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Tree-ring proxies and the divergence problem

What the science says...

The divergence problem is a physical phenomenon - tree growth has slowed or declined in the last few decades, mostly in high northern latitudes. The divergence problem is unprecedented, unique to the last few decades, indicating its cause may be anthropogenic. The cause is likely to be a combination of local and global factors such as warming-induced drought and global dimming. Tree-ring proxy reconstructions are reliable before 1960, tracking closely with the instrumental record and other independent proxies.

Climate Myth...

Tree-rings diverge from temperature after 1960

Actual reconstructions "diverge" from the instrumental series in the last part of 20th century. For instance, in the original hockey stick (ending 1980) the last 30-40 years of data points slightly downwards. In order to smooth those time series one needs to "pad" the series beyond the end time, and no matter what method one uses, this leads to a smoothed graph pointing downwards in the end whereas the smoothed instrumental series is pointing upwards — a divergence (Climate Audit).

Tree growth is sensitive to temperature. Consequently, tree-ring width and tree-ring density, both indicators of tree growth, serve as useful proxies for temperature. By measuring tree growth in ancient trees, scientists can reconstruct temperature records going back over 1000 years. Comparisons with direct temperature measurements back to 1880 show a high correlation with tree growth. However, in high latitude sites, the correlation breaks down after 1960. At this point, while temperatures rise, tree-ring width shows a falling trend. This divergence between temperature and tree growth is called, imaginatively, the divergence problem.

The divergence problem has been discussed in the peer reviewed literature since the mid 1990s when it was noticed that Alaskan trees were showing a weakened temperature signal in recent decades (Jacoby 1995). This work was broadened in 1998 using a network of over 300 tree-ring records across high northern latitudes (Briffa et al. 1998). From 1880 to 1960, there is a high correlation between the instrumental record and tree growth. Over this period, tree-rings are an accurate proxy for climate. However, the correlation drops sharply after 1960. At high latitudes, there has been a major, wide-scale change in tree-growth over the past few decades.

Figure 1: Twenty-year smoothed plots of tree-ring width (dashed line) and tree-ring density (thick solid line), averaged across a network of mid-northern latitude boreal forest sites and compared with equivalent-area averages of mean April to September temperature anomalies (thin solid line). (Briffa et al. 1998)

Has this phenomenon happened before? In other words, can we rely on tree-ring growth as a proxy for temperature? Briffa et al. 1998 shows that tree-ring width and density show close agreement with temperature back to 1880. To examine earlier periods, one study split a network of tree sites into northern and southern groups (Cook 2004). While the northern group showed significant divergence after the 1960s, the southern group was consistent with recent warming trends. This has been a general trend with the divergence problem - trees from high northern latitudes show divergence while low latitude trees show little to no divergence. The important result from Cook 2004 was that before the 1960s, the groups tracked each other reasonably well back to the Medieval Warm Period. Thus, the study suggests that the current divergence problem is unique over the past thousand years and is restricted to recent decades.

This suggests the decline in tree growth may have an anthropogenic cause. A thorough review of the many peer reviewed studies investigating possible contributing factors can be found in On the ’divergence problem’ in northern forests: A review of the tree-ring evidence and possible causes (D’Arrigo 2008). Some of the findings:

  • Various studies have noted the drop in Alaskan tree-growth coincides with warming-induced drought. By combining temperature and rainfall records, growth declines were found to be more common in the warmer, drier locations.
  • Studies in Japan and Bavaria suggest increasing sulfur dioxide emissions were responsible.
  • As the divergence is widespread across high northern latitudes, Briffa et al. (1998) suggests there may be a large scale explanation, possibly related to air pollution effects. A later study by Briffa proposed that falling stratospheric ozone concentration is a possible cause of the divergence, since this observed ozone decline has been linked to an increased incidence of ultraviolet (UV-B) radiation at the ground (Briffa et al. 2004).
  • Connected to this is global dimming (a drop in solar radiation reaching the ground). The average amount of sunlight reaching the ground has declined by around 4 to 6% from 1961 to 1990.
  • One study suggests that microsite factors are an influence on whether individual trees are vulnerable to drought stress. Eg - the slope where the tree is located, the depth to permafrost and other localised factors (Wilmking & Singh 2008). This paper amusingly refers to the divergence problem as the "divergence effect" so as "to not convey any judgement by the wording" (you wouldn't want to offend those overly sensitive Alaskan trees).

