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Global Temperatures, Volcanic Eruptions, and Trees that Didn’t Bark

Filed under: — mike @ 6 February 2012

My co-authors and I have just published an article in Nature Geoscience (advance online publication here; associated press release here) which seeks to explain certain enigmatic features of tree-ring reconstructions of Northern Hemisphere (NH) temperatures of the past millennium. Most notable is the virtual absence of cooling in the tree-ring reconstructions during what ice core and other evidence suggest is the most explosive volcanic eruption of the past millennium–the AD 1258 eruption. Other evidence suggests wide-spread global climate impacts of this eruption [see e.g. the review by Emile-Geay et al (2008)]. We argue that this–and other missing episodes of volcanic cooling, are likely an artifact of biological growth effects, which lead to a substantial underestimation of the largest volcanic cooling events in trees growing near treeline. We speculate that this underestimation may also have led to overly low estimates of climate sensitivity in some past studies attempting to constrain climate model sensitivity parameters with proxy-reconstructed temperature changes.

Tree rings are used as proxies for climate because trees create unique rings each year that often reflect the weather conditions that influenced the growing season that year. For reconstructing past temperatures, dendroclimatologists typically seek trees growing at the boreal or alpine treeline, since temperature is most likely to be the limiting climate variable in that environment. But this choice may also prove problematic under certain conditions. Because the trees at these locations are so close to the threshold for growth, if the temperature drops just a couple of degrees during the growing season, there will be little or no growth and therefore a loss of sensitivity to any further cooling. In extreme cases, there may be no growth ring at all. And if no ring was formed in a given year, that creates a further complication, introducing an error in the chronology established by counting rings back in time.

We compared simulated temperature of the past millennium derived by driving theoretical climate models with estimated natural (volcanic+solar) and anthropogenic forcings for the past millennium. We employed two different climate model simulations: (1) the simulation of the NCAR CSM 1.4 coupled atmosphere-ocean General Circulation Model (GCM) analyzed by Ammann et al (2007) and (2) simulations of a simple Energy Balance Model (EBM). While the GCM provides a more comprehensive and arguably realistic description of the climate system, the computational simplicity of the EBM lends itself to extensive sensitivity tests. As the target for our comparison, we used a state-of-the-art tree-ring based Northern Hemisphere (NH) mean temperature reconstruction of D’Arrigo et al (2006). The reconstruction was based on a composite of tree ring annual ring width series from boreal and alpine treeline sites across the northern hemisphere, and made use of a very conservative (“RCS”) tree-ring standardization procedure designed to preserve as much low-frequency climatic information as possible.

Interestingly, the long-term variations indicated by the model simulations compared remarkably well with those documented by the tree-ring reconstruction, showing no obvious sign of the potential biases in the estimated low-frequency temperature variations that have been the focus of much previous work (see e.g. this previous RealClimate review). Instead, the one glaring inconsistency was in the high-frequency variations, specifically, the cooling response to the largest few tropical eruptions, AD 1258/1259, 1452/1453 and the 1809+1815 double pulse of eruptions, which is sharpy reduced in the reconstruction relative to the model predictions. Indeed, this was found to be true for any of several different published volcanic forcing series for the past millennium, regardless of the precise geometric scaling used to estimate radiative forcing from volcanic optical depth, and regardless of the precise climate sensitivity assumed.

Following the AD 1258 eruption, the climate model simulations predict a drop of 2C, but the tree ring-based reconstruction shows only about a 0.5C cooling. Equally vexing, the cooling in the reconstruction occurs several years late relative to what is predicted by the model. The other large eruptions showed similar discrepancies. An analysis using synthetic proxy data with spatial sampling density and proxy signal-to-noise ratios equivalent to those of the D’Arrigo et al (2006) tree-ring network suggest that these discrepancies cannot be explained in terms of either the spatial sampling/extent or the intrinsic “noisiness” of the network of proxy records.

However, using a tree growth model that accounts for the temperature growth thresholding effects discussed above, combined with the complicating effects of chronological errors due to potential missing growth rings, explains the observed features remarkably well.

Show in the above figure (Figure 2d from the article) is the D’Arrigo et al tree-ring based NH reconstruction (blue) along with the climate model (NCAR CSM 1.4) simulated NH mean temperatures (red) and the “simulated tree-ring” NH temperature series based on driving the biological growth model with the climate model simulated temperatures (green). The two insets focus on the response to the AD 1258 and AD 1809+1815 volcanic eruption sequences. The attenuation of the response is produced primarily by the loss of sensitivity to further cooling for eruptions that place growing season temperatures close to the lower threshold for growth. The smearing and delay of the cooling, however, arises from another effect: when growing season lengths approach zero, we assume that no growth ring will be detectable for that year. That means that an age model error of 1 year will be introduced in the chronology counting back in time. As multiple large eruptions are encountered further back in time, these age model errors accumulate. This factor would lead to a precise chronological error, rather than smearing of the chronology, if all treeline sites experienced the same cooling. However, stochastic weather variations will lead to differing amounts of cooling for synoptically distinct regions. That means that in any given year, some regions might fall below the “no ring” threshold, while other regions do not. That means that different chronological errors accumulate in synoptically-distinct regions of the Northern Hemisphere. In forming a hemispheric composite, these errors thus lead to a smearing out of the signal back in time as slightly different age model errors accumulate in the different regions contributing to the composite.

Including this effect, our model accounts not only for the level of attenuation of the signal, but the delayed and smeared out cooling as well. This is particularly striking in comparing the behavior following both the AD 1258 and AD 1809 eruptions (compare the green and blue curves in the insets of the figure). Our model, for example, predicts the magnitude of the reduction of cooling following the eruptions and the delay in the apparent cooling evidence in the tree-ring record (i.e. in AD 1262 rather than AD 1258). We have also included a minor additional effect in these simulations. While volcanic aerosols cause surface cooling due to decreased shortwave radiation at the surface, they also lead to increased indirect, scattered light at the surface. Plant growth benefits from indirect sunlight, and past studies show that e.g. a Pinatubo-sized eruption (roughly -2W/m^2 radiative forcing) can result in a 30% increase in carbon assimilation by plants. This effect turns out to be relatively small because it is proportional in nature, and thus results in a very small absolute increase when growth is suppressed i the first place by limited growing seasons. However, not including this effect results in a slightly less good reproduction (purple dashed curves in the two insets of the figure) of the observed behavior.

