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Tree Rings and Climate: Some Recent Developments

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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.
M08

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.

References:

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).

Reader Interactions

79 Responses to "Tree Rings and Climate: Some Recent Developments"

  2. 35 physicist said that a = GM/r^2 . I’m sure you must’ve really meant that the acceleration of the sun towards the earth is a = Gm/r^2 and doesn’t depend on the mass of the sun and that therefore the orbit depends only on the Earth’s mass but not the sun’s.

    (occasionally one or the other of you find my sense of humor to be lacking in the funniness stuff. this is another one of my feeble attempts at it).

  5. Is there anyone doing research on using high frequency ultrasound to measure tree ring density? Would anyone like to? IMHO an mechanically scanned ultrasound reflectometer should be straightforward to implement and less expensive than Xray technology.

  6. Another thing about the authors’ press release: even if it’s true that the Roman warming globally (as opposed to their one regional sample) is of the same order as that of today or even bigger, their graph shows a cooling trend for the last 2000 years, which should have been emphasised in recent years by an unusually deep solar low. Yet the right hand side of the graph has an upward shift. This does not in any way suggest that anthropogenic warming is not a real or significant effect. On the contrary, it goes against a trend over 2000 years (“a cooling of -0.3°C per millennium”), and I don’t suppose they’ve shown a change in the sign of the solar forcing trend. Without that cooling trend, we would be 0.6°C warmer now and remember also that we have not stopped the underlying cause of warming so it will continue.

    I’m curious as to why the error bars are no bigger the older the data gets. Even if techniques have improved since the early reconstructions, wouldn’t there be significantly more uncertainty with older measurements?

    Mike, I can see why you may be a tad annoyed that they are making such unsupportable claims after all you’ve gone through. I doubt somehow they are going to be hauled before a political kangaroo court and accused of faking their results, scientific incompetence or dishonesty.

  13. I think the downward trend line presentation is a good one. Good to see where things were going before mankind became fully dedicated to burning fossil fuels. Adding the data after 1900, I think, would somewhat distort the picture.

    Also, what does that trend line say about the pop theory that mysterious oodles of unknown origins are causing a recovery from the LIA.

  14. Sue, people reading the press release don’t see “1900” anywhere.
    Look again at the text and the picture in the press release

    You say it “states the trend which stops around 1900” — that’s in the paper, not in the press release. The text of the press release is all about cooling.

    You know better. I’m just pointing out the information is omitted from the press release text and the image provided with it.

    Yes, the red line stops before the end of the squiggle in the picture in the press release. You know what that means. But it’s not stated in the release.

    Most of the blog/press coverage reads like they only saw the press release.
    The press release is incomplete. They usually are. This one’s especially bad.

  16. I’m a physicist who admittedly knows little about climate change .. but I’m trying to learn. I wanted to thank the authors for the excellent article and the good discussion in the comments. It’s enormously helpful to me. I had the same question many others raised as well … a better understanding of the technique used by Esper et al. to overcome the divergence problem. I then read the Esper paper and found that the discussion at this site helped me to follow the arguments in the paper. They also have an excellent description of the Maximum Latewood Density technique. If you were confused by the discussion here, it’s explained well in the paper.

    [Response:The idea that this paper was somehow an improvement on, or solution to, the divergence effect, or that the authors developed some kind of new approach to deal with it, is not supported by the paper or the larger collection of existing studies. They make nothing of that issue, barely even mentioning it in passing, and then never again. It’s likely a fortuitous result, possibly related to use of density data and possibly not, but not one that is new–there have been numerous studies in which divergence at decadal scales was weak or absent. If you look at their Fig 3 you can see that the difference between density and ring width data is not great in recent times–they track each other fairly well but diverge as you go back in time. Nor do I agree that the authors were really making any adamant claim regarding the global prevalence of this orbital forcing–though they could have been clearer on that point. But they do provide some good evidence from widely differing sources regarding trends in far N Scandinavia and the sub-Arctic more generally (e.g. their Fig S1), and that ring widths and densities give different trend estimates. Their larger point, that a lot of reconstructions include a lot of northern tree ring sites in them, and these could well be affected by any weaknesses in the ability of existing analytical methods to capture long term negative trends in ring widths. That’s important, because a large number of studies are based on ring widths rather than density data.–Jim]

    I have a question that I’m really puzzled about. From the discussion above:

    “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 …”

    Can someone please explain to me .. or at least point me in the right direction … what change in the earth’s orbit are we talking about that occurs on the 1000 year time interval? Is this the accepted explanation for both the Roman and the medieval warm periods? Finally, what is this orbital forcing doing now? Thanks!

