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Icing the Medieval Warm Period

Posted on 4 March 2011 by Daniel Bailey

"It's cold out!"

Not strange to hear that during the winter, here in the Northern Hemisphere.  But strange to hear that raised as an objection in the warming world in which we live today.  How much warmer it's going to get and what are the related impacts is what science is currently debating.

One of the commonly raised objections from those who would have us debate even the existence of gravity is that "It was warmer in the Medieval Warm Period."  This is an innocent, but untrue, claim clearly unsupported by the available literature.  Indeed, Martín-Chivelet et al  reveals the 20th Century as the time with highest surface temperatures in Northern Spain in the last 4,000 years.

The Medieval Warm Period (MWP) was a period of supposedly warm climate during the early part of the past thousand years. How long it lasted, what areas were affected and even if it existed have been questioned.  Some areas seem to have been affected more by changes in precipitation than in temperature.

A study has been recently published which sheds new light on this debate.  Koch & Clague, in their paper Extensive glaciers in northwest North America during Medieval time, provide new evidence showing that several glaciers in western North America and elsewhere in the world advanced during Medieval time and that some of these glaciers achieved extents similar to those at the peak of the Little Ice Age, a very cold period many hundreds of years later.

What the authors' research finds is that these glacial responses could not have happened in a world with a climate similar to ours today. Indeed, recent studies (here and here) by Mauri Pelto show that glaciers without a consistent accumulation zone (where the glacier "packs on weight") will not survive.  This helps explain why today's glaciers (responding to today's warming world) are retreating to their smallest areas in many thousands of years, exposing their longer histories in the form of buried datable material for scientists like Koch & Clague to decode.

What this means for the MWP is that if summers were as warm then as today, glaciers globally should have retreated significantly.  Changes affecting glaciers around the world require global effects.  Since that did not happen, global temperatures then could only have been warm for a part of the MWP.

So what else could explain these glacier advances in a supposedly warm world?  The most likely answer is that changes in factors besides temperatures played a significant role.  Koch & Clague find a linked response between:

  • increased winter precipitation
  • changes in solar activity
  • changes in the El Niño/La Niña (a Pacific Ocean weather pattern affecting the entire globe) in response to variations in solar activity

As a result of these linked responses, Koch & Clague suggest that glaciers were able to advance during the MWP because warm regional areas were interrupted by periods of cooling, interspersed with variable patterns of precipitation.  Given the length and breadth of the information we now know, Koch & Clague make a substantial case that the MWP should be more aptly named the Medieval Climate Anomaly.

 

Acknowledgements:

To glaciologist Mauri Pelto, for being kind enough to lend his perspectives and expertise.  To the many other contributing authors at Skeptical Science for their valued efforts as well.  Many thanks, all.

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Comments 51 to 83 out of 83:

