Progresos en reconstruir el clima en los últimos milenios

Una traducción en español está disponible aquí.

119 comments on this post.
  1. Spencer:

    Excellent paper, but we are still stuck in NH chauvinism, if not North Atlantic chauvinism: most of what you hear about Medieval warming, the Little Ice Age, etc. refers to the weather in the regions where most climatologists have lived… Still badly needed is more info on the Southern Hemisphere.

    Indeed the authors say, “Given the uncertainties, the SH and global reconstructions are compatible with the possibility of warmth similar to the most recent decade during brief intervals of the past 1,500 years.” So as you say, this result is not the sort of thing that should be trumpeted as saying, See, the world really is warmer now than ever.

    My guess is that there is a “see-saw” (probably in ocean circulation) that tends to make the SH colder when the NH is warmer and vice-versa, so the Medieval warming and Little Ice Age will hardly show up at all when we get truly global measures. But that remains a big challenge.

    [Response: Thanks, as always, for the insightful comments Spencer. The above quote ‘Given the uncertainties’ was perhaps not delivered with the appropriate context or nuance. All that I was saying was that the uncertainties are substantially greater in the Southern Hemisphere (and therefore for the globe as well) because of the dearth of long-term proxy data in the Southern Hemisphere (for example, the entire extratropical southern ocean). So we can’t really draw meaningful conclusions regarding the comparison between current and past warmth for those cases. Its not because there is evidence of any greater Medieval warmth for the SH and globe—its just that the uncertainties are large enough to preclude a tight comparison w/ current instrumental temperatures. That having been said, the same radiative forcings that have driven NH mean temperature changes, have driven global temperature changes as well. And I’d be really surprised if the true global mean temperature (if we knew it as precisely as the NH mean temperature) looked all that different. I don’t for example thing that changes in ocean circulation would impact SH mean temperatures all that much, and certainly in the global mean they largely cancel out, though not entirely (for example, Knight et al, GRL ’05 show that the so-called “AMO” in the context of the UK Met Office model, is associated with very modest, i.e. tenth or so of a degree C or so, changes in global mean SST. -mike]

  2. coby:

    Past decade is warmest in at least 1300 years

    Well, not many changes in the big picture, no pressing need to update this article I guess. If Climate Audit can be the least bit fair they will have to drop the majority of their talking points wrt the MWP and proxies in general.

    The real test of a scientific argument vs a political one is whether or not the argument changes in the face of new information.

  3. Chris Colose:

    Well Steve Mc is already complaining about some of the proxies on his site– don’t expect to get off easy on this one!!

    In all seriousness, congratulations on this new piece, as it looks like the best single paper on a global reconstruction over the last millennia. It still looks like we got more digging to do in South America, Africa, and Australia.

  4. Richard Pauli:

    Forgive my simple question: How is it that the zero value is set?

    Temperature Anomaly 0 degrees C Looks like it should be in the middle of the spaghetti. Is it inherited from a smaller data set?

    [Response: Everything is baselined to the period 1961-1990. – gavin]

  5. David Garen:

    Anybody care to comment on the (contrarian) argument that most of the global warming since ~1800 is due to natural recovery from the Little Ice Age rather than greenhouse gases? Here is a rather lengthy essay on this topic (beware, the link is split on two lines):

    Syun Akasofu

    My simplistic summary of the argument is that there has been a ~0.5 degree per century increase of temperature that Akasofu attributes to a natural recovery from the Little Ice Age. The IPCC says there has been overall a 0.6 degree per century increase, therefore only 0.1 degree per century could be attributable to greenhouse gases.

    This argument raises some red flags in my mind, but I am not a climate expert, so I would be really interested if anyone can comment on this.

  6. PHE:

    Two dumb questions:
    1. Why is it that the most recent proxy results (1995) are exceeded by historical proxy results?
    2. Given that the recent instrumental record exceeds the nearest proxy results by around 0.6 degC, can we assume this ‘difference’ could not have ocurred at any time in the past?

    [Response: It depends on the method – the CPS results with tree-rings have the worst fit in recent decades (the ‘divergence’ problem), but that is much less to non-existent with EIV or if the tree-ring data are screened out (see Figure 2 in the paper). -gavin]

  7. coby:

    Hi David Garen,

    Try this article that addresses the “we are just recovering from the LIA” argument.

  8. Mauri Pelto:

    Spencer good comment on the SH. Alpine glacier terminus behavior does not provide a good annual temperature proxy, but do provide a good temperature proxy for decadal periods and more. In the southern hemisphere ubiquitous retreat of glaciers on South Georgia, throughout the Andes, New Guinea and in New Zealand at the end of the LIA, indicate that this was not just a NH phenomenon.

  9. Alexandre:

    What´s the methodology to get a world or hemispheric mean temperature from spot proxies (or instrumental measurements)? How do we know it´s reliable?

    These are questions I get a lot from denialists.

    [Response: The actual methods are quite complicated, but the basic idea is to see how any of the individual proxies lines up with temperature patterns in the instrumental period and then use that to go back in time. The reliability is tested by leaving off some of the instrumental data and seeing how the reconstruction matches climate at that known time. The reconstructions can then be tested against documentary sources of climate information that go back centuries, or can be compared with times of known volcanic eruptions etc. The best sense of overall reliability is to see what reasonable but different methodologies, with different proxy choices give – if they vary widely, then it wouldn’t be very reliable, if they are more consistent, then you’d weight them a little more strongly. Uncertainties increase going back in time, so for 1000 years ago they are a few tenths of a degree. However, you need to ask your correspondents what they want to know and why they think it important. Much of the misinformation that surrounds this issue is because people mistake a reconstruction of the past with an attribution of changes right now. – gavin]

  10. Steve Reynolds:

    The CPS and EIV methods seem to product results that disagree by much more than the uncertainties shown (sometimes by a full degree). How should that be interpreted?

    [Response: That’s a measure of the structural uncertainty – and it’s often larger than any formal error estimate (think about the difference in trends between RSS and UAH MSU records, or the various radiosonde products). – gavin]

    [Response: The EIV and CPS approaches, as described and shown in the manuscript, produce results that are (i) remarkably similar back to AD 1500 or so, (ii) certainly consistent within estimated uncertainties back to AD 1000 and which finally (iii) do differ outside the uncertainties prior to that. Where the estimates differ, it means the data are sparse enough that the differing assumptions underlying the two methods really do matter, and not surprisingly the answer does depend on those assumptions. This is all discussed in the paper, and even more extensively addressed with parallel ‘pseudoproxy’ experiments in the supplementary information which test the sensitivity of the two different methods to increasingly sparse data networks. – mike]

  11. Blair Dowden:

    Can you explain why the ‘Composite and Scale’ (CPS) method diverges so strongly from the other proxies around 1200 years ago?

    [Response: See the two inline response to the immediately previous commenter. – mike]

  12. Joseph Romm (Climate Progress):

    This new article is important and thorough. I have turned some of the PDF figures to JPEGs — including the Southern Hemisphere temperature reconstruction — here:
    “Sorry deniers, hockey stick gets longer, stronger: Earth hotter now than in past 2,000 years.”

    The bottom line from Mann is important to remember: “You can go back nearly 2,000 years and the conclusion still holds–the current warmth is anomalous. The burst of warming over the past one to two decades takes us out of the envelope of natural variability.”

    Kudos to Mann et al.

  13. John Mashey:

    re: #5 Garen

    I don’t know why this is resurfacing now – which blog mentioned it?

    I went through this at Open Mind, May 17, 2007 5:03 am. Go over there and search down, beyond some earlier posts. Also, look at Marion Delgado’s comments later in that thread.

    As noted elsewhere, part 2, for climate expertise, I’d rate myself 2 on a scale of 10, but that was plenty good enough to take care of that paper.

    ALWAYS be very careful when a well-published (100s of articles in this case) scientist:

    a) Retires, then
    b) Starts opining strongly about a different topic,
    c) Directly contradicting a large body of established science
    d) Making simple mistakes
    e) And does it in OpEds, newspaper interviews, web pages, but NOT in peer-reviewed literature, even though they know the ropes well there.

  14. Andy:

    You had a review of Craig & Lohle on this blog this year, with quite a many critical comments. Doesn’t the same comments apply to Mann et al as well? Usage of some uncalibrated mineral ratios as linear proxies for temperature etc?

    [Response: The problem with the Loehle method is that there was no validation of his results and no attempt to adjust for fidelity of any proxy – they were all uniformly thrown in (and that’s distinct from all the dating and interpretation mistakes that were made). Both the methods used in Mann et al have calibration steps and cross-validation and only CPS requires a local response to temperature. – gavin]

  15. John Dodds:

    Did I misread the charts or is Richard Alley’s GISP2 ice core data NOT included in the charts above? If not then WHY?