    [Response: The orbital forcing being referred to is (mainly) the shift in precession over the last 8000 years. This is a cycle that has a periodicity of about 19,000 years and relates to the position of the perihelion (closest approach to the sun) with respect to the seasonal cycle. (There is a shift in obliquity (tilt) that also plays a role over this time period too). Currently, perihelion is Jan 3, while in the early Holocene it was in August. The expectation is that if you are closest to the sun during the summer, the summer will be hotter. Thus NH summers are expected to have got slightly cooler over that 8000 year period, and will be expected to start warming up again in a couple of thousand years. But note that these are very slow long-term trends, and cannot be explanations for the much more rapid ups and down around that trend. Thus the Esper et al claim is *not* that the Medieval period climate anomaly was caused by orbital forcing, but rather that with more accurate low frequency variability in tree rings, the medieval period might have been warmer than existing reconstructions show. – gavin]

  17. I looked quickly for mention of other papers mentioning high latitude temperature, comparing that to other locations. Here’s one:
    http://www.agu.org/pubs/crossref/pip/2012PA002291.shtml
    A precise search for drastic temperature shifts of the past 40,000 years in southeastern Europe

    “… the geographic extension of temperature anomalies is largely uncontrolled due to the scarcity of quantitative records of sufficient time resolution on the European continent. Here, we propose, based on a recently developed temperature proxy (TEX86), a reconstruction of millennial-scale temperature variations in a Black Sea sediment archive for the last 40,000 years….. In notable contrast to observations from nearby archives, Heinrich events imprinted our glacial temperature record, consistent with a strong reorganization of oceanic circulation and a large spreading of the temperature anomaly from the North Atlantic toward the south-east. Furthermore, in contrast to high latitude records, our Black Sea record lacks of the signatures of Dansgaard-Oeschger interstadials suggesting a decreasing temperature gradient away from the North Atlantic.”

  18. Gavin — Thank you very much for taking the time to clear that up for me. I completely misunderstood the context in which orbital forcing was being used, especially when the title of the paper is “Orbital forcing of tree-ring data”. This is an incredibly valuable service you are providing.

  19. JoeT — I recommend reading The Discovery of Global Warming” by Spencer Weart:
    http://www.aip.org/history/climate/index.html
    if have not yet done so. I also recommend studying Ray Pierrehumbert’s “Principles of Planetary Climate”
    http://geosci.uchicago.edu/~rtp1/PrinciplesPlanetaryClimate/index.html
    which I found fascinating, although more difficult than even QM. After a reading of the entire book and then careful study of chapters 1 through 6 I know think I have a fairly decent grip on climatology.

  20. Update 6/12/12: Media Matters comments on the latest misrepresentations of the Esper et al study discussed in our article: ‘Surprise: Fox News Fails Paleoclimatology’
    Update 6/13/12: 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’”

    Your update dates are wrong by a month.

    [Response: Indeed. Thanks. – gavin]

  21. Just been on a 2 week holiday and although I followed some of the discussion on my phone, I was not able to make any comments. As Jan Esper and other co-authors have been silent, I feel it would help if I clarified a few issues:

    1. It has not been a” tough few months for tree-rings” at all. This comment is likely related to Mike Mann’s NG paper earlier this year. A response is in review and I am sure more will be discussed on this topic over the coming weeks/months.

