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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 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 1998)

Has this phenomenon happened before? In other words, can we rely on tree-ring growth as a proxy for temperature? Briffa 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 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 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 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 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.

Last updated on 26 June 2010 by John Cook.

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Comments 51 to 66 out of 66:

  1. dawsonjg - I have to say that your posts have been a fascinating review of many of the popular myths of climate denial, with this most recent one being that somehow climate scientists would make money falsifying data in some huge conspiracy.

    Once again you have demonstrated that you are not a publishing scientist, as anyone in the field knows how silly an accusation that is. The fame and position in science go to those who publish solid data that stands out in the field as being objectively correct. Those caught falsifying or plagiarizing data (Wegman), publishing poorly written or clearly incorrect papers (McIntyre, Spencer), or just making fantastical stuff up (Gerlich and Tscheuschner) receive no rewards in the field of science. The risk, the downside to writing bad papers or falsifying data - that's huge.

    The rewards may indeed include being able to get additional grants, or rather having a slightly higher percentage of your grant applications go through - success breeds success. But please note that grants do not tend to increase the income of the scientists involved. Very few practicing scientists are in the upper 1% income levels...

    On the other hand, if you wish to write advocacy papers for particular industries (Tobacco Institute, Clean Coal, Exxon, etc.), promoting a position that is not based on the science but is instead thinly disguised propaganda contrary to facts (Michals, Singer, Soon), you can get a lot of $$$ for it in "consulting fees". This is apparently driven by the billions in profits that some industries see at risk if they have to change their course of business.

    This is not to say that someone writing from an "advocacy" position will always write worthless stuff - but you have to consider the motivation behind it, and judge the material accordingly.
  2. dawsonjg... Think tanks pay some heavy coin. [link]

    You might note that almost all the lead AGW skeptics have think tank positions. Spencer, Michaels, Lindzen, etc.
  3. dawsonjg:

    What is the basis of your assertion that climate scientists have received billions of dollars in renumerations?
  4. Rob H,

    And there sure are a lot of these so-called 'think' tanks. The denial business must be berry-berry good.
  5. Also dawsonjg, who do you think is paying these climatologists to promote AGW? Governments? If so, can you explain why so many of these same governments have been steadfastly ignoring or downplaying the findings of their bought-and-paid-for experts? While handsomely subsidizing the extraction industries, no less?

    For that matter, why couldn't these global conspirators get meaningful emissions cuts passed during the eight years when their alleged ringleader Al Gore was in the White House? The payoff for all the expense, lawbreaking and secret bullying it'd take to create a phony scientific consensus seems pretty paltry to me, no matter who's footing the bill.

    Let's look at things realistically. You've got the oil industry, which is one of the most profitable businesses in the world, pumping huge amounts of money into government, media and pro-industry thinktanks. That's not subject to dispute. You've also got governments downplaying or ignoring scientific concerns relating to AGW. You've got hard-right politicians threatening to prosecute high-profile climate scientists like Michael Mann. And you've got large media outlets that tend to blur the distinction between, say, Lord Monckton and mainstream climate scientists.

    And yet, despite all of these phenomena -- which are actually demonstrable and quantifiable -- many people still believe that someone's paying scientists all over the world "billions" to advance a theory in which the laws of physics cause the climate to behave in pretty much the way that those laws predict. For reasons no one actually knows, on the basis of evidence no one actually has. And in this narrative, amazingly, it's the climate scientists who are bullies and ideologues; the powerful interests who are casually slandering them as incompetents, frauds or worse are somehow their victims.

    The theory that certain industries have spent a large amount of money to manipulate opinion for their own benefit is consistent with all of the phenomena I've brought up here. The theory that someone or other has been paying climatologists to support AGW for some nefarious reason or other is incoherent, in addition to being totally unsupported by evidence.

    If you disagree, I'd love to know why.
  6. Dawsonjg, your challenge to me is misplaced. You're the one who claimed "billions." Show me the references. Sources, serious ones. Break it down to how much that represents per researcher, per paper, then we can talk. The links I provided contains itself numerous links.

    Considering that M&M have managed to cough up a couple of papers of little interest over several years, whatever money they got, from whatever source can already be labeled as a waste anyway. McIntyre does not deserve anything remotely comparable to the attention he gets.

    I'm also waiting for your comment on the computer code from M&M that sorts out upward hockey sticks on top of the pile and saves them as representative samples. How about Wegman copy and paste method? What shall we call that?
  7. Dawson, you're also ignoring how the money is used. Scientists use money to do science. Organizations such as The Heartland Institute use money to change public opinion and to do so without supporting evidence. That's all they do. Any science these organizations support directly (through funding or publication opportunities) is not meat-(or tofu)-and-potatoes, everyday science; it is scientific or, rather, statistical work attempting to find a rhetorically exploitable weakness in the prevailing theory.

