Oh Mann! Paper demonstrates that tree-ring proxy temperature data is 'seriously compromised'

Michael Mann won’t be happy about this.

A new paper now in open review in the journal Climate of the Past suggests that “modern sample bias “has “seriously compromised” tree-ring temperature reconstructions, producing an “artificial positive signal [e.g. ‘hockey stick’] in the final chronology.”

Basically, older trees grow slower, and that mimics the temperature signal paleo researchers like Mann look for. Unless you correct for this issue, you end up with a false temperature signal, like a hockey stick in modern times. Separating a valid temperature signal from the natural growth pattern of the tree becomes a larger challenge with this correction.

Here is a relevant excerpt:

Much of the work in dendrochronology, and dendroclimatology in particular, relies on accurate, unbiased reconstructions of tree growth long into the past. As a result, a great deal of effort has been put into trying to isolate important trends and identify potential 5 biases. However, one major bias called “modern sample bias”, first identified by Melvin (2004), is still largely neglected in applied studies, despite its potential impact on all regional curve standardization chronologies (Brienen et al., 2012a).

Dendrochronologists observed that the older a tree was, the slower it tended to grow, even after controlling for age- and time-driven effects. The result is an artificial downward signal in the regional curve (as the older ages are only represented by the slower growing trees) and a similar artificial positive signal in the final chronology (as earlier years are only represented by the slow growing trees), an effect termed modern sample bias. When this biased chronology is used in climate reconstruction it then implies a relatively unsuitable historic climate. Obviously, the detection of long term 15 trends in tree growth, as might be caused by a changing climate or carbon fertilization, is also seriously compromised (Brienen et al., 2012b). More generally, modern sample bias can be viewed as a form of “differing-contemporaneous-growth-rate bias”, where changes in the magnitude of growth of the tree ring series included in the chronology over time (or age, in the case of the regional curve) skew the final curve, especially 20 near the ends of the chronology where series are rapidly added and removed (Briffa and Melvin, 2011).

A likelihood perspective on tree-ring standardization: eliminating modern sample bias

J. Cecile, C. Pagnutti, and M. Anand

University of Guelph, School of Environmental Sciences, Guelph, Canada

Abstract

It has recently been suggested that non-random sampling and differences in mortality between trees of different growth rates is responsible for a widespread, systematic bias in dendrochronological reconstructions of tree growth known as modern sample bias. This poses a serious challenge for climate reconstruction and the detection of long-term changes in growth. Explicit use of growth models based on regional curve standardization allow us to investigate the effects on growth due to age (the regional curve), year (the standardized chronology or forcing) and a new effect, the productivity of each tree. Including a term for the productivity of each tree accounts for the underlying cause of modern sample bias, allowing for more reliable reconstruction of low-frequency variability in tree growth.

This class of models describes a new standardization technique, fixed effects standardization, that contains both classical regional curve standardization and flat detrending. Signal-free standardization accounts for unbalanced experimental design and fits the same growth model as classical least-squares or maximum likelihood regression techniques. As a result, we can use powerful and transparent tools such as R2 and Akaike’s Information Criteria to assess the quality of tree ring standardization, allowing for objective decisions between competing techniques.

Analyzing 1200 randomly selected published chronologies, we find that regional curve standardization is improved by adding an effect for individual tree productivity in 99% of cases, reflecting widespread differing-contemporaneous-growth rate bias. Furthermore, modern sample bias produced a significant negative bias in estimated tree growth by time in 70.5% of chronologies and a significant positive bias in 29.5% of chronologies. This effect is largely concentrated in the last 300 yr of growth data, posing serious questions about the homogeneity of modern and ancient chronologies using traditional standardization techniques.

The full paper is here: http://www.clim-past-discuss.net/9/4499/2013/cpd-9-4499-2013.pdf

h/t to The Hockey Schtick

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Leo Morgan

I’m not clear as to how this works. For example, the ring pattern of twenty-year-old trees would be unaffected by this effect no matter what year their seed sprouted… are they proposing that modern trees tend to be younger trees than the average of sampled trees? Why would this be so?

rogerknights

The truth is finally getting its boots on.