    The GISP2 data clearly supports the “NEWS” that current day temperatures are the highest since the year zero (AD, not absolute!), let alone for the last 1300 years.
    BUT the GISP2 data also shows that we reached higher temperatures 8000years BP, 7000years BP, 3000 years BP, and 2100years BP.

    The data also shows that 16 (SIXTEEN!!) times in the last 10,000 years we have had 300 years periods when the temperature rose or fell by 1 to 2 degrees C. which is comparable to the 1.5 degrees since 1700. (or 0.8 since 1850)
    The rapid rise in the last 300 years is by no means unnatural.

    [edit OT]

    [Response: Please read the SI and the paper itself. GISP2 isotopes and accumulation are used. Your rather excitable interpretation of the Greenland record over longer time scales, at best, is a reflection of local conditions, not hemispheric or global, and, at worst, are mostly noise. The whole point of multi-proxy reconstructions is to find the signal that goes beyond a single favoured record. – gavin]

  16. Danny Bloom:

    Excellent paper, but at the same time, while we’re splitting hairs, Dr Lovelock is still trying to pull the wool off our eyes…

    See: “Wake Up, We Are On The Brink of Extinction! Says Ecoactivist” at

  17. Barton Paul Levenson:

    David Garen says:

    Anybody care to comment on the (contrarian) argument that most of the global warming since ~1800 is due to natural recovery from the Little Ice Age rather than greenhouse gases?

    Sure. What is the mechanism of action by which “recovery from the Little Ice Age” affects the global mean annual temperature? Where is the energy coming from?

  18. David W:

    When was the last time the Northern Hemisphere had a “burst of warming” similar to 20th century?…Then when was the last time before that…The proxy data is impressive but what does it mean?

  19. Ethan:

    “…around 1350 AD which may make interesting case studies for attribution to solar or volcanic forcings in future”

    Black Death and re-forestation in Europe? (Ruddiman, 2005)

  20. Matt:

    Could you please explain where the CRU temp comes from. It’s showing a current temp anomoly of over 0.8 deg C yet none of the usually quote sources (GISS, HadCrut3, RSS, UAH) are at this level. They are currently in the range of 0.1 to 0.4.

    Without the red line (CRU temp), it doesn’t look much like a hockey stick!

    [Response: Read the caption and the paper – this is the smoothed NH CRU (land) data. Note that 2005 and 2006 have raw anomalies of 0.898 and 0.894ºC (and similar in the CRUTEM3v data). – gavin]

  21. Andrew Davison:

    David Garren:

    >Sure. What is the mechanism of action by which “recovery from the >Little Ice Age” affects the global mean annual temperature? Where is >the energy coming from?

    Why surely it’s the same mechanism that caused the LIA in the first place. And what might that have been?

    [Response: Volcanic and solar most likely with a somewhat uncertain balance. But neither explain recent trends, and frankly, this is completely orthogonal to the ‘natural recovery’ idea that you started with. If you agree that climate change is largely determined by external forcings, then the anthro GHG increase is by far the most compelling cause for recent trends. – gavin]

  22. Timothy:

    [17, Ethan] – The decrease in black carbon from wood-burning might be important.

  23. Mark Zimmerman:

    Can anyone tell me where I can learn how close the various proxies come to the present day? I’ve already had a contrarian dismiss this paper as a hoax because the various proxies don’t reflect the warming of the past three decades.

    [Response: A lot of them actually do. The raw data (before any infilling) is also available on the SI site, and so you can look for yourself. But even if not many did, how can that be a hoax? Obviously the more calibration/validation data there is, the better, but what we are talking about is degrees of skill. Talk of conspiracies and hoaxes is juvenile paranoia in the extreme. – gavin]

  24. Lawrence Brown:

    By using various proxies with and without using tree rings and getting similar results makes it harder to dismiss proxy methods for obtaining results prior to instrumental recording. The many curves though different show approximately the same trends, especially over the past 1000 years.

    As Teddy Roosevelt might have said- Mann et al speak softly, but carry a big stick.

  25. Magnus Westerstrand:

    And while we are on proxies… I have a few colleagues that or on a trip to the Arctic looking at the methane situation (I have a quick translation of one of their blog posts here). So I was wondering if any one on here know anything about what the ice cores say about fast concentration increases of methane during the “history” and what effect that might have had?

  26. Alexandre:

    Hi Gavin,

    Thanks for your useful answer, but that wasn´t quite the one I was looking for.

    I´ll try to rephrase my question: each tree, each ice column, shows the temperature behaviour of only one spot on the researched territory. Yet you´re able to calculate a single temperature time series representing the “mean temperature” of a big and hetherogeneous territory (a country, a hemisphere, or the whole world). I assume there is some specific procedure for that, and not a simplistic arithmetic average of all proxies. How is this done? And how do we know it reflects reality?

    [Response: You should read the paper, and in particular the supplementary information (p3). Both methods used are described pretty concisely. The reason why this works overall is that temperature anomalies are correlated over quite large distances and you can check the results by withholding some instrumental data to validate against. – gavin]

  27. Timo Hämeranta:

    Re 21 Gavin etc “Where is the energy coming from?”

    Please see:

    Johnson, Gregory C., Sabine Mecking, Bernadette M. Sloyan, and Susan E. Wijffels, 2007. Recent Bottom Water Warming in the Pacific Ocean. Journal of Climate Vol. 20, No 21, pp. 5365-5375, November 2007, online

    “…estimates of the change in ocean heat content suggest that the abyssal warming may amount to a significant fraction of upper World Ocean heat gain over the past few decades.”

    The oceans heat the continents, please see:

    Compo, Gilbert P., and Prashant D. Sardeshmukh, 2008. Oceanic influences on recent continental warming. Climate Dynamics, in press 2008, preprint online

    “Evidence is presented that the recent worldwide land warming has occurred largely in response to a worldwide warming of the oceans rather than as a direct response to increasing greenhouse gases (GHGs) over land…”

    [Response: Again what is your point? An AMIP experiment cannot be used for attribution of warming trends because the warming (however caused) is built in to the ocean temperatures – as Compo et al clearly state.Your continued use of this paper to insinuate that the oceans have been releasing heat into the atmosphere is completely contradicted by the increase (not decrease) of ocean heat content in recent decades, and indeed by the first paper you cite! Try to have a little internal consistency. – gavin]

  28. John Mashey:

    re: #19

    Ruddiman has a long 2007 article at Reviews of Geophysics. Section 10 further explores the plague/reforestration hypothesis, with some newer material than in the book or early articles.

  29. Dave Clarke:

    Much of the past controversy involved the specific implementation of Principal Component Analysis (PCA) in MBH98 (see McIntyre and McKitrick, 2005, GRL). In my opinion, Amman and Wahl effectively addressed and resolved that particular objection, insofar as an adjustment of the PCA centering did not materially affect the reconstruction, assuming reasonable rules for incorporation of principal components.

    Nevertheless, I’d be curious to know how the analysis and its implementation in the latest reconstruction differs from the MBH98 PCA. If you could provide a quick summary on that point, or point us in the right direction, I’d be most grateful.

    [Response: There is no separate PCA stage in this analysis. Both the CPS and EIV methodologies have their own ways to deal with statistical redundancy (i.e. making sure that several nearby and similar records don’t get overweighted in the final reconstruction). For CPS, it is done through gridding onto a 5×5 grid prior to the CPS procedure, while EIV takes account of the co-variance of individual proxies directly. This is better explained in the SI (linked above). – gavin]

  30. BillS:

    Re: #25

    You might have a look at and some of the papers cited there.

  31. Hank Roberts:

    Magnus, you’ve likely read these, but perhaps the search on your question will interest others. Lots of studies, over 2000 hits from this search:

    Picking just one off the first results page for example:

    Atmospheric Methane and Nitrous Oxide of the Late Pleistocene from Antarctic Ice Cores
    R Spahni, J Chappellaz, TF Stocker, L Loulergue, G … – Science, 2005 –

  32. David Garen:

    Many thanks to those who responded (comments #7, 13, 17, 21) to my somewhat off-topic inquiry about the Little Ice Age (#5, 3 September, 2:36 pm). Since this article was forwarded to me by a colleague, these comments help me a lot in knowing what to think about this.

    John Mashey — enjoyed reading the posts you linked to.

  33. Magnus Westerstrand:

    …And many thanks to those who responded to my post to, this is not my area… but I’m giving some classes and the students are bound to ask about the trip… however I should have taken some time to go through the article archives, a bit stressed at the moment :)

  34. Hank Roberts:

    > go through the article archives

    Won’t that task be assigned to the students who ask?

  35. David B. Benson:

    David W (18) — I’m an amateur at this, so I’ll just tell your about what the GISP2 central Greenland ice core suggests. Using just this one proxy means the whole northern hemisphere may not have responded as indicated just for Greenland.

