[Note: My first post in which I had written commentary mysteriously lost all of its content, posting nothing but white space. This is some sort of internal wordpress error, but has never happened before. I have some elements restored below, but my original commentary is lost. -Anthony]
I’ve written before about the difficulties associated with extracting a valid temperature signal due to all of the confounding variable in Liebigs law of the minimum, which I describe in detail here: A look at treemometers and tree ring growth
Now a new confounding variable has been introduced that does not bode well for tree ring studies such as Mann et al.
A new paper by Brienen et al in the journal Global Biogeochemical Cycles suggests that there may be a whole new set of biases in tree ring studies.
Tree ring analysis allows reconstructing historical growth rates over long periods. Several studies have reported an increasing trend in ring widths, often attributed to growth stimulation by increasing atmospheric CO2 concentration. However, these trends may also have been caused by sampling biases. Here we describe two biases and evaluate their magnitude. (1) The slow–grower survivorship bias is caused by differences in tree longevity of fast- and slow-growing trees within a population. If fast-growing trees live shorter, they are underrepresented in the ancient portion of the tree ring data set. As a result, reconstructed growth rates in the distant past are biased toward slower growth. (2) The big–tree selection bias is caused by sampling only the biggest trees in a population. As a result, slow-growing small trees are underrepresented in recent times as they did not reach the minimum sample diameter. We constructed stochastic models to simulate growth trajectories based on a hypothetical species with lifetime constant growth rates and on observed tree ring data from the tropical tree Cedrela odorata. Tree growth rates used as input in our models were kept constant over time. By mimicking a standard tree ring sampling approach and selecting only big living trees, we show that both biases lead to apparent increases in historical growth rates. Increases for the slow-grower survivorship bias were relatively small and depended strongly on assumptions about tree mortality. The big-tree selection bias resulted in strong historical increases, with a doubling in growth rates over recent decades. A literature review suggests that historical growth increases reported in many tree ring studies may have been partially due to the big-tree sampling bias. We call for great caution in the interpretation of historical growth trends from tree ring analyses and recommend that such studies include individuals of all sizes.
Presumably, this new source of bias applies just as much to tree ring studies where the increase in growth is ascribed to temperature.
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Here is the abstract from GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 26, GB1025, 13 PP., 2012 doi:10.1029/2011GB004143
Detecting evidence for CO2 fertilization from tree ring studies: The potential role of sampling biases
Key Points
- Observed increases in tree ring widths may be caused by sampling biases
- Standard sampling methods lead to spurious trends in historical growth rates
- Reported increases in ring width may often not be due to CO2 fertilization
Roel J. W. Brienen
School of Geography, University of Leeds, Leeds, UK
Programa de Manejo de Bosques de la Amazonía Boliviana, Riberalta, Bolivia
Emanuel Gloor
School of Geography, University of Leeds, Leeds, UK
Pieter A. Zuidema
Programa de Manejo de Bosques de la Amazonía Boliviana, Riberalta, Bolivia
Ecology and Biodiversity, Institute of Environmental Biology, Faculty of Science, Utrecht University, Utrecht, Netherlands
Forest Ecology and Forest Management, Centre for Ecosystem Studies, Wageningen, Netherlands
Tree ring analysis allows reconstructing historical growth rates over long periods. Several studies have reported an increasing trend in ring widths, often attributed to growth stimulation by increasing atmospheric CO2 concentration. However, these trends may also have been caused by sampling biases. Here we describe two biases and evaluate their magnitude. (1) The slow–grower survivorship bias is caused by differences in tree longevity of fast- and slow-growing trees within a population. If fast-growing trees live shorter, they are underrepresented in the ancient portion of the tree ring data set. As a result, reconstructed growth rates in the distant past are biased toward slower growth. (2) The big–tree selection bias is caused by sampling only the biggest trees in a population. As a result, slow-growing small trees are underrepresented in recent times as they did not reach the minimum sample diameter. We constructed stochastic models to simulate growth trajectories based on a hypothetical species with lifetime constant growth rates and on observed tree ring data from the tropical tree Cedrela odorata. Tree growth rates used as input in our models were kept constant over time. By mimicking a standard tree ring sampling approach and selecting only big living trees, we show that both biases lead to apparent increases in historical growth rates. Increases for the slow-grower survivorship bias were relatively small and depended strongly on assumptions about tree mortality. The big-tree selection bias resulted in strong historical increases, with a doubling in growth rates over recent decades. A literature review suggests that historical growth increases reported in many tree ring studies may have been partially due to the big-tree sampling bias. We call for great caution in the interpretation of historical growth trends from tree ring analyses and recommend that such studies include individuals of all sizes.
