RealClimate logo

Tree Rings and Climate: Some Recent Developments

Filed under: — mike @ 8 July 2012

by Michael E. Mann, Gavin Schmidt, and Eric Steig

Update 7/12/12: Media Matters comments on the latest misrepresentations of the Esper et al study discussed in our article: ‘Surprise: Fox News Fails Paleoclimatology’
Update 7/13/12: Further comment from Bob Ward of the Grantham Institute in Huffington Post UK “The World’s Most Visited Newspaper Website Continues to Regurgitate Nonsense from Climate Change ‘Sceptics'”
Update 7/14/12: Some additional context provided by this LiveScience article

It’s been a tough few months for tree-rings, perhaps unfairly. Back in April, we commented on a study [that one of us (Mike) was involved in] that focused on the possibility that there is a threshold on the cooling recorded by tree-ring composites that could limit their ability to capture the short-term cooling signal associated with larger volcanic eruptions. Mostly lost in the discussion, however, was the fact–emphasized in the paper—that the trees appeared to be doing a remarkably good job in capturing the long-term temperature signal—the aspect of greatest relevance in discussions of climate change.

This week there have been two additional studies published raising questions about the interpretation of tree-ring based climate reconstructions.

The first of these by Steinman et al (Mike is again a co-author) appeared in PNAS, and compared evidence of winter precipitation changes in the Pacific Northwest over the past 1500 years using a physical model-based analysis of lake sediment oxygen isotope data to statistical reconstructions of drought based on tree ring data. Steinman et al note that the tree-ring and lake estimates track each other well on multidecadal timescales, but show some divergence in their lower frequency (i.e. centennial and longer timescale) trends. They conclude that this divergence may simply reflect the differing and, in fact, complementary seasonal information reflected by the two proxy records, noting in the abstract:

differences in seasonal sensitivity between the two proxies allow a more complete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.

The authors amplified this point in their press release (emphasis added):

Tree ring and oxygen isotope data from the U.S. Pacific Northwest do not provide the same information on past precipitation, but rather than causing a problem, the differing results are a good thing, according to a team of geologists.

Nonetheless, some of the coverage (e.g. “Scientists see ancient climate patterns in lake-bottom ‘muck” by Lauren Morello, E&E/Climatewire, July 3, 2012) emphasized conflict between scientists over the discrepancies, rather than the more positive message about making use of complementary strengths of diverse sources of information about past climate change. In other words the ‘signal’ (moving forward with the science) became buried in the ‘noise’ (scientists on record arguing with each other).

The principle that different types of proxy data are complementary in the information they provide is in fact the motivation for the development of “multiple proxy” (multiproxy) reconstructions of climate (see e.g. this commentary by Mike from a decade ago). A new paper today is worth discussing for just this reason.

Jan Esper and colleagues have an article in Nature Climate Change that introduces a new reconstruction (N-Scan) of high-latitude (Fennoscandian) summer temperature changes over the past two millennia based on Maximum Latewood Density (‘MXD’). The most exciting–and in our view important–development is that they seem to have greatly ameliorated the “divergence problem” that has plagued some surface temperature reconstructions based on these types of data; given that the revised MXD data appear to be able to track the most recent warming provides increased confidence in the estimates they provide of past temperature changes.

Another interesting finding is that N-Scan exhibits a substantially larger pre-industrial (pre 1900) millennial cooling trend (around -0.31C/1000yr) than a tree ring width (TRW) based summer temperature reconstruction from the same trees. The authors interpret this finding as indicating that TRW reconstructions may be unable to recover millennial-timescale temperature trends owing to non-biological impacts on growth and limitations of detrending procedures used to separate climatic and non-climatic growth components. This seems a plausible conclusion, arrived at through a thoughtful and elegant case study. Yet the article extrapolates quite a bit, in terms of its conclusions regarding proxy-based temperature reconstructions more generally. The authors make much of the importance of long-term radiative forcing due to the changes in the earth’s orbit for millennial timescale temperature trends. They argue that TRW data which fail to record this forced long-term cooling might therefore underestimate variability on millennial timescales more generally, and potentially underestimate the warmth of past warm periods (e.g. medieval and Roman periods).

Orbital forcing is indeed substantial on the millennial timescale for high-latitudes during the summer season, and the theoretically expected cooling trend is seen in proxy reconstructions of Arctic summer temperature trends (Kaufman et al, 2009). But insolation forcing is near zero at tropical latitudes, and long-term cooling trends are not seen in non-tree ring, tropical terrestrial proxy records such as the Lake Tanganyika (tropical East Africa) record (Tierney et al, 2010) (see below).

Long-term orbital forcing over the past 1-2 millennia is also minimal for annual, global or hemispheric insolation changes, and other natural forcings such as volcanic and solar radiative forcing have been shown to be adequate in explaining past long-term pre-industrial temperature trends in this case (e.g. Hegerl et al, 2007). Esper et al’s speculation that the potential bias they identify with high-latitude, summer-temperature TRW tree-ring data carry over to a bias in hemispheric temperature reconstructions based on multiple types of proxy records spanning tropics and extratropics, ocean and land, and which reflect a range of seasons, not just summer (e.g. Hegerl et al, 2006; Mann et al, 1999;2008) is therefore a stretch.

Indeed, there are a number of lines of evidence that contradict that more speculative claim. For example, if one eliminates tree-ring data entirely from the Mann et al (2008) “EIV” temperature reconstruction (see below; blue curve corresponds to the case where all tree-ring data have been withheld from the multiproxy network), one finds not only that the resulting reconstruction is broadly similar to that obtained with tree-ring data, but in fact the pre-industrial long-term cooling trend in hemispheric mean temperature is actually lessened when the tree-ring data are eliminated—precisely the opposite of what is predicted by the Esper et al hypothesis.