As noted earlier, our main conclusions are insensitive to the precise details of the forcing estimates used, the volcanic scaling assumptions made, and the precise assumed climate sensitivity. They were also insensitive to the details of the biological tree growth model over a reasonable range of model assumptions. The conclusion that tree-ring temperature reconstructions might suffer from age model errors due to missing rings is bound to be controversial. A few points are worth making here. First of all, our conclusion is quite specific to temperature-sensitive trees at treeline, and it does not imply more general problems in the larger discipline of dendrochronology. Secondly, the conclusion at this stage simply a hypothesis, a hypothesis that can account for these key enigmatic features in the actual tree-ring hemisphere temperature reconstruction: the attenuation, and the increasing (back in time) delay and temporal smearing of the cooling response to past volcanic forcing. Were an equally successful and more parsimonious hypothesis to be provided for these observations, I would be the first to concede and defer to this alternative explanation.

One argument against the specific conclusion of missing growth rings is that trees are carefully cross-dated when forming regional chronologies, and this precludes the possibility of chronological errors. That, however, assumes that there are at least some trees within a particular region that will not suffer a missing ring during the years where our model predicts it. Yet our prediction is that all trees within a region of synoptic or lesser scale where growing season temperatures lie below the growth threshold will experience a missing ring. Thus, cross-dating within that region, regardless of how careful, cannot resolve the lost chronological information. It is my hope that dendroclimatologists will reassess raw chronologies more carefully and critically assess the extent to which the predicted features might indeed be present in the underlying tree-ring data. Again, this paper presents a hypothesis for explaining some enigmatic features of existing tree-ring temperature reconstructions. It is hardly the last word on the matter.

Finally it is worth discussing the potential wider implication of these findings. Climate scientists use the past response of the climate to natural factors like volcanoes to better understand how sensitive Earth’s climate might be to the human impact of increasing greenhouse gas concentrations, e.g. to estimate the equilibrium sensitivity of the climate to CO2 doubling i.e. the warming expected for an increase in radiative forcing equivalent to doubling of CO2 concentrations. Hegerl et al (2006) for example used comparisons during the pre-industrial of EBM simulations and proxy temperature reconstructions based entirely or partially on tree-ring data to estimate the equilibrium 2xCO2 climate sensitivity, arguing for a substantially lower 5%-95% range of 1.5–6.2C than found in several previous studies. The primary radiative forcing during the pre-industrial period, however, is that provided by volcanic forcing. Our findings therefore suggest that such studies, because of the underestimate of the response to volcanic forcing in the underlying data, may well have underestimated the true climate sensitivity.

It will be interesting to see if accounting for the potential biases identified in this study leads to an upward revision in the estimated sensitivity range. Our study, in this regard, once again only puts forward a hypothesis. It will be up to other researchers, in further work, to assess the validity and potential implications of this hypothesis.


56 Responses to “Global Temperatures, Volcanic Eruptions, and Trees that Didn’t Bark”

  1. 1
    MMM says:

    Do reconstructions that exclude all tree-based information show the volcanic forcings better than the dendrochronological estimates? Or if you exclude trees, do you not have enough high-frequency information to detect said volcanic events?

    [Response: We discuss this in the supplementary information (see here). The bottom line is that tree-rings are the only annually-resolved proxy data available over wide regions of the globe six centuries or more back—we cannot really estimating a meaningful annual mean hemisphere temperature that far back w/out them. So its a bit of a conundrum. – mike]

    -MMM

  2. 2
    Salamano says:

    Are these the same sorts of tree-rings that were deemed not-really-that-valuable (for lack of the proper words) for validly/reliably evaluating temperature proxy prior to 1400 AD (I believe that was the cut-off)?

    Also, it seems from what you are saying that it is difficult to explicitly quantify prior to receiving actual core data and looking what the data shows whether a tree is a ‘good’ or ‘bad’ indicator of temperature. If this is the situation, how can one escape the charge that tree x vs y is ‘good’ only if we say it is? Or, tree x vs y is ‘good’ only if it gives a signal that has some elements in there that comes across as plausible in the mind, regardless of what the “actual actual” might be?

    [Response: Nope–you are thinking of the MXD (tree-ring density) data of Briffa et al, I believe. We are using a tree-ring width based reconstruction (D’Arrigo et al ’06) which displays a high degree of skill back to at least AD 1200 according to the validation statistics provided by the original authors. We cut off the analysis at AD 1200 owing to the dropoff in skill before that date. Fortunately, that still allowed us to analyze the AD 1258 eruption response. – mike]

  3. 3
    Pete Dunkelberg says:

    Thanks for posting this, and for doing the research. It is good to learn more about the fine points of any proxy, and this is a good time to talk of tree rings again. I’ve been wondering what experts think of this new study of tree growth responses and inferred temperatures in Scandinavia?

    Esper et al. (2012) – “Variability and extremes of northern Scandinavian summer temperatures over the past two millennia”

  4. 4

    where was the 1258 AD eruption?

    [Response: Ah, great question. We don’t know for sure. Like many earlier eruptions, we know they were tropical eruptions if volcanic deposits are found for the same year in ice cores at both the North and South Poles–that only happens if the aerosol makes it into the tropical stratosphere (where large-scale air currents ultimately distribute the aerosol cloud into both hemispheres—there are some really nice satellite images of this process for the ’91 Pinatubo eruption) . A nice review of the evidence for the eruption, which includes historical anecdotal information, is provided in the Emile-Geay et al (2008) article linked. – mike]

  5. 5
    Salamano says:

    Thank you for your response. It seems that the D’Arrigo et al (2006) data set may also have that “divergence problem” in recent decades, does it not? If the validation statistics aren’t grand for the most recent observed data, then would that appear to be problematic for the paleo? I mean, I realize that there are caveats that can be pointed to for modern dendro series (I’ve heard it posited that there’s a muted dendro sensitivity when CO2 or temperature elevates above a threshhold, etc)…but could that not mean that there are other sorts of caveats (to adjust in one direction or the other) for various things that have occurred in the past as well (presumably the very volcanic signatures that you are researching)? I’m not saying that it is impossible to know/learn/understand anything from dendro…It just seems like there’s a lot of caveating that has to go along with the actual data for bold conclusions.