  1. @ Tom Curtis Unfortunately it seems to me that you are a person, arguing ad hominem: "silly" "look a fool". That's a real shame. You are not that kind of person I want to talk to at all. So this is the last post addressed to you personally. "However, you identify as one of your regionally cooling regions New Zealand." No, I just identify some glaciers in NZ advancing in the current (until 1998), global WP. "... of just one or two glacier records for only decade (instead of the whole century) is cherry picking. If you don't like the term, don't do it." Come on. The chart starts 1977 and not 1900. Your argument "overall decline" is only valid for the period 1998 until now, with the exception 2002-2005. If you will call this a "trend" than do so. I am fine with it. You can find a "trend" downwards on the scaled mass balance of New Zealand glaciers, starting 1998. That is true. "Finally ..." My post Nr. 33: "Gehen Sie von den möglichen Steuergrößen aus. Was bleibt übrig? Alles läuft auf dieser Skala auf die Sonne hinaus. Wenn dem so ist, müsste man sicher von einer globalen Anomalie ausgehen dürfen; natürlich durch interne Oszillationen modifiziert" My translation: Assuming potential actuating variables. What has been left? Everything leads on this scale to the sun. If this is the case, a global anomaly can be assumed; of course modified by internal oscillations. @ MarkR Scherler et al., Abstract: In contrast, more than 50% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability, or global sea level You are right but I don`t think this is an argument against my statement that we have increasing glaciers in the Karakorum.
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  2. Why did you delete my post regarding Global Warming, MWP the IPCC errors ?
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    Moderator Response: [Dikran Marsupial] Your post was deleted because it violated the comments policy (specifically the first item on the list). Keep to the science and your posts will not be deleted (provided it is not off-topic for the thread of course).
  3. Currently the Scientific Data shows the global satellite data at 0.02 below the 30 year average. http://www.drroyspencer.com/latest-global-temperatures/
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    Moderator Response: [Dikran Marsupial] This appears to have little relevance to the article under discussion. Climate is the study of long term averages and trends; observations about current temperatures are essentially just weather noise and don't give a reliable picture of what the climate is actually doing. The same was true of the peak in 1998. I've made your link clickable to encourage others to view the data.
  4. As climate is the study of long term averages and trends, the MWP is essentially just noise. The earths temps have been documented to be much higher at times in the past. http://en.wikipedia.org/wiki/File:Holocene_Temperature_Variations.png.
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    Moderator Response: [Dikran Marsupial] By long term averages/trends, I meant sufficiently long to average out the weather noise. Climatologists generally take that to mean starting at about 30 years. The MWP ocurred over a long enough timescale to be 'climate' rather than 'weather' (noise).
  5. The entire premise of "Icing the MWP" and suggesting that the name be changed to " the Medieval Climate Anomaly" must be based on changing temperatures/climate is an .. Anomaly. The climate is always changing. If climate stayed the same that would be an Anomaly. The Climate changed long before the any effect from manmade activity.
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    Moderator Response: [Dikran Marsupial] See the response to the second most used skeptic argument Climate's changed before.
  6. @ stevee I personally think that talking about a 'Medieval Climate Anomaly' does not really make sense. What does it mean to say this is a normal this an abnormal state of climate? We do not have the "view from nowhere" to do so. If we take the last 2000 years for example, than we find in some places evidence for a so called "Roman Warm Period" in most places evidence for a "Medieval Warm Period" in almost every places evidence for a "Little Ice Age" and at least a "Current" Warming (WP), starting something about 1850. Example: Are these periods anomalies? If we take a look on the last 100 years we see a warming from 1910 until 1940, a cooling until the 70s and a warming until the "peak" 1998, then a flattening of the curve. So I am convinced it is better to talk about a MWP and not a MCA.
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    Moderator Response: [DB] Actually, not-so-flattened:
    5 datasets, exogenous factors removed (Tamino)
  7. On various periods of ice expansion in NZ. Is this because NZ got colder? No. The alps perpendicular to the predominantly westerlie flow (same in Argentina). A warmer Tasman sea increases snowfall and hence the advance. But the terminus warms too so this doesnt go on forever. In a Franz Josef response study, it was calculated that needed in long term to increase precipitation by 40% for each 1 degree of temperature rise in ablation zone (from memory - paper at work). Again, read glacial data with care.