Dennis Wingo (@wingod) says:
March 20, 2012 at 9:30 am
Color me crazy but an interesting experiment would be to place a high resolution thermometer by a tree for about 20 years with a data recorder and then compare the derived vs actual temperatures?
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QUite so.
However, we almost certainly know the result since this is, in effect, the subject of hide the decline. As from 1960 onwards, tree ring data does not match the instrument temperature record post 1960 when the tree ring proxies are tuned to the instrument temperature data for the period about 1905 to 1960. This comfirms that tree rings are not good temperature proxies and ‘the Team’ were well aware that they were not good temperature proxies but still pushed ahead with their reconstruction of temperutures going back to 1300AD then back to about 1000AD.
One thing that occurs to me is whether Mann’s work should be brought up to date following all the revisions to the GISS and Hadcru data sets. As I understand matters, his (and Briffa’s) tree ring proxy was tuned to the instrument record for the period ~1905 to ~1960. Well these temperatures have been revised quite a bit this last decade. They have been revised downwards (and post 1980s upwards). How has this affected the calibrtation that he used? And would the decline, ie the difference between tree ring results and instrument record for the period post 1960 be even greater? The hide the decline could now be worse than we think!!
At the back end of my garden, I have 3 tallish pine trees growing within about a metre of each other. One is considerably older but two are about the same age, within a year or so. I recall them as small saplings about 10 a foot high when I bought the property.
These two small trees are broadly similar height but one has a trunk which is about 50% bigger than the other. The reason would appear to be that the smallest is in the shade of the other two. This has not adversely affected its height, but has had a dramatic effect on its girth. Of course one might be a slow grower and the other a fast grower, but I suspect that they both seeded from the same tree and are therefore likely to have similar DNA. My guess is that it is a light issue, although it is possible that there is also competition for rainfall/soil moisture. The stark differenc ebetween these two trees does illustrate why trees are unlikely to be good proxies for recording temperature.
The plot looks hetroscedastic.
What? No comments by Hugh Pepper? Certainly he’ll say Mann has already covered this in the book that I (and everybody else) should read. Mann the tree ring circus hero–again performing a death-defying leap over logic and verifiable science.
At least it makes great theatre for those hooked on the CAGW circus.
But no, Hugh–I don’t have to read Mann’s most recent book to know everything I need to know about Mann. That has been made perfectly clear to me from what other honest researchers have said about his methodology.
And no, I don’t believe Mann has invented another type of statistics–well, let me clarify that: He’s “invented” something alright, but it isn’t statistics.
There are plenty of reasons for people (here) to continue to study trees in climate.
Backpack the high passes in the Rockies, and above 11,000 feet or so, you’ll see the skeletons of trees that lived and died. Take a moment to realized why they stand out: they are high above current timberline. Some of the bigger relict bristlecones are more than two feet in diameter, sunbleached, lying where they dropped, probably several hundreds years ago, like pieces of driftwood thrown up by the high tide of the last millennium. Whether it was warmer temps or moister soil that allowed them to achieve that elevation, their living descendants can’t and don’t – the present forests of bristlecones are hundreds of vertical feet below.
I cored a few of the standing trees (mostly smaller than the those that died higher up) and, one dated to 1440 at the pith. Some of the dead trees had undoubtedly been there for centuries, so their lives may have overlapped, but the clear space – of scrub oak and tundra – between them is telling. If the fallen giant had comparable age when it fell, it was a seedling around the 1200’s. You don’t need to argue the significance of tree ring widths for this – just count the absolute years (it takes a hand-held magnifier) and use your reasoning to infer what it means.
Maybe someone here can tell me I’m crazy, because otherwise I consider the implications of this clear.