    That said, the last time there was a roughly similar burst of warming was during recovery from the 8.2 kybp event. The time before that was the recovery for Younger Dryas. Both might be thought of as episodes of extremely fast warming during the transition from LGM to the Holocene.

    What all this means to me, using also other data such as

    is that we have now left the Holocene on our climate adventure into the unknown and are in serious danger of leaving agriculture behind.

  36. JamesG:

    “The problem with the Loehle method is that there was no validation of his results and no attempt to adjust for fidelity of any proxy – they were all uniformly thrown in (and that’s distinct from all the dating and interpretation mistakes that were made). Both the methods used in Mann et al have calibration steps and cross-validation and only CPS requires a local response to temperature.”

    I thought that Loehle’s proxies had all been calibrated and validated to the local temperatures by the original authors. Isn’t that better than wiggle matching a local proxy to a global record? One thing that Loehle’s reconstruction and this one have in common is that none of the individual proxies looks remotely like the combined result. Isn’t that a pretty good argument for local temperature calibration? Those interpretation/dating problems were apparently minor and didn’t affect the results BTW.

    [Response: No it isn’t better – as I explained at the time. Loehle threw in data that was a century scale mean with local records (actually included in that same mean) and weighted them equally. He weighted one record in South Africa the same as a whole China average. He used data sets that weren’t dated to better than +/- hundreds of years. Etc. None of those issues were dealt with, nor was there any attempt to weight for areal representation or for how good any one proxy was. Neither was it calibrated to any modern record, so no comparison between his MWP and the modern was possible (not that this has stopped various people claiming it was). – gavin]

  37. Richard Sycamore:

    Re: #26

    Gavin inline reply says: “that the oceans have been releasing heat into the atmosphere is completely contradicted by the increase (not decrease) of ocean heat content in recent decades”

    I must have missed something. If the oceans were out of radiative balance (e.g. “warming in the pipe”) then could they not be releasing heat more slowly than they are absorbing it (or better “have absorbed it in the past”)? Thus causing the atmosphere to warm, even while OHC rises? Why must OHC drop just because there is a net Heat transfer from Ocean to Atmosphere?

    [Response: Please think before you write. – gavin]

    (Also, but secondarily, what proportion of the ocean volume is sampled? Is it possible to know total OHC and basin OHCs with reasonable certainty?)

  38. Magnus Westerstrand:

    Frank at 34

    >> go through the article archives

    > Won’t that task be assigned to the students who ask?

    :) Oh I wish!

    In Sweden we do as the students tell us! ;)

  39. John Mashey:

    This has been mentioned before, but one of my favorite historical indicators (albeit limited in geography, and not a serious proxy) is viticulture in Britain, and it is quite consistent with the new reconstructions, within the usual uncertainties.

    “They used to grow grapes in England, so it used to be warmer” is a common denialist meme. My (British) wife is from Yorkshire, and did her PhD at Imperial College, London, so:

    Richard Selley is an emeritus geologist at IC and also wine person.

    His book, The Winelands of Britain: Past, Present & Prospective, 2004, includes detailed archaelogical and geological studies of where vineyards have been located in UK for 2000 years, with detailed vineyard maps given for each of the major periods, showing the ebb and flow with (mostly) temperature.

    The map (but without all the details in the book) is here.

    In that map, modern vineyards were already a bit North of Medieval ones, and heading North, but it’s slightly out of date. Actually, there are already several vineyards in Yorkshire, which is about on his projected 2050 line. (I know the area … vineyard in Leeds! Wow!)

    He expects there to be a good vineyard post-2100 on the North shore of Loch Ness, which will likely increase sightings of Nessie, but worries that it will get too warm for good viticulture in Southern England.

    Viticulture location actually makes for an interesting indicator, since:

    a) Wine grapes are a high-value crop, so people do try to grow them where they can.

    b) Temperature ranges are *important* to grape-growing.

    [Response: I spent a lot of time going over this argument and discussed it in two posts that you might find interesting. Here and here. – gavin]

  40. tidal:

    @ John Mashey,

    I know you often point to William Ruddiman’s theories on “early”-AGW. There is an extensive article in the current New Scientist on this, including comments by critics like Broecker. This link is “subscription”, but it should also be on the newstands now or imminently:

  41. Lab Lemming:

    So how do we get a good mid latitude SH proxy? I assume that forams dissolve at abyssal depths down there. How long to sea turtles and whales live? Do they record anything?

  42. John Mashey:

    Yes, (sorry, I should have put the links in my first sentence).

    By happy coincidence, in response to a claim by a poster at Hot Topics that the Romans were growing grapes in Northumberland (!, Northeast corner of England), I asked Selley what he thought:

    a) He “does not know of any Roman vineyards as far nNorth as Northumberland. North Thoresby in Lincolnshire is the most northern to the best of his knowledge.”

    North Thoresby is about 25 miles South of Leeds (where vineyards exist now), but nearer the coast, about 60m lower, and should be roughly 1-2C warmer than Leeds, in 2080, according to Selley’s latest map.

    North Thoresby would be in the pink (Merlot) area near the coast, whereas Leeds would be in the yellow (Chardonnay) area in the middle, as should Northumberland be, by then (top right corner). SO, Northumberland will get its chance.

    Also, by happy chance, Selley has a new edition of his book out, with more material on temperatures and Roman period in England.

  43. John Mashey:

    #40 Tidal

    Thanks for pointer, and maybe you can say a little more about what’s there?
    Specifically, does it include long 2007 paper and CHina & rice, 2008.

    I wouldn’t go off on this, except that it does bear on the general topic.

    a) Of course, that as Ruddiman does, I generally reference these as hypotheses, and a chunk of talsk about that, i.e., as hypotheses that might or might not get accepted as theories.

    b) Like I said there, one of the reasons I’m especially fond of this is:

    “Current example of real science one can watch happening: my favorite example of scientific process in visible action can be found in William Ruddiman’s “Plows, Plagues, and Petroleum” plus surrounding papers, arguments, counter-arguments, modifications, to-and-fro-ing. Bill offers several somewhat surprising hypotheses (early CO2, early CH4, and more recent plague effects on CO2), with enough evidence from a highly-regarded researcher to make it to S2 (hypothesis), but not yet (and maybe never) part of S3 (established theory).

    Unlike many arguments, this set is actually understandable to non-experts (like me), and one can actually watch science in progress. These hypotheses may linger with insufficient evidence to confirm or deny, may get refuted later, or may turn out to be brilliant multidisciplinary theories accepted as the best explanations for otherwise puzzling data.”

    Maybe Bill can be talked into a return visit to do an update, last one here was 2005 .

  44. Chris Crawford:

    I don’t know if this is significant, but I recently heard from a friend that her father, while canoeing at a lake at the base of Mt. Mazama, saw on an embankment a number of recently exposed logs buried in about ten feet of ash. Mt. Mazama erupted around 7,000 years ago. Most of the logs he saw were burned, but one was not burned. I wonder if that unburned log might contain tree ring data of interest?

  45. Timo Hämeranta:

    RE 27 Gavin,

    please notice that the warmth from the abysses (below 2000-3000 m) is NOT in current ocean temperature measurements or in ocean heat content calculations.

    About oceans, in general, please see:

    “…Observational and computational progress in physical oceanography, however, over the last 30 years has rendered obsolete the old idea that the fluid ocean is a slowly changing, passive, almost geological system. Instead, it is a dynamically active, essentially turbulent fluid, in which large-scale tracer patterns arise from active turbulence and do not necessarily imply domination of the physics and climate system by large-scale flow fields. To the contrary, oceanic kinetic energy is dominated by the time and space-varying components. The complexity of the resulting fluid pathways is an essential part of any zero-order description of the system. Thus general circulation models are the essential tool for understanding past, present and future climate states. Quantification of the likely major errors in using oversimplified models with inadequate turbulence closures and undersampled data becomes the main issue. Determining the past and future circulations is not easy, but hiding the difficulties is not a viable option.”

    Ref: Wunsch, Carl, 2007. The Past and Future Ocean Circulation From a Contemporary Perspective. Chapter in Schmittner, Andreas, John Chiang, Sidney Hemmings, Editors, 2007. Ocean Circulation: Mechanisms and Impacts. AGU Geophysical Monograph Series Vol. 173, 2007, online

    Certainty proves false confidence only.

    btw, when you will admit that yr assertion about raw data until 1995 “(before any infilling) is available” is flat out wrong?

    [Response: So now you are arguing that global warming is being driven by a mysterious heat emanating from the bottom of ocean? Right… And perhaps you have an inability to click on links? Try this, and then apologise. – gavin]

  46. popoff:

    I´m reading carefully the article. First, I’m impressed by the big amount of data they have worked with. I’m not an expert in statistics, so my first thinking was similar to that in 6, which was answered yet. My intuition tells that the accuracy of proxies must be determined by comparing them with well known present temperature data so it will be continously developed and no doubt this article will be a reference one for those developments.