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  8. My question..was deleted !! If at 5pm tonight, you were provided with Peer reviewed, non-cherry picked, Scientific evidence that there is no mad made global warming...Would you be a)Happy ? or b) Angry ? Why was there cooling in the 70's, the C02 models demand that temp rise with C02, yet they did not, as you stated. http://en.wikipedia.org/wiki/File:Holocene_Temperature_Variations.png Why stop at 2,000 years, try the period in the above link. Why are the Polar Ice caps on Mars melting ? Solar Activity.
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    Moderator Response: [Dikran Marsupial] I would be delighted. However it might be a good idea if you were to acquaint yourself with the responses to common skeptical arguments already documented on this site. Firstly models do not demand temperature rises with rising CO2 because CO2 is not the only driver of climate, you also might want to look at Why did climate cool in the mid-20th Century?. Secondly, we know it has been warmer in the past, see What does past climate change tell us about global warming? . Lastly, the climate change on Mars is thought to be due to changes in albedo, see Global warming on Mars, ice caps melting . It is unlikely to be solar activity, see the response to the number one skeptic argument it's the sun. The reason your question was deleted was that, again, it violated the comments policy, for the same reason your previous post was deleted.
  9. @ DB Actually a bit "flattening": For example UAH Satellite-Based Temperatures until Feb. 2011:
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    Moderator Response: [Dikran Marsupial] Images (hopefully) fixed. BTW, the difference is the "exogenous factors" that Daniel's caption mentioned. The article by Tamino is well worth reading.
  10. "If at 5pm tonight, you were provided with Peer reviewed, non-cherry picked, Scientific evidence that there is no mad made global warming...Would you be a)Happy ? or b) Angry ?" I'd wonder why my CO2 laser still works, personally.
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  11. "If at 5pm tonight, you were provided with Peer reviewed, non-cherry picked, Scientific evidence that there is no mad made global warming...Would you be a)Happy ? or b) Angry ?" Well first I would be skeptical, because a claim that strong would need the evidence tested rigourly by other workers. Then I would be ecstatic. Who wouldn't be? But what has this got to do with glaciers and MWP?
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  12. The variations of the Donne Glacier, Tasman Glacier and Murchison Glacier in New Zealand highlight the general response of NZ glaciers.
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  13. williambaskerville, you may well like to look up the definition of "ad hominen". My comments indicated that a certain argument was silly, not that a particular person was. I also indicated that a particular activity was foolish. It is not ad hominen to say it is foolish to bang your head against the wall, even in the presence of somebody who is doing it. It is of course open to anybody to judge the foolishness or wisdom of that man if, after you have reminded them how foolish their activity is, they do not stop. Now on to your head banging ... The chart may only start in 1977, but you already have the comment of scaddenp @30 that the glacier's are in long term retreat. Also the comment of nigelj @45 (after your cherry picking post, but before your futile attempt to defend it) that, "Over the last 100 years the trend is most glaciers are retreating, or a nett decrease in ice mass. From NIWAS website." And, of course, and most crucially, you had from NIWA at the link provided with the graph where you foolishly cherry picked the upward trends, the comment that, " NIWA’s snowline surveys show an overall decrease in the glacier mass balance", and that:
    2. Despite the sensitivity of New Zealand glaciers to changes in both precipitation and temperature, the volume of ice in the Southern Alps dropped by roughly 50% during the last century. New Zealand’s temperature increased by about 1 °C over the same period. I would also point out that as you are trying to find an equivalent case to that studied by Koch & Clague, you needed a case in which there was wide spread regional glacier growth over the course of the century (such as they found in the MWP), and should yourself have checked whether NZ's glacier growth was century long, or just a few years or decades. Had you done so, you would have found out about the overall retreat of NZ glaciers: So, what I can find is not a trend downward that starts in 1998, but an overall 50% loss of NZ glacier mass balance starting in the early 1930's. Out of that massive decline, you were only able to find a few cherry picked upward slopes, the longest of which is less than 10 years long. Do you think it might be wise to stop banging your head yet?
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  14. Williambaskerville @59, to answer my question of the prior post - evidently not. Trust you when shown the overall temperature trend once the influences of the solar cycle and ENSO to show the raw figures, and cherry pick the bit between the largest El Nino on record and a near record La Nina and talk about flattening.