    [Response: The divergence problem is far less severe for these data than for the density (MXD)-based reconstructions. D’Arrigo et al do however note that there is some divergence after the mid 1980s. The point of our article, of course, is that it is important to understand the sources of bias specific to particular types of proxy data as well as possible. We allude to possible ways forward in the concluding section of the paper. In particular, the combination of process models of proxies incorporated in data/model fusion exercises via data assimilation. We’re actually doing some work on that problem. – mike]

  6. 6
    TheATHiker says:

    With this study and the one Geophysical Research Letters
    http://www.agu.org/pubs/crossref/2012/2011GL050168.shtml

    “Here we present precisely dated records of ice-cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430–1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium”

    The two seem to overlap however, I am a little confuse with the relative forcing shown in your graph above compared to the data from the icecap growth in GRL article. Is the Arctic change exclude in modeled or is it accounted for in the above model.
    Is it possible that you could combine a new topic or amend this one to include that report and explain how they integrate better together.
    Thanks

    [Response: Thanks for your question. The NCAR CSM 1.4 was driven by the radiative forcings (volcanic+solar natural and anthropogenic ghg + aerosol) developed in Ammann et al (2007). We use the Ammann et al (2007) forcings for our standard EBM simulation. However, with the EBM we were able to test the sensitivity of the results to a wide range of past published forcing estimates. Both model simulations represent the full Northern Hemisphere (land+ocean, tropics, mid-latitudes, and poles). This is explicit with the GCM and implicit in the case of the EBM. The forcing series used, other data and results, are provided in the supplementary information (see here). – mike]

  7. 7
    Arthur Smith says:

    If the missing rings are real but vary by region then you should be able to do regional reconstructions that show dramatic responses to the volcanic events in some regions (that didn’t miss), and much smaller response in others (where the ring was missed). Has anybody looked at that yet?

    [Response: Actually, we did look at this closely in the pseudoproxy experiments referred to in the paper (and described in detail in the supplementary information–link provided in earlier comments). It turns out that for the signal-to-noise ratios estimated for the individual tree-ring records, one would be hard pressed to cleanly isolate the volcanic cooling signal in individual records or even regional composites. Its only in the hemispheric mean because of the cancellation properties of proxy noise and regional climate noise, where the signal for individual volcanic cooling events becomes clear. Unfortunately, however, the signal that does emerge appears to be strongly attenuated for the reasons discussed in the paper. – mike]

  8. 8
    Russell says:

    I’m chagrined to report that I sailed across the site of the 1452 eruption in 1975, but did not recognize the Kuwae caldera’s island remnants as such, as I was searching the New Hebrides (now Vanuatu) for archaeological lithic sources, not palaeoclimate clues.

    After an uneventful landfall on Epi, we sailed on to Ambrym, to spend a night ogling the lava jets in the crater of the Mount Benbow caldera, which rather disconcertingly went into pyroclastic eruption the next, day, covering the boat in ash.

  9. 9
    Martin says:

    Mike,

    Do regional sets of tree ring data exist which contain not only trees close to the alpine tree line but also at lower elevations?
    Would it then be possible to compare the two and thereby identify a missing year?
    How would you go about to further validate (or falsify) your hypothesis?

    Cheers,
    Martin

    [Response: Great point–and have thought about that. This IS indeed possible for alpine treeline because the lateral scale of variation in temperature is short and such a near-neighbor comparison can be done between trees near and distal from treeline. The effect, if it is present, could be eliminated for alpine-treeline limited trees in this way. And in cases where that *has* been done, I suspect that the bias we are describing is NOT present. Its more difficult to see how it is eliminated however for the boreal treeline case, where the scale of temperature variation is the synoptic scale, and nearby cross-dating along a sharp temperature gradient is not possible. Many of the trees that contribute to the D’Arrigo et al reconstruction are of the boreal, rather than alpine, variety. – mike]

  10. 10
    Christopher Hogan says:

    I have no expertise here, and so hesitate to be a skeptic, but this strikes me as somehow too neat. Or I’ve misread it. You *only* get dormant summers at the time of these volcanic eruptions? And this is attributed to temperature alone? But … weather varies. If the trees were that close to the border of remaining dormant or not for a year, shouldn’t year-to-year temperature variation result in randomly skipped years throughout the record, forcing the tree-ring record further out-of-sync as you go further back? Or are these volcanically-induced cooling events, at those altitudes, so large as to be unique or nearly so in the weather record for those locations?

    [Response: As explained in the paper, it doesn’t happen just for volcanic eruptions, but for any temperature excursion of more than about 1C relative to the reference period baseline. Typically though, its only volcanoes that will do that, but in the NCAR simulation, there are some natural events e.g. La Nina-type cooling events, that also place the trees below the threshold, and you can see this in the figure. – mike]

  11. 11
    David B. Benson says:

    Wow. Just wow.

  12. 12
    KeithWoollard says:

    Re : Mike’s response to Fred @#4 – is there a chance that 1258 was not a single tropical eruption but two or more more localised ones? And if so, could that change things

    [Response: There are a number of researchers who have looked closely at the ice core data (and tephra evidence), etc. and conclude that there was a single very strong tropical eruption in AD 1258. See e.g. Stother (2000) the abstract of which reads Somewhere in the tropics, a volcano exploded violently during the year 1258, producing a massive stratospheric aerosol veil that eventually blanketed the globe. Arctic and Antarctic ice cores suggest that this was the world largest volcanic eruption of the past millenium. According to contemporary chronicles, the stratospheric dry fog possibly manifested itself in Europe as a persistently cloudy aspect of the sky and also through an apparently total darkening of the eclipsed Moon. Based on a sudden temperature drop for several months in England, the eruption’s initiation date can be inferred to have been probably January 1258. The frequent cold and rain that year led to severe crop damage and famine throughout much of Europe. Pestilence repeatedly broke out in 1258 and 1259; it occurred also in the Middle East, reportedly as plague. Another very cold winter followed in 1260-1261. The troubled period’s wars, famines, pestilences, and earthquakes appear to have contributed in part to the rise of the European flagellant movement of 1260…. But while AD 1258 is the most prominent example of the phenomenon we’re addressing in the paper, we find a very similar story for AD 1452 and AD 1809 + AD 1815 (Tambora) sequence, where we know the volcano that erupted, and there is little doubt of a large and explosive tropical eruption. In in each of these cases, the tree-rings show greatly attenuated and delayed cooling relative to what any of the climate model simulations show, using any of the published volcanic forcing estimates and differing assumptions of the optical depth/radiative forcing relationship. There doesn’t appear to be any simple way out of the conundrum. We’ve offered one hypothesis, and we argue that it yields a prediction that bears a striking relationship with the actual tree-ring reconstruction. -mike]

  13. 13
    John Pollack says:

    Nice work! I really like your approach combining proxy data with modeling, and respect for trees as living systems not always responding in a linear manner to growing season temperature.