    Some critics have already begun, [edit]

    In fact, as a non-expert, but interested reader I’m just waiting for the different arguments, because I suppose it will be interesting to learn a lot about the, no doubt difficult and complicated, methodology.

  47. Steve McIntyre:

    #45. Gavin, look at the dates of the directory that you linked to in #45. This directory was not available when you made your original comment and was only placed online yesterday after criticism at Climate Audit. When you made your original comment, the online “data” included “infilling”. Your original statement was incorrect. You’re the one who should apologize.

    [Response: How about you think about it for a second? Why would I link to a non-existent directory? Here’s an alternative hypothesis: I put in my link when I’d verified that the data was there (which is not the time that the comment was received, but when it was approved). And you were so convinced I couldn’t possibly be correct you didn’t even check. Hence your comment (at September 4th, 2008 at 11:33 pm) accusing me of being ‘flat out wrong’ (echoed by Timo above) and ‘full of crap’ (September 5th, 2008 at 6:50 am), were in fact many hours after the data was there (ftp file date: 04-Sep-2008 15:14). Your continued and persistent accusations of bad faith based on nothing more than your personal prejudices and apparent paranoia are not particularly surprising, but you might want to consider what impression that leaves. Since you appear to want to be treated professionally, I would suggest you act accordingly. – gavin]

  48. Marcos Mattis:

    Are there any/enough studies that measure temperature from the end of the last glacial period roughly 8000 YBP [rather than from 2000 YBP]?

    How do temperatures in the last interglacial (circa 120,000 YPB) compare to now, agian are there any/enough studies to construct an accurate temperature record.?

    If we have ice cores [artic and antartic] showing the last few glacial periods, can we extrapolate global temperatures from the temperature swings at the poles seen during the last interglacials?

    My first post here so be gentle! Superb site but takes a hell of a lot of concentration to get through all the material!

  49. Timo Hämeranta:

    Re 47 Marco Mattis,

    please see

    Huang, Shaopeng, Henry N. Pollack, and Po-Yu Shen, 2008. A late Quaternary climate reconstruction based on borehole heat flux data, borehole temperature data, and the instrumental record. Geophys. Res. Lett., 35, L13703, doi:10.1029/2008GL034187, July 4, 2008, online

    Figure 1 presents surface temperature history over the past 20,000 years.

  50. Briso:

    I would like to congratulate the authors on such a magnum opus. A tremendous amount of work has been done.

    Now a really stupid question. It looks to me like the only lines which go above the early highs generated from proxy data (~960 AD) are the instrumental record data. Does this not show that the proxy data suggests warmer times in the past than during the more recent proxy period? Comparing that to instrumental data is apples and oranges, no?

    [Response: No. The proxies are calibrated to the instrumental target just so that they will be comparable. – gavin]

  51. Greg:

    Gavin, what is your opinion on the Finnish Lake sediment proxies? They certainly jump out when looking at the series used. Would you consider them quality temperature proxies?

    [Response: I have no particular opinion, but leaving them out (and a few other potentially problematic ones) was tested as described in the SI (p2 and fig S8). It makes a little difference, but nothing particularly striking. It’s worth pointing out that in these kinds of projects there are no guarantees – all you can do is attempt to find out how robust answers are to various issues (this is one, but so is the dependence on tree ring data, reconstruction methodology, etc, etc.) and seeing what decisions make a significant difference to the final answer. If they do, then obviously you are constrained about what can be concluded, while if they don’t matter, it’s not worth delving deeper. – gavin]

  52. Timo Hämeranta:

    Re 49

    for those who don’t know what multi-proxy reconstruction statisticians do:

    it’s subjectively to select from real-world paleoclimate raw proxy data suitable material to pseudo-proxy data used in climate models.

    Further reading e.g.

    Lee, Terry C.K., F.W. Zwiers, and M. Tsao, 2008. Evaluation of proxy-based millennial reconstruction methods. Climate Dynamics Vol. 31, No 2-3, pp. 263–281, August 2008

  53. Mark Zimmerman:

    In the graphs presented here, the proxies appear not to reproduce the 20th century temperature rise. Why is this? If the 20th century temperature rise is not reproduced by the proxies in the 20th century, why would we expect them to reproduce a temperature rise in the past similar to today’s, if such a temperature rise did occur?

    [Response: Please read the actual paper and look at the figures (particularly fig 2) and you will see a close up of what the 20th century calibration/validation looks like. There is a well known problem with tree-ring divergence (in the CPS method), but that is much less of a problem if the tree-rings aren’t used, and even less if one uses the EIV method. Thus if your criteria for acceptance is a good late 20th C trend, then I’d suggest focusing on the EIV method. However, in the absence of a good explanation of the modern divergence issue (which is not universal), it seems prudent to keep track of the differences in the two methodologies as a heuristic estimate of the structural uncertainty. – gavin]

  54. Timo Hämeranta:

    Re 50

    The authors of the study explicitly state

    “. Natural variability in the sediment record was disrupted by increased human impact in the catchment area at AD 1720.”

    Ref: Tiljander, Mia, Matti Saarnisto, Antti E. K. Ojala and Timo Saarinen, 2003. A 3000-year palaeoenvironmental record from annually laminated sediment of Lake Korttajärvi, central Finland. Boreas, Vol. 26, pp. 566–577. Oslo. ISSN 0300-9483, December 2003

    When the study was published I discussed with Mia Tiljander and she stated that human impact, especially land cover change, agriculture, accelerates erosion, more material flows to lake sediments, and thereafter the lakes are of no use in proxy reconstructions anymore, Korttajärvi from 1720 on, most of the other 180.000 Finnish lakes from 1850s on.

    [Response: That is, I presume, why Mann et al did a test that didn’t include them. – gavin]

  55. Bob Clipperton:

    Re #40, New scientist article on Ruddiman:-

    I read this article 2 days ago at my local library. Although I scanned it quickly, my initial conclusion was that Broecker thinks Ruddiman’s hypothesis (early agriculture responsible for much climate change) is weak, to put it politely.

  56. Dave Andrews:


    JM, I know nothing about viticulture but I assume, perhaps wrongly, that the grapes grown in the UK in the MWP were somewhat different to those grown now, today’s having been selected over time for resistance to cold/diseases etc. So the fact that vineyards are now able to be located in more northerly locations in Britain than was apparently the case in the MWP doesn’t necessarily relate to temperature at all.

  57. Iain:


    DA, Assuming todays grapes are, as you say (more) disease and cold resistant compared to those grown during the MWP then wouldn’t that indicate even warmer MWP temperatures?

  58. JamesG:

    Actually at Loch Maree in Northern Scotland they grow semi-tropical plants outside at Inverewe Gardens. All due to the gulf-stream, good cover and mild Winters apparently. So you can get pockets of temperate climate in unpromising places without it having to mean anything at all.

  59. popoff:

    [edit] Evidently most of the discussion is in comparison between present proxies and present instrumental measured temperatures. The other is in the contradictions that sometimes happens between proxies. If you have 2 and minus 2, the statistical medium is 0. But reality is 2 and minus 2.

  60. Mark Zimmerman:

    re post 53, I guess another way of phrasing my question would be why aren’t the proxies carried through the 20th century in the figures reproduced here?

    [Response: I think it just the plotting – download the reconstructions for yourself (here) and see what they look like in close up. – gavin]

  61. David B. Benson:

    Timo Hämeranta (49) — Thank you for the link. Highly revealing.

  62. Richard Sycamore:

    #49 is inactivism. #61 is ignorance. How nicely they go together. The cited paper incorrectly refers to the MCA as “MWP”, which is an outdated concept.

  63. John Mashey:

    re: #56, 57, 59

    Please take a look at the longer discussion at Hot Topics that I mentioned in #42, with the various caveats around the interpretations of the data. However, it really helps to have the actual book, with the detailed archaeological maps, to assess whether or not this is meaningful, or not. I’m looking forward to Selley’s 2nd Edition, as he says it has more temperature data.

    Regarding cold-climate grapes:
    1) I’m no oenologist or viticulture expert.
    I do live in Northern California, vineyards are in the hills near our house, Napa/Sonomoa are an easy day trip, and we spend a few weeks a year in the Okanagan, the wine area of British Columbia.

    2) However, despite the existence of various kinds of grapes, look again at Selley’s Expected grape regions in 2080.

    See that he shows that different kinds of grapes grow different places, and the zones move, at least in part by temperature. As far as I know (but if an expert knows something, please post), one might breed more cold-resistant grapes, but generally, grape *types* can be very geography/climate/soil/temperature limited. I glaze over when the vintners around here wax enthusiastic about the relatively growability of this grape type versus that type in areas 5 miles apart. {We have a lot of microclimates around here.]

    3) The area around Lake Okanagan in B.C. has become a pretty decent wine area, especially in the last few decades. We’re up there 2-3 weeks/year and always try the local wines. Even the WSJ wrote about the vintners there betting on milder climate (correctly).