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  15. Tom : "So, what I can find is not a trend downward that starts in 1998, but an overall 50% loss of NZ glacier mass balance starting in the early 1930's. Out of that massive decline, you were only able to find a few cherry picked upward slopes, the longest of which is less than 10 years long. " But this is NOT what you would expect, if this decrease were caused by the anthropic forcings that are supposed to exceed significantly the natural ones only after 1970 ? about flattening : what is certain is that there is no sign of significant acceleration since 1970. Do you have an idea of how much acceleration is needed before the warming becomes "dangerous" ?
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    Moderator Response: [DB] Enabled inline graphic.
  16. @ Moderator [ - Complaining about moderation removed - ]
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    Moderator Response: [Daniel Bailey] As it was pointed out to you previously, it is not ad hominem to show an argument as being silly, which was done in relation to your statements. Complaining about having to comply with the Comments Policy adds nothing to the dialogue here & thus begs for moderation. I will leave this up for a time so you have a chance to see it & will then remove it. No one wants to stifle dialogue that's on-topic and complies with the Comments Policy. Thanks!
  17. Gilles @65, yes, that is exactly what I would expect. New Zealand temperatures, after being flat for the first part of the 20th century started to rise in the 1930's. Glaciers do not care what causes the temperature to rise, they only respond to that rise. Further more, in 1930, the CO2 concentration was 306.6 (Law Dome), generating a total additionaly forcing in excess of 1750 levels of 0.54 w/m^2, or half that in 1980 (or a third that in 2000). For comparison, the net incease in solar forcing from its lowest point in the 19th century to its highest in the twentieth is just over 0.3 w/m^2. Once again, the claim that because global temperatures do not fall out of the confidence interval of natural forcings alone until after 1980, that therefore anthropogenic forcings are inconsequential before that is a fallacy, and shows the person making the claim does not know what a confidence interval is. In fact, it is transparently a fallacy because the models also show that: 1) The temperature does not fall outside the confidence interval of combined anthropogenic and natural forcings over the same period; 2) The combined anthropogenic and natural forcings predict a higher temperatures than natural forcings alone (thus shown the anthropogenic forcings to be significant); and 3) The temperature tends to lie near the center of the anthropogenic and natural forcings, but on the extreme limit of the confidence interval of natural forcings alone. The natural interpretation of that is that, on the evidence: 1) Both anthropogenic and natural forcings are significant in the early part of the 20th century; 2) Natural forcings have a 5% or less chance of explaining early twentieth century temperature alone; 3) Natural forcings have virtually no chance of explaining temperatures in the late twentieth century alone; 4) Anthropogenic forcings combined with natural forcings have a better than 66% chance of explaining temperatures across the whole twentieth century without involving any other factor; 5) There is a 33% chance or less that some other factor is involved, but it must explain only a small part of the variation; and 6) Anthropogenic forcings are dominant in the late 20th century. Once you accept these 6 facts, you will find you no longer need to distort science to accommodate your world view.
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  18. DB @66, may I suggest you leave it up until WB posts his screenshot at his blog, then I can post a screen shot of your response at his blog.
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  19. Tom, If I may add in support of your graph (NZ temp. anomaly), the NASA/GISS temp anomaly's for 3 distinct latitudes. The leveling or slight decline of temperatures in the northern hemisphere from 1940 to 1970 is attributed to uncontrolled sulfate aerosol emissions. Those aerosols being very short-lived in the atmosphere certainly would not be as prevalent in the southern latitudes and would not mask the CO2 effects of increasing CO2 nearly as much.
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  20. RickG, "aerosols being very short-lived in the atmosphere certainly would not be as prevalent in the southern latitudes " Good point. In addition, those graphs illustrate that claims of 'flattening and 'no sign of significant acceleration since 1970' are utterly devoid of contact with observations.