The wider hypothesis doesn’t get much support from looking at the pre-industrial millennial-scale temperature trends in published proxy reconstructions of Northern Hemisphere mean temperature, all of which indicate cooling of varying magnitudes. Ordered from smallest to largest cooling trend, we have:

Moberg et al (2006): -0.06 ºC/1000yr (0-1900)
Esper et al (2002): -0.11 ºC/1000yr (831-1900)
Hegerl et al (2007): -0.14 ºC/1000yr (558-1900, 30º-90ºN land, Chblend-dark)
Ljungqvist (2010): -0.18 ºC/1000yr (0-1900, 30º-90ºN)
Mann et al (1999): -0.19 ºC/1000yr (1000-1900)
Mann et al (2008): -0.23 ºC/1000yr (300-1900, nhcru_eiv_composite):

This can be loosely compared to the -0.31 ºC/1000yr estimate derived for N-Scan and trends of -0.10 and -0.19 ºC/1000yr at that latitude in summer seen in two model estimates discussed – though note that the model simulations will have smaller trends for the whole hemisphere and for the annual mean.

There are a few rather interesting observations here. One is that the Moberg et al (2006) reconstruction, which–unlike all of the other reconstructions listed above–uses no tree-ring proxy data at all to estimate centennial and longer-timescale temperature variations, shows the smallest cooling trend of all. That is in contrast to Esper et al’s hypothesis that including tree-ring data leads to reduced long-term cooling trends. Another interesting observation is that trends calculated from Ljungqvist (2010), Mann et al (1999), and Mann et al (2008) are quite similar to the theoretical cooling trends cited by Esper et al (based on forced multi-millenial GCM experiments; in fact the Mann et al 2008 trend is substantially greater than the model estimates). So there is no support, at least with these reconstructions, for any systematic underestimate of forced millennial temperature changes, if the climate models–and forcings used to drive them—are indeed correct.

Finally, as these latter reconstructions target full hemispheric mean temperature, including tropics & extratropics, and annual mean conditions, the impact of orbital forcing is expected to be far smaller than for high-latitude summer reconstructions, such as this new N-Scan reconstruction. So the fact that a larger millennial cooling trend is seen in this latter case is hardly surprising.

In any case, the Esper et al paper represents a valuable contribution, suggesting important steps forward in the science of dendroclimatology. The paper provides a promising approach to at least reducing the vexing “divergence problem”, and it suggests prospects for tree-ring reconstructions of extratropical summer temperature with substantially greater fidelity at millennial timescales.

Only by understanding the relative strengths, weaknesses, and limitations of various sources of proxy evidence can we continue to refine proxy estimates of past climate change, something that is of interest not just to the paleoclimate community, but to the broader climate research community which relies, in part, on paleodata as a benchmark for testing and evaluating our mechanistic understanding of the climate system.


Esper, J., Cook, E. & Schweingruber, F. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295, 2250-2253 (2002).

Hegerl, G. C. et al. Detection of human influence on a new, validated 1500-year temperature reconstruction. J. Clim. 20, 650-666 (2007).

Kaufman, D. S. et al. Recent warming reverses long-term Arctic cooling. Science, 325, 1236-1339 (2009).

Ljungqvist, F. C., A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia. Geogr. Ann., 92A, 339-351 (2010).

Mann, M. E., Bradley, R. S. & Hughes, M. K. Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophys. Res. Lett. 26, 759-762 (1999).

Mann, M. E. et al. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proc. Natl Acad. Sci. USA 105, 13252-13257 (2008).

Moberg A, Sonechkin DM, Holmgren K, Datsenko NM, Karlen W. Highly variable Northern Hemisphere temperatures reconstructed from low- and high resolution proxy data. Nature 433:613-617 (2005).

Tierney, J.E. et al. Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500, Nature Geoscience, 3, 422-425 (2010).

79 Responses to “Tree Rings and Climate: Some Recent Developments”

  1. 1
    Larry Saltzman says:

    As always your material is invaluable to lay people like myself who care to get educated. The tragedy right now is the scientific illiteracy that is prevalent in this country. Nuances are far to much for a lot of the media to grasp. And the forces of global warming denial are expert at playing on the public’s inability to grasp the the basic science behind global warming.

    Sadly, as climate change influences weather more and more people will increasingly “get it”. I hope we take action soon.

  2. 2
    Martin says:

    Why does Maximum Latewood Density avoid the “divergence problem”?

    [Response: You seem a bit confused. The divergence problem primarily impacts Maximum Latewood Density (MXD). (there are similar, but less pronounced issues with TRW). This paper purports to have largely gotten around the traditional divergence problem seen in past studies using MXD to reconstruct temperatures. -Mike]

  3. 3
    bjchip says:

    “are complimentary in the information”

    See difference from complementary – fix please.


    [we had *just* fixed it–but thanks anyway -editors]

  4. 4

    It seems to me that the warming/cooling trend is even more complex.

    As mankind grows and utilizes more of the fossil fuels, the mass of the earth decreases. However, the earth is collecting space dust in massive amounts as it moves throught its orbit around the sun. some years it accumulates very large amounts of dust, thus increasing the earth’s mass.
    This loss of mass from burning fossil fuels and the accumulation of space dust varies from year to year.
    Thus when we pass through a “clean” area of space, and at the same time burn condsiderable fossil fuels, the Earth mass has a sum decrease. This allows the Gravatational pull of the Sun to pull the earth just a litle closer in its orbit….thus Global warming. As mankind population grows and we utilize more and more fossil fuels, we must get warmer.