Jon

There are so many factors affecting tree Growth that it’s ideal for policy based science? What ever you want to produce you will find it if you search enough?

Mike Bromley the Kurd

Fer chrissake, dendrochronology is rife with assumptions and bias sources….wait for it….like a model! OK, so old trees grow more slowly. How much of a correction do we apply? And which one? Bob’s? Mike’s? Eddy’s? Jennifer Juniper’s? For there to be so much egomaniacal buffoonery over whose DC record is the most ‘robust’, whose derobustification correction was applied to the quasi-robust records to rule them out? While all of this makes stimulating reading, especially when we find a paper that bashes ol’ Mann, does any of it really mean anything, or is it just more make-it-fit-the-assumption trainwreck detritus? The fact that there’s a journal ‘Climate of the Past’ suggests that it’s an important branch of climate druidity, but whose correction got applied? Watching Mann defend his smarmy position is bad enough, but now we have an out-and-out display of just how flawed the whole exercise is. A black hole for funds.

Paul Martin

All you can tell from tree rings is whether the environment round that tree was favourable (or not) to that tree’s growth. The amount of rain, sunlight and nutrients (including ppCO2) available to the tree will be as much of an influence on its growing rate as the prevailing temperature.

ralfellis

>>Unless you correct for this issue, you end up with a
>>false temperature signal.
But will you end up with a temperature signal at all? Here are a few scenarios for you…
a. Tree experiences a very cold summer, with no growth. (a temperature signal?)
b. Tree experiences a very hot but too dry summer, with no growth. (a temperature signal?)
c. Tree experiences a cool but nicely moist summer, with good growth. (a temperature signal?)
d. Tree experiences a very hot and wet summer with good growing conditions but a rampant pest infestation, with no growth. (a temperature signal?)
Just what are they measuring, when looking at tree rings? Temperature? Moisture? Infestations? Micro-climates? Forest canopy competition?
.

Alan Mackintosh

This is well known to foresters. Young trees have a rapid growth which is reflected in wide growth rings,which continue until limiting factors kick in. These could be light, moisture stress, nutrient availability. Different species have different growth rates, eg, Birch is relatively short lived reaching maturity in as little as 70-80 years, whereas with Oak, it is several hundred years. As the trees move through Juvenile to mature their growth rate slows, much like everything else i suppose. In commercial forestry, to maximise returns, harvesting is timed to occur after the fastest accumulation in volume, which can be as little as 40 – 50 years for Spruce. This of course depends on the end market specifications.
I would point out that dendrochronology is useful in its primary purpose as a tree-timescale-indicator, but its torturing into making up temp series, without proper recognition of the impacts of the restriction of water, nutrients, access to light and the potential for human impact, thinning, grazing(and therefore manure enrichment) etc is a step too far.

ralfellis

And I will say this again…
If tree rings are actually measuring micro-climates, micro-moisture content, micro-canopy-competition, and micro infestations, then how on earth can dendro-chronology work?
Yes, your bristle-cone pine tree ring analysis may sort-of mimic the climatic record. However, your wood sample for dating was grown in a completely different location with completely different local conditions – with local weather, local pests, local water sources and local canopy cover. How on earth is your wood sample going to be an equivalent of the bristle-cone reference sample? And if it is not an equivalent, then how can you derive a date from the width of its rings?
.

micky

So to summarize it appears the volume of the tree ring might be a more useful measure than the width

Henry Galt

“Of course I pre-adjusted for all these supposed faults. Do you take me for a fool?” MM
“No, you cannot see my code and methodology. You probably just want to pick holes in it. I have stored it all in the ‘censored’ folder. You are not a climate scientist.” Same Guy.

gopal panicker

to measure temperature you need a thermometer, not a tree ring…when does a tree ring measure the temperature ? in January, May, or September ? in the morning, afternoon, or midnight ? Proxies are rubbish.

Txomin

Quick, Mr. Obama, tweet it.