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  21. " 1) The temperature does not fall outside the confidence interval of combined anthropogenic and natural forcings over the same period;" First you should recognize that it is not the "temperatures" but the "anomalies" with respect to the first half of the XXth century - this by no means surprising that ANY anomaly computed with respect to the same period are comparable, irrespective of the model - the opposite would be surprising ! so the agreement in the first part is of low significance. " 2) The combined anthropogenic and natural forcings predict a higher temperatures than natural forcings alone (thus shown the anthropogenic forcings to be significant); and" actually the graphics has always been unclear for me, but may be you can explain me : is the blue curve a "best fit" obtainable without anthropic component ? or is it a "best fit" WITH anthropic component, after substraction of the anthropic component? that's quite different. If you optimize the model with an anthropic component, and AFTER substract anything , you are sure that the result is worsened. But that doesn't prove that another equally good fit could be obtained with other hypothesis. " 3) The temperature tends to lie near the center of the anthropogenic and natural forcings, but on the extreme limit of the confidence interval of natural forcings alone." see above. How "natural forcings alone" have been adjusted ? " Once you accept these 6 facts, you will find you no longer need to distort science to accommodate your world view." I will not accept these 6 facts before having a clear answer to the previous questions ....
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  22. " New Zealand temperatures, after being flat for the first part of the 20th century started to rise in the 1930's" Really, I don't see anything like that in your curve. Why do you say that temperatures started to rise in 1930 ? I see a shift in the 1950's, and your glaciers has experienced already most of their melting at this date. Aren't you distorting mere facts ?
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  23. G: "I don't see anything like that in your curve." We are discussing the bottom graph here. Is there not a conspicuous rise from 1930-mid 40s? Is the northern hemisphere 1970s low present in this graph? Please avoid accusations of factual distortion -- that is a violation of the Comments Policy.
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  24. Gilles "I see a shift in the 1950's, and your glaciers has experienced already most of their melting at this date. Aren't you distorting mere facts ? " No. I see this little factoid as a possible signal of other things. That initial leaning into a downward slope on the glacier mass balance trend indicates 2 possibilities. Firstly, solar irradiance was higher for much of that period and that might just have been a blip in a long term trend if other things had not changed. Secondly, and more importantly from my point of view, I have a lurking suspicion that much of the warming-that-didn't-show-up-until-the-70s went into a couple of dark corners that seemed insignificant at the time. Night-time minimum temperatures. Ice melt. And that ice melt would have predominated in land terminating glaciers rather than those supported by sheets of sea ice buffered by the oceans. Even where the edges of those ice sheets were retreating, however erratically, they didn't lose enough to damage the glaciers at their landward edge. It certainly wasn't large enough or fast enough to sound the kind of alarm bells we're now seeing for Arctic sea ice. But I think there's almost enough data around for there to be a century long trend when we look at the right indicators.
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  25. Gilles - The second graphic Tom Curtis shows here is the result of multiple model runs using various forcings. As per the caption, the blue bar represents the 2-sigma variation range around model runs that do not include anthropogenic forcings (but do include insolation, volcanic aerosols, orbital effects, etc.), the red bar represents the variation range around models run with our forcings, and the black line is the actual temperature record. These are models shown to have considerable agreement with historic temperatures (including some pre-industrials) using recorded forcings. The point made in this graphic is that we have some idea of how forcings drive temperatures, and that we cannot account for current temperature rise without the anthropogenic contribution as part of the physics.
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  26. Mucounter : Tom and myself were obviously talking about the first graph (the NZ trend), because he introduced the sentence just before showing it. Concerning the "conspicuous rise " in the high southern latitudes between 1930 and 1940, I don't see how to blame anthropic forcings for it : did anthropic forcings really rise more in this period ? KR : as you probably know, all these models have free parameters, especially for clouds, and the outputs are selected by some kind of "best fits". So it's not the same to do a "best fit" without anthropic forcings, and to do a best fit with anthropic forcings and THEN substract them. Now you may also know that if I had added new free parameters, I would always improve the fit. I could improve the fit with natural forcings only by adding the position of Jupiter, Saturn, and Pluto, and then argue that if I substract these influences, the fit is worse. That is mathematically perfectly correct - and yet physically absurd. This is not a correct way of reasoning.