    [Response: Yes–I’m looking forward to this, but no the comment wasn’t referring to that study so much as it was mainly referring to your paper and comments both in it and in press release issued about it that call into question the reliability of tree-ring widths (TRW) for all long-term reconstructions. Indeed, if it is true that the multi-century/millennial trends in TRW data are more generally not reliable, then it has deep implications for long-term drought reconstructions from TRW as well, and the discussion that was included in the RealClimate piece (in part, for this reason) about the recent Steinman et al PNAS article. The apparent discrepancies between the lake-based precipitation reconstructions presented in that study and tree-ring drought reconstructions becomes even more interesting and relevant in this context as, in part, the two agree well on decadal and multidecadal timescales and it is only the long-term trend which is fundamentally in disagreement (of course there are other factors such as seasonality that come into it). As Gavin has noted (and our piece makes quite clear) the claim in your paper that this finding has implications for long-term annual hemispheric temperature reconstructions however appears without support. You can expect some discussion of this in the peer-reviewed literature too. In any case, thanks for dropping by! -mike]

    2. Many commentators have been getting their apples and oranges mixed up between regional and large scale reconstructions. The N-SCAN reconstruction represents a new regional reconstruction of JJA temperatures for the northern Scandinavian region and comparison to larger scale composites and assertions of global climate changes need to be made with extreme caution. The basic observation is this: MXD data, measured from tree samples from northern Scandinavia, when appropriately processed (using Regional Curve Standardisation) to capture trends longer than the mean length of the samples, portray a long term decline in values which agrees with the expected orbitally forced trend for this location which is also seen in other long-term non-tree-ring proxies for high latitudes (Fig S1). The RW data when similarly processed do not show this trend. This has potentially massive implications, as Jim rightly realises, for larger scale hemispheric proxy temperature composites which utilise high latitude RW series. So that is the hypothesis – simply put – if this MXD vs. RW bias exists for all high latitude regions, then all larger scale composite reconstructions which have utilised high latitude RW data may underestimate temperatures during earlier periods. As we clearly state in the paper, quantification of this potential bias is not really possible as there are (1) few high latitude well replicated tree-ring records which have both RW and MXD measured from the same samples; (2) different large scale composite series are calibrated against different temperature targets (full NH, extra-topical NH temps, summer vs. annual etc etc) and (3) large scale composite series normally utilise a mix of high vs. lower latitude proxies with different seasonal interpretation (summer vs. annual) – the weighting of the individual input proxy series is often unclear. Therefore, looking at long term linear trends in most large scale composite series is not really very useful and does not help to assess our hypothesis at all. We specifically only compared our series to the Kaufman et al. (2009) study for this reason (Figure 3). If the bias exists using high latitude RW data, it WILL have implications for attribution studies as a few tenths of a degree (which does not sound much) will have an effect over the Medieval period for example. Reference to the Hegerl (2006/07) studies is not really relevant as their attribution analysis does not even include the Medieval period. The fact of that matter is that prior to ~1250, large scale reconstructions and model output do not agree well at all (see AR4). Even Mike Mann only showed the post 1200 period in his recent NG article.

    [Response: Rob, thanks for coming by. The difference between TRW and MXD is very interesting and it will be interesting to see how this plays out in the tree-ring reconstructions. However, for the large scale reconstructions, it is not at all obvious that the result automatically means that the very lowest frequencies are underestimated. As mentioned above, Moberg et al only uses tree rings for the high frequency component, and more recent reconstructions use a mix of sources. The orbital trends are expected to be most important in the northern high latitudes and in the summer, and any differently targeted reconstruction will have less of an effect. That they all have negative trends over this period – that are frequently larger than the modelled trends you showed – indicates that it might not be as severe a problem. – gavin]

    3. The Divergence issue is not as big a problem as may be perceived. It is very much a bandwagon which many jump on to and there are different issues at both high and low frequencies and for different regions. Certainly in Europe, Divergence is hardly seen. In my opinion, the region where “divergence” is greatest is central/northern Alaska and the Yukon and recent studies have shown that divergence is expressed more in RW than MXD. The large scale studies of Briffa et al. which showed post 1960 divergence are slowly being re-addressed at regional scales by multiple groups and much of the divergence noted in these earlier larger scale studies can be minimised by targeting of an appropriate season (not the fixed season used by Briffa et al for all locations), reduction of detrending biases (see papers by Melvin and Briffa), and not compositing over too large regions. More on these results and updates will come out over the coming years.