Brian H

The volume of two rings of equal width will be different: the outer (later) one is longer (larger diameter), with more mass. Conversely, if two rings have the same mass, the outer one will be narrower.

MJA

@Leo Morgan
I’m not a dendrochronologist so I don’t know details, but I can easily imagine that trees from more than 300 years ago might disproportionally rot from the outside inwards, destroying the latest 20 years of growth rings. So maybe 20 year old trees are never found to sample from that long ago.

SasjaL

So, climate and enviroment affects tree ring width/thickness, which is old school knowledge, also time/age is an important factor. Good years with old trees gives thin rings, that can be interpreted as harsh years, vice versa and all in between, if not careful or deliberate …
Tree rings = circular evidence ? 😉

wayne Job

Tree rings are perfectly accurate to tell the age of a tree, or the age that a tree died, every thing else is mysticism, the druids would be proud.

Mike McMillan

I’m not quite sure which way this modern sample bias would affect chronologies, up or down, or more or less hockey stickiness.
The MBH98 hockey stick treated Graybill and Idso 1993 as a treemometer, but the main purpose of the bristlecone pine paper was to document CO2 fertilization, not temperature. They explicitly stated that there was a lack of strong, consistent temperature response in the subalpine chronologies, and that while there may be some temperature signal pre the mid-1800’s, the signal becomes obscure with the increase in CO2.

If this is the basis of debunking Mann’s hockey stick, then one can only say it has been recreated using different methods with the same conclusions”
The truth behind Mann’s hockey stick!
[ http://www.youtube.com/watch?v=r9jtVZ3RUCU ]

Keith

Not entirely clear what is being stated here (other than that using tree rings to determine temperature trends is perhaps a fool’s errand).
Old trees were young once, so they would’ve had the same rate of growth in their early years as modern trees would in their own early years, all other things being equal. What I think is being suggested is that long-lived trees are more likely to be the ones that grow more slowly than the ‘average’ tree throughout their lives. Think of it as the slow-moving Galapagos tortoise versus the scurrying mouse.
While the sample of modern trees will include some that are destined for a long life and are therefore growing slowly (and would be discarded from the sample by the Team…), the suggestion seems to be that there will also be many faster-growing, shorter-lived trees. By definition, there wouldn’t be any of this type of tree among the 700-year-old trees. Therefore, the average growth rate according to a study sample would be slower in the past and faster in the present.
That would make sense to me, but I’m not totally clear that is what is being said in this paper.

steveta_uk

Keith, that was also my reading, but it certainly isn’t clear.

Greg

“Much of the work in dendrochronology, and dendroclimatology in particular, relies on accurate, unbiased reconstructions of tree growth long into the past. ”
Dendrochronology means measuring time (chronology) or establishing a time-scale using tree rings. This does not require knowledge, detailed or otherwise of past growth rates, it requires counting rings and matching patterns.
This a precise and well established science.
There is a wilful attempt by many involved in dendroclimatology and particular dendrothermometry to confound all these terms in the hope that unscientific attempts to use trees as thermometers will get a free pass and be granted the hard-earned credibility of dendrochronology.
Unfortunately the authors of this paper , while doing a valuable job of pointing out the fallibility of dendrothermometry are tacitly accepting this abuse of the term dendrochronology as encompassing anything done with a tree.
It is time this abuse of the reputation of dendrochronology stopped and those that want to do dendrothermometry set out to prove it to be a valid method without falsely relying on the credibility of another branch of science.

richardscourtney
Eric H.

Tree rings are being used to estimate past temperature, as are sediments and ice cores. Although there are known accuracy problems, I don’t think that tossing out all tree ring proxies is the way forward. This paper, if the science and statistics are sound (I am not one to judge), takes a step forward in improving the accuracy of the proxies. I am eager to see what the paleo temperature record looks like after the changes suggested in this paper are incorporated. If Dr. Mann is the scientist that he claims himself to be then he should be foaming at the bit to incorporate this new method into his next paper, regardless of the political implications that come from the results. Too idealistic?