    There might be the potential to identify missing tree rings at the boreal tree line near Hudson Bay where a sharp temperature gradient exists away from the Bay.

    Does the GCM capture the tendency for a large volcanic eruption to induce a La Nina?

    [Response: Thanks. Ah, well this gets at another long-time pet hypothesis of mine, e.g. Adams et al (2003) and Mann et al (2005). And sadly, no, this GCM does not reproduce that response, but some others do. I think we’ll see some interesting work published on that by several groups in the year ahead. -mike]

  14. 14
    David B. Benson says:

    From
    http://www.volcano.si.edu/world/largeeruptions.cfm
    the only reasonable candidate for a 1258 CE eruption is
    QUILOTOA Ecuador G 1280 (?)
    of VEI 6, about the same as Pinatubo. The G denotes a corrected radiocarbon date but the (?) indicates an unknown uncertainty in that dating.

    [Response: VEI is a geologically-based index and as such is not favored in quantitative reconstructions of radiative forcing. Those are typically based on volcanic aerosol deposits in ice cores, from which estimates of optical depth (under certain assumptions) and ultimately radiative forcing (under additional assumptions) are typically derived. See e.g. the Robock, Gao et al, Ammann et al, and Timmreck et al papers referenced in the article for further details about this. For none of the derived radiative forcing estimates and neither the linear or geometric (2/3) scaling assumptions is the radiative forcing estimate for the AD 1258 eruption smaller than -8.1 W/m^2 (roughly 4 x Pinatubo!). But to echo an earlier comment above, the finding of this paper goes well beyond the AD 1258 eruption . Equally vexing are the attenuated and delayed cooling found for the other two largest eruptions (AD 1452/53 and AD 1809+1815 sequence–both of which are reproduced in our analysis as a result of predicted tree-growth related artifacts. -mike]

  15. 15
    David B. Benson says:

    VEI is a logarithmic index
    http://en.wikipedia.org/wiki/Volcanic_Explosivity_Index
    and so 4xPinatubo is still VEI 6. However,
    http://en.wikipedia.org/wiki/List_of_large_volcanic_eruptions
    lists the 1258 CE event as VEI 7? with source unknown. To further confuse matters, the 1452–3 CE eruption is listed as Kuwae, Vanuatu, with but a VEI of 6. This just indicates that a logarithmic scale, obtained from geological data, isn’t that accurate. However, large events do leave a record which can be read by geologists so Quilotoa, Ecuador, remains the best candidate AFAIK. That would change if the Quilotoa eruption also showed up in ice core data at around 20+ years later than the still-a-bit mysterious 1258 CE event.

  16. 16
    Slioch says:

    #12 Response: (Stother, 2000), … the 1258 eruption “appear(s) to have contributed in part to the rise of the European flagellant movement of 1260…”

    Not to mention the global warming denial movement of the early years of the third millennium.

  17. 17

    The study is conceptually quite beautiful–I hope this idea holds up.

    [Response: Thanks, us too. But like I’ve said elsewhere, if the paper does nothing more than stimulate a closer examination of this particular hypothesis, and generate additional good science, then I’ll still be happy. -mike]

    I also wonder if someone out there has a plot of young trees for which growing conditions can be controlled enough to see if it is, in fact, possible to induce zero seasonal growth and check whether the annual growth ring is completely suppressed in such a case. A brute force approach, I know, but “straightforward” can be good–though it’s easy to imagine that negative results might be less than conclusive.

    [Response: Yes–this is quite doable and I suspect someone has actually done something like this. It will be interesting to see if someone comes out of the woodwork now w/ results from such previous studies. -mike]

    [Response: Thanks. Used it in the original paper draft but apparently it didn’t conform to title format guidelines :( -mike]

    And as for the post per se, I must say I relished the combined Holmes reference and pun in the title!

    [Response: Thanks. Used it in the original paper draft but apparently it didn’t conform to title format guidelines :( -mike]

  18. 18
    TheATHiker says:

    By the way Mike great work on your article!
    “13: Does the GCM capture the tendency for a large volcanic eruption to induce a La Nina?
    [Response: Thanks. Ah, well this gets at another long-time pet hypothesis of mine, e.g. Adams et al (2003) and Mann et al (2005). And sadly, no, this GCM does not reproduce that response, but some others do. I think we’ll see some interesting work published on that by several groups in the year ahead. -mike]”

    As if on demand. “Multi-centennial tree-ring record of ENSO-related activity in New Zealand”
    http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1374.html
    Which goes back to 1300 BCE. It not answering you question but it is interesting none the less

    [Response: Thanks for the heads up! -mike]

  19. 19
    Christopher Hogan says:

    Regarding my comment at 10 above, I did a little homework, and now I get it. I had naively thought that mean summertime temperatures might vary fairly substantially from year to year. Not true. I picked three high-altitude stations and summarized the June-July-August mean temperature data from NOAA. Over the last century, the range from coldest to warmest was 3.3C (Flagstaff, AZ), 4.0C (Gunnison, CO), and 4.3C(Tahoe, CA). What that as context, a drop of a couple of degrees C looks pretty large.

    More broadly, this runs counter to the notion that average warming of (say) 1C doesn’t much matter at any one location. For the station nearest my home, the range (max less min) in annual average temperature for the past century is just 3.3C. Can’t say whether 1C warming matters in any tangible way, at this particular spot, but it’s certainly large compared to the variation observed over the last century.

    [Response: Thanks for the comments Christopher. -mike]

  20. 20
    Ted says:

    Dear Mike, [apologies if this is a repeated post … looks like the first did not go through on this end.]