    A potential solution is to quickly develop more solar energy systems, wind, and clean thorium atomic energy systems; instead OF BURNING MORE OF THE COAL, OIL, AND GAS.

    Hopefully we can solve the Warming, so that our civilization can survive.

  5. 5
    Unsettled Scientist says:

    Harry Francis, you are very confused.

    If the Earth has a mass decrease, the Sun will pull on the Earth less, not more. The force of gravity is essentially…

    Pull of Sun on Earth = Mass of Sun * Mass of Earth / Sun-Earth Distance Squared

    Less mass for Earth = Less pull of Sun on Earth

    Mass & Energy are two sides of the same coin and can be converted between one another. When we burn fossil fuels, yes the exhaust has less mass, but the energy in put back into the Earth system, it is not lost to Space. I believe the main drivers of Mass/Energy loss to Space are the blackbody radiation, and evaporation (so to speak) of very light elements at the top of the atmosphere as individual molecules gain escape velocity. I can’t think of other ways Earth is losing mass or energy, but there might be some.

    You’re conclusion is right, we need to develop renewable sources of energy, but the science is a mixed up.

  6. 6
    David B. Benson says:

    …, but show some divergence in their lower frequency trends. [Emphasis added.]

    Lower frequency than a multidecadacal timescale is, say, centennial or millennial timescale. I think it should read higher frquency, such as seasonal timescale.

    Or I am completely lost.

    [Response: No, you are reading it correctly. The sediment record only resolves multidecadal variations, and on that timescale the two records tend to agree. It is only at the lower frequencies (i.e. longer timescales), namely centennial and longer, where the records diverge. But I added some additional wording to clear up any potential confusion on this point. Thanks! -Mike]

  7. 7
    Dave E says:

    Harry Francis (#4) and Unsettled Scientist (#5) – No. Combustion of fossil fuels does not reduce the mass of the earth, if we assume that the atmosphere is part of the earth. Combustion does NOT convert mass into energy. Nuclear fission/fusion do, but chemical reactions do not.

    Now, maybe you’re trying to be more subtle, and I’m missing something… but what you wrote is going to confuse the heck out of people!

  8. 8
    KeithWoollard says:

    Harry @ 4,
    This is a joke isn’t it? I am a bit scared that anyone might actually think that burning anything reduces the mass of the earth.
    The only energy source that will do that is nuclear and I can’t imaging a few kg will make much difference

  9. 9
    sue says:

    Was this their ‘response’ to your feb/2012 paper, or is that somewhere else? I’ve been curious about that.

    Rob Wilson says:
    8 Feb 2012 at 12:03 PM

    Dear Mike,
    Your paper has certainly generated a lot of discussion over the last few days between some dendroclimatologists.

    You would appreciate that we are somewhat sceptical of your hypothesis and analyses and are drafting an appropriate measured response to your work.

    With kind regards,
    Rob, Rosanne, Ed, Kevin, Keith, Tom, Jan, Dave, Ulf, Brian, Håkan and Paul.

    [Response: Thanks Rob. Looking forward to your alternative explanation of why these trees are missing/greatly underestimating all of the largest eruptions of the past millennium. I’d be open as anyone to a better hypothesis, and a rigorous reanalysis of the chronologies to insure beyond doubt that the effect we hypothesize is not present. This is the way science works, and I’m looking forward to a constructive discussion. As I’ve stated before, if the paper generates nothing more than that, then I would consider it a positive development. Note also–as I alluded to in an earlier coment above–that with all of the emphasis on the post-volcanic cooling, many observers are missing an equally important conclusion that we draw in the paper (and mentioned in the press release): that if the models we compared to have things about right, then RCS is doing a remarkably good job of reconstructing the long-term variability. This is a positive implication of our study that many are glossing over in their emphasis only on the thing that the reconstruction doesn’t appear to be getting right (large post-volcanic coolings). -mike]

    [Response: Stay tuned for more on that. -Mike]

  10. 10
    Marcus says:

    Sue reminds me very much on JeffID

    Kind regards,

  11. 11
    Edward Greisch says:

    What is N-Scan?

    [Response:It’s what Esper et al are using as shorthand for their Northern-Scandanavian reconstruction. – gavin]

  12. 12
    sue says:

    Marcus, hmm. not sure what JefflD has commented on previously so I don’t know what to make of that. But Mike’s comment seems cryptic… Was there an official response from those scientists? And if so, I would very much appreciate a link to it.

    [Response: Didn’t intend to be cryptic, just requesting patience. There is no published response at this time. These things need to work their way through the system (i.e. peer review), and it is possible that a comment/reply will ultimately be published. Nature journals have strict embargoes, and there is little more that I can say right now than that. -Mike]

  13. 13
    Martin Vermeer says:

    Dave E #7:

    Combustion does NOT convert mass into energy. Nuclear fission/fusion do, but chemical reactions do not.

    Actually neither type of reaction does… mass is not CONVERTED to energy, it IS energy (times c^-2).

    Chemical reactions release on the order of an eV per atom, atoms weighing several GeV apiece. That makes the mass loss, if the energy is radiated out to space, on the order of 10^-9 of the mass of the fuel burned. Uranium fission releases some 200 MeV per nucleus, which weighs over 200 GeV. Mass loss order 10^-3 of the mass of the fuel.

    … and “Harry Francis” surely is a Poe… as the saying goes, you cannot make this up

  14. 14

    “… and “Harry Francis” surely is a Poe… as the saying goes, you cannot make this up”

    But if ‘Harry Francis’ is indeed a Poe–which seems quite possible to me!–then he IS in fact “making this up!”