There is evidence for both local and regional causes (e.g. drought stress) as well as global scale causes (e.g. global dimming). It's unlikely there's a single smoking gun to explain the divergence problem. More likely, it's a complex combination of various contributing factors, often unique to different regions and even individual trees.

One erroneous characterization is that scientists have been hiding the divergence problem. In fact, tree-ring divergence has been openly discussed in the peer-reviewed literarure since 1995. A perusal of the many peer reviewed papers (conveniently summarised in D’Arrigo et al. 2008) reveal the following:

  • The divergence problem is a physical phenomenon - tree growth has slowed or declined in the last few decades, mostly in high northern latitudes.
  • The divergence problem is unprecedented, unique to the last few decades, indicating its cause may be anthropogenic.
  • The cause is likely to be a combination of local and global factors such as warming-induced drought and global dimming.
  • Tree-ring proxy reconstructions are reliable before 1960, tracking closely with the instrumental record and other independent proxies.

Intermediate rebuttal written by John Cook

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Additional videos from the MOOC

Interviews with various experts

Expert interview with Tim Osborne

Last updated on 29 October 2016 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

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Comments 1 to 25 out of 88:

  1. I'm guessing that acid rain had a *lot* to do with the sudden decline in tree-ring width post-1960. It certainly doesn't prove any kind of global cooling post 1950. It does, however, highlight the need to use more than a single proxy for determining climate in the absence of direct temperature measurements
  2. Mr Cook: Regarding: "The divergence problem is unprecedented, unique to the last few decades, indicating its cause is anthropogenic." One thing I don't understand is this assumption that if we don't know what causes something in the climate system to happen, it must be us. I think the best way to explain this is the study on the links between telephone poles and cancer, where scientists found that in areas with high per-capita telephone poles, there were higher rates of cancer. Thus, telephone poles must be causing cancer! Now, this sounds silly, because there are other factors which may cause an increase in both factors (something to do with industrialization/urbanization, etc.), but it is quite comparable to current climate science. We assume that because we can't find any other natural factor that could cause the current warming (or tree ring divergence), it is automatically us. But we can't assume that. We have to assume it is natural until it is proven to be caused by us.
    Response: It's not definitely proven that the divergence problem is caused by us but the fact that over the last 1000+ years, it's only occured in the last 40 years is highly suggestive, particularly when likely causes like global dimming are anthropogenic. However, the key point is not that divergence is anthropogenic but that the evidence indicates divergence has not occured before recent decades so tree ring proxies are reliable before 1960.
  3. I am confused. THere seems to be a logic problem here. If the proxies are incorrect post 1960 or there is a divergence at one time and you don't really KNOW the cause for that divergence then how can anybody conclude that there weren't other divergences you didn't understand in the past? Just because we don't see divergence between north and south there could be something which affected tree ring data over any period of time in the past either depressing or increasing temperatures that actually ocurred. You really can't have any confidence in this proxy until you understand the cause. What if the cause is caused by droughts? What if there was a large drought over the areas north and south covered by these trees? What if there was a huge flood or volcanoes or some other co-incidence like a increase in acidification due to some bacterial or animal or plant extinction or proliferation? The point is not the specifc thing but the logic being used here which is flawed by you guys. The fact is that the "science" is still very nascent and major things like what is affecting tree ring densities and widths is not really understoof even TODAY let alone 1,000 years ago. It is hard to have confidence in you "scientist" proclamations when a simple computer scientist from MIT can see the logical flaws in your arguments. Maybe because I was trained in math and physics my expectations as to logic and proof are much more stringent. For instance, the idea you can use these proxies and eliminate trees so easily, use so few and not have a well understood rigorous process for these things leaves me totally skeptical. This doesn't seem to qualift as science but more like "social science" which is based on surveys and self-reported