JackT

Just one more reason that there can never be a scientific concensus on climate. There are far too many contributing factors. Those that have “already decided” are the true deniers, as the true science of the Earth is not just mysterious, but ever changing.

cRR Kampen

“Unless you correct for this issue, you end up with a false temperature signal, like a hockey stick in modern times.”
So show the correction and find a better description of reality – being the ‘hockeystick’, of course.

I have followed, to date, publications Håkan Grudd’s (http://hakangrudd.blogspot.com/p/publications.html).
In the last two sentences are those:
“Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century.”
“The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century.” (second to last).
“The use of 'growth-rate' based multiple RCS curves is recommended to identify and mitigate the problem of 'modern sample bias'.”
“The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c . ce 900–1100) and the latter half of the 20th century.” (last – intentionally I moved the sequence sentences).

knr

Question given the high reliance on trees and there growth , how many of ‘the Team ‘ are actual experts in this area , whose background gives them a good understand of ‘tree growth ‘
Answer , none but this does not stop them being able to make absolutists statements on this subject and attacking others , some with more knowledge than them , for failing to agree with them.
Arrogance, massive ego , hubris are the three thing s that will never be lacking in this area no matter how bad the shortfall in scientific ability and honest gets .
It’s really basic stuff that your data is only ever as good as the tool you use to collect it, that this ignored by those ‘leaders’ of this area . Which tell us a great deal about the standards seen in this area.

JimS

J. Cecile, C. Pagnutti, and M. Anand will be unceremoniously kicked out of the consensus, in spite of the fact that they got to the “root” of the problem.

Not a long ago M. Mann claimed he discovered the AMO, or at least the name for something which was already known.
As far as Yamal is concerned, he could have done more convincing job if he compared natural geomagnetic changes there with the Atlantic Oscillations as shown here :
http://www.vukcevic.talktalk.net/YAMAL-GMF-AMO.htm
Unlikely to be coincidence, since it is known that the AMO is related to the volume ice flow through Fram Straits etc.. (see link above).

Unless I missed something, I scanned the paper and saw no reference to Yamal YAD061:
http://wattsupwiththat.files.wordpress.com/2009/10/briffa_single_tree_yad061.png
There are lots of complicated, fancy formulas, which are way over my head, but isn’t the crux of the matter (the Mann hockey stick) – is that it is based on this one tree?
Am I missing something here?

There are simply too many variables involved to use the width of tree rings alone as a proxy for anything other than the general (but important) concept of “growth,” and “growing season.”
There is some prospect of learning more specific things through the painstaking process of taking tiny samples from each ring, and studying the chemistry and radioactive isotopes of each ring.
Unfortunately political bullying and balderdash has so blotched dendroclimatology that it will take time to recover. There is little dignity in being part of a “tree-ring-circus.”
However “dendroclimatology” remains a cool word to casually use at a cocktail party. Much cooler than “looking at the rings.”

Don K

Greg says:
August 16, 2013 at 3:04 am

It is time this abuse of the reputation of dendrochronology stopped and those that want to do dendrothermometry set out to prove it to be a valid method without falsely relying on the credibility of another branch of science.
============================================
That sums it up pretty well. AFAICS Dendrothermometry is in roughly the state that Phrenology had reached by the early 19th century when there were numerous bright people trying to determine the relationship between cranial bumps and other human characteristics. There were Phrenology societies and journals publishing learned papers. By the 1840s Phrenology was fading. It simply didn’t work.
I suspect that Dendrothermometry may have a little more to offer than Phrenology. But maybe not a lot. The claims made for it strike me as being mostly being motivated by folks who can torture tree ring data into yielding the answers they want to see. “C’est magnifique, mais ce n’est pas la science.”

Geoff Sherrington

Is there a similarity with ice core dating, where very young core parts cannot used if they are still firn? Does the present firn property influence ice core selection of test areas in way similar to that proposed in the paper for trees? What can be deduced from a core whose firn sometimes melts before compaction, leaving a thinner ring or an occasional series of no rings?