  15. 15
    B Eggen says:

    “Harry Francis” reminds me of “Archimedes Plutonium” (see URL ); the Earth’s mass is approx 5.9736×10^24 kg, so even if we could sequester the CO2 into space it would only remove the tiniest of fractions of this mass, some 20×10^12 kg per year, that’s of order 1ppt (parts per trillion). On a LinkedIn forum some similar comment wasted a lot of time & bandwidth – don’t let the same thing happen here (:-)

  16. 16
    Corey S. says:

    differences in seasonal sensitivity between the two proxies allow a more compete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.

    I hate to nitpick, but shouldn’t it read ‘a more complete understanding of the climate system…’?

    [Response: Yes it should. Fixed. Thanks. – gavin]

  17. 17
    Martin says:

    Answer to #2
    Hi Mike,
    then I will reformulate my question:

    “How did they get around the divergence problem by using MXD to reconstruct temperatures?”

  18. 18
    CM says:

    “Dendro for Dummies” request: Could you clarify what they did in order to get around the ‘divergence problem’? If this is the major advance, as you assess, the method should be of interest. I see that they state they found no evidence for divergence, but if there’s an account of why not, I’m missing it — is it simply the extent and replicatedness of the record? or the inclusion of living lakeshore trees along with the subfossil ones? or statistical smarts that I missed? or…?

  19. 19
    Hank Roberts says:

    35 hits with Scholar on this search, including other papers that mention latewood density.

    Reading several different authors’ discussions of the method is often helpful.“maximum”+”latewood+density”+”reconstruction”

  20. 20
    JCH says:

    CM – just a dummy guess here, the article says they included living trees in close proximity to the shoreline of the lake(s) from where the subfossil samples came.

  21. 21
    John Mashey says:

    Good stuff, but a wish,k maybe even for a short tutorial here at RC:

    1) Reconstructions can differ by algorithms applied to same data.
    2) Data selection can be land or land+ocean.
    3) A reconstruction can claim to represent:
    100% a) Earth (SH+NH)
    50% b) SH
    50% c) NH (0deg-90deg N)
    or parts of NH:
    30% d) 23.5deg N – 90deg N (Tropic of Cancer, I’ve seen that somewhere.
    25% e) 30degN – 90degN
    18% f) 40degN – 90degN
    6.5% g) 60degN – 90degN [which after all has some interesting proxies)

    4) Spaghetti diagrams can unintentionally mislead people , as I discussed on p.142 of Strange Scholarship inn the Wegman Report:
    ‘- Different reconstructions cover different geographies, and in particular, those focused on (land-dominated) NH extratropics are expected to vary more than the entire NH, which in turn varies more than global.
    – Human eyes tend to notice the outer edges of the spaghetti graph more than the density of lines between.”
    (I think that’s right, tell me if wrong, but these explain why I like the IPCC AR4 density graph as a display technique.)

    Also, I’d add that people sometimes interpret the multiplicity of lines to think that researchers have no idea what was going on and must disagree (more than they do).

    5) SO, I’D WISH:

    a) For a short tutorial to set people’s expectations about expected differences among geographies.

    b) For any listing of reconstructions or graph thereof to be explicit about land/ocean and geography claims.

    6) I also give a plug for Nick Stokes’ inhteractive visualization of spaghetti.

  22. 22
    Dave E1 says:

    Several people have already commented on #4 and #5, but even if the mass of the Earth did change (from solar dust, shooting probes to other planets, etc.) that would not effect it’s orbit–Galileo showed long ago that different objects fall at the same rate regardless of their mass–the same is true of objects orbiting the sun.

  23. 23
    Louis Hooffstetter says:

    This paper raises the following questions:

    1. How accurately do density variations in tree rings reflect changes in solar insolation through time?

    2. How do we know solar insolation changes over the last two millennia unquestionably result from long-term oscillations of orbital configurations?

    3. Is there any other evidence to support the claim that the forcing of solar insolation changes over the last two millennia was up to four times as large as the net anthropogenic forcing since 1750?

    4. Is there any way to know if the wood density data supports the general circulation models (to conclusively demonstrate summers have cooled substantially), or if the models were tweaked to match the data?

    Mike & Gavin, comments please.

  24. 24
    AIC says:

    Re #4:
    Harry is wrong. The biggest problem is that we have epidemic obesity in the USA, greatly increasing the mass of the Earth, so it is being gravitationally attracted closer to the Sun, causing global warming. So we all need to go on a diet. (Sorry, couldn’t resist, even if it is not April 1.)

  25. 25
    AIC says:

    Michael Mann, thank you for your book. I’m working my way through it, increasing my understanding of the science, and posts like this make it clear that that you are real scientists interested in new information, that there is real science being done, still developing in some ways, separate from the political theater.

    [Response: Thanks very much AIC :) -mike]

  26. 26
    PDG says:

    Why ‘anthropogenic forcing since 1750’? Isn’t a hundred years later more accurate, with the greatest increase in human emissions two hundred years later – ie post WW2?

    [Response: It’s a point in the pre-industrial era. CO2 between 1750 and 1850 rose about 10ppm, CH4 went up etc. It doesn’t make much difference though given the much larger rises since. – gavin]

  27. 27
    Toby says:

    Michael Mann,

    Let me add my agreement with what AIC said in #25. Your book was the first downloaded to my Kindle, and I found it a thoroughly satisfying read. I know you would rather immerse yourself in the science, but your public work (interviews, talks, articles, books) is very important also, so please keep it up. It’s been thrust upon you, but you’re bearing up fine! :)

    About to N-Scan chart above, what do you say to someone who jumps on it and say “Aha! so it was warmer than today in Roman times!”? At least as high latitudes. Will the N-Scan results be incorporated into a global study?

