RealClimate logo


Technical Note: Sorry for any recent performance issues. We are working on it.

Progress in reconstructing climate in recent millennia

Filed under: — gavin @ 3 September 2008 - (Español)

What makes science different from politics?

That’s not the start of a joke, but it is a good jumping off point for a discussion of the latest publication on paleo-reconstructions of the last couple of millennia. As has been relatively widely reported, Mike Mann and colleagues (including Ray Bradley and Malcolm Hughes) have a new paper out in PNAS with an update of their previous work. And this is where the question posed above comes in: the difference is that with time scientists can actually make progress on problems, they don’t just get stuck in an endless back and forth of the same talking points.


We discussed what would be required in an update of these millennial reconstructions a few months back and the main principles remain true now. You need proxies that are a) well-dated, b) have some fidelity to a climate variable of interest, c) have been calibrated to those variable(s), d) that are then composited together somehow, and e) that the composite has been validated against the instrumental record.

The number of well-dated proxies used in the latest paper is significantly greater than what was available a decade ago: 1209 back to 1800; 460 back to 1600; 59 back to 1000 AD; 36 back to 500 AD and 19 back to 1 BC (all data and code is available here). This is compared with 400 or so in MBH99, of which only 14 went back to 1000 AD. The increase in data availability is a pretty remarkable testament to the increased attention that the paleo-community has started to pay to the recent past – in part, no doubt, because of the higher profile this kind of reconstruction has achieved. The individual data-gatherers involved should be applauded by all.

The increase in proxy records allows a whole bunch of new things to be done. First off, the importance of tree rings can be tested more robustly. With the original MBH98 proxies, there was only enough other data to go back to 1760 if you left out the tree rings. The match was pretty good over multi-decadal periods, but the interannual variability was much larger without tree-rings. Now though, the Northern hemisphere land temperature reconstructions without tree rings can go back to 1500 AD or 1000 AD depending on which of two methodologies are used. For the NH land and ocean target, it’s even possible to get a coherent non-tree ring reconstruction back to 700 AD! As before, there are some differences (notably in the 17th Century where the tree rings indicate colder temperatures), but the recent warming is anomalous regardless.

Secondly, you can screen records and pick targets more finely: do you want only records that match local temperatures? Done. You want to get a handle on global and southern hemisphere means as well as the northern hemisphere? Done. Other screens could easily be implemented.

The two methodologies used themselves span the range of different approaches that people have used. ‘Composite and scale’ (CPS) is perhaps the simplest method – it is basically an average of all the temperature proxies scaled to the target time series. The other method is denoted ‘Error in variables’ (EIV) in this paper, but is really a simplified application of the RegEM climate field reconstruction method used in a couple of more recent papers. It is essentially a fancy multiple regression to the target time series that can incorporate non-local proxies as well. The point of using two methods is to demonstrate what is, and what is not, robust, and to give an idea of what the structural uncertainty in these estimates is – something not easily calculated using standard statistics. That uncertainty is clearly larger as you go back in time, and larger still for the southern hemisphere.

Other improvements over previous work are that more proxy data sets go past 1980, and so calibration up to 1995 is possible. That allows more of the recent trends to feed into the calibration and highlights the so-called divergence problem in some (but not all) recent tree-ring records. That divergence is significantly lessened without tree-rings or using the EIV method.


Figure: Spaghetti plot of the new reconstructions over a) 1800 and b) 1000 years
along with selected older ones for comparison.

So what does it all mean? First off, this paper (like MBH98 before it) is not an attribution study. That means that the reasons for any of the ups-and-downs in the records are not demonstrated by these papers alone. Attribution of the recent trends (as discussed in IPCC AR4) to anthropogenic effects has mostly focussed on the last 150 years and did not use any paleo-data. Nonetheless, there have been a couple of key studies that have used this kind of data along with simple energy balance models (Crowley, 2000; Hegerl et al, 2006 for instance) and it will be interesting to see if this new reconstruction will make any difference to their conclusions.

Secondly, in comparison with previous reconstructions, the current analysis does not provide many surprises. Medieval times are warmer than the Little Ice Age as before, and a little warmer using the EIV method than was the case in MBH99. The differences in the 11th Century are on the order of a couple of tenths of a degree – well within the published error bars in IPCC TAR though. Interestingly, there are quite rapid and strong drops in temperature near 1100 AD and around 1350 AD which may make interesting case studies for attribution to solar or volcanic forcings in future. Overall, there are a few more wiggles than before, but basically nothing much has changed. (Though one should always be aware of the maxim that one person’s noise is another person’s signal).