Bob Kutz

It appears to me that actual scientists have decided to look into dendroclimatology.
The tea-leaf readers ought to be looking into what phrenologists did for a living after they were found out. Could well inform their career decisions going forward.

DCA

Do I have this right? Trees make very poor temperature proxy for several reasons and a large portion (>50%) of temperature paleo data is based on tree rings. High CO2 sensitivity calculations are based on paleo data. Therefore high sensitivity calculations are highly uncertain.

jlurtz

In the tree ring growth chemistry, is it possible to find a difference between cold growth/wet growth/dry growth/hot growth. Look up, there’s grant money here!

chris y

The paper’s conclusions suggest that this is a clever way to hide the decline by adjusting it rather than erasing it.
“Contrary to prevailing opinion (Brienen et al., 2012a), modern sample bias does not always impart a positive bias on the standardized chronology but depends instead on the complex ecological interactions dictating survival and the vagaries of sampling. In fact, in 70.5% of the chronologies analysed, it had a negative effect instead.”
“D’Arrigo et al. (2008) suggest that modern sample bias may be responsible for the
“divergence problem” in dendroclimatology, the widespread reduction in temperature sensitivity of tree-ring chronologies in recent decades. The generally negative trend induced by modern sample bias in recent years certainly suggests that this may be at least part of the problem.”

oakwood

Oh, those pesky Canadians again!

Gary Pearse

Another good example of non experts in the basic source of the data – forestry knowledge happily boring holes in trees and measuring rings. It seems from comments, that foresters already knew that mature tree growth slows down. Rapid root development slows down, the area of canopy exposed to the sun gets constrained by neighbors’ competition, water resources more and more competition, nutrients in the soil lesser (trees can’t walk around. Wow and I’m only an engineer.
Now what would an engineer do. Note the general narrowing of the rings and asking what could cause this. Most of these possibilities have been answered in above posts and maybe I made a small contribution in paragraph#1. It’s clear if you are LOOKING FOR A TEMPERATURE SIGNAL, and you are a dummy about the actual botany of a tree, then other possibilities don’t interfere with your work.
Now how could an engineer discover this new finding if contracted to do so. Why, by drilling holes in a few dozen older trees and a few dozen younger trees in the juvenile stage and doing a statistical analysis of the difference in ring thickness for the same years. I haven’t read the paper so I’m not sure yet how they did it, but it must be something like this. Possibly also measuring the exposed leaf area of trees of various ages might give supplementary info.
Now perhaps, from this, my discovery of the reason for the “divergence problem” the GACS (golden age climate scientists) have much mused over.
http://en.wikipedia.org/wiki/Divergence_problem
Alas, history will record some other guy’s name for this discovery.

Tom in Florida

J. Philip Peterson says:
August 16, 2013 at 4:45 am
“Unless I missed something, I scanned the paper and saw no reference to Yamal YAD061:
http://wattsupwiththat.files.wordpress.com/2009/10/briffa_single_tree_yad061.png
There are lots of complicated, fancy formulas, which are way over my head, but isn’t the crux of the matter (the Mann hockey stick) – is that it is based on this one tree?”
Yeah but by using one tree you don’t get conflicting data so that has to count for something.

izen

So all those guesstimates of multidecadel climate cycles like the PDO & NAO that are invariably based on tree ring proxys are also rendered uncertain.
Of course the fact that every other method of deriving past temperature from geological or biological proxys (boreholes, corals sediments etc) confirms the tree ring record helps to support the contention that errors in deriving temperature from tree rings are not a significant factor in generating uncertainty otherwise the divergence between tree rings and other proxy measurements would be much greater.

François GM

“It appears to me that actual scientists have decided to look into dendroclimatology”.
I don’t see it that way. Don’t forget that the recent decline in temperatures from dendro reconstructions was hidden by – the now infamous – “Mike’s trick”. The discordance between instrumental records and recent paleo data was a huge stain on the credibility of dendroclimatology. They’re now simply trying to gain back some credibility by claiming that trees are good temp proxies except for the last 20 years, and that therefore there was no LIA or MWP and that there is a hockey stick after all. It’s all about their credibility, funding and keeping the Hockey Stick alive.
To make matters worse they are now setting themselves up to justify “adjustments” – and we all know what that means.

