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
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?
The truth is finally getting its boots on.
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?
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.
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.
>>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?
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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.
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?
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So to summarize it appears the volume of the tree ring might be a more useful measure than the width
“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.
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.
Quick, Mr. Obama, tweet it.
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.
@Leopold Danze 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.
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 ? 😉
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.
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 ]
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.
Keith, that was also my reading, but it certainly isn’t clear.
“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.
Greg:
re your post at August 16, 2013 at 3:04 am
http://wattsupwiththat.com/2013/08/16/oh-mann-paper-demonstrates-that-tree-ring-proxy-temperature-data-is-seriously-compromised/#comment-1391840
YES! Well said!
Richard
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?
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.
“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.