I received the following note from Paul Williams. It is an interesting critique of the use of bristlecone pines as an indication of past temperatures. In the note, Williams explains the pines may be a better proxy for carbon dioxide (CO2) than temperature. So, Williams concludes, the famous hockey stick graph may not be a ‘temperature hockey stick’, but rather a ‘CO2 Hockey Stick’:
“The “Hockey Stick” is the famous graph showing the results of studies done by Michael Mann, Raymond Bradley and Malcolm Hughes.
Mann, ME; Bradley, RS; Hughes, MK: ‘Global-scale temperature patterns
and climate forcing over the past six centuries’, NATURE |VOL 392 | 23 APRIL 1998Mann, ME; Bradley, RS; Hughes, MK:‘Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations’, Geophysical Research Letters, Vol. 26, No. 6, 1999
These papers are often called MBH98/99.
The Hockey Stick depicts relatively constant temperatures from 1000AD up to 1900, (the shaft of the “hockey stick”), followed by a sharp rise in temperature, (the “blade”). This graph is used extensively to support the argument that humans are causing global warming by emitting large quantities of CO2 and other “greenhouse gasses” into the atmosphere.
Steve McIntyre and Ross McKitrick, (MM), challenged the statistical basis of MBH98/99, claiming that the conclusion, (that the 1990s were likely to have been the warmest years in the last millennium), was not supported by the data and statistical workings described in the papers.
Subsequently, MMs claims were verified by independent statistical analysis, as detailed in the Wegman Report and summarised in a Factsheet.
One of the aspects of MBH98/99 that the Wegman Report touched on, was the use of Bristlecone pines as indicators of temperature, or “temperature proxies”.
I have no expertise or qualifications in this field, but the use of bristlecones can be understood without knowing maths and statistics, and it is one of McIntyre and McKittrick’s main objections to the hockey stick, yet it gets little discussion compared to the obscure statistical arguments.
Bristlecones are pine trees living at altitudes up to about 4,000 metres in the dry mountains of California and Utah. Some of them are very old, over 4,000 years. They live in soils that are very low in nutrients, in areas with low precipitation. They have developed a survival system known as “dieback”. When a tree is stressed due to lack of nutrients or moisture, part of the tree will die, thus lowering the requirements for the nutrients or water for the tree as a whole. The tree survives by maintaining a strip of viable bark that carries nutrients to the surviving branches and canopy. Thus a large, old tree 18 metres tall, may be sustained by a 40cm strip of bark up one side.
Because of their great age, Bristlecones tend to dominate dendrochronologies, or climate records based on tree rings, that extend back in time for long periods. So they are very important in the data that MBH98/99 used to draw their conclusions. They are also important in some of the other studies that support the Hockey Stick. As the “Hockey Team” said in their post at Real Climate, “The Missing Piece at the Wegman Hearing”, in which they show that doing the statistics differently still leads to a Hockey Stick shape,
“Why doesn’t it make any difference? It’s because the PC analysis was used to encapsulate all of the statistically relevant information in the N. American tree ring network and so whatever patterns are in there they will always influence the final reconstruction.”
But is the pattern that’s in the Bristlecones a true reflection of temperature? As Wegman mentions, it is known that Bristlecones have reacted to increased atmospheric CO2 since about 1850. This CO2 fertilisation was allowed for in MBH99, but only by using the 19th century CO2 figures, as though the increase in CO2 that happened in the 20th century had no additional effect on the Bristlecones. This may in fact be correct, as they react more to CO2 increases at lower levels than at higher, but it is a point that needs to be verified.
It’s worth noting that MBH98 was the first “multi proxy” temperature reconstruction to include Bristlecones. Several of the following studies that support the Hockey Stick also use Bristlecones. Before MBH98, Bristlecones were not considered useful for temperature reconstructions.
Thus Bristlecones react strongly to atmospheric CO2 levels. They may be a better indicator of CO2 than of temperature. So the underlying pattern is a Hockey Stick, but not a temperature Hockey Stick, instead it is a CO2 Hockey Stick. And this pattern shows up in other studies that use them.