The other Phil

Perhaps there are some good answers, but a couple things gave me pause:
Dendrochronologists observed that the older a tree was, the slower it tended to grow, even after controlling for age- and time-driven effects.
So there’s an age effect, even after controlling for age? What does that mean?
similar artificial positive signal in the final chronology (as earlier years are only represented by the slow growing trees)
Presumably, the problem is that the final chronology has an over-representation of fast, not slow growing trees. It is confusing to call these earlier year. Don’t you mean later? Or maybe you mean earlier in the life of the tree. In any event, very confusing.

Chris B

wayne Job says:
August 16, 2013 at 2:45 am
Tree rings are perfectly accurate to tell the age of a tree, or the age that a tree died, every thing else is mysticism, the druids would be proud.
__________________________________
Amen!

The other Phil

I am far from an expert on the subject, but it appears that some respondents haven’t even glanced at the literature.
Of course tree ring growth, in general, is affected by many variables other than just temperature. Water, sun and nutrients to name some of the main ones. This is why dendroclimatologist cannot simply do a study in a nearby forest, they need to identify locations where temperature is viewed to be the main limiting factor.
This is challenging and imperfect. Reasonable people can differ on how well the goal has been accomplished. However, some of the commenters act as if they are raising these issues for the first time. They are well-known, with attempts to control for them.

Mike B.

Way too early to declare victory here. The “Mann” who gave us “upside down Tijander” is likely to contend that the bias identified in this paper steepens the blade, and thus that modern warming is “worse than we thought”.

Pete Brown

These people should do the environment a favour and stop cutting all the flippin’ trees down to look at their rings…

Mickey Reno

My appreciation for the wonderful senses of humor of WUWT readers has certainly grown in reading this block of comments. “Tree ring circus” is hilarious, but I think “derobustification correction” is my favorite. 😉
Greg’s comment about Dendrothermometry or Dendroclimatology being distinct from, but leeching credibility from the long-established study of Dendrochronology makes great sense. There does not appear to be a series of careful, step-wise scientific building blocks that lead to an objective, scientific confidence in Dendrothermometry, and we shouldn’t accept terminology that confuses these two branches of tree-ring studies. We seem to have gone quite quickly from the idea of trees as thermometers to having a full-blown “community” (h/t Lewandowsky) which presumes its own accuracy, relies heavily on Gnostic style mysticism to anwer the many confounding complexities outlined in this thread, and which seems not the least bit interested in testing the foundational questions of this new so-called science.

Marcos

even if temps and precipitation stayed the same, wouldnt trees have increased growth due to CO2 fertilization alone? it seems like the increase in CO2 since the Industrial Revolution would explain the additional growth, no?

aliunde

I don’t think you presented the conclusions of the paper accurately in the first paragraphs you quoted. Here’s the relevant conclusion with respect to a modern period positive temperature bias (unrelated to “modern sample bias”)–the paper says there is none–that the proxy bias is actually negative and may account for the “divergence problem” (the infamous “hide the decline” problem):

     “Contrary to prevailing opinion (Brienen et al., 2012a), modern sample bias does not always impart a positive bias on the standardized chronology but depends instead on the complex
ecological interactions dictating survival and the vagaries of sampling. In fact, in 70.5 %
of the chronologies analysed, it had a negative effect instead….
.
     “D’Arrigo et al. (2008) suggest that modern sample bias may be responsible for the “divergence problem” in dendroclimatology, the widespread reduction in temperature sensitivity of tree-ring chronologies in recent decades. The generally negative trend induced by modern sample bias in recent years certainly suggests that this may be at least part of the problem.

The paper actually supports the idea of a modern warm period as shown in instrumental temperature records–the opposite of what I think your bolded first paragraphs are intended to suggest.