    4. Lastly, I want to highlight the importance of the N-SCAN record for dendroclimatology. This record is the new Gold Standard for TR-based temperature reconstruction and all dendroclimatologists need to aspire to attaining such high sample replication. This is of course not easy. Esper et al. state that the “record was developed over three years” – that is actually only relevant for the measurement of the MXD data. In actually fact, the sub-fossil samples for the study were collected by Finnish colleagues over ~20 years – a difficult time frame to keep funding turning over to acquire such replication. RCS really only works when replication is high (> ~30 series) and periods of low replication almost always behaves “oddly” when different detrending methods are used. The Tornetraesk record is a good case in point where multiple studies over the last 20-years have come up with differing estimates of Medieval temperatures. This is hardly surprising when replication prior to ~1300 is only about 10 series.

    So – let’s not get too hung up on misinterpreting what our paper says. This is a regional study which shows a decreasing trend for almost 2000 years. The Roman, Medieval and present periods are above this long term trend – these trends are not relevant for southern Europe and are certainly not relevant for the globe as a whole. This record should not be used to debunk global warming and people should not get their knickers in a twist if the medieval is warmer than present in this regional record. This is exactly what would be expected from orbital forcing of summer temperatures at this latitude.

    [Response: That would all be fine and well if Esper’s press release hadn’t extrapolated wildly from that narrow specific conclusion to calling into question the IPCC consensus about large-scale temperature reconstructions, etc. As Gavin has explained above, those arguments really don’t appear to hold up when you look at the various reconstructions, the trends in them, comparisons with model simulations (taking into account the greatly reduced orbital trend in annual hemispheric temperatures), and the actual effect that removal of tree rings from that data has on them—which apparently goes in the wrong direction relative to what your paper would predict. Its a pity that you didn’t do that analysis in the paper, because it puts some of the papers comments in a curious light. For example, the paper oddly singles out Mann et al (1999) and Mann et al (2008) for citing possible impacts on large-scale temperature reconstructions, though these reconstructions show among the largest long-term cooling trend. In fact the cooling trend in Mann et al (2008) is larger than the models predict for high-latitude summers! And it is actually the Esper et al (2002) reconstruction, which is based purely on tree-rings (from the summer and largely extratropics), that shows among the smallest long-term trends of all. It sort of surprising that this wasn’t mentioned in the paper, isn’t it? Thanks again for stopping by in any case! -mike]

  22. > difference between TRW and MXD

    I’d think this might be interesting to forestry/wood industry/woody agriculture people too, now that it’s been pointed out.

    Has anyone tried forcing trees (bonsai science?) in lab conditions to see what correlates with making an annual ring wider, or denser? I wonder if something like isotopic tracers would show where cells were being formed for example.

    Can you do really slow positron emission tomography for example, looking at where and when for biological activity — cell division? cell size? — as a tree ring is formed over a year?

    Structurally in a tree trunk, what’s the difference between a trunk with wider or narrower rings, or denser or less dense rings? Stiffer or more flexible/stretchable in windy conditions for example?

  24. Re- Comment by Hank Roberts — 16 Jul 2012 @ 9:08 AM:

    I am also interested in more information about the microscopic structures that underlie the measurements and how they translate to tree and wood qualities. Steve

  25. Steve, see the first few pages of results from that search, or do the same search with Scholar for the last couple of years. There’s an amazing wealth of info, good summary and review papers.

  27. @69 David B. Benson: This was just what I was looking for. Thanks for the suggestions!

  28. Dear RC

    Once again thanks for your amazing correspondence on all aspects of ACC/AGW. This article is timely and some of the answers from you guys in the thread makes me realise for many years now that if it was not for you lot here and a few other good websites then the medias approach to senstaionalism and hype would have won out over me and I would be questioning evrything wrongly.

    Brilliant work as per usual.