    [Response: Thanks Toby, appreciate the kind words and support. Re N-Scan, well sure cherry-picking and selective citation of individual records is a favorite past time of climate change deniers. Just have to call it out for what it is. -mike]

  28. 28
    Dean S. says:

    RE #22

    Actually what Galileo supposedly proved or expanded on was that two objects of different insignificant masses would fall at approximately the same rate in the presence of a significant gravitational field as long as the medium through which they were falling causes insignificant differences to the objects…because the main gravitational force would completely overshadow all the lesser forces working on the two masses differently. The “two masses fall at the same rate” idea is a rough rule of thumb that works good enough in most situations, but not exact or proper law of physics. In a vaccuum this rule of thumb is generally very close for objects without significant differences in mass or centrifugal force, because the next most significant force we usually experience is air resistance, so then we can for example make a feather fall as fast as a small sphere…however the magnitude of difference between the compared object’s masses affect the balance/difference of centrifugal force/gravitational pull of said objects between each other and any other nearby objects, such as between bodies orbiting in a solar system.

  29. 29
    Steve Fish says:

    Because we are already off topic re the mass of the earth and orbits, I can’t help bringing up the fact that there is no such thing as a centrifugal force. It is a fiction of a rotating frame of reference. This is why it is referred to as a fictive force. Problems of understanding physical reality can result from this confusion.

    or just check wiki. Steve

  30. 30
    Jim Pettit says:

    Of course the Esper et al N-Scan reconstruction and accompanying chart have been all the rage across the denialosphere today, with many headlines trumpeting (for perhaps the thousandth time) that, finally, here is definitive “proof” that the planet is cooling, not warming.


    I find it extremely telling that none will of course comment on the rapid post-1900 warming reflected in both the reconstruction and the instrumental record; “ignore the incline” is the new denialist motto, I suppose. But I’m nevertheless with most here in welcoming the article and having a chance to dig deeper into it, especially given that it seems it will help with both small-scale fidelity and the divergence problem. But on first read, Esper & gang do appear to be “extrapolating quite a bit”, since the long-term cooling trend they found is not reflected at all in proxy data such as those from Lake Tanganyika. A “stretch”, indeed.

  31. 31
    Hank Roberts says:

    > Lake Tanganyika.
    Latitude, 6:0S.
    Any correlation of trend with latitude?

  32. 32
    Chris Colose says:

    Thanks Mike (et al) for the interesting update.

    It’s a shame that the authors of the Esper paper were so irresponsible in talking about the “radiative forcing” of orbital insolation changes. I already have found some people on the internet claiming that sensitivity is low because a ~6 W/m2 forcing only produced 0.3 C or so temperature change; some of the blame on that misunderstanding goes to the paper.

    It was just a sound byte that takes away from an otherwise interesting and probably very useful study. Hopefully it helps out the dendro community!

    [Response:I don’t think they were irresponsible at all Chris. The 6 W/m2 figure is not an estimate of theirs–they cite Berger & Loutre (1991) Quat. Sci. Rev. 10:297-317 for that. The main finding of the study is that ring widths and densities can return different long term trends. Three different types of evidence (their Fig. 3, Fig. S1, and considerations involving detrending specifics of the two ring variables), argue that it’s the density values that are most likely to be correct. Given that ring widths are far more commonly measured (and hence used in reconstructions), that’s potentially real important.–Jim]

    [Response:As noted in our piece, where the authors can rightly be criticized is for vastly overstating the implications of their findings w.r.t. to multiproxy reconstructions of annual hemisphere-mean temperature. As we explain in the piece, that hypothesis appears to fall apart when you look at the actual reconstructions. Its disappointing the authors didn’t go the extra step that we did to check if their hypothesis holds up in that context. Indeed, this has already led to some misleading headlines e.g. New Scientist “Tree rings suggest Roman world…warmer than thought” (No it doesn’t!).” -Mike]

    [Response:Since we are all chiming in, I agree with Chris on the forcing statements – the local/seasonal insolation forcing from orbital calculations shouldn’t be compared to the annual global mean radiative forcing over the 20th Century – it’s just not an apples-to-apples comparison – and as Chris noticed, it is easy to be mislead over what that means. – gavin]

  33. 33
    Hank Roberts says:

    > Lake Tanganyika

    At six degrees off the Equator, that’s nowhere near “high-latitude (Fennoscandian)”

  34. 34
    Hank Roberts says:

    > ring widths and densities can return different long term trends.

    Jim, you’re the tree biologist, can you give or point to an 8th-grader-comprehensible take on what makes the differences people see, in terms of the actual tree’s life? Something that would help with questions like these that occur to me? (Pointer’s fine, I’m too busy for a while to try to find the best way to get into this whole area.)

    What’s the individual tree doing internally when it’s making a thicker or thinner annual ring? What’s it doing when it’s making the denser or less dense ring? And if you compare trees growing together, how well correlated are they year by year.

    [Response:To make a long and complex story overly simplistic, the tree is attempting to optimize water transport efficiency and mechanical strength/flexibility relative to carbon expenditure needed to do so. Much of this is driven by hydraulic considerations at very small scales, i.e. the internal diameter of a wood cell (i.e. conifer tracheid). As seasonal water stress increases, as it usually does over the growing season, the tree has to keep from having its cells cavitate (form an embolism) due to the enormous tension on the water columns in the xylem (a tension which typically shrinks the diameter of the tree during mid-day). Once cavitated, that cell is useless for water conduction, weakening the overall transpiration stream. If you decrease the internal diameter of the tracheids, the water column will not cavitate nearly as easily (exponential function there due to support from the cell wall adhesion), and increased cell wall width and lignification increase the mechanical strength so they don’t collapse. Therefore, maximum density culminates near the end of the growing season.]