data. A lot of the climate science stuff strikes me as having these logical errors. For instance, the models are built and calibrated, initialized to match historical data, then "scientists" say they match history good and predict the future. That's just falacious reasoning. I can construct many milllions of models which match history but which fail immediately to predict the future. In fact that looks to be exactly the case. Modelers admit (from what I've read) that they cannot explain current 10 year trends, whether down or flat or even up a little they don't correspond to any models output. WHereas these models are more accurate in past temperature predictions. Of course they are, they are not used UNLESS they fit past temperature records. But that does not give them ANY credibility in saying they "predict" the future. That would have to wait for confirmation that they actually DO predict temperatures and so far they aren't. Doesn't that pretty much say the models are flawed? Even if you say the models represent almost all relevant factors the errors introduced by their mathematical nature means that the ability to say with any assurance the predictions have a numerical preciseness is almost impossible. The temperatures could be -10 or +30 and still be in the models error bars. So there is no predictive power possible even if the models were correct. We would all be better reverting to a simplification and look at the earth as one giant thermodynamic entity than trying to break it into pieces and understand interelationships in the pieces at a micro level. THis just seems numerically implausible to return any level of accuracy useful over even a few iterations.
  4. I think some of the statements in this article are worded overly strongly. Tree rings can also be sensitive to moisture or other constraints on growth. This article makes it sound like tree rings are only ever sensitive to temperature, but they aren't. If I understand it, the trick is to find those trees that are primarily reporting temperature. Also, you don't know what's unprecedented in the time before the thermometer record picks up around 1850-1880. Maybe divergence has happened before; you'd only know by checking against other proxies. But other proxies come with their own uncertainties. So I'd suggest softening some of the language a bit.
  5. This is the most comprehensive explanation I've seen anywhere on this topic, and it's not too strongly worded. I have been looking into tree sensitivity to pollution ever since I realized that the trees are not only growing more slowly, they are actually dying at a rapidly accelerating rate. This is being reported from all over the world, not just around my farm in New Jersey. Every species of every age is in decline, as is the understory of the woods. Ozone interferes with the ability of vegetation to photosynthesize by damaging the stomata of foliage and needles. Last year, even annual plants showed the unmistakeable symptoms of exposure to toxic greenhouse gases, which is a stippling of the leaves and loss of chlorophyll, in extreme cases turning leaves into brown webs. Crop losses were disguised by the USDA and blamed on weather, but if we do not recognize this problem, famine will be the result. Photographs and links to research are posted at
  6. I don't think this is good enough. The "divergence" is evidence, surely, of some severe environmental stress, one that could at any moment start to impact on our food supplies. If this was any other branch of regular science, an expermiment would be rigged up, 6 or 16 greenhouses in a row, one with increased CO2, one with reduced CO2, one with artificial acid rain, one with extra ultra-violet and so on and so forth. We'd have at least parts of the answer in 12 months. Why is nobody treating this problem as a matter of urgency, surely we need to know what's going on?
  7. MalcolmMcDonald, "surely we need to know what's going on?" i agree but it's not up to climate scientists. I'd be tempted to ask for more research funds, but you know, people would think it's just the standard complaint from scientists.
  8. Malcolm, the research you wish for is very much in play. Simply bounce over to Google Scholar, try searching on "plant metabolism anthropogenic C02" or "plant metabolism acid rain" and you'll see what I mean. The first term produces some 7,000 results, the second (being the subject of earlier interest) over 45,000. Plugging in better terminology for search terms will yield more and better results.
