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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 201 to 225 out of 271:

  1. I agree that the few barley grains found could have been imported.  It is logical that the earliest Vikings would have experimented with cereals.  These experiments might well have failed given the short growing season both then and now.  Henrickssen, however, believes that these grains were locally harvested.  

  2. "My interpretation of D'Anrea's studies is that he believes in present day CO2 induced global warming."

    Meaning what? That therefore his data must be wrong?

  3. What about these studies using senentments ect indicating the MWP was global?

    http://wattsupwiththat.com/2013/10/31/new-paper-shows-medieval-warm-period-was-global-in-scope/ Shows a NYT reporter intervieweing 2 scientist who measured sentiment samples around Indonesia.  Look at 11:15

    http://www.sciencemag.org/content/342/6158/617

    MWP in China http://www.co2science.org/subject/m/summaries/mwpchina.php

    http://scienceandpublicpolicy.org/images/stories/papers/originals/mwp_asian_countries.pdf

  4. aristotelian...  Making the rounds at all the climate denial sites, I see.

    What WUWT, CO2Science and Science&PublicPolicy are not going to tell you is that there are many places where no MWP shows up at all. Such as, the Tibetan Plateau.

  5. re: "aristotelian... Making the rounds at all the climate denial sites, I see.

    What WUWT, CO2Science and Science&PublicPolicy are not going to tell you is that there are many places where no MWP shows up at all. Such as, the Tibetan Plateau."  

    Where talking about 1000 and 2000 (Roman Warm Period) years ago.  I don't see how the absence of evidence is evidence of absence in this case.

  6. Also, one of these links is a video of a NYT reporter interview scientists. As a layman, who am I more likely to believe, some random website I found the immiediatly lables someone "a denier" which is a clear reference to the holocost deniers so is a gross ad hominin attack or some scientists interviewed by the NYT?

    Response:

    [TD] Nearly every claim on Skeptical Science is backed up with references to, and often links to, scientifically peer reviewed publications.  Those are even more reliable than interviews with scientists, because some scientists make claims in interviews,  editorial writings, and web sites that they are unable to make in peer reviewed publications, due to the strict quality controls over peer reviewed publications.

  7. aristotelian, you misinterpreted Rob's reply.  When he wrote "where no MWP shows up at all" he did not mean absence of evidence of the temperature.  He meant there is solid evidence of the temperature, and that temperature was not anomalously warm. That is existence of evidence, not absence of evidence.  Click the Intermediate tab at the top of the original post on this page, to see more evidence.

  8. Thanks, Tom. That makes more senese.

  9. Aristolean,

    This Skeptical Science article describes an article written by 78 climate researchers who combined data from around the world to make a graph of the temperature for the last 2000 years.  The MWP does not show up in analysis that combine all the world data.  The MWP was evident in some areas, but other areas were colder at the same time so the global temperature was not warmer.  By contrast, in the current warming everywhere is warmer.  It is possible to find warm areas somewhere in the World at almost any time in the past you choose, they are just balanced by other cold areas.  Do you believe a careful analysis by 78 scientists or a blog post on WUWT?  (note this confirms Mann's Hockey stick data, again)

    In general, WUWT leaves out most of the data.  You can make anything appear to be true if you leave out enough data.  The IPCC report does not leave out any data and they find no MWP.

  10. aristotelian...  Three of the four references you provided are well known AGW denial sites. I've spent a large amount of time going through the MWP material shown on CO2 Science, in particular. Many of their written synoses of the research actually contradict the actual research they present. And they, curiously, do not (though they are clearly capable) endeavor to sythesize the data into any kind of multiproxy reconstruction.

    The other thing you'll notice at CO2 Science is that, what they call the MWP does not occur at the same time in all the datasets. And this is clearly pointed out in most of the research, but CO2 Science goes to great lengths to hide this fact.

    When you read the actual research papers what you find is that the MWP was "heterogeneous." That is, it's not homogenous. It occurs at different times, at different places. And it doesn't occur at all in still other places.

    When people go to such lengths, as CO2 Science does, to hide this elemental fact, then yes, I call those denial websites.

  11. aristotelian... Whenever you hear someone say (or see it stated somewhere) that the MWP was global, ask one simple question: "Was the MWP heterogeneous?"

    That's all you have to say.

  12. Is the current Global Warming trend hetetrogeneious?  If the artice shelf melting was not a indicator of global warming 1000 yeears ago, why is it considered an indicator, today?

  13. Aristotelian, SkS is not some random website and if you believe so, you have not paid any attention. It won the Eureka Prize of the Australian Musem for advancement of climate knowledge. That's not a popular online vote in which everybody who has no clue can click and bots can click automatically. Every SkS article is backed up by peer-reviewed science papers, readers are encouraged to explore them, so the kind of dissimulation that happens at Idso's site can't happen. Several of the SkS moderators have had publications in high impact science journals in the past 3 years. In comparison, it took years for Watts to publish one paper, and when he did, it failed to confirm the very premise to the existence of his website. It also brought nothing new to previous publications by others that had already done that.