    How’s each tree allocating the resource available among making trunk, leaves, roots, new growing branches? And I’m guessing the “resource available” to the individual tree corresponds to what the climatologists want to derive — total and peak/minimum available light and water for photosynthesis; maybe wind, if the tree grows differently over years or decades in windy or more protected locations.

    [Response:The question that keeps plant physiological ecologists employed! Trees are nothing if not the world’s greatest optimizers. For the purposes of dendro-climatology, the lower stem radial growth is on the hierarchy of a tree’s carbon allocation priorities, the better. That means it will be responsive to limitations in resources, which is what you want.]

    And, long shot — seeing the new topic on Andrill — given that leaf waxes survive in sediments, is there anything in tree rings that correlates with leaf waxes? Anything you could say about leaf constitutents that would vary along with ring size or density, that might end up in annual layers near a site?

    [Response:Don’t know about waxes. Leaf stomatal density would be the one that comes immediately to mind as the best possibility.–Jim]

  35. 35
    Physicist says:

    @ #4, #5, #7

    Dave E is wrong is saying that “Combustion does NOT convert mass into energy”. Einstein’s equation E = mc^2 applies to chemical reactions just as much as to nuclear reactions. So the products of combustion do weigh less than the fuel + oxygen that went into them. It is just the the amount of mass lost is too small to be easily measurable in a chemical reaction.

    The heat generated still has mass (weight) but assuming that eventually radiates into space, the earth will lose a small amount of mass due to burning fossil fuels.

    In 2011 world energy use was about 5.15×10^20 J ( of which 87% was fossil fuels. Doing the maths that comes to about 5000Kg of mass converted into energy.

    However that will not alter the Earth’s orbit, because the mass of the Earth cancels out when calculating the acceleration caused by the sun’s gravity:

    F = ma = GmM/r^2

    (F = force, m = mass of earth, M = mass of sun, r = earth-sun distance, G = Newton’s constant). So:

    a = GM/r^2

    independent of the mass of the Earth. (The gravitational force gets less if the Earth loses mass, but the Earth’s inertia is less too, and the effects cancel, leaving the orbit unchanged. The interplanetary dust we pick up may or may not change the orbit, depending on whether it carries net momentum – I would have to check on that. But the increase in mass of itself does not).

    And of course, none of the above has anything to do with global warming :-)

  36. 36
    Hank Roberts says:

    > resources available
    Oh, and, are tree rings responding to air temperature, or to soil temperature? Is there a lag time reflecting how long the root zone temperature changes?

  37. 37
    Hank Roberts says:

    and a pps — just a nudge toward where to start, if there’s a good recent review/summary/textbook I can get from Interlibrary Loan would help. Looking just at 2010, just at the brief excerpts, I see the answers are out there:,5

  38. 38
    timg56 says:

    RE Hank Roberts @ 34 & 36

    Good questions.

    I’ve been involved in forest based science education for 15+ years and every forester and biologist I’ve asked has said trees are far more likely to be indicators of percipitation and soil nutrients than temperature.

    Maybe it shouldn’t be surprising that we have papers utilizing tree ring data that indicate warming and other papers that indicate cooling.

    [Response: Then apparently you were misinformed. There is literally decades of research behind the field of dendoclimatology, and it is well known and understood that one seeks different environments depending on what parameters might be of interest. When seeking to reconstruct temperature, one looks for environments (e.g. the boreal or alpine treeline) where temperature is indeed a limiting factor determining growth, and specific sites where stand dynamics etc. are not an issue. There is a wealth of literature on this that you might want to direct your colleagues to. In fact, there are books written on this. Start with Ray Bradley’s “Paleoclimatology” in the side bar of this site (or my book “The Hockey Stick & The Climate Wars” for that matter!). -mike]

  39. 39

    “Aha! so it was warmer than today in Roman times!”?

    Important to keep in mind, actually, since it completely scuppers another common denialist meme, namely, “Of course the planet is warming, we’re just coming out of an Ice Age!”

    Of course, we’re not: AFAIK, the highest post-glacial temps were about 8 kyr back (updated info gladly welcomed.) The 1999 “hockey stick” showed cooling over the ‘handle,’ Kaufman 2009 showed 2 kyr of cooling in the Arctic (prior to the ‘blade’, of course), to name just two examples of studies showing/suggesting ‘elevated temps in Roman times.’

    Folks who make these types of arguments aren’t always much concerned with logical consistency, of course, but if they try to make both sides of the argument in the same conversation, that too becomes persuasive–though not necessarily in the way they wish.

  40. 40
    dhogaza says:


    When seeking to reconstruct temperature, one looks for environments (e.g. the boreal or alpine treeline) where temperature is indeed a limiting factor determining growth, and specific sites where stand dynamics etc. are not an issue.

    Perhaps part of the problem here is that foresters aren’t trying to grow commercial timber near the boreal or alpine treeline, and therefore are ignorant of the fact that in such environments temperature can be the limiting factor.

  41. 41
    Manuel Lerdau says:

    2 points:
    A) very clear piece. thanks.
    B) re:#32, this exchange highlights some of the best of RC. Readers and bloggers disagreeing constructively in a way that increases understanding.

  42. 42
    Hank Roberts says:

    that’s why I asked Jim for an 8th-grader level resource, there’s so much science done that the ordinary reader can get lost.

    Trees can handle _some_ temperature variation (and while they can, other factors would be the limiting ones). And the trunk (rings) and leaves (that waxy buildup) handle temperature differently — within whatever range the temperature isn’t the limiting factor.
    trunks (old 1966 info):

    Got that part. And that’s why the paleo/dendro work isn’t done in the forest plantations but at the edge — edge of an ice cap, latitude or elevation or both — where they know the temperature was the limiting factor.