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    Moderator Response: [Dikran Marsupial] No subtraction is being done. The breadth of the error bars tends to be due to the variability in model runs, not the variability in parameters. GCMs are not statistical model fits, they are models of the physics, that allow you to find out the consequences of a set of assumptions regarding climate physics. The degree to which they can be tuned is fairly limited. If you (or anyone else) think the observed 20th century climate can be modelled without anthropogenic climate by adding a few parameters, go for it, the source code of the GISS model E is publically available as a starting point. If you had to include the positions of Jupiter etc. to do so, that would just indicate that anthropogenic forcings are important; there is good evidence that carbon dioxide is a greenhouse gas and affects climate; that is not the case for the position of Jupiter!
  27. "The second graphic Tom Curtis shows here is the result of multiple model runs using various forcings. " Another strange thing : it is unusual to cover an experimental curve with a set of DIFFERENT models. Through the use of multiple wrong models, I can always cover everything in their interval. What does it prove ?
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    Moderator Response: [Dikran Marsupial] It isn't at all unusual (in fact it ought to be standard operating procedure). The purpose is to show the uncertainty in the projection/hindcast based on our current understanding of climate physics. Climatologists are generally very happy to talk about model uncertainty; if they only showed the best model fit, they would be accused of cherry picking. BTW GEP Box said "all models are wrong, but some are useful"; you have asserted they are wrong, but have provided no evidence that they are not useful.
  28. Daniel: with reference to your moderator replies to comments #15 and #24 - here is the url for the original temperature anomaly graphics: earthobservatory.nasa.gov Of relevance to modern temps vs MWP is the recent paper Recovery mechanisms of Arctic summer sea ice. S. Tietsche,1 D. Notz,1 J. H. Jungclaus,1 and J. Marotzke1 The paper states clearly that temperatures in the Fram Strait are higher than at any time in the last 2ky - including the MWP. Tietsche_GRL_2011.pdf
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    Moderator Response: Hey, Welcome to Skeptical Science! Thanks for the link to the graphic (I haven't yet had the time to search for it, so many thanks)! Far be it for me to correct you, but I think you may have meant Spielhagen et al, 2011 wherein they present a 2,000 year marine sediment proxy record showing the unprecedented warming of which you speak. BTW, if you want to link to your website, please do so as many of the regulars here would find it invaluable. Thanks again!
  29. "No subtraction is being done. " By "subtraction", I mean switching off the anthropic forcings in the runs. Is it what is done, or no? If yes, I repeat : the fact that you worsen a fit by switching off a component AFTER you have adjusted your fit to data is not surprising : it is always true. And it is hence not very useful to "prove" anything on the validity of your model. "GCMs are not statistical model fits, they are models of the physics, that allow you to find out the consequences of a set of assumptions regarding climate physics." DM : we saw in the post about sensitivities that there was still a fairly large range of different sensitivities compatible with the data (or they would be dismissed by them). Do the "different curves" used to generate this interval include all these values of sensitivities, yes, or no ? and if yes, how can they all be "equally true" ? If you can explain the same data with different physical models, you're pretty sure that they are almost all (but one) wrong. And you cannot exclude they are all wrong. This is not a situation usually associated with a "well understood" theory.
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    Moderator Response: [Dikran Marsupial] You are assuming that the model with only non-anthropogenic forcings was not independently "tuned" (as far as tuning is possible). I doubt that is the case. As I said, the GISS Model E code is freely available, so you can always go and check for yourself. Secondly, the diagram shown was not meant to "prove" anything on the validity of the model. If there is a large range of sensitivities that are compatible with the data, that suggests that the models would be relatively insensitive to their adjustment. Lastly, no, all models are wrong (GEP Box); nobody expects any of them to be exactly right. The question is, are they sufficiently right to be useful. YOu have yet to provide any evidence that they are not useful, we all know they are "wrong", that is entirely uncontraverisal.
  30. Gilles, everyone - might I suggest taking the models conversation to the Models are unreliable thread, where this topic is relevant? I don't believe it's a core part of a MWP discussion.
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    Moderator Response: [Dikran Marsupial] Agreed, further discussion of models should be conducted on that thread.