Finally, while the headline numbers ‘likely warmest since XXXX’ are of some contextual value, they aren’t the real point of this kind of study. Most of the interesting work – looking for patterns associated with solar forcing say – will start when the spatial patterns of temperature change start to be discerned – and that is still a work in progress.

So, onto the inevitable discussion! One test of whether that discussion is more political than scientific will be the extent to which people acknowledge the progress that has been made. Repetitions of tired and oft-debunked one-liners will be telling!


119 Responses to “Progress in reconstructing climate in recent millennia”

  1. 1
    Spencer says:

    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. 2
    coby says:

    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. 3
    Chris Colose says:

    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. 4

    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. 5
    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? Here is a rather lengthy essay on this topic (beware, the link is split on two lines):

    IS EARTH STILL RECOVERING FROM THE LITTLE ICE AGE?
    Syun Akasofu
    http://people.iarc.uaf.edu/~sakasofu/pdf/Earth_recovering_from_LIA_R.pdf

    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. 6
    PHE says:

    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. 7
    coby says:

    Hi David Garen,

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

  8. 8
    Mauri Pelto says:

    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. 9
    Alexandre says:

    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. 10
    Steve Reynolds says:

    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. 11
    Blair Dowden says:

    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. 12

    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. 13
    John Mashey says:

    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. 14
    Andy says:

    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. 15
    John Dodds says:

    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. 16
    Danny Bloom says:

    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
    http://www.rushprnews.com/2008/09/03/wake-up-we-are-on-the-brink-of-extinction-says-ecoactivist

  17. 17

    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. 18
    David W says:

    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. 19
    Ethan says:

    “…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. 20
    Matt says:

    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. 21
    Andrew Davison says:

    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. 22
    Timothy says:

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

  23. 23
    Mark Zimmerman says:

    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. 24
    Lawrence Brown says:

    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. 25

    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. 26
    Alexandre says:

    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. 27
    Timo Hämeranta says:

    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 http://www.pmel.noaa.gov/people/gjohnson/gcj_3m.pdf

    “…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 http://www.cdc.noaa.gov/people/gilbert.p.compo/CompoSardeshmukh2007a.pdf

    “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. 28
    John Mashey says:

    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. 29
    Dave Clarke says:

    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 5x5 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. 30
    BillS says:

    Re: #25

    You might have a look at cdiac.ornl.gov/trends/atm_meth/lawdome_meth.html and some of the papers cited there.

  31. 31
    Hank Roberts says:

    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:

    http://scholar.google.com/scholar?hl=en&lr=&safe=off&q=%22ice+cores%22+%2Bconcentration+%2Bincreases+%2Bmethane+&btnG=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 – sciencemag.org

  32. 32
    David Garen says:

    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. 33

    …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. 34
    Hank Roberts says:

    > go through the article archives

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

  35. 35
    David B. Benson says:

    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

    http://news.softpedia.com/news/Fast-Melting-Glaciers-Expose-7-000-Years-Old-Fossil-Forest-69719.shtml
    http://news.bbc.co.uk/2/hi/science/nature/7580294.stm

    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. 36
    JamesG says:

    “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. 37
    Richard Sycamore says:

    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. 38

    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. 39
    John Mashey says:

    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. 40
    tidal says:

    @ 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:
    http://environment.newscientist.com/channel/earth/mg19926721.600-the-ice-age-that-never-was.html

  41. 41
    Lab Lemming says:

    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. 42
    John Mashey says:

    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. 43
    John Mashey says:

    #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. 44
    Chris Crawford says:

    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. 45
    Timo Hämeranta says:

    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 http://ocean.mit.edu/~cwunsch/papersonline/present_pastocean_rev_2april.pdf

    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. 46
    popoff says:

    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. 47

    #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. 48
    Marcos Mattis says:

    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. 49
    Timo Hämeranta says:

    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 http://www.geo.lsa.umich.edu/~shaopeng/2008GL034187.pdf

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

  50. 50
    Briso says:

    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]


Switch to our mobile site