    This is why it’s easy to find tree records that _don’t_ show temperature effects; because there are large areas of the world where they won’t because the temperature isn’t what’s making the tree hit its limit.

    I know this is an area where there are lots of different possible causes/limiting factors affecting what’s observed (rings, wax in sediments).
    And every one a likely PhD topic :-0

    Science is rugby, not a lovefest, as Peter Watts says.

    [Response:I have a lot to say on the topic Hank, but no time to say it. That goes for this Esper et al paper, also. I hope to get some time to elaborate fully on why their detection of a long term trend difference between density and width data is real important, why it arises, and what it implies for tree ring analysis. I have a very different take on this study than Mike/Gavin/Eric do. It will require a separate post to do so.–Jim]

  43. 43
    Hank Roberts says:

    A rather wonderful overview essay:

    TREWAVAS, A. (2009), What is plant behaviour?. Plant, Cell & Environment, 32: 606–616. doi: 10.1111/j.1365-3040.2009.01929.x
    † This manuscript is part of the special issue on plant behaviour.

  44. 44
    Jeff Rubinoff says:

    No reward for guessing what The Daily Mail have done with the story. Esper might want a word with them.

  45. 45
    freemike says:

    any response to the daily mail article we can share with the web and those seeking to misinform?

    [Response: Presumably this Daily Mail article? Tree ring study proves climate WARMER in Roman and Medieval times than it is now – and world has been cooling for 2,000 years… Well, the study is from a single area in Northern Scandinavia, it shows cooling only until 1900 (and then warming). Whether regionally or hemispherically or globally it means that either Roman or Medieval times were warmer than now would require including this data in the multi-proxy reconstructions and seeing if it changed that. Judging from the analogous cooling trends in the reconstructions already, it doesn’t seem likely. Overall, the Mail is just wildly overreacting – possibly something to do with them having had to retract the last inaccurate article they wrote about medieval climate because of a ruling from the Press Complaints Commission. – gavin]

  46. 46
    Kleber Costa says:

    Journalists should only be partially blamed for the bad coverage of the latest Jan Esper paper. Some of them wrote stories without interviewing the authors, which is wrong, but the press release issued by JG University in Mainz helps the denialist fringe by including a couple of odd quotes from Esper himself. Take a look at what he says:

    “We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low,” says Esper. “Such findings are also significant with regard to climate policy, as they will influence the way today’s climate changes are seen in context of historical warm periods.”

    [Response: This line should have made it clearer that this was a single location, and I’m not sure what previous estimates he is referencing. The impact on climate policy is a stretch, though this certainly isn’t the only press release that has over-egged that pudding. – gavin]

    and also

    “This figure we calculated may not seem particularly significant,” says Esper. “However, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia.”

    [Response: I think this is also a stretch. Is it ‘likely’ that any new NH reconstruction using N-Scan will have a big difference from the range of trends in the existing reconstructions? I’m not sure, and absent any actual test (which would be relatively easily do-able), I don’t know how he is so confident. – gavin]

    [Response: The reference to “THE large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC)..” is most peculiar indeed. IPCC AR4 report showed A DOZEN such reconstructions, each indicated that modern warmth is unprecedented as far back as they reconstructions go (some more than a millennium). In at least ONE case (Moberg et al, 2005) NO TREE RING DATA WAS USED to construct the long-term trend. So the statement in the Esper press release is not only vague but necessarily false! -mike]

    [Response: I think Gavin is a bit too generous here. We’ve shown in the article that his hypothesis actually fails—it isn’t even consistent with the variation in the long-term cooling trend between studies that do and do not use tree-rings. Curiously, Esper cites two full hemisphere (not extratropical or high-latitude), annual (not summer), multiple-proxy (not tree-ring only) studies (Mann et al ’99 and Mann et al ’08) as examples of the impact this high-latitude, summer, tree-ring effect could have, and yet those two studies have the largest long-term cooling trend, and show no support for the effect he is talking about. Curiously unmentioned in the paper is the ironic fact that Esper’s own previous Northern Hemisphere temperature reconstruction (Esper et al ’02), which is indeed entirely based on tree-rings, and reflective of extratropical summer temperatures–shows among the least cooling of all the reconstructions, and is perhaps most likely to have suffered from the very bias mentioned in this study. Isn’t it surprising that this isn’t mentioned in the paper? The number of speculative, unsubstantiated—and indeed easily falsifiable–claims in the paper is rather stunning, and detracts greatly from what otherwise looks like a decent contribution. -mike]

    I wonder if you guys could please comment on this press release, because it’s very hard for journalists to deal with such vague statements. Do you really think Esper is advocating lowering the tone of the IPCC reports?

    [Response: I have no idea. I’d say it was more related to emphasizing the potential implications of one’s own work over anyone else’s – a frequent occurrence in press releases. I generally find it prudent to wait for the work on the implications to be done (for instance). – gavin]

    [Response: Gavin is again quite generous. It would appear that Esper’s misleading statements and overstatement of larger implications directly fed the sort of denialist frame represented in the Daily Mail article. It is of course impossible, and unwise, to guess at whether or not that was his intent. -mike]

  47. 47
    Robert says:

    “Moberg et al (2006): -0.06 ºC/1000yr (0-1900)
    Esper et al (2002): -0.11 ºC/1000yr (831-1900)
    Hegerl et al (2007): -0.14 ºC/1000yr (558-1900, 30º-90ºN land, Chblend-dark)
    Ljungqvist (2010): -0.18 ºC/1000yr (0-1900, 30º-90ºN)
    Mann et al (1999): -0.19 ºC/1000yr (1000-1900)
    Mann et al (2008): -0.23 ºC/1000yr (300-1900, nhcru_eiv_composite):

    This can be loosely compared to the -0.31 ºC/1000yr estimate derived for N-Scan and trends of -0.10 and -0.19 ºC/1000yr at that latitude in summer seen in two model estimates discussed – though note that the model simulations will have smaller trends for the whole hemisphere and for the annual mean.”