  31. Re: comment #78 - moderator response. Many thanks for the warm welcome! Yes, I did mean the Spielhagen et al paper. I mistakenly copied from my recent urls list the one above Spielhagen. The link I mistakenly posted is a paper suggesting the possibility of sea ice recovery in 2 years after an ice free summer. I have been 'lurking' here for far too long and thought it was about time I signed on/in/up. Best regards, Patrick Lockerby http://www.science20.com/chatter_box
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    Moderator Response: [DB] Patrick, I think your html for the no follow code string in your SkS Sign-on ID is hashed (plus I'm not sure that the no follow string will work there anyway, but it's worth a try; for some reason it shows up as logicman w/ no link in the Recent Comments window, but it's hashed up on the actual threads). Between the left-arrow symbol and the right-arrow symbol should be this string: a rel="nofollow" href="http://www.science20.com/chatter_box"
  32. Moderator @#22, "For posterity and context, current northern hemisphere temps greatly exceed those of the MWP." I suggest you can only come to that conclusion by using current instrumental temperatures and then comparing them with 1000-year old proxies. This methodology can be incorrect. The correct methodology should be to compare current proxies with past proxies. Let me explain further by utilising Ljungqvist's (2010) reconstruction which is shown in Figure 1 below. Figure 1: 2000-Year NH Temperature Reconstruction (after Ljungqvist, 2010) I have plotted the MWP peak temperature as the red line and the modern proxy peak as the blue line on the reconstruction in Figure 1. The instrumental data are deleted for clarity. It is evident from Figure 1 that the MWP peak is 0.11 °C warmer than the modern proxy peak decadal mean (1980-1989). However, Tom Curtis in #136 @ SkS Hockey Stick Own Goal pointed out that the GISS data show that 2000-2009 mean temperature to be 0.18°C higher than 1990-1999 decadal mean. Therefore, I show Ljungqvist's data in greater detail for the modern instrumental period in Figure 2. The data for Figures 1 and 2 is available from NOAA (here). Figure 2: Modern NH Temperature Reconstruction 1860-1999 (after Ljungqvist, 2010) I have restored Ljungqvist's instrumental data in Figure 2. I have also added 0.18°C as an estimate of the 2000-2009 instrumental decadal mean. This is shown as the dashed red line. Several features are evident from Figure 2, the most important of which are as follows:
    1. There is a very good correlation between Ljungqvist's proxy temperatures and the whole of the pre-1990 instrumental data. Thereafter, they diverge. This is a good illustration of the aptly-named "divergence problem."
    2. The MWP is approximately 0.11°C warmer than the modern-day proxy temperatures, including the 1980-1989 decadal peak.
    3. The modern instrumental data for 1990-1999 of 0.39°C and 2000-2009 of 0.57°C exceed the MWP proxy peak temperature by 0.19°C and 0.37°C respectively.
    4. Notwithstanding item (3), the MWP peak is 0.14 °C higher than the 1990-1999 proxy temperature. Therefore, it is possible to infer that an "instrumental" MWP peak would be approximately 0.14°C higher than the 1990-1999 instrumental temperature, if there were a linear response between proxy and instrumental data. However, to be safe, it may be better to use the 0.11°C difference between the MWP and modern proxies. This would correspond to a MWP peak "instrumental" temperature anomaly of 0. 39 + 0.11 = 0.50°C.
    From (4) above the difference between the MWP "instrumental" temperature and the 2000-2009 actual temperature would be 0.57 - 0.50 = 0.07°C. This is hardly unprecedented warming and is about one-seventh of the 0.5 °C value stated in SkS Hockey Stick Own Goal. However, my assumption that proxies would increase linearly from the 1990's to the 2000's is questionable because,"…recent proxy data does not emulate the recent instrumental data" (Ljungqvist, 2010). However, my assumption is no less questionable than adding modern-day instrumental temperatures to proxies that are based on a 1961-1990 mean when the proxies did emulate Ljungqvist's instrumental record. Consequently, what we need urgently are up-to-date proxies so that we can compare their data with the current warm period. If these proxies were below the MWP then we can conclude that the MWP was warmer than today. Alternatively, if the proxies were above the MWP then our conclusion would be that the MWP was cooler than today. I contend that the only prudent conclusion that we can make from the current data is that it is likely that current temperatures are higher than MWP. However, we cannot accurately determine by how much. Statements such as that in SkS Hockey Stick Own Goal that current temperatures exceed the MWP by 0.5°C are not supported by current proxy temperatures.
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  33. Re: my comment #78 - html link fixed, thanks. The profile needs a no tags url, unlike the comments pages. By putting in tags, I seem to have provoked your site's software into adding a no-follow.
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