    I have a question about your conclusion. (Well, it’s not a conclusion per se … you characterize it more as ‘food for thought.’) You write that climate sensitivity to 2XCO2 may be higher, because past sensitivity is shown here to be higher than is captured in the tree-ring data.

    This is a different definition of climate sensitivity than I have seen in, for example, the IPCC reports. There, they define climate sensitivity as how strong an effect doubling CO2 will have on average global temperature. This effect is generally calculated based on the change in recent temperatures as compared to the change in CO2 levels (minus, of course, natural variability). That is, we don’t know enough about the physics: the known or potential feedbacks, and the end-result of all the forcings, for our complex global climate. So we use the data to calculate the sensitivity, not the physics.

    You seem to be arguing, however, that climate “equilibrium sensitivity” is known based purely on the physics. I make this inference because you conclude that previously underestimated sensitivity from one forcing – volcanoes – implies present-day underestimated sensitivity from another forcing – CO2.

    In fact, if the physics-based understanding of “equilibrium sensitivity” to any forcing is too low, then not only will CO2 have a greater effect, so too will all other forcings, such as: changes in the sun, in cloud cover, in albedo, etc. The ratios of these forcings, in other words, would be no different in present-day studies, if the climate is thought to be more sensitive to forcings in general.

    Conversely, if “climate sensitivity” for a doubling of CO2 is based on recent measurements and CO rates, and past natural variability is underestimated – as you’ve shown here – then this implies our estimates of sensitivity per CO2 doubling is too high, not too low. That is, natural variability is more than we thought, which leaves less of the 20th century variability (increase) to be attributed to CO2.

    So, if climate sensitivity is based on physics, then all forcings retain the same ratios. If climate sensitivity is based on recent measurements of temperature and CO2, with a discount for natural variability, then showing natural variability (volcanoes, in this case) to be higher in the past, draws the conclusion that natural variability is higher now. This reduces, rather than increases, the effect on global temperatures from a doubling of CO2.

    Does this make sense? I look forward to your response.

    Cheers,

    -Ted

    [Response: Hi Ted, thanks for the question. Actually, we’re using the term climate sensitivity in the same sense, the equilibrium response of mean temp to the surface radiative forcing associated with CO2 doubling. That forcing is just under 4W/m^2, so put differently, equilibrium climate sensitivity is the equilibrium expected surface warming for a radiative forcing of 1W/m^2, divided by 4. That radiative forcing could be longwave (GHGs) or shortwave (solar/volcanic). In principle then one can tease out the climate sensitivity to CO2 doubling from the past response to natural shortwave radiative forcing. That’s what past studies like Hegerl et al (06) linked at the end of my article have done. There are of course a number of assumptions in such an approach, not the least of which is that all forcings (longwave and shortwave) can be treated the same way. And this isn’t necessarily so. -mike]

  21. 21
    Russell says:

    This is wonderfully edifying for so short an exchange, Mike, but your candor as to variability suggests tree rings leave a lot to be desired as temperature proxies.

    Natural selection is more in evidence on wilderness hillsides than flatland orchards growing clones. Clearly a lot of sampling skill is needed to discount trees that have taken root where subsoil variability comes into play, lest that inhomogeneity of nutrient and water availability blur or wash out the temperature component .

    What do you say to those who, having read this thread so far, still think the subject doomed to controversy by intrinsic variability compounded by subjective expertise?

    [Response: Thanks for your comments Russell. I think there is an important context here that is easy to lose in all of the emphasis on the thing that the trees don’t appear to be doing well w/ (i.e. the response to the high-frequency cooling events associated primarily with explosive volcanic eruptions): that’s, the thing that the trees appear to be doing remarkably well with, i.e. capturing the long-term trends and low-frequency variability that is predicted by the climate model simulations. Its of course possible that they agree for fortuitous reasons, but I think it adds additional support to the notion that methods like RCS, championed by folks like Briffa, D’Arrigo and co, etc. are indeed doing quite well in preserving the low-frequency signal. So, forgiving the mixed metaphor, its easy to lose the forest for the trees here ;) -mike]

  22. 22
    Russell says:

    How large a sample is needed to overcome subsoil nutrient and water variability issues at a given elevation ?

    Edifying and candid as this thread is, It leaves some doubt as to operator subjectivity in choosing which trees to core.

    I realize proxy data splicing may be as necessary as it is statistically hazardous, but as few prefer proxy data to the real McCoy, further candor is suggested.

  23. 23
    Russell says:

    Thanks are due Mike for his candid and edifying replies, but proxy data splicing still seems at times as subjective as the history on which it partially depends.

    To what degree could re-measurement with Increased sample sizes and subsoil drilling beside at least some cored trees to measure subsoil nutrient and water flux variability increase confidence intervals, and reduce controversy ?

  24. 24
    David B. Benson says:

    This fine paper seems to resolve to minor mysteries of paleovuncanology, of interest to at least me. The highly accurate ice core data sets rathr precise dates for three major (and tropical) eruptions for which previous studies by traditional methods of paleogeology gave only poorer approximations. These are found in
    http://www.volcano.si.edu/world/largeeruptions.cfm
    with the oldest being
    Quilotoa, Ecuador @ 1258 CE (a distinct sharpening of the uncertain radiocarbon date)
    next being
    Sourfriere Guadaloupe, West Indies @ 1452 CE (removing a +-100 year uncertainty)
    and the youngest being
    Pago, New Britian @ 1809 CE (an improvement of the uncertain tephrochronolgy date).

    As for VEI, both a sharpened considerrable via the ice core results. The Quilotoa eruption needs upgrading to at least VEI 7 while the Sourfriere Guadaloupe and Pago receives a first VEI of at least 6.

    In any case I’m satisfied with the indentifications of the particular eruptions which gave rise to the (most impressive) ice core records and reconstruction offered in this thread.

  25. 25
    Russell says:

    David: I must be the least skilled 1452 eruption site observer on record-

    Besides not noticing the Kuwae caldera, I drove clear across Basse Terre on Guadeloupe too impressed by the greenery to give a thought to its past pyroclastic potential. Historical records and geochemical coverage of Caribbean and Central American eruptions is extremely spotty- I couldn’t find any colonial records for eruptions of the east-west line in Guatemala.