    Not trying to nitpick but showing different trends from different time periods takes away from your point there. How do we know that any of the shorter series would have similar trends without even covering the same length as the trend they reported.

    [Response: Fair point. The trends above were for the maximum length of the reconstructions. If I take the 901-1900 and 601-1900 periods, then I get: Moberg: -0.50/-0.28ºC/1000yr, Ljungqvist: -0.53/-0.31ºC/1000yr, Mann08: -0.62/-0.38ºC/1000yr, Hegerl: -0.28/-0.14ºC/1000yr. All quite negative. I can’t find the N-scan numbers anywhere so I can’t do the trend for these exact periods for that yet (I’ll update if I find them) but note that these are for wider regions than just the N-scan location. The Supp Info has a good graph (fig. S13) showing the differences you get in the long-term orbitally-driven trend (land-only?) for different latitude bands in the models they used. For 30-90N, the trend is expected to be smaller that for 60-70N – by roughly half in ECHO-G and by even more in ECHAM5 (eyeballing, I estimate ~0.1ºC/1000yr and -0.03ºC/1000yr respectively). For the NH as a whole, the modelled trends are smaller still, and actually slightly +ve in ECHAM5 (-0.04ºC/1000yr and +0.01ºC/1000yr). These are just an indication of the latitudinal issues though. Overall, the reconstructed trends are dependent on the exact period looked at, but I don’t see any obvious discrepancy with the N-scan or the modelling results. – gavin]

    [Response: Gavin is once again being extremely charitable. The calculations above, frankly, *falsify* the claims by Esper et al regarding the supposed impact of their finding on Northern Hemisphere annual mean temperature reconstructions. There is neither any observational OR theoretical evidence to support their claim that there is a fact. Will Nature Climate Change require a retraction of the associated statements in the paper? -mike]

    [Further Response: I looked at the trends for different time scales with the N-Scan data and get -0.48 and -0.52ºC/1000yr for 901-1900 and 601-1900 respectively. Given the model results on the latitudinal relationships, I don’t see that any contradiction between this one study and the larger-scale reconstructions. – gavin]

  48. 48
    Hank Roberts says:

    > Given that ring widths are far more commonly measured
    > (and hence used in reconstructions), that’s potentially
    > real important.–Jim]

    Jim, how is the density information captured?
    Is it possible to capture after the fact?
    Can it be done from archived photographs, for example?

    If you’re given a dendro sample, and you’re the grunt worker/postdoc/tech — how much more is involved in getting density along with width from what you’re given?

    And, same as the first question really, if you want to go back and get density info, what do you need? If it’s done on actual wood in storage, does that change over time in storage?

    [Response:X rays! You cut a thin transverse section (i.e. perpendicular to the cell axes), and X ray it. How the actual data reading and recording is done I have no idea. It’s a specialized set of procedures and equipment that most labs don’t have. Fritz Schweingruber, from the WSL-Birmensdorf was the one who really put density data on the map in a big way, through an incredible, northern hemisphere collection effort in the 1980s and 90s, about 500 sites total. I read a paper a couple of years ago by some folks who claimed that a blue staining technique combined with regular light produced good correlations with the x-ray data. If so, that makes such data much more accessible, although some of the high end software for image analysis is still exorbitantly expensive, though really powerful also, some of it capable of measuring cell and cell wall characteristics automatically. As for old photographs I’ve never heard of anyone doing that; I doubt if you could estimate density with them but I don’t know. Even with ring size, you could run into problems with optical distortion issues if you didn’t know the lens that was used. I don’t believe the density deteriorates with time as long as the wood is kept dry but not sure on that. The answer to your question is, it’s a LOT more work and money to measure density; you either set your lab up to handle it big time, or you don’t do it. It’s enough work just to do regular dendro especially if measuring early and latewood width in each ring. Also, gonna get to your earlier question in a second here–Jim]

  49. 49
    Hank Roberts says:

    > old photographs
    Might be surprising. I recall years back, when digital astronomy was taking over, that a project was taking a huge archive of old astronomical photos — they’re negatives, big thick sheets of emulsion with three-dimensional arrays of silver where the photons went through them. And by doing some kind of scanning (X-rays? nuclear magnetic resonance? I dunno) it was possible to basically do a 3-D image of the photon tracks in the emulsion for each star on each photograph. Turns out those long exposures on old film had accumulated far more information than the positive prints (flat 2-D images). Dunno what became of it.

    But it wouldn’t surprise me if old photos have info in them. Negatives are so often used, well, were so often used for science — and remembering those are thick slabs not flat planes.

    [Response:Interesting. Old photos would definitely have information in them, I just don’t know how many exist that would really contain all the information you’d need to make them useful in the context where you want quantitative information from each ring. You’d need to know the site location, species, lens focal length, distance from camera to object, and also that the camera was held ~ perpendicular to the item. If you’re just doing demography (counting rings), then they’re potentially much more useful, because those last 3 items then don’t matter.–Jim]

  50. 50
    Bob Ward says:

    I have been in touch with one of the co-authors of the study to ask about the article on the website of the ‘Daily Mail’ and other wilful misrepresentations of its contents:

    This shows yet again how the echo chamber of climate denial transmits distortions of climate research from the ‘sceptic’ blogosphere to its cheerleaders in the mainstream media.