    4) Anyway, I make no claim this is an authoritative proxy, but it is one where there is historical and archeaological information that actually goes back ~2000 years, in one area. The ebb and flow of wineries is an interesting indicator, because people always seem to try to grow grapes as Pole-ward as they can, even if the results are … not so good :-)

  64. David B. Benson:

    Richard Sycamore (62) — To relieve your ignorance and inactivism:

    “Palaeoclimatologists developing region-specific climate reconstructions of past centuries conventionally label their coldest interval as ‘LIA’ and their warmest interval as the ‘MWP’.” from

  65. Hank Roberts:

    tasty bait! google “brucew4yne aka John Holliday”

  66. Gareth:

    John Mashey (#63):

    The ebb and flow of wineries is an interesting indicator, because people always seem to try to grow grapes as Pole-ward as they can, even if the results are … not so good…

    There’s an old adage in the wine business: the best wines are made from grapes grown on warm slopes in cool climates. Winemakers in the newer growing regions have been searching for those sorts of sites. In France and Germany they found them a thousand years ago (or more).

  67. gerda:

    that obvious dip at 1350 is the black death surely?

    [Response: Surely? Why? Maybe the plague spread faster in cooler conditions? There have been some suggestions that reforestation after the outbreak could have had a climate effect, but it’s all very speculative. – gavin]

  68. Rob Huber:

    #67 –

    The world population in 1350 must have been around 500 million. To believe that 500 million pre-industrial humans had even a slight impact on the climate is just insane.

    [Response: Insane is too strong. Speculative is better. And there was a great deal of deforestation in medievel Europe and China. – gavin]

  69. David B. Benson:

    Also the impact of agriculture (including deforestration) in North America, Mesoamerica and South America.

    [Capthcha cryptically proclaims “and lacked”.]

  70. Rob Huber:


    I was trying to follow the discussion from #47, but I can’t find the referenced posts at September 4th, 2008 at 11:33 pm and September 5th, 2008 at 6:50 am. Are they under another topic?

  71. Hank Roberts:

    During the same time span:
    “… The population figure for the entire hemisphere [before contact from Europe] is thought to fall between 48 and 53 million…. no one disputes the sharp downward trajectory in population density …. Estimates of population decline following contact with the Europeans range from 70 to 90 percent and what happened has been called a ‘demographic catastrophe.'”

  72. John Mashey:

    re: #55 Bob
    Yes, I know Broecker’s earlier objections. I think Bill thinks he’s addressed those via the latest papers, but I don’t know if Broecker has come back with further comments in reply. I stopped in Stanford Bookstore today, but it still only had the previous issue of NS, so I’ll have to wait, unless someone can say how up to date the article is. I.e., does it use Ruddiman’s latest, and Broecker’s comments to that.

    Again, as noted, all this stuff is speculation/hypothesis, not strong theory. I really do recommend that people go read the various papers, because it illustrates *real* science as practiced by *real* scientists, where in fact serious experts take different viewpoints. Watching such in action [as opposed to after the dust has settled, however] is extremely educational, especially since:

    a) This one is actually reasonably accessible to the non-expert.
    b) One need read only a relatively small number of papers to keep up.
    c) It helps people learn the difference between real scientific arguments and fake ones.

    It makes no sense to argue the issue without reading the papers and the earlier commentary here.

    re: #66 grapes gareth
    I can believe that [warm slopes, cool climates, which also fits Selley’s North shores of lakes].

    My comment was perhaps unfair, but it derives from having spent my early years in PA/NJ, and for many years, thought that “wine” meant pink Catawba grown further North in NY :-).

  73. John Mashey:

    re: #68 Rob

    So, have you read the book, or any of the articles? Bill may or may not be right, but he has done some fairly detailed analyses of the number of hectares/person required at various levels of technology, plausible deforestration/reforestration rates, methane generation from rice paddies, and they don’t seem contradictory with other things I’ve seen, like Brian Fagan’s books.

    Reforestration rates are certainly compatible with my personal experience. Our family farm (PA) had a pasture cut from forest in the 1840s, and within 20 years of stopping farming,the forest had reclaimed it to the point you’d never think it had been anything else.

  74. Barton Paul Levenson:

    Hank writes:

    “… The population figure for the entire hemisphere [before contact from Europe] is thought to fall between 48 and 53 million…. no one disputes the sharp downward trajectory in population density …. Estimates of population decline following contact with the Europeans range from 70 to 90 percent and what happened has been called a ‘demographic catastrophe.’”

    Some people dispute the high figures, especially for North America, on the basis that hunter-gatherer economics can only sustain a very low population density. There certainly was a bad die-off from European contact, primarily due to the spread of smallpox, as I remember, but I think the 90% figure is controversial. Remember that that the Black Death only killed off 25% of Europe.

  75. dhogaza:

    Some people dispute the high figures, especially for North America, on the basis that hunter-gatherer economics can only sustain a very low population density

    Those hunter-gatherers who were growing corn and squash in new england in the 1600s? Or the hunter-gatherers living in the SW Pueblo culture?

    Our TV indians were all hunter-gatherer types, but the reality extends far beyond the stereotype.

  76. David B. Benson:

    My understanding is that about 90% of the population of the Americas secumed to the three waves of smallpox; they had essentially no immunity whatsoever.

  77. Barton Paul Levenson:

    Thank you, Dhogaza, I am of course aware that they weren’t all hunter-gatherers. I don’t think you can get up to several tens of millions with family-plot farming, either, especially since we know that most of the territory was in fact forest.

  78. Jim Eager:

    Barton, you seem to be thinking only about North America. Don’t forget about the large agricultural civilizations of Central and northern South America. Even in the North America there were the mound builders of the south/southeast.

  79. John Mashey:

    re: #74 BPL
    This isn’t like you…

    dhogaza gave a few examples, but:

    patuxet plague (of ~2000 natives, 1, Squanto, survived, because he wasn’t there. Almost 100% die-off.)
    squanto (who taught Pilgrims how to plant corn)
    mound builders (Hopewell was earlier, but it’s worth looking at)
    Tenochtitlan (~200,000 people in 13.5 km^2, in 1519, one of the largest cities in world, 5X bigger than London. “hunter-gatherers” :-); people guess 1M people in the Mexico Basin.
    Inca empire

    A few references:

    Jared Diamond, Guns, Germs and Steel … is a must.

    Tim Flannery, The Eternal Frontier- An Ecological History of North America and Its Peoples, 2001. Chapters 18-19. Also:

    p.289: “And the settlers were to become like Indians in other ways: nearly every colony established itself on the site of an old Indian village. They grew Indian crops such as corn and squash, and according to European observers they even started to farm like Indians.”

    p. 303: “In 1634 a violent epidemic of smallpox broke out among the Pequots living inland along the Connecticut River. ‘it pleased God,’ William Bradford wrote, ‘to visit these Indians with a great sickness and such a mortality that of a thousand, above nine and a half hundred of them died.” [Sounds like 95% die-off.]

    [Not too far North of you is a town called Bradford Woods; our family farm was next to that. I still have arrowheads. History of Pittsburgh is worth reading; especially track down the lifestyles of Adena and Hopewell cultures which preceded Europeans by 1000+ years. I.e., of course they hunted & fished (they lived in PA, after all!) and farmed.

    p.304 “The Mandan tribe, which lived along the upper Missouri River, was blighted by smallpox in the fall of 1837…. cost the lives of 1569 out of 1600 Mandan.” [That’s ~98%. of course, people argue abotu whether this was accidental or on purpose.]

    p.305 “But which-the guns, the germs, or the steel-was the most important?”

    The answer, Diamond has no doubt, is germs. Although estimates vary widely, by 1492 the human population of the Americas may have reached 57 million, of whom 21 million lived in Mesoamerica. Eighty years later it had shrunk to 18 million….indeed, it is thought that disease had wiped out 90 percent of the people living in Mesoamerica, Peru, and the Caribbean by 1568.” [57->18 ==> ~70% die-off in 80 years, if that’s accurate.]

    The likely truth, when Europeans arrived:
    a) *Most* native Americans lived in societies that farmed at least some of the time. Given corn (teosinte)origin in Mexico & south, if they were growing corn around Boston (and they were), it’s hard to believe there weren’t extensive agricultural communities in the much-easier farming areas all over the Mississippi river basin.

    b) Given that native Americans didn’t have cows, sheep, goats, etc [see Diamond], many still hunted/fished, but people living in large villages who raise crops, but also hunt/fish are a far cry from “hunter-gatherers”. [And they were: look up “mound builders”.]

    c) California had a bunch of tribes who gathered acorns in preference to raising crops.

    d) Unlike Europeans, native Americans didn’t keep the domestic animals that act as disease reservoirs, and had almost no immunity to the diseases when they came.