Disclaimer:
Much of this material I gleaned from Steve McIntyre’s site, Climate Audit.org. I could find literally no discussion about Bristlecones on Mann’s site, Real Climate.org.
Additional links:
Ross McKitrick’s presentation to the Australian APEC Study Group, 2005. A non-technical summary of the main issues that McIntyre and McKitrick have raised about the Hockey Stick.
http://www.uoguelph.ca/~rmckitri/research/McKitrick-hockeystick.pdfClimate Audit website. (Steve McIntyre)
http://www.climateaudit.orgRealClimate website. (Michael Mann and others)
http://www.realclimate.org/Bristlecones
http://www.sonic.net/bristlecone/home.htmlTree rings
http://web.utk.edu/~grissino/default.html“
Jennifer Marohasy BSc PhD is a critical thinker with expertise in the scientific method.
Intial point of order: Is it really fair to call Realclimate “Mann’s site”. Gavin and others seem to be the main commentators? The RC style is fairly different also with substantial position papers on a variety of topics from different authors.
Luke, we could take your comment seriously if Mann were to disown RealClimate.
Well summarised Paul, That is as good a description of “The story so far” as I have seen.
When you think about it, reading the bristlecone pine rings as a proxy for WORLD temperatures is a bit like reading tea-leaves to predict the future! But only SELECTED tea leaves!
PS. I wonder what happened to all those tea-leaf readers with the introduction of tea-bags?
http://www.realclimate.org/index.php/archives/2006/02/a-new-take-on-an-old-millennium/
12. Do you have information on why Osborn and Briffa used the controversial bristlecone pines when even the people who did the sampling (Graybill and Idso) made it clear that these measurements did not correlate very well with temperature?
It seems that whatever filtering Osborn and Briffa did make for proxy selection, it appeared to be rather flexible in terms of what got in and what was excluded.
[Response: Perhaps you need to read and/or re-read the paper and our description of the paper. Osborn and Briffa quite clearly describe an objective screening process which eliminated proxy series that did not correlate significantly with local instrumental temperature measurements over the 20th century. So what you mean by “rather flexible” is unclear. Their use of western U.S. tree-ring composites (which includes “Bristlecone Pine” data), as we understand it, specifically accounts for potential non-climate related effects originally proposed by Graybill and Idso, and explicitly estimated by Mann et al (1999). Corrected for these effects, the data correlate highly with local temperature measurements over the 20th century. They also use an independent, 2nd western U.S. tree-ring record that shows very similar behavior (compare panels 1 and 3 in their Figure 1) to the first. Furthermore, their conclusions, as described quite clearly in the paper, are robust to the exclusion of both of these, or in fact, any 3 of the 14 proxy series used. – mike]
17. “Their use of western U.S. tree-ring composites (which includes “Bristlecone Pine” data), as we understand it, specifically accounts for potential non-climate related effects originally proposed by Graybill and Idso, and explicitly estimated by Mann et al (1999).”
How, specifically, was this estimate of non-climate related effects done?
My understanding is that the MBH99 estimate of non-climate related effects for the B_cone pines increased towards the end of the 19th century, then decreased in the 20th century. This would seem to be inconsistent with the known path of increasing CO2 concentrations in the atmosphere through both the 19th and 20th century, and the associated CO2-fertilization effect on vegetative growth.
[Response: Actually, not. It is well understood by those who study terrestrial ecosystem dynamics that there are multiple limiting factors on growth. For example, once a tree has essentially as much CO2 as it can use, other conditions such as soil nitrogen availability, will become limiting. In addition, the longer the stomates remain open (to try to take in the additional CO2), the more vulnerable the tree becomes to water loss through evapotranspiration. So one would only expect a significant impact of Co2 fertilization only until these other limiting factors kick in. Any subsequent increase in ambient CO2 concentrations would have little incremental value to the tree once this happens. A plateau in the observed response is the rule, not the exception. There is a vast scientific literature on this sort of stuff. We’ll leave it at that. – mike]
Luke, it’s true that Mann is not the only contributor to RC. They discuss a lot of topics at RC, what they don’t talk about are Bristlecones.