    If the opposition to the theory of AGW provided an alternative theory that covers the evidence and physics even half as well as AGW, this would be an interesting conversation. There is no such alternative theory, yet some people continue to insist there must be--there must be. What do you call people who believe in a "how things work" that relies on an absent physical mechanism(s) and ignores a range of evidence?

    Phillipe: Molière would salivate over the deeply hypocritical figure of Wegman and his ripe-for-satire situation.
  8. I kind of had this discussion with a someone on FB just after climetEmailTheft1.0 regarding grants at CRU... I looked at some leaked spreadsheet and was astonished at the low level of funding people where counting as "grand larceny"... especially when you consider that in the UK Universities charge grants <40% overheads, stipends include tax, NI, pensions etc.
    I've seen better grant capture in many other disciplines - with nothing going into the pockets of academics.
    Running a unit in a university is both extremely expensive (compared to grant levels available) and highly audited. The only way an academic can end up with some cash is through consultancy (you could easily pay 80% or 100% overheads on that also, before income tax).
  9. DSL,
    No doubt he woud make a great modern Tartuffe...
  10. The "billions" almost all go into satellites. Climate science remuneration (or for that matter any public funded scientist) is not closely tied to their level of funding. You can't spend research funds on fancy living. Not so with misinformation money.
  11. Also note that public science is interested in finding out what we don't already know. You can't get funding "to prove AGW" - you can only get funding to further understand the climate system. Science follows whereever the data leads you.

    Not so for "industry science". FF companies have the resources to run climate models (I work in the industry) but what would be the point? Disinformation is much cheaper. Do you think Fred Singer is going to publish data in support of AGW if that is where his "research" took him?
  12. Has anyone looked at a correlation divergence with UV-B? There are several studies that suggest reduced growth with increasing UV exposure as trees increase the production of phenolics and flavonoids at the expense of growth. The timing would seem to fit the increased use of ozone depleting compounds.
  13. Regarding tree ring proxies and the divergence problem, this paper claims that some trees such as western juniper are growing more rapidly in response to elevated CO2

    http://libres.uncg.edu/ir/uncg/listing.aspx?id=7935

    Chris Shaker
  14. cjshaker#63: Your link has nothing to do with the 'divergence problem,' although there is a passing mention of it.

    This is clear evidence of warming. But it is not good news:

    The outlook may be less favorable for the vast interior forests that ring the Arctic Circle. Satellite images have revealed swaths of brown, dying vegetation ... Evidence suggests forests elsewhere are struggling, too. In the American West, bark beetles benefitting from milder winters have devastated millions of acres of trees weakened by lack of water.
  15. This article from Columbia University says the opposite, that trees next to the Tundra are thriving

    “I was expecting to see trees stressed from the warmer temperatures,” said study lead author Laia Andreu-Hayles, a tree ring scientist at Columbia University’s Lamont-Doherty Earth Observatory. “What we found was a surprise.”

    http://www.ldeo.columbia.edu/news-events/trees-tundras-border-are-growing-faster-hotter-climate

    Chris Shaker
  16. cjs#65: It's the same article. And it still is off-topic. See CO2 is plant food.

    Let's try less selective quote-mining and more reading for content.
  17. We should add an article about the Yamal stuff, I've seen it a lot, for instance on Forbes.com from James Taylor
  18. Why? The warming in the reconstruction is robust, with or without tree ring data. See Mann et al 2008.
    "Recent warmth appears anomalous for at least the past 1,300 years whether or not tree-ring data are used."
  19. jsmith, countering James Taylor with comment is folly. Taylor is not concerned with credibility. He is paid by Heartland to say whatever needs to be said to promote doubt in the target audience. This is not baseless accusation. I've tried discussion with him. It's a game of mumble mumble fade away, much like Joe Bastardi, though Bastardi has more at stake. Taylor knows he has an audience that just needs some sciencey looking stuff with a few links (that no one in the target audience will follow). He's a minor league James Delingpole. I go there now and then to try to goad him into engaging in conversation, but he's not interested in discussion.
  20. Me again, my opponent sent me this message and was wondering if I could recieve a proper answer to it. I know tree rings dont ONLY respond to cold temperatures, but I dont have everything I need to make a compelling case.

    What he said: 

    "if you look at the tree ring proxies, they are only effective during "cold periods. They chose to exclude MWP because the data model breaks down."

    http://wattsupwiththat.com/2013/08/16/oh-mann-paper-demonstrates-that-tree-ring-proxy-temperature-data-is-seriously-compromised/

  21. tkman0 @70, the following graph from the supplementary material of Mann 09 shows reconstructions using all data (red), excluding tree rings (blue), excluding seven particular proxies "skeptics" have objected to (brown), and excluding both all tree rings and the seven particular proxies (green):

    You should note that excluding the tree rings, or excluding the 7 particular proxies makes virtually no difference to the reconstruction.  Excluding both does, at various locations, but does not consistently do so in any particular direction.  Sometimes it is warmer and sometimes colder.  That may simply be a function of the low sample size.  The fewer the proxies, the more erratic the signal both because regional temperatures vary more than global (or hemisperic) temperatures; and because the fewer the proxies, the greater the probability actual noise, ie, a non-temperature signal, will survive rather than being cancelled out by averaging.  Despite this greater erraticness, however, even excluding both tree ring and the seven particular proxies does not result in any major change in recent reconstructions, nor lift the reconstructed temperature above that in the late twentieth century at any point.  Its biggest change where there are enough proxies to consider the reconstruction informative (ie, where it has a solid line) appears to be an elimination of the cooling coincident with the Maunder Minimum, ie, a clear error.