    I’m no expert on the population estimates, and the experts do argue, but many of the more current estimates are much higher than they were, say 20 years ago, because the older estimates simply didn’t factor in the decimation that went on. By the time Europeans got to some areas, the natives were dead and forest had regrown. Of course, it’s way more comfortable to think that Europeans settled an empty land sparsely populated by a few hunter-gatherers…

    Given the difficulty of estimates, none of this *proves* there were 50M+ pre-Columbian natives.

    But for sure, the evidence is overpowering that many natives did agriculture (whether by planting crops or gathering acorns), and hunting/fishing, based in well-established settlements.

  80. David B. Benson:

    Furthermore, in many parts of at least North AMerica, they routinely burned the forests even if they didn’t practice planting; the Californians did so to preferentially promote the nut bearing trees. “The Ancient Forest” expllains this for the lower 48 in detail.

  81. David B. Benson:

    Also, deforestation in the Amazon, for example “‘Pristine’ Amazonian Region Hosted Large, Urban Civilization”:

  82. Hank Roberts:

    Barton, you really need to read this, horrifying as it is to know. Look at the link I gave you; that gives the range of estimates, and I didn’t pick an extreme.

    The “Eden” the whites described was rich because it was farmed, or morelike gardened. Not using horses, not with plowed straight lines, not with Medieval land use patterns, not recognized as such by those who marched through it and took it, telling themselves it was untouched wilderness, just happened to be incredibly rich with things people eat and use — and yet almost empty of people. They’d already died. The diseases moved ahead of the Europeans.

    There was plenty of human agricultural activity, and it ceased when the diseases crossed the Americas.

  83. dhogaza:

    The Willamette Valley in Oregon was routinely burned, too. That’s why it was mostly cleared by the time the first white settlers arrived to farm. In fact, that’s one reason why it looked like such fantastic farm land to them (and, they were right).

  84. John Mashey:

    Ruddiman, 2007 writes:

    “By 1500 an estimated 55 million people lived in the Americas, mostly in naturally forested areas [Denevan, 1992]. The per capita amount of land in cultivation or in fallow was 0.4–1.4 ha/person [Hurt, 1987; Krech, 1999], equivalent to the estimate by Gregg [1988] for Neolithic Europeans. Assuming an average footprint of 0.9 ha, the 55 million people would have cleared 0.5 million km2 and emitted 6.5 Gt C (at an average carbon density of 13 Gt/km2). [153] This estimate, however, omits an important consideration: fire. In contrast to Neolithic Europeans most indigenous Americans kept no livestock (except for Andean llamas and alpacas). Instead, to supplement their needs for nutrition, they repeatedly burned vegetation to maintain grassland, attract game, and promote growth of berries and other foods. The additional ‘‘clearance’’ by burning, generally thought by paleoecologists to have been extensive, would have added considerably to the footprint based only on land under cultivation. [154] On the basis of the added clearance by burning the human forest footprint in the Americas is (arbitrarily) assumed to have been 2 ha, compared to 3 ha in Neolithic Europe. For a population of 55 million the total amount of deforestation just before European arrival would have been 1.1 million km2. Faust et al. [2006] arrived at a nearly identical value. At a mean carbon density of 13 Gt C per million km2 [Houghton, 1999], total deforestation by 1500 would have produced 14 Gt C.”

    So, that’s a rough estimate of the C released by deforestration.

    For the reforestration/sequestration, Table 7 compares the various cases, giving: area, mortality (millions), Tons/sq km, Total Gt Carbon:

    Americas (1500-1750)
    North Am: 3.5M, 1630T, 1.1 Gt, deciduous
    Mexico: 15M, 800T, 2.4 Gt, seasonal/dry
    Amazon: 15M, 2190T, 6.6 Gt, tropical wet
    Andes: 14M, 1300T, 3.7 Gt, montaine
    Total: 47.5, …, ~13.8 Gt

    That compares to an estimate of ~8.2Gt of carbon sequestration for Black Death (1350-1450) and ~7.7 Gt from Late Roman pandemics (200-600).

    Ruddiman has pages of analysis talking about China issues, whether specific areas would have gotten refarmed quickly (some did, some didn’t), etc, ending that part with:

    “In summary, pandemic-driven reductions in atmospheric CO2 can explain half or more of the 7-ppm drop between 1200 and 1700. Depending on the highly uncertain size of the global mean cooling during this interval, this anthropogenic forcing could account for anywhere between 16% and 66% of the total cooling. In view of the uncertainties in pandemic mortality and in the size of human forest footprints, however, the simulations in Figure 21a should be viewed as a demonstration of the first-order plausibility of the pandemic CO2 hypothesis rather than as a detailed simulation of actual changes.”

    Like he says, a demo of first-order plausibility… Unlike statements we hear like “Effect X is caused by Y”, we get “It looks like 16-66% of effect X could be caused by Y”, with large amounts of careful detail to create plausible estimates and error bars.

    Main message: if you’re interested in this topic, go read that 37-page paper. I’m no expert to know if it’s right, but it certainly looks like careful work, real science in progress of chewing on hypotheses.

  85. Barton Paul Levenson:

    Barton, you really need to read this, horrifying as it is to know.

    What makes you think I haven’t?

    I’m astonished by the attitude that if I just studied the subject, I’d change my mind, and that I don’t know anything about the Central and South American empires or the smallpox epidemics. For the record, I’m not disputing

    A) that there were lots of people in the New World
    B) that the Europeans introduced smallpox, or
    C) that the diseases resulted in a big human die-off.

    I’m simply and solely disagreeing with the upper-end numbers quoted.

    I’m waiting for someone to quote Stannard’s figure of 100 million dead, which is probably about four times the actual population of the New World back then.

    I’d also like someone to give me an example of a continent-wide plague other than the American one which killed off 90% of the population. The Black Death in Justinian’s time? The Black Death in the 14th century? Why is it that this kind of die-off happened only in America?

    Estimates of the precolumbian population of American range from 8 to 145 million. And as one demographer put it, all of them are “completely unscientific.” We just don’t know.

  86. Barton Paul Levenson:

    Dhogaza writes:

    The Willamette Valley in Oregon was routinely burned, too. That’s why it was mostly cleared by the time the first white settlers arrived to farm. In fact, that’s one reason why it looked like such fantastic farm land to them (and, they were right).

    Ah. They know the local Indian population was wiped out there because they didn’t find any evidence of them — the wiping-out had been too efficient.

  87. gerda:

    74;Barton Paul Levenson:

    “Remember that that the Black Death only killed off 25% of Europe.”

    are you sure? i was taught at school pop was reduced to half, and this says 30% to 60%

    25% was the settlements abandoned, there are rings of them like on a bathtub in some valleys up north.

  88. Kevin McKinney:

    Regarding the whole sub-thread about AmerIndian mortality, I tried Googling “Huron mortality agriculture,” since no-one had mentioned the Huron/Iroquoian nations–both of which groups are well-known as agricultural. (Slash & burn cultivation of corn.) Evidence is indeed sparse, though I did find some support for the 85-90% mortality figure. But not much help with the central question we have in hand, which is, what was the climatic impact of pre-industrial humanity? (BTW, the desertification of the Sahara has occurred since Roman times–what is known about this process, esp, human contributions?)

    However, another point about the AmerIndian experience arose which is perhaps relevant to our larger concern–the effects of AGW on humanity’s future. This point is that the effects of European contact were often synergistic. In the case of the Huron/Iroquioan conflict, for example, it is believed that the effects of disease, the increased lethality of weaponry improved by European technology, and intertribal warfare (also entangled with European colonial rivalries) were mutually reinforcing. And in a more specific case I found discussed, a group of Brazilian Kayapo were wiped out over period of a few decades; one significant episode was a measles epidemic which killed 34% of *vaccinated* individuals, including all but two adults over 40. This latter was decisive, because cultural transmission was predominantly grandparent to grandchild; thus social cohesion was severely undermined, and the ground was prepared for much indirect mortality via conflict, dispersal, and loss of technical information (in the sense of agricultural, hunting, food-handling, and other cultural techniques.)

    It would certainly appear that *our* social cohesion will be at severe risk with the various predicted physical effects of AGW. “If the sea rise doesn’t get you, the food riots will.” (Not trying to be “alarmist,” just an illustrative comment!) Not for nothing has the U.S. Defense Department begun to evaluate AGW as a security threat. And of course, conflict will make it much harder to deal with the problem: the conflict in Iraq, for one obvious example, has sucked up massive amounts of resources and nearly monopolized years of attention, as well as complicating efforts at cooperation in many other areas. Future conflicts will certainly have parallel effects, presumably at times when we can afford it still less than at present.

    (Spooky Captcha: “choose the”)

  89. Kevin McKinney:

    Slight clarification: “all but two adults over 40” would be clearer had I written “all but two *of the* adults over 40.”