  9. How's this for a simple explanation. Certain high latitude trees are adapted to ice age conditions-ie most of the last million years or so. They will respond to slightly warmer condtions and follow a warmer temperature record to a point, but with too much warmth, accompanied by a decrease in moisture and rainfall, these trees reach a 'threshold', and start to diverge from the temperature record, as in the last few decades. Once this threshold is passed growth rates and tree rings start to diverge from temperatures. Because the last several hundred years, prior to the late 20th century, has been below this threshold, tree rings closely follow measured temperatures (eg back to ~1800s), and also various other proxy temperatures back to the ~end of the Medieval Warm Period. At about 1960 the 'threshold' was passed and these tree rings diverged from the temperature record, unique in the last several hundred years at least. If this simple explanation is the case, there is no way you can use tree rings to ascertain temperatures on longer time scales (eg past the end of the Medieval Warm Period) because they will diverge from any warm enough period once the threshold, mentioned above, is passed; obviously any warm temperatures beyond such a threshold wont show up in the tree ring data, you will get a flat line regardless of warmer temperatures, as in Manns 1998 hockeystick, which lacks a Medieval Warm Period. Furthermore, it is not enough to show that tree rings are consistent with other proxies, because it depends on which proxies you pick. Trees ring proxies are consistent with some, and not consistent with others. It is also not much use to 'average' out the various proxies to get an 'average' trend, because any inherant bias in the proxies wil simply become enhanced. For example, if 30% proxies dont pick up warmer temperatures well, coupled with a decrease in both measurable response the further back you go and a reduction in quality of data the further back you go, then 'averaging' out the proxies will produce a flattened/cool bias in the data, as in Mann's more recent papers. It's similar to the averaging out the 'gaps' in the fossil record, you simply get more 'gaps', and the further back in time you go, the more 'gaps' you get. This is a reflection of the imperfection in the fossil record, and not a reflection of the constant evolution of life. It is a preservation/measurability problem, and averaging out the imperfection of proxies over time also gives unreliable/distorted results.
  10. Trees adapting to changing conditions more likely would happen over generations of trees rather than a single generation being able to adapt. Certain individual trees will flourish as they handle conditions whilst others do poorly, so natural selection would create a bias as the improved growth of succeeding generations becomes perhaps a measurement of the ability of the trees to adapt better to existing conditions rather than the conditions themselves necessarily changing.
  11. I think that this is the real reason for the decline in tree-ring data. An increasing amount of CO2
  12. Here are links to every article I've been able to find so far about ozone, CO2 and vegetation:
  13. With regard to tree ring growth diverging from warming. I understand from other sections here that solar activity and cosmic radiation have also declined while temperatures have increased over the past few decades. Is there a possible causal effect due to this correlation?
  14. @ Philip Shehan (13) Welcome to Skeptical Science! There is an immense amount of reference material discussed here and it can be a bit difficult at first to find an answer to your questions. That's why we recommend that Newcomers, Start Here and then learn The Big Picture. I also recommend watching this video on why CO2 is the biggest climate control knob in Earth's history. Further general questions can usually be be answered by first using the Search function in the upper left of every Skeptical Science page to see if there is already a post on it (odds are, there is). Or you can search by Taxonomy. Forcings, except for CO2, have been flat for nearly 40 years. Temperatures continue to climb, and that rate of climb is still increasing (as are CO2 levels). Hope that helps, The Yooper
  15. Thank you Daniel for the welcome. My original post may have been ambiguous. I was specifically wondering if ther was any causal link between reduced solar (or cosmic) radiation and reduced tree ring growth.
  16. @ Philip Shehan @ 15 Tree rings are not my area of expertise, but a re-read of the post above makes it clear there is still significant uncertainty (see the linked summary above for details). I suspect the cause is indeed multi-factorial (with the other factors mentioned in play, tree growth response to the rising temps [we are now at temps equal to that of the Holocene Maximum of 8,000 years ago] of the latter half of the 20th Century may be discontinuous to established response). Certainly the experts in this field are continuing to look at all possible factors (a scientific "race to glory" for bragging rights). My two cents. The Yooper