  26. 26
    MangoChuthey says:

    If the LIA was caused by erupting volcanoes to some extent and we could cancel out the LIA, is it possible that the current warming is just one long continuation of the Medieval Warming Period, Roman Warming Period, all the way back to ….. ?

    [Response: No. -mike]

  27. 27
    Paul S says:

    Interesting ideas. By coincidence I’ve recently been looking at the response to the 1809 and 1815 eruptions in the GHCN weather station database, and came upon Emile-Geay’s paper just a few days ago.

    There is surprisingly good coverage at this time across Europe and the picture that emerges across the region is that it’s very difficult to pin down when the eruptions happened by looking at the thermometer data. With the caveat that this is essentially just based on data from Europe, I’m not sure the timing of changes in the tree ring record is beyond what might be expected when factoring in normal natural variability (or enhanced, e.g. your volcano-induced El Nino idea. There appears to be an anomalously warm year in 1811 for instance – possibly a very strong El Nino – and an anomalously cool year in 1812). I’m not sure it’s clear there is a problem to be solved by this solution. Of course, if there is smearing in the data as you describe it might not be immediately obvious due to being hidden within this same natural variability.

    [Response: I guess it depends on how much you trust the models to capture the natural variability. The climate model simulation shown in the figure in the main article above is a coupled OAGCM which does exhibit El Nino-type variability,etc. In no case does that variability obscure the profound cooling associated with the major eruptions. Keep in mind that a big El Nino or La Nina event doesn’t change global mean temperature by more than about 0.2C, and the eruptions we are talking about are associated with global mean cooling about 2C, an order of magnitude larger! -mike]

  28. 28
    MangoChutney says:

    @Mike #25

    [Response: No. -mike]

    because…..

    [Response: Because of nearly two decades of science showing that modern warming cannot be explained by natural factors. I suggest you go to the “start here” tab at the top of our page and read some of our background material on this. Thanks for your comment. -mike]

  29. 29
    Ray Ladbury says:

    MangoChutney,
    The main question your answer ignores is that the energy must come from somewhere. The planet does not simply “warm naturally”. There must be a change in either energy out or energy in.

    Add to this the fact that tropospheric warming is occurring at the same time as stratospheric cooling, that we can actually see the increased bites in the IR spectrum, that the warming matches the characteristics expected for greenhouse warming… Seriously, there is a whole lot more evidence than the temperature record.

  30. 30
    Rob Wilson says:

    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]

  31. 31
    Chris Colose says:

    Mike,

    This seems like a rather eloquent hypothesis in its simplicity (surprising it has not been analyzed before) and strikes me as something that is unlikely to be completely wrong. Thanks for posting this, and keep up the good work. A couple inquiries:

    -I don’t study tree rings at all, so this will be quite naive, but I took a class on paleoclimate (actually with one of your old office-mates, Mathias Vuille) discussing the standarization procedure and why this tends to remove some of the low-frequency variation, though we never got into quantitative detail. Can you comment on why this is the case and to what extent this has been improved by newer methods (i.e., specifically what we can’t see with dendro methods)?

    [Response:I’ll take that one Chris, because I have been intensively working on that very topic for the last year and a half. There are two commonly used detrending (=standardization) methods. The first of these fits some type of empirically determined curve to each individual tree ring time series (tree core), from which residuals are computed, typically by division. The residuals from each ring are then robustly averaged, for each year, giving a time series of yearly estimates of the departure of the climatic variable of interest over the full time span. This is fine for estimates at annual and decadal time scales, but as you approach the timespan covered by each core, you run a serious risk of starting to remove part of the climatic variation, thereby under-estimating the total amount that occurred, with the loss being greatest at the longest time scales (i.e. lowest frequency variation). Ed Cook described this problem in detail in 1995, referring to it as “the segment length curse”, and it is a potentially very serious problem. It is also possible to get the opposite problem (artificially introducing climatic variation that is not real), if you use too flexible of a curve during the fitting. This issue led Keith Briffa and colleagues to re-introduce some time ago a method known as regional curve standardization (RCS), that attempts to solve this problem by fitting curves to some larger collection of ring series, thereby producing a more robust estimate of the ring response expected for a given tree age. This method has its own issues, but in general, it succeeds in its objective, or at least gives a better result than not using it. I’m about to submit a paper describing a completely new approach that attempts to solve the various problems of both methods, accurately returning the true environmental signal at all time scales, which I am confident it does.–Jim]

    – Another naive question. In the future (e.g., by 2100), it seems to me that there could be a substantial temporal lag between the development of a “new treeline” (one that has equilibriated to a new climate) and the timescale over which local climate conditions could increase in temperature by several degrees; do you suspect that areas on the modern treeline will fall into a regime where they never cross below the growth threshold, either owing to increased temperature, or for example more diffuse sunlight owing to more water vapor in the atmosphere (which some have argued can dominate a global dimming signal in the future, regardless of aerosol trajectories)?

    [Response: Thanks Chris, great questions. Regarding the first one, there is a nice recent literature on RCS, etc. and how it seeks to retain low-frequency variability. The D’Arrrigo et al (2006) paper is actually a great source for background and previous related studies. As for the 2nd comment–potentially for moving treeline and lag therein–is very incisive. This is indeed something I’ve thought about and worried a bit about–but I don’t have a simple way to include it in a process-based model. We often think of this process as being so long timescale that its not a factor, but I’m not convinced that this assumptions holds up for the rapid and dramatic high-latitude warming of the past century. My guess is some of the dendro folks reading this (if they don’t hate me too much now) might be willing to comment further on this issue here–folks? -mike]

    [Response: You can in that situation start to run into other issues, in particular heat and water stress. So, it your question is whether [ring response = f(climate)] can itself vary with time, the answer is yes, it certainly can. This is one of a number of things that makes tree ring interpretation difficult.–Jim]

  32. 32

    Not sure if you are familiar with this result or if someone has brought it up already, but a quick search on the subject yields this paper :
    http://www.agu.org/pubs/crossref/2009/2009GL040083.shtml

    This modeling project can be found : HERE, and I believe the first two pages of this particular paper can be had online for preview if you search around.