  90. dhogaza:

    Ah. They know the local Indian population was wiped out there because they didn’t find any evidence of them — the wiping-out had been too efficient.

    In your overeager attempt to humiliate me, you seem to have failed to notice that I never said the early settlers in the Willamette Valley didn’t find any natives living there.

  91. Muff Potter:

    Hi Gavin & friends,

    this is slightly OT, but could someone of you (maybe Stefan) have a look at this scholarly looking (german) website: . The guy gives in his Holocene time table some rather high values for temperatures for “Roman Optimum” (“Roemisches Optimum”), MWP and LIA. Numbers are 4K, 1 to 1.5 K and -2K resp., for the periods, globaly on average. Rather high I’d say. He presents a lot of informations and it’s unclear to me if he is confused about the numbers, or it’s denialist’s stuff. Anyway, his website is cited by denialist’s so maybe someone could have a look and maybe comment on it.

  92. Hank Roberts:

    > Why is it that this kind of [contiental] die-off happened only in America?

    No other continent had been geographically isolated for that long and had the population without immunity and the population density for efficient transmission of smallpox when it was introduced. You need all three.

    But note the epidemics introduced into many geographically isolated areas, in many species.

  93. Hank Roberts:

    > didn’t find any evidence

    You can look it up.,M1

  94. Hank Roberts:

    Kevin McKinney asked what caused the Sahara:

    A good recent starting point would be this article, its footnotes, and citing papers. It reminds me of Gavin’s point after returning from China a while back about the need for more cross-correlation among people working on paleo core drills. Look at the earlier literature on the Sahara and you’ll find this is quite an update, I think, on what used to be thought.

    Excerpt below is from the accompanying Podcast interview — see main link above.:

    Interviewer – Robert Frederick
    So what did your team look at in order to describe how the Sahara region evolved over
    the past 6,000 years?

    Interviewee – Stefan Kröpelin
    There’s two ways. I mean, one approach we did – it was in a Science paper two years ago where we went into the big surfaces – that means we checked an area of, of much more than 1 million square kilometers for prehistoric sites. And we were locating them and dating them, and this gave us a quite nice picture on the evolution of the eastern Sahara, over the time. But there is practically no more information about the past 3,000 years because even all the humans had left because there was no more rain. So, the only other way is to look into geological deposits. And the most important climate and environment archives are lake deposits. And the good thing about the deposits of Lake Yoa in Ounianga in the extreme northeastern part of the Republic of Chad is that there are not only deposits deposited, accumulated to the present day – that means while we are speaking there are still some thin deposits of the present day sediments and pollen and other indicators. But, the most important thing is that they are vast – that means they are annually or even subannually eliminated, which gives very, very detailed information about environmental and climatic change up to the present day.

    Interviewer – Robert Frederick
    Do these indicators at all contradict one another as to the condition at this Lake Yoa?

    Interviewee – Stefan Kröpelin
    Yes, the data, in direct opposition to interpretations, taken from the ocean cores of the western Saharan coast. And these data were interpreted as an arid abrupt change to very arid conditions, which were supposed to have concerned all of the Saharan desert. And this was always not very convincing to people who work in the Sahara because the people from the marine community, of course, they don’t know the terrestrial situation on the continent. So, there was always a suspicion that they might be wrong in their interpretation because they were only using data which they found in marine cores, which so far could not be compared to any data from the continents. And now we are lucky to have this permanent core at a extremely high temporal resolution, well the best you could wish for as a paleoclimatologist. And these data are in clear contrast to the destruction or to the claim of the hypothesis of an abrupt change of the humid period, which was called by some authors African Humid Period. And a change to arid conditions as they are today. It was a rather gradual change, which took some time, and which was very slowly, that means the Sahara turned green around 12,000 years ago. But then, after it had passed the peak of humidity, then it was very slowly becoming a desert today – and this is a gradual process which keeps on to the present day….


  95. Jim Eager:

    Re Barton @ 85: “Why is it that this kind of die-off happened only in America?”

    Because the Amerindian population was never exposed to the domesticated animal vectors that harboured the diseases they were exposed to when the Europeans arrived, and thus had developed no natural immunity through natural selection and adaptation. No oxen, no cattle, no horses, no sheep, no swine, no chickens, and no Asian rats and their fleas, and thus not the pathogens that those animals hosted.

    Ominous captcha: fighting soon

  96. Hank Roberts:

    Ice age climate and solar variability, long term

  97. Dave Clarke:

    #47 and 70:

    Gavin’s response appears to reference two of Steve McIntyre’s posts at his blog, not here at RC. I actually participated in that thread and tried to patiently explain that Steve had almost certainly gotten it wrong, and owed Gavin an apology (a couple of others expressed the same opinion).

    It all reminds me of what my high school math teacher would say when confronted with lack of comprehension of the obvious: “Eyes that see not.”

  98. David B. Benson:

    Hank Roberts (94) — Wonderful find, thank you.

    I’m now moderately certain that precession, controlling the thermal equator, is the major influence on the latitude of the ITCZ. (Obviously there are other influences, since in many areas the ITCZ isn’t on the thrmal equator.)

    Now for the long-range predictions: since precession is moving the thermal equator to the north, and has been for about 2000 years, the southern portions of the Sahara will revert to (largely) savanna once again; so will the Amazon Basin. I’m much more confident of the latter; Dr. Jose Mendoza writes so and it seems there was a paper at last winter’s AGU meeting along these lines; the last time the precessional cycle brought the thermal equator to the north the Amazon Basin was mostly savannah and also the time before that.

    Now there is nothing to do but adapt to this. However, it probably will take about 5-6 thousand years, so there is plenty of time. That said, possibly anybody attempting to do a regional climate forecast for 2200 CE might do well to take this precessional effect into account.

    Hope I’ve correctly represented this effect.

    [Cartcha states “of hoping.”]

  99. Nathan Kurz:

    Dave #97,

    Apart from the question of who needs to apologize, I’m not sure that anyone is disagreeing about the facts, just the apportionment of blame. Is this your understanding of the time line?

    1. An original version of the data was posted that included some degree of infilling. Steve McIntyre downloaded the data, and griped publicly about the infilling.
    2. At some point after this (04-Sep-2008 15:14, likely UTC, but definitely after McIntyre’s initial download), Gavin provided a link to the newly availableraw data at the SI site.
    3. That night, McIntyre posts a vituperative comment asserting that the data is available only in an infilled form. At the time of the comment, this is false, as the raw data has become available since his download.
    4. The next morning, McIntyre posts a slightly more polite comment #45 here repeating his outdated accusations, to which Gavin responds inline suggesting a timeline of events consistent with this one.

    My read is that McIntyre was caught unaware when the data was changed at some point day, and Gavin is offended by the accusations that anything he posted was false at the time it was posted. Does this agree with your interpretation of events?

  100. Dave Clarke:

    #97 Nathan
    I agree that the timelines are pretty much as you and Gavin described them, subject to the comments below (edited from previous posts at ClimateAudit).

    It well may be that the raw proxies only appeared after objections from Steve McIntyre and others. But, of course, that still does not support the specific allegation that was made against Gavin Schmidt by Steve McIntyre, and nor does it excuse the vituperative attacks that are all too common at CA.

    Now it’s clear that there was some sort of problem with the first Sept. 4 “originalallproxy” archive, and so it was updated on Sept. 5. My personal opinion is that these kinds of issues should be documented in a directory “readme” and resolved if necessary by respectful, professional dialogue between investigators.

    The continual unwarranted presumption at CA of bad faith, expressed publicly in unprofessional and abusive language, is a severe impediment to meaningful dialogue about these kinds of issues, or indeed any scientific issues.

  101. Kevin McKinney:

    Ask (Hank) and ye shall receive. A different picture than the last one I encountered–thank you!

  102. for4zim:

    #91 Muff Potter: Kehl is a well known denialist, and his pages are a prefered choice for his alike, because of the appearance of a serious collection of sources and data. A closer look reveales, that the collection is very one-sided (“Mann’s hockey-stick was found to be false”, “Moberg et al and Loehle provide a correct impression”)

  103. Nathan Kurz:

    #100 Dave

    Thanks for your responses, Dave. I agree that a presumption of bad faith is an almost insurmountable impediment, and one needs to remove this obstacle if one hopes to start communicating again.

    Gavin, I realize it’s not your responsibility to patrol the skeptic hordes, but could you offer a quick summary of how the data set has been updated and where these changes are recorded? Is there a “readme” file somewhere of the sort that Dave refers to? I think (hope?) that McIntyre would happily “move on” and apologize after a clear statement that you were acting in good faith. It’s sad that it’s necessary to make such statements, but I think it is worth it if it helps people to concentrate on the science rather than the accusations.