  17. #15: "any causal link between reduced solar (or cosmic) radiation and reduced tree ring growth." This was suggested by Suess 1980, who was the author of one of the greatest one-liners in all of science: "The line was not drawn by computer - I drew the line by `cosmic schwung'. More recently, Kulmala et al 2009 and Dengel et al 2009 provided some fuel for an uptick of chatter in the deniersphere. The usual crowd made the usual misrepresentations of an otherwise valid study (you can find this by googling 'cosmic rays tree ring growth'; I don't link to sources of disinformation). The basis of the argument is that increased cosmic ray flux stimulates cloud formation (that's not proven) and clouds control tree growth rates (somewhat), so that cosmic ray flux helps explain tree ring anomalies (indirect, uncertain, unsettled). There are some vague resemblances of tree ring cycles to the solar cycle (see Dengel), but that's hardly a smoking gun for cosmic rays as a cause. But with evidence like the figure shown below (Kulmala), it will take more than cosmic schwung to draw a line. The larger subject of cosmic ray flux is discussed in the thread It's cosmic rays.
    Response: [Daniel Bailey] Fixed broken linked image URL.
  18. John, either your inability or choice not to respond to JMath (1/31/10) does not instill confidence. He makes a very valid point of criticism; one I have voiced myself on various forums. The utility of Dr. Mann’s temperature reconstruction is entirely dependent upon an assumption that is neither provable nor disprovable; thus is unscientific. The assumption is, as JMath pointed out, that the tree ring divergence problem only pertains to recent years. Since there are no recorded temperatures throughout most of the time span of the hockey stick graph, there is no way to verify the correctness of this assumption. This in turn casts grave doubts upon the accuracy of the entire reconstruction.
  19. Don Schneider, start by reading Hockey Stick or Hockey League, then look at this RealClimate Wiki, then look at this RealClimate Thread. After that, you should find that Mann's reconstruction is not at all the only reconstruction that gives those results. If you don't like Mann's, what's your problem with all the rest ?
  20. JMurphy, thanks for your response and the links. It is seldom that I have such a quick turnaround in thought, so I find this astonishing. While driving somewhere just a few minutes ago, and mulling over the point I just made on this forum, I had an epiphany of sorts. How ironic I found it to be that the very first comment posted at the first link you provided—a comment made by one ProfMandia—was exactly the thought that hit me! How strange it would seem that if the tree ring proxies were somehow wrong in the prerecorded temperature times of the Mann chart that they should be both wrong and so closely correlated with other reconstructions using various other proxies as opposed to tree rings. As ProfMandia points out, what are the odds of that? What are the odds that they should all be not just inaccurate, but so closely so in the same way? Therefore, unless the skeptics can somehow prove collusion on the part of all these researchers who have presented temperature reconstructions based upon various proxies, then at least the part of the theory that we are living in an anomalously warm period has been proven to my satisfaction. Being convinced that we are responsible for this, and that the result will be necessarily deleterious to a significant extent, might take me some more time Perhaps I shall mull over these latter points while shoveling out of yet our next snowstorm! (G) By the way, I realize it is hard to sell this theory while so many people in the Northern Hemisphere have been enduring two brutal winters in a row; thus the falling poll numbers as to how many take the theory seriously right now. However, adherents do have a point about warmer air causing more evaporation from lakes and such and thus more snow. It hasn’t been, by and large, the temperatures that have been so brutal, just the snow and sometimes ice. When I was a teenager and young man in the 70s here in Philadelphia, I can recall it was not at all uncommon for the temperature in the winter to frequently dip to single digits and even hit zero (F) occasionally. We haven’t experienced too much of that for many years and the ponderous amount of snow we have been getting has largely been (backbreaking!) wet snow as the temperatures unfortunately have a penchant to remain just below freezing. So you might be right on this point as well, as odd as it might seem to others and me. If AGW is true and we must endure it and its potential consequences, I just wish it could at least get us over 32 here! I wasn’t expecting the 70s in January.
  21. Don Schneider, there are plenty more pages on here that will give further information as to the cause of our current warming. Try these two for a start : Newcomers Start Here The Big Picture Also, while I understand that parts of America are experiencing lots of snow and cold temperatures, that isn't the case for the rest of the Northern Hemisphere - as far as I'm aware. Temperatures in Europe, at least, are normal for the time of year, although they are going up and down all the time. And, the last I heard, Canada and the Arctic were pretty mild, comparatively ? All that snow digging will keep you fit !