    So really ,the choice is between a website with established expertise demonstrated by winning awards from scientific organizations and a record of publications in the field, against some random interview in the NYT. That is a much better representation of reality in this case. 

  14. Aristotelian...  This is definitely not a 1000 year representation, but it gives you a better idea of the changes in spacial distribution of temperature on the planet for the past 150 years.

    http://www.youtube.com/watch?v=X8XqHwSrakA

  15. aristotelian...  RE: "artice shelf melting"

    It would be helpful if you could provide links to what you're referring to.

  16. And to answer your question, modern warming is homogeneous, whereas the MWP was heterogeneous. Peaks of MWP warming occur during a span of about 600-800 years. Some places it was as early as 1400 years ago, some places as recent as 600 years ago.

    If you go back to the CO2 Science site and look closely at all the graphs they present, you can clearly see this.

  17. MWP has warming occuring in different places at different times and not at all in some places. Warming now is occuring at different rates in different places, but the whole planet is warming. eg see here spatial distribution for warming since 1980. Compare that with spatial distribution during the MWP shown in the intermediate version of this article.

  18. Aristotelian,

    This Wikipedia article describes the break up of the Ward Ice shelf on North Elesmere Island in the Canadian Arctic from 2002-2010.  It was a remnant of the Ellesmere Island Ice Shelf which was 3,000 years old when discovered in 1875 (dated using drift wood on the inner surface of the shelf).  Virtually all of the Arctic ice shelves have melted since their discovery, this is only the first hit of a Google search.  The shelf survived the MWP and has melted with modern AGW.  Since the ice shelf was 3,000 years old, how could the MWP have been warmer, even in the Canadian Arctic where it was presumably the warmest?  

    Please provide a link for your supposed Ice shelf melting during the MWP.  Perhaps your sources of information are feeding you fabricated data.  That is why they are called deniers.

  19. http://www.ldeo.columbia.edu/res/div/ocp/drought/medieval.shtml

    THE MEDIEVAL MEGADROUGHTS WERE LONGER THAN ANY RECENT DROUGHTS THOUGH. HOW DO YOU EXPLAIN THIS.?

     

    Response:

    [JH] The use of all-caps is strictly prohibited by the SkS Comments Policy. Please read the Policy and adhere to it.

  20. Myles, the existence of megadroughts in the past does not disprove the human-causational component of the modern warming period nor does it offer support for the denier meme "the Medieval Warm Period was warmer".

  21. Myles - You've pointed to some data on the American West, which is a region representing less than 2% of the globe. Regional variations are weather, not climate, and if you examine the global data the Medieval Warm Period was just not as warm as the world is today. 

  22. Myles... Just to clarify, the MWP is generally believed to be a heterogeneous event, meaning it didn't happen globally and at the same time. We know that there have been periods of local warmth that exceeded global average temperature today.

    What you don't find is evidence that the global temperature during the middle ages was warmer than global temperature today.

    This is a very common trick pulled by fake skeptics. They will point you to a regional event during the middle ages, call it the MWP, and then compare it to global average temperature today.

  23. Just assume Myles is correct of a moment.  That is, assume the MWP was warmer than it currently is, and that it caused series of one hundred year droughts in the SW of the USA.  It would follow that as the world continues to warm with AGW, we can expect the start of such a series of megadroughts in the SW of the USA.  That should make Myles want to do all he can to avoid ongoing warming.  Is that right Myles?

    In fact, we already see the signs of the in the SW of the USA.  From the site to which Myles linked we have this graph:

     

    Attentive readers will notice that the drought extent in 2002 extended 80% of the area, far more than the at most 60% seen in the MWP.  I understand there have been worse droughts in the area since then.  Using SW US drought as a proxy for global temperature, we therefore must conclude that it is already warmer than at any time during the MWP.  And expected to get warmer so that we can expect worse droughts still.  

  24. Hi, 

    I encountered the following argument:

    <statement>The earth is now definitely cooler than during the Roman Warm Period.

    There are passes in the Alps that the Romans used, today they are covered in ice.

    The tree line was hundreds of meters higher 3000 years ago, we still find remnants of trees under glaciers.</statement>

    I have no idea if these statements are correct, can anyone point me to the relevant literature ?

     

  25. bvangerven--

    1. "The earth is now definitely cooler than during the Roman Warm Period"

    It's fairly clear from global Marcott et al. 2013 that the Roman Warming Period was not globally warmer than the present.  

    2. "There are passes in the Alps that the Romans used, today they are covered in ice.  The tree line was hundreds of meters higher 3000 years ago, we still find remnants of trees under glaciers."

    Note regional Marcott.  The Northern hemisphere (north of 30) was likely warmer than present (though Marcott's uncertainty for 20th c. temps is large).  It may be that the Alps were responding to a greater forcing during the RWP.  How about the rest of the globe?  Take a look at Funder et al. 2011 and see if the RWP is noticeable (note that Funder ends in 2000).

    Regardless, what's the point?  Even if the RWP were warmer than present, what would it mean, other than the possibility that climate sensitivty is greater than the current mainstream range?  If the RWP was warmer, does it mean that global warming is going to stop? 

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