    This is not the only reconstruction without tree rings, and none of the reconstructions without tree rings show a significantly different story to that from other reconstructions.  Ergo, whatever the merits of Cecile et al (the paper referred to above), it has little bearing on the overall picture of past temperatures.

     

  22. tkman0 @70, the following graph from the supplementary material of Mann 09 shows reconstructions using all data (red), excluding tree rings (blue), excluding seven particular proxies "skeptics" have objected to (brown), and excluding both all tree rings and the seven particular proxies (green):

    You should note that excluding the tree rings, or excluding the 7 particular proxies makes virtually no difference to the reconstruction.  Excluding both does, at various locations, but does not consistently do so in any particular direction.  Sometimes it is warmer and sometimes colder.  That may simply be a function of the low sample size.  The fewer the proxies, the more erratic the signal both because regional temperatures vary more than global (or hemisperic) temperatures; and because the fewer the proxies, the greater the probability actual noise, ie, a non-temperature signal, will survive rather than being cancelled out by averaging.  Despite this greater erraticness, however, even excluding both tree ring and the seven particular proxies does not result in any major change in recent reconstructions, nor lift the reconstructed temperature above that in the late twentieth century at any point.  Its biggest change where there are enough proxies to consider the reconstruction informative (ie, where it has a solid line) appears to be an elimination of the cooling coincident with the Maunder Minimum, ie, a clear error.

    This is not the only reconstruction without tree rings, and none of the reconstructions without tree rings show a significantly different story to that from other reconstructions.  Ergo, whatever the merits of Cecile et al (the paper referred to above), it has little bearing on the overall picture of past temperatures.

     

  23. Thanks Tom, also the guy sent this back which I found more hilarious than frustrating:

     

    Again this is a very sophisticated but deceptive argument.

    What Mann has done is he has thousands of tree samples from the cold periods. He has 7 (or some other similar number) from the IGW period.

    So yes, applying the samples from the IGW period doesn't change the results - because his his data is not valid or representative.

    Do you get the point? I'll explain it more if you dont.

  24. tkman0...  I think it would be a good idea to get this person to cite his references so people can see where he's getting his information.

    This last comment is as bizarre as I've seen, and I've seen a lot.

  25. my thoughts exactly

  26. To summarize, supporters of Michael Mann are annoyed at the dust up about him splicing direct NOAA data on the end of 960 years of proxy data when we know now that many climate scientists knew about this, called it "the divergence problem," and were OK with Mann not clearly labeling what he was doing for the world policy makers.  The big story about 2009 is need for better email security and need to catch science hackers.  Because the "data trick" to "hide the decline" got miscontrued by many due to bad non-scientists like Sarah Palin.  All real scientists know that the Briffa data set was perfectly valid until 1960 when modern interferences caused it to be invalid from there on.  And, we all know that warming must be occuring becuase atmosheric CO2 in known to cause rapid warming (not gradual or minimal) because we see in the ice core record that warming is always associated with higher CO2, even though the "lag problem."  The CO2 increased always after the warming but this is an even more alarming observation because it means that warming can be triggered by a small amount of CO2 that then causes a run-away spiral of more and more CO2.  And we don't know how the warming stopped, the CO2 declined and the oceans and land recovered.  Conclusion: we need to take action to freeze industrialization or better, reverse it along with population but currently lack necessary political structure to do so.

    Response:

    [PS] Welcome to Skeptical Science. Please take time to review the comments policy. Comments that do not conform will be deleted. In particular, please note the "No sloganeering" and "No Accusations" section. You are very welcome to discuss the science and article content, supporting your arguments with appropriate data and references. However, it would pay to first read the articles.

  27. That was thoughtful of you to protect your blog from my post.  Surely most of the reader already know that CO2 has a shielding limit once the critical concentration is reached that completely blocks the IR the narrow band areas not overlapped by water vapor.  But this post only makes sense if you allow my prior post back. 

    Response:

    [PS] Offtopic. If you want to comment here, you must abide the comments policy. If you do not, then posting rights will be forfeited. The right place to comment (after you have read the science and can back your comment) is "CO2 effect is saturated". Use the search function on top left to find appropriate place. Repetition of long-debunked myths without new data is simply sloganneering.

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