Here is the what I found at RC when I searched for “bristlecone”
http://www.realclimate.org/index.php/archives/2005/02/dummies-guide-to-the-latest-hockey-stick-controversy/
http://www.realclimate.org/index.php?p=111
http://www.realclimate.org/index.php/archives/2006/02/a-new-take-on-an-old-millennium/
If you know of any sites where Mann, Bradley or Hughes discuss Bristlecones, I’d be grateful for a link. The one glaring fact when I researched the above post was that their side of the story was not available, at least to my limited skills.
A fairly simple question is: “Do other climate reconstructions that don’t use the bristelcone series show a hockey-stick – that is, warmer temperatures in the latter half of the 20th century compared to the preceding millennium?”. Using different experimental methods, are MBH results reproducible? My limited understanding is that “yes” is the answer to both questions.
Paul – I assumed the Mike above was “that” Mike.
Incidentally on the central thesis of CO2 – CO2 ain’t Jack’s beanstalk – sure you can get some impressive increases in growth in the lab from 2x CO2 – you need other things to be optimum e.g. temperature and water. Given these pines high live up alpine environments in the USA, temperature would be limited and nowhere near a C3 optimum. The soils are likely to be poor mountain soils – low in depth – i.e. low water holding capacity. Of course fog or regular rainfall can make up for low soil water stores.
But Wiki says “Because of cold temperatures, dry soils, high winds, and short growing seasons, the trees grow very slowly”
Sounds like good candidates for temperature proxies a priori but I know much has been written to disagree.
Is there an ecophysiologist in the blog house?
dang, the steve who just commented was not me. i’m the steve who has been commenting on this blog since it started.
Might need to look into a new moniker so that nobody gets confused.
Luke, Just the other day I posted the following on this blog:
“Subsequently, McIntyre testified to the Whitfield Sub-Committee that ‘the first Briffa version of the Polar Urals series said that the early 11th century was among the coldest of the millennium: updated sampling in 1998 showed the opposite, but BRIFFA DID NOT REPORT IT. Instead he SUBSTITUTED ANOTHER SERIES from a site 70 miles away with a hockey stick shape. This substitution had a dramatic impact on the medieval-modern relationship in the Briffa (2000) reconstruction and nearly all other subsequent studies’ (CAPITALS added).
I said that it was beyond my comprehension that the mainstream science community could remain silent while one of their number refused to share data, failed to report material information and substituted one series for another without explanation.
You said that you had not read this – you got bored along the way. Now you’ve reproduced a long posting on the RealClimate site in which Michael Mann patronisingly tells an inquirer to read or re-read O&B or RealClimate’s description of O&B.
Well, I’m bored with Mann’s tedious efforts on RC. I won’t be reading and re-reading O&B, let alone RC’s take on it, until the researchers in question offer an explanation for what Steve McIntyre has sworn under oath that they’ve done. Perhaps there’s a valid reason for this apparently extraordinary behaviour. I’ll keep an open mind on that, and if a good explanation is now given or has already been given, I’ll look at Mann’s claims again.
OK Steve (the original Steve on this blog), my name is changed. I am now New Steve. Damn common names!!
Gads a post from Ian under 3 pages. Let’s just stay mutually bored Ian – saves time.
“But Wiki says “Because of cold temperatures, dry soils, high winds, and short growing seasons, the trees grow very slowly.”
This mightn’t be quite right on the growing seasons. Sitka spruce is an ace aircraft timber because summer and winter growth rings aren’t all that different in thickness, producing the equivalent of a ply material made of thin laminates.
Ian B – yes but CO2 wouldn’t do much on top of an already limited alpine environment. Depends how “limited” summer growing time is. Plant types tell me you’d be going hard to demonstrate growth effects on plants at even 2006 levels of CO2, given error levels involved.
The issue with CO2 fertilization of Bristlecones is with the strip-bark form.