    [Response: What is the point? The presumption will be that I’ve just made something up and even if I didn’t, I’m a bad person in any case. I have no interest in communicating with people whose first and only instinct is to impugn my motives and honesty the minute they can’t work something out (and this goes back a long way). Well, tough. You guys worked it out already, and I have absolutely nothing to add. If McIntyre was half the gentleman he claimed to be, we’d all be twice as happy. – gavin]

  104. John Mashey:

    re: 103 Nathan

    Are you familiar with the Data Quality Act, and what it was really intended to do?

    Put another way, there’s a line between asking questions, poking at data, looking for code, to do science as normal science …

    and engaging in activities designed to use up scientists’ time so they *can’t* do science. The data Quality Act falls in the latter… that’s what it was for, and certain people follow that strategy as well.

    [Chris Mooney’s “The Republican War on Science” and David Michaels’ “Doubt is Their Product” are good sources on the DQA.

  105. Hank Roberts:

    Those who believe in demons _do_ see them them. Look at the result — stuff like the Data Quality Act:

  106. Philip Machanick:

    Several people want to know if any other continent suffered a similar indigenous die-off to the Americas (or claimed that it happened nowhere else).

    Australia did, with estimates of at least 50% being wiped out by smallpox and other imported causes. The actual numbers are not accurately known, with a wide range (see e.g. WikiPedia).

    There were also select indigenous groups in South Africa that were almost wiped out by smallpox.

    These were all relatively isolated populations without exposure to domestic animals that may have co-evolved diseases and immunities with people.

  107. RichardC:

    I’d like to see two things – first, a summation graph with perhaps 95% error ranges (ie, a three-line graph), and second, the same thing for the entire planet. Anyone have anything like that?

  108. Barton Paul Levenson:

    Philip, I didn’t say die-offs due to smallpox didn’t happen anywhere else. Kindly don’t put words in my mouth. What I said was that the 90% figure strikes me as very unlikely — and your citing 50% elsewhere does nothing to contradict that; it fact it strengthens my argument.

  109. Mango:

    Any chance you could update your article on the Hockey Stick for Dummies to include the latest findings?

  110. dhogaza:

    Philip, I didn’t say die-offs due to smallpox didn’t happen anywhere else. Kindly don’t put words in my mouth. What I said was that the 90% figure strikes me as very unlikely — and your citing 50% elsewhere does nothing to contradict that; it fact it strengthens my argument.

    Not really. In most the US, native population densities were higher than in Australia, which has a much higher percentage of desert than the US. I base this on the fact that desert population densities of native americans here in the US were much lower than elsewhere, and I see no reason for it to be have been different in aboriginal Australia (indeed, it’s still true today, in both countries).

    Lower population densities make it more difficult for disease to spread.

    These were all relatively isolated populations without exposure to domestic animals that may have co-evolved diseases and immunities with people.

    Regarding smallpox, I think one can replace “may have” with “must have”. After all, the world’s first vaccine came about because an astute englishman noticed that dairy farmers (or whatever you want to call them) had a lower incidence of smallpox than typical populations, and connected this with exposure to cowpox. Intentional vaccination with live cowpox was then introduced to successfully combat smallpox.

  111. Manny, in Moncton:

    Did you censor my question?

    Three days ago, I asked why I could not reproduce the CRU red line with the data from the CRU itself. Where is it?

    I have another question: how can the red line, smoothed over 40 years, reach 2006? Shouldn’t it stop at 1986?

    [Response: Your question was answered way above (the target was CRU NH Land) and how the smoothing was done was explained in the paper (see Mann (2008)). – gavin]

  112. R James:

    I’d like to see this graph with an additional 2,000 years of history on it. I’d also like to see a global plot, rather than just the northern hemisphere. As it stands, it doesn’t show the complete picture. I believe it would put things better into perspective, and further show that the current pattern is nothing unusual.

    [Response: The global picture is very similar, but with a little more noise due to less data availability in the south, and we’d all like to see another 2000 years – unfortunately the analogous data is just too sparse. – gavin]

  113. Philip Machanick:

    Barton I don’t know why you think I’m attacking you. I was just adding in some additional data.

    The 50% figure I quoted is an absolute minimum. The more likely range is significantly higher but I only have time to look this up at WikiPedia, where the article is currently a mess. There is evidence that the Aboriginal die-off in Australia could have been as high as 90%. Stats from that era are poor, as aboriginal Australians people were official fauna until the 1960s (if you can believe that). We are pretty sure of such numbers from isolated populations like the Khoi people in South Africa who were similarly isolated from diseases that had migrated to Eurasians from domesticated animals, so it is plausible that it could happen on a continental scale.

  114. dp:

    Interesting about that drop around 1350. Scientists should look ar historical records. They state that temperature drops around that caused famines and left the population of Europe weakened when the Black Death struck. In England the fall in temperatures at the time are attested to in monastic chronicles.

  115. llewelly:

    Interesting about that drop around 1350. Scientists should look ar historical records. They state that temperature drops around that caused famines and left the population of Europe weakened when the Black Death struck. In England the fall in temperatures at the time are attested to in monastic chronicles.

    Brian Fagan covers this in two of his books – I think The Long Summer: How Climate Changed Civilization. and The Little Ice Age: How Climate Made History .
    Both highly recommended, wonderful discourses on the connections between climate and history.

  116. M. Potter:

    Since the spaghettis start at about 200 A.D, what about the time period sometimes called “Roman Optimum” (200 B.C. to A.D. 400)? Any chance to get upper and lower bounds for global or NH temperature? And I dare to ask, what about the so called Holocene climatic optimum (9000-5000BC) ?

    [Response: These are collections of data that is either annually or decadally resolved and well-dated. Unfortunately that kind of data gets rarer as you go back in time and so whether the Roman Optimum is real and what it’s spatial extent was will remain uncertain for some while. On longer time scales (multi-millennia) such dating accuracy isn’t as important and so coarser and less well-dated proxies are useful. There is a review paper on exactly this from Heinz Wanner and colleagues in press at the moment. – gavin]

  117. Briso:

    >Now a really stupid question. It looks to me like the only lines which go above the early highs generated from proxy data (~960 AD) are the instrumental record data. Does this not show that the proxy data suggests warmer times in the past than during the more recent proxy period? Comparing that to instrumental data is apples and oranges, no?

    >>[Response: No. The proxies are calibrated to the instrumental target just so that they will be comparable. – gavin]

    I’ve been looking at the paper again and trying to understand it. First, an important quote in the context of the AGW issue.

    “Because this conclusion extends to the past 1,300 years for EIV reconstructions withholding all tree-ring data, and because non-tree-ring proxy records are generally treated in the literature as being free of limitations in recording millennial scale variability(11), the conclusion that recent NH warmth likely** exceeds that of at least the past 1,300 years thus appears reasonably robust. For the CPS (EIV) reconstructions, the instrumental warmth breaches the upper 95% confidence limits of the reconstructions beginning with the decade centered at 1997 (2001).”

    Further down on the same page (italics added by me):
    “Peak Medieval warmth (from roughly A.D. 950-1100) is more pronounced in the EIV reconstructions (particularly for the landonly reconstruction) than in the CPS reconstructions (Fig. 3). The EIV land-only reconstruction, in fact, indicates markedly more sustained periods of warmer NH land temperatures from A.D. 700 to the mid-fifteenth century than previous published reconstructions. Peak multidecadal warmth centered at A.D. 960 (representing average conditions over A.D. 940–980) in this case corresponds approximately to 1980 levels (representing average conditions over 1960–2000). However, as noted earlier, the most recent decadal warmth exceeds the peak reconstructed decadal warmth, taking into account the uncertainties in the reconstructions.”

    OK, some questions.

    1. Does the EIV reconstruction represent a forty year moving average as suggested by the part I italicized?

    2. Does the instrumental record shown on the graph in Fig 3 represent a forty-year moving average? I say no, because such a plot would have an end point in 1987. It looks like a five year moving average perhaps?

    3. It is true that the upper 95% confidence level of the peak warmth centered at A.D.960 of the EIV land-only reconstruction is approximately 0.4. I assume that this means that peak of the five year average temperature at that time would have been considerably higher?

    4. Is it not true that whatever the red line in figure 3 is, it is an apple being compared to a pear?

    5. “Peak multidecadal warmth centered at A.D. 960 (representing average conditions over A.D. 940–980) in this case corresponds approximately to 1980 levels (representing average conditions over 1960–2000).” Corresponds approximately? Shouldn’t that be exceeds significantly? If my figures are right, 1960-2000 HadCrut NH 40 year average – 0.06 (98-08 app 0.17), 960 PMW central – app 0.25, 960 PMW upper 95% – 0.4?

    6. Does this paper really show that “recent NH warmth likely** exceeds that of at least the past 1,300 years”?

  118. Briso:

    In point 5 of my previous post I should have written “(68-08 app 0.17)”. Sorry about that.

  119. Barton Paul Levenson:

    Briso writes:

    Does this paper really show that “recent NH warmth likely** exceeds that of at least the past 1,300 years”?