There is unrest in the forest
There is trouble with the trees
For the maples want more sunlight
And the oaks ignore their pleas
Words from a song by Canadian rockers Rush. Tree proxies have many more variables than temperature alone. Makes me suspicious that they’re proxies for a cause, not a climate.
dorsai123 says:
March 20, 2012 at 8:32 am
by my count tree growth is effected by at least 4 variables: sunlight, temperature, water and soil/nutrients
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Plus disease, fungi, parasites, fire, lightning, windstorms, (natural) acid rain, and likely several more variables. It is beyond ludicrous to believe you could reconstruct local temperatures based on tree rings, and completely absurd to use them as a proxy for global temps.
I thought it was well known that slower growing trees were more “dense” than fast grown trees. Surely, it is therefore likely that slow-grow trees will show up more in the archaeological record than faster trees.
Another thing is that far more trees were cultivated in the past than people are willing to admit. So, even in apparently “natural” woodlands, trees will be cultivated with an effect on tree growth rate.
Neophyte says:
March 20, 2012 at 3:12 pm
There are plenty of reasons for people (here) to continue to study trees in climate.
Backpack the high passes in the Rockies, and above 11,000 feet or so, you’ll see the skeletons of trees that lived and died…..
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Tree line advance and retreat at high altitudes is a decent proxy for temp/growing conditions. It is a heck of a lot better proxy than tree rings. Also used is the extent of other plant species. Orange groves in Florida for example. History of Florida’s Citrus Groves
Before dismissing all tree-ring studies, it might be worthwhile to consider the recent study published by the Chinese. Their study of trees on the Tibetan plain showed a different result than what Mann found. They DID pick up the LIA, and the MWP. What they did NOT pick up was a hockey stick.
And this means nothing to global warming believers and their politics. The Mann Hockey stick will still be said to be very solid science.
Get ready for the poorly thought out rebuttals to this work to be posted on some web site and quoted verbatim everywhere on the web.
Another factor that impacts tree ring growth is the proximity of other trees and plants. And like the other variables mentioned, there is simply no way to take that into account over tens or even hundreds of unobserved years.
Dendrochronology is a vegetarian form of Extispicy, and works just as well.
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Extispicy
The term is derived from extra and spiere, meaning to view or consider. It applied cheifly to the inspection of entrails for the purposes of augury.
The practice of extispicy came to the Etrurians from the Babylonians as most forms of divination were derived from Mesopotamia. The practiced also was employed throughout Greece where the priesthood was confined to two families.
The Roman auspieces had four distinct duties: to examine the victim or animal before it was opened, to examine the entrails, to observe the flame of the sacrificial fire, and to examine the meat and drink offered in accompaniment of the sacrifice.
It was a fatal sign when the heat of the fire was wanting. This occurred when two oxen were immolated on the day Caesar was killed.
Signs predicting a potential instant disaster were if the priest let the entrails fall, if there was more bloodiness than usual, or they entrails were of a livid color.
One phenological phenomenon going on right now in Washington DC is the Centennial (100-year) Cherry Blossom Festival.
http://www.nationalcherryblossomfestival.org/about/bloom-watch/
The Japanese trees sent in 1912 are reaching “full blossom” this week. The claim is that the warmer the springtime and preceding winter, the earlier the blossom. So, with an unusully early bloom this year, the warmists are making the most of the moment.
The buds typically appear in late February – early March, and petals bloom fully by early April. But over the last century, there have been at least six early blooms comparable to or earlier than this year’s: 1920, 1926, 1944, 1976, 1990, and 2000.
http://www.washingtonpost.com/blogs/capital-weather-gang/post/dcs-cherry-blossoms-have-shifted-5-days-earlier-in-last-90-years-what-about-the-future/2012/03/16/gIQARvtTPS_blog.html
With no record of the blossom dates between 1912 and 1919, and the recent warm-weather blooms, this Washington Post environmental writer apparently feels justified in extrapolating his declining line out into the 22nd century when cherry trees will be blossoming at Christmas.
It never ceases to amaze me that there are people in the world whose florid fantasies are fuelled by the banal and beautiful coming of springtime. This year’s relatively early bloomtime seems to have unleashed people’s imaginations.
Reports last night on the News Hour, and in various Washington papers predict more and more precocious blossoms… on into the future.
dorsai123 says:
by my count tree growth is effected by at least 4 variables: sunlight, temperature, water and soil/nutrients … since we have no historical data for sunlight, water or nutrients it is impposible to calculate historical temperature using tree growth rates … impossible … Why is this still being debated by anyone ?