Graybill and Idso, (“Detecting the Aerial Fertilization Effect of Atmospheric CO2 Enrichment in Tree-Ring Chronologies”, Global Biogeochemical Cycles, Vol. 7, No. 1, 1993), showed that strip bark Bristlecones displayed increased ring width from about 1850 that was unrelated to the local temperature record. They related this to experiments with CO2 enrichment of sour orange trees, and showed that the strip-bark forms put the extra fertilization into tree-ring growth while the full-bark forms put it into total biomass, and so didn’t display such a large trend in tree-ring growth.
The point is that the older trees are going to be strip-bark forms, and these are the ones that introduce the Hockey Stick into MBH98/99 (and some other studies)
How do sour orange trees in perhaps less limited environments relate to alpine Bristlecone pines ??? Be good to gets some isotopic analysis on sequestered carbon.
Just asking.
Want to hear some ecophysiological advice before believing any of this. Otherwise it’s the same as signing off on PC analyses when one has no idea what they’re about.
I’ve given you the reference. Why don’t you try reading it?
Another eminently-plausible influence on climate in general – and on trees – is the Sun. Solanki S.K. et al 2004 (“Unusual activity of the Sun during recent decades compared to the previous 11,000 years”, Nature 431, 1084-7) said: “Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendro-chronologically dated radiocarbon concentrations. … According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago.
A paper with some of the same authors (Usoskin I.G. et al 2005, “Solar activity, cosmic rays, and Earth’s temperature: A millenium-scale comparison”, JGR 110 A10102 10p.) looks at the last 1800 years: “Comparisons of the Sun-related data sets with various reconstructions of terrestrial Northern Hemisphere mean surface temperature reveals consistent positive correlation coefficients for the sunspot numbers and consistently negative correlation for the cosmic rays. Both papers see a “hockeystick” about where Mann does – in solar activity.
Of yes – sure Paul – I’ll just get my copy of the shelf !!
There are other explanations too! Far from all over Red Rover.
http://biology.usgs.gov/s+t/noframe/r108.htm
“According to our reconstruction, the level of solar activity during the past 70 years is exceptional” when solar output at the top of the atmosphere hasn’t changed in last 30 years and PAR at plant level has has the global dimming dip !
Here it is, Luke.
http://www.climateaudit.org/pdf/graybill.idso.1993.pdf
Luke, the “solar activity” I am talking about is not total solar irradiance (TSI) which, as you know, fluctuates only by fractions of a percent. It is (inertially-driven) solar eruptive activity, which fluctuates by orders of magnitude. Among things the variable solar wind does is modulate the penetration of galactic cosmic rays into the heliosphere; and probably thereafter, their entry into Earth’s magnetosphere is further modulated.
You mention the last 30 years (during which the Sun has gone a bit off the boil). The Great Pacific Climate Shift of 1976/7, reversing a similar cooling event in the mid-40s, is an inertial event on Earth. It appears also related to inertial impacts on the Sun. Probably, the resumption of a higher level of upwelling (of cold water) in the eastern equatorial Pacific is only a year or two away. A “quiet Sun” should be discernible by about 2020, and the next Little Ice Age cold period – barring surprises, of course – will be fully developed by about 2030. Climate change: Sun yes – TSI no.
fosbob – having asked about such things before, I have been told that such other solar influences should be picked up in the ozone modelling studies and it has not.
In any case – how many watts and/or effecting what?? You need a reasonable mechanism of interaction to cinch your hypothesis. As I have said before the problem with any solar stuff is that lots of quasi-periodic phenomena are out there like El Nino and Interdecadal Pacific Oscillations – on deep stats analysis or on prediction the solar correlations don’t hold up.
Quite often, a great deal of useless discussion can erupt when fundamental assumptions do not get the scrutiny they deserve.
Jennifer opened things with an observation by Paul Williams suggesting that “the famous hockey stick graph may not be a ‘temperature hockey stick’, but rather a ‘CO2 Hockey Stick’.”
If CO2 is a proxy for warming, then CO2 “proves” CO2-driven warming–but only if you like circular arguments.
Circular arguments are very seductive, as they can make correlation seem like causation