    [Response: Yes, Timmreck et al is an interesting paper and indeed one of the articles that does focus attention on the enigmatic AD 1258 eruption and apparent response. Its the 5th reference in our article. -mike]

  33. 33
    David B. Benson says:

    Russell @25 — Thank you for your interest in paleovulcanology. In my formative years I lived on the outer (east) side of the Valle Grande, now the Valles Caldera National Preserve. I assure you that most residents of Los Alamos were completely uninterested in the geology, so I doubt your claim of being the least observant.

  34. 34
    Dave Werth says:

    Thinking about missing growth rings I wondered if radiocarbon dating might provide evidence for a chronological discontinuity. I know its accuracy isn’t that sharp but presumably each ring’s age is reflected in its radiocarbon and there might be a big enough change across a discontinuity to show something’s missing. I don’t know enough about it though to know if that’s even feasible or not.

    [Response:No, C14 can’t resolve ages to individual years.–Jim]

  35. 35
    Nick Barnes says:

    A very interesting summary; I wish I could read the paper.

    [Response:Thanks for your questions Nick, some responses below. You can find the paper here -mike]

    A few questions:

    1. Do the high-frequency signals from these eruptions show up in other paleo records, specifically other T proxies? (corals, lake-bed sediments, etc)? Or do we just have the ice cores? You say “ice core and other evidence”; what other evidence?

    [Response:The ice core evidence regards the magnitude of the eruption, not the climatic impact (though in principle, ice core isotopes in some locations would record the cooling signal). There is wider-scale evidence of a large climate signal in the wake of the eruption, and that is discussed in the Emile-Geay paper linked (and in further detail in the Stother paper linked in an earlier comment). However, are pseudoproxy analysis suggests that the volcanic cooling signals of interest will only emerge in coarse-scale averaging over multiple records in multiple regions, because of the obscuring impact of proxy noise and regional climate noise. I.e. its hard to clearly detect the signal in isolated records. see the Supp Info for the details (there is brief discussion in the paper). Herein lies the conundrum, because it is only tree-rings which offer widespread spatial sampling at annual resolution. So that is a bit of a condundrum, and again this is discussed at length in the supp info. -mike]

    2. Some trees, at more favourable sites, will continue to grow so should still show some signal in an eruption year. So drilling down into individual dendro series to find such signals may offer a test for your hypothesis. Has this been done?

    [Response:Yep, I discussed this a bit in one of the earliest comments. In places where there is a sharp lateral temperature gradient it should be possible to do this–this should work for alpine treeline sites. Its much more difficult for boreal treeline sites because temperature gradients occur in this case at the synoptic, rather than micro-scale. -mike]

    3. Do you quantify your prediction of a ‘smearing’ effect?

    [Response:I’m simply referring to what you can see in the Figure insets, i.e. that the apparent signal is delayed, attenuated, and smeared relative to the much sharper signal produced by the climate model simulations. -mike]

    I look forward very much to a constructive exchange between yourselves and the dendro community. Their low-frequency record is excellent, and I hope that this might lead to improvements at higher frequencies.

    [Response:Thanks–that’s my hope too, that this leads to constructive progress in this field. Our analysis shows, as you allude to, that the RCS tree-ring reconstructions appear to be doing quite well at the low-frequency. That’s a feather in the cap, in my view, of the dendro folks who have developed and applied these standardization methods. -mike]

  36. 36
    Ranyl says:

    Fascinating stuff.

    The recent paper by Knutti in Nature geo. also suggsts a higher climate sensitivity than 3C as do a multitude of papers looking at the past climates and there must be a lag in temperature rise and tree invasion of regions which might well mean that Northern areas are actually hotter than they have been for a long time, it just taking time for the proxies to catch up by growing.

    If climate sensitivity is higher than thought as so much evidence keeps suggesting recently, when do actually take this seriously and just stop using fossil fuels on a personal and general level?

  37. 37
    steven mosher says:

    Tight. Well argued. Circumspect.

    [Response: Thanks for your comment. -mike]

  38. 38

    Would it be useful to take tree ring readings from the tree line and on a transect down slope, say every 200m of altitude. Presumably, if a ring is missing at the tree line, it will be present at some elevation below this. Not only would this correct the chronology but there are a few other interesting results that would come out of the data.

    [Response: Thanks for your comment. Yep–this was discussed a bit further up in the thread. This could certainly be done (and perhaps has been done in some cases) for the trees at/near alpine treeline. The problem is doing the same for boreal treeline case. No obvious way to do that, unfortunately. -mike]

    [Response:Salzer et al. (2009) did something similar to this a couple of years ago, though not for that reason if I remember right.–Jim]

  39. 39
    Tony says:

    1258 and 1453 hit me as historic dates (Mongols/Constantinople). Any thoughts?

    [Response: Well, interestingly the Stother (2000) article does allude to some human societal impacts, e.g. from the abstract: The frequent cold and rain that year led to severe crop damage and famine throughout much of Europe. Pestilence repeatedly broke out in 1258 and 1259; it occurred also in the Middle East, reportedly as plague. Another very cold winter followed in 1260-1261. The troubled period’s wars, famines, pestilences, and earthquakes appear to have contributed in part to the rise of the European flagellant movement of 1260…. -mike]

  40. 40
    Dave Werth says:

    @34: Jim, I realize that C14 dating can’t resolve individual years but I thought maybe there could be a detectable difference across a 2 or 3 year chronological discontinuity in the tree rings that would be evidence for the discontinuity. I suppose that’s still too short a period though.

  41. 41

    Just poking around, and found this:

    Some tree species, for example larch, are notorious for having incomplete or even (locally) absent rings, most often due to high levels of defoliation by insects. In this case wood may be laid down only on whichever side of the tree has sustained the least defoliation, as photosynthesis might only happen there. Marker chronologies are very important in detecting absent rings; the only evidence that a ring is missing from a sample is the desynchronization of the following marker years with respect to the reference chronology.

    It’s not in primary literature, but in a popular exposition:

    http://www.envirothon.org/pdf/CG/dendrochrono.pdf

  42. 42

    So if I have this straight, post-volcanic cooling may be a problem in tree-ring reconstructions, but how big a deal is that? Unless it’s a mega multi-year eruption, isn’t this usually an effect that clears in a year or two?