Tree rings only record growth on one part of a tree. Trees can vary in growth of leaves, branches, fruit, etc from year to year.
KNR says:
It’s worth noting that for all their self declared ‘vast knowledge ‘ there is not one amongst ‘the team’ who actual understand plant physiology well.
We been here before of course with statistics, were people with far more expertise in an area can show us how the ‘the Team’ can be totally out of its depth and its only the arrogance ,that seems to be part of being a ‘climate scientists ‘, that stops them admitting it and accepting advice form those outside their little club.
As shown by the attitude “If your not a climate scientist you are not qualified to comment at all”.
The obvious question then being what is “the team” actually well qualified in…
My personal favorite “un accounted tree ring bias” is the Bear Poo Problem…
In the Pacific Northwest, at least, a very large part of the nitrogen delivered into the forest is from bears eating fish. And bears, after catching fish, like to take them over near a tree, where they eat some and leave some. Then, time passes, and so does the, er, bear dinner… when the bear is even further from the stream.
Bears like to, ur, “use” a tree then….
So, if a bear poos in the forest, does anyone counting tree rings notice?
As folks don’t like to be eaten by bears, we tend to kill them. Bear populations vary inversely with human hunting.
All this means that to properly calibrate your tree rings, you really must now what the bear population was and how much “bear poo” was “deposited” on those particular trees. Which varies with human population, fish runs, water in the streams, bear populations, and weather.
Or, in summary, “Poo Happens” and you need to know just how much it happens…
Yup that is what we need. Another adjustment, from Mannian forces, to the tree ring data to show how much near the boiling point of water, we will soon be.
Neophyte says:
March 20, 2012 at 3:12 pm
I noticed the high percentage of dead pinion pine trees in several mountainous areas where I’ve worked as a geologist and upon close inspection, found the vast majority displayed blackened trunks and limbs–obviously the victims of lightning strikes. I’ve also scurried from these locations several times as summer thunderstorms generated such intense lightning displays that it sounded like one continuous peal of thunder–no wonder there were so many dead, charred trees. I also wonder what impact all that lightning has on the growth rate of trees–obviously those receiving the full brunt are terminated but those around the strikes would also be affected.
Anthony,
RE: WordPress dumping your original post.
I’ve noticed this happening to my posts recently, too. In my case, I have discovered that if I leave a New Post page up overnight and then use that same New Post page to write a post in the morning, the entire post gets eaten by the “NETherworld monster.” The solution, at least for me, has been to start out with a fresh New Post page whenever writing. If I am writing something late at night and I haven’t finished it, I save it as a Draft and then edit that draft in the morning when I wake up. So far, that seems to have kept the NETherworld monster at bay—knock on wood rings…
Cheers
RE: WordPress : the rule is always write in a backed up way, by either typing into a text editor ..preferably with autosave on, or something like Yahoomail which also autosaves drafts as you type. (actually Chrome is quite good at keeping form data, often when I click back the typing is still there)
sorry for you Antony…. it’s an awful feeling when it disappears & you have to type it all in again.
[Note: My first post in which I had written commentary mysteriously lost all of its content, posting nothing but white space. This is some sort of internal wordpress error, but has never happened before. I have some elements restored below, but my original commentary is lost. -Anthony]
Yeah, that just happened to me twice on comments. I don’t think I am going to post on my blog for a day or so…
Steve Garcia
This seems to argue that the Divergence Problem doesn’t exist – that the tree-rings still tell everybody the temps are going up. WTF is with that? Don’t they pay attention to the actual dedroclimatology crisis?
Steve Garcia
@Stew Green (@stewgreendotcom):
“RE: WordPress : the rule is always write in a backed up way, by either typing into a text editor”
Yeah, but for the entire time WUWT has been here – YEARS – it never blanked stuff out before. It gave a sense of false security.
It is probably as well with my comment on another thread, because I ripped into a guy a LOT. Not my style really…
So everyone here can still think I am not an a**hole. So some good came of it!
Steve Garcia
@Anthony, feet, steve;
If you are writing with the Firefox browser, install Lazarus, and it saves all typing in its own data base, searchable and with page links. Use version 2.x; less flexible but still effective 3.x versions for Safari and Chrome. The 2.x version allows modification of the default 14-hr ‘save’ period. I set mine to 54 weeks. 🙂
A true a**-saver.