  22. About the only thing anthropogenic about the divergence "problem" are the thermometers.
  23. Hello, Concerning the divergence problem, I suggest you should be more cautious in the formulation you choose. Two examples from d’Arrigo 2008, whose review is your main source : D’Arrigo 2008 : « Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations » Your choice : « The divergence problem is unprecedented, unique to the last few decades, indicating its cause may be anthropogenic. The cause is likely to be a combination of local and global factors such as warming-induced drought and global dimming. » You are quite more affirmative than your source. D’Arrigo 2008 : « However, the relative scarcity of ring width and density records from the lower mid latitudes, tropics and Southern Hemisphere precludes making definitive conclusions about the spatial extent of this phenomenon, and more research is needed to more fully evaluate the extent of the divergence problem worldwide » So clearly, it’s impossible to say for the moment that DP is limited to circumpolar forest (your « mostly high tlatitude »). For example a well-known scientist of the field observes that mid-latitude tree-rings are also of concern, and that the phenomenon is quite widespread except for low-latitude : « Evidence for reduced sensitivity of tree growth to temperature has been reported from multiple forest sites along the mid to high northern latitudes and from some locations at higher elevation. This alleged large-scale phenomenon reflects the inability of temperature sensitive tree-ring width and density chronologies to track increasing temperature trends in instrumental measurements since around the mid-20th century. » Hereafter, another example at mid latitude with a very recent analysis showing a divergence in Alpin Larch (since 1990) and rising some problems for temperature-calibration in order to reconstruct past climate variations. IPCC AR4 also mentions : "Others, however, argue for a breakdown in the assumed linear tree growth response to continued warming, invoking a possible threshold exceedance beyond which moisture stress now limits further growth (D’Arrigo et al., 2004). If true, this would imply a similar limit on the potential to reconstruct possible warm periods in earlier times at such sites. At this time there is no consensus on these issues (for further references see NRC, 2006) and the possibility of investigating them further is restricted by the lack of recent tree ring data at most of the sites from which tree ring data discussed in this chapter were acquired." It means that if some trees are affected by such a non-linear treshold related to temperature/humidity pressure (no specifically anthropogenic-induced factors), the DP is not necessarily limited to modern era and that tree-growth may have been affected in some locally warmer period in the past. So I suggest you're a bit overconfident and selective in some preliminary conclusions of this very active field of research. Best
  24. I noticed that in early December, McIntyre was again/still attempting to sow doubt about reconstructions. In this case, it was the Briffa et al 2001 line in Figure 1 of Mann et al (2003). In short, that line of the plot appears to end around 1940. Someone asked about this over at RealClimate and I think it might be useful to provide a link to the question and Gavin's response Dr. Schmidt's key points are -
    It's interesting to add that in Briffa et al, 1998, they state: "Over the hemisphere, the divergence between tree growth and mean summer temperatures began perhaps as early as the 1930s;".
    The divergence issue as a recognised problem predates this paper by years (Briffa et al, 1998), and was discussed in the almost contemporaneous Jones and Mann (2004) paper. That paper was a little clearer about what was done and why (i.e. fig 5): The various other (smoothed) NH reconstructions shown in the enlargement to Figure 5a have been scaled by linear regression against the smoothed instrumental NH series over the common interval 1856–1980, with the exception of the ‘‘Briffa et al.’’ series, which has been scaled over the shorter 1856–1940 interval owing to a decline in temperature response in the underlying data discussed elsewhere [Briffa et al., 1998a].
  25. Dawsonjg, Who says including those data sets was in error? Mann uses specific criteria to exclude problematic data sets. Deniers then add data sets they like and claim Mann is in error for including ones that they do not like. Mann responds by showing that deleting the supposed problematic sets does not affect the result. You are confusing unsupported blog criticism with actual peer reviewed criticism. Keep in mind that the Mann hockey stick has been reproduced by numerous other peer reviewed studies. Can you provide a reference to a peer reviewed study that does not show a hockey stick? If everyone gets a hockey stick, how can you claim that Mann is in error?

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