    [Response: You can see the estimated impacts of the eruptions from the figure above (the red curve). And since the large eruptions are the largest radiatively forced signal of the past millennium prior to the anthropogenic era, these very large coolings constitute an important component of any sensitivity estimates derived from the data. So, yes, this is potentially quite important. – mike]

  43. 43
    Michael Doliner says:

    Perhaps somebody can help me with this puzzlement. In the past something besides CO2 starts a warming period. This causes a positive feedback that releases CO2 which is the cause of further much greater warming. OK, I got that. What troubles me is the reverse process. Since a small amount of warming tiggers release of CO2 and more warming, it should be too warm for any small reverse process to take hold. What gives?

  44. 44

    “Since a small amount of warming tiggers release of CO2 and more warming, it should be too warm for any small reverse process to take hold.”

    I don’t follow you. Cooling temperatures would still cause atmospheric CO2 to decline over time, since the capacity of surface water to ‘hold’ CO2 is an inverse function of temperature.

    Perhaps it’s worth mentioning that other feedbacks can come into play, too, such as water vapor and albedo changes.

  45. 45
    Hank Roberts says:

    > CO2 … reverse process

    The processes are called “biogeochemical cycling”

    http://www.enviroliteracy.org/article.php/478.html

    The recommended resources linked at the bottom of that page will be helpful

  46. 46
    Jim Eager says:

    Michael, think about what would happen as small variations in earth’s orbit changed such that the northern polar region received increasingly less sunlight each summer, preventing all of the previous year’s snowfall from melting.

    Would it not build up and eventually compress into permanent ice?

    And as that permanent ice cap grew it would cover an increasingly larger area and reflect an increasing amount of incoming sunlight back to space (less energy in), causing earth as a whole to cool.

    As the atmosphere cooled it would then be able to hold less water vapour, which would reduce the greenhouse effect (more energy out). And eventually as the far more massive ocean cooled it would be able to hold more dissolved CO2, so atmospheric CO2 would be drawn down, thus reducing the greenhouse effect further (even more energy out).

    The feedback loop would continue until the decline in incoming energy slowed and then stopped, leaving earth in a new stable climate regime with continental glaciation.

    It’s the same series of an initial forcing (change in insolation due to Milankovitch orbital cycles) being amplified by reinforcing feedbacks (change in albedo, change in temperature and partial pressure regulating both CO2 and H2O), but in reverse from an exit from a glacial period.

  47. 47

    #43-44) None of what you are discussing happens on the annual basis relevant to tree rings and volcanism.

  48. 48
    Jim Eager says:

    Quite true, Thomas, pardon my adding to the off topic excusion.

  49. 49
    Thomas says:

    Mike, I want to throw out a couple of ideas. Perhaps they’ve been studied I’m certainly no expert in this area.

    [Response: Thanks very much for your comments. Some short responses below. – mike]

    (1) I’m guessing near treeline samples are preferred because they are more sensitive to temperature, and under normal conditions exhibit lower variability. Or could it be because the natural lifetime of trees is too short in warmer climes?

    [Response: Its because temperature sensitivity can be assumed at the treeline. – mike]

    The following are some speculative microclimate effects that *might* be applicable near alpine treeline.
    (1) How is the growth of alpine trees affected by the data when snowcover melts? I would think that years with early snowmelt might be locally warmer near treeline. Could the first year or two after an eruption have anomlously early snowmelt, with respect to synoptic conditions, because ash fall dirties the snow and aids its melting? Could this be the cause (or a cause), for a weak or missing signal shortly after an eruption?

    [Response: Snowmelt (and associated latent heating) represents an interesting issue. Ultimately, it would be nice to see followup forward modeling studies which attempt to use all potentially relevant variables, rather than the simplified model form we use (which is generally justified at treeline–but perhaps only to first order approximation, i.e. some of the details of changing hydrology and solar insolatino might still matter in these environments.). Ash should not be an issue as fallout is only significant in regions proximate to an eruption–there is no hemispheric-scale ash deposit associated with these eruptions. If there were, it would be far easier to establish volcanic eruption chronologies, i.e. we wouldn’t have to go to polar ice cores. – mike]

    (2) An argument against sulphate injection geoengineering is that a reduced land ocean temperature gradient during the warm season can weaken monsoons. I assume this contrast is caused by the different cooling timescales of ocean and land, i.e. rapid global cooling for any reason could weaken monsoons, until the ocean has time to catch up to the cooling trend. Perhaps the local weather -especially summer cloudiness is affected by altered monsoon strength. At least some mountain sites experience monsoonal conditions, notably the Himalayas and souther rockies. Could this effect be introducing an additional signal into the alpine datasets?

    [Response: Certainly, failed monsoons, etc. appear to be one significant impact of very large eruptions, but the effect is a bit mercurial. Indeed, one complicating issue is that many volcanic eruptions are shortly followed by large El Nino events (e.g. Emile-Geay argue strongly for such a response to the AD 1258 eruption). We’ve argued that there is a causal connection here e.g. Adams et al (2003) and Mann et al (2005). Since El Nino also has an important impact on the Asian Summer Monsoon in particular, its hard to know precisely what large-scale changes in atmospheric circulation are due to the radiative forcing of the eruption itself, and the secondary response to that eruption of ENSO. As you might imagine, we have written a paper on this too, Fan et al (2009). -mike]

  50. 50
    AlaskaHoundish says:

    Lookinbg at the water content of the tree’s in question should be an important factor (IE: the moisture purity & filtering of constiuents).
    It may be a worthy endeavor to look at how well the trees in question compare to what we haven’t been looking at; the rate of decay of the constiuents held within the trunk and the conditions that allow absorbtion of inpurities or lack thereof. The rings themselves offer a baseline but without knowing how the moisture content via root systems and absortion through the skin reacts to different conditions and contaminents may subtract from our understanding of this phenomena.
    The speed of plant growth relies only partially on temperture and photon impact with the remaining factors attributed to it’s moisture content intake and what is contained in that source.
    All The Best!

    [Response: Thanks. Yes, this is all well known and discussed in the paper, where we discuss the models for tree growth in their full generality, and then note the assumptions that typically allow one to assume nearly exclusive sensitivity to temperature alone at temperature-limited treeline locations. Its precisely because temperature becomes the leading limiting fact at treeline that such locations have been used for past temperature reconstructions, just like trees growing near their hydrological limits are chosen for drought reconstructions, etc. But see my reply to the commenter above. There would still be some utility for followup studies using more elaborate and complete process models for tree-growth. – mike]


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