling going on, simply statements that say new evidence seems to point to a hotter more global MCA. As to peer review working fast for M&M, that is doubtfull, but those doing the new construction are inside the climate community. IMHO the best course is to be open to new information. Some recent mann research says that some of the cold is not csptured well in the tree ring studies, as no rings grow in these years. Esper's criticism of previous TRW proxies has to do with underestimating cold years. There is room in science. A hotter MWP does not mean that today's hot temperatures do not have strong correspondence to ghg, and the sensitivity appears to be 2.5-4 degrees c for a doubling of concentration of carbon dioxide. Nailing down solar forcings, volcanic forcings, and ocean oscillations is helped not hindered by better historical models.
  25. You are still calling for an "audit". Feel free. Still off-topic, so let it go on this thread. Or I will have to drop out of discussion into moderation for the remainder. Other moderators are of course free to step in right away & excise the offending bits as they see fit. The facts: -You opined that the MCA was global. -You were called on it & challenged to present evidenciary support. -You presented three sources. -You received strikes on all three. -You continue to ignore those strikes. -You continue to opine that some places today are cooling while some are warming, thus painting a picture inconsistent with the modern record. All without evidenciary support. -You still fail to provide a cohesive, evidenciarilly-supported framework that the MCA was global. You furnish ample rhetoric. Substance is needed to constitute intellectual victuals, however. No matter the spin, calling a dog's tail a 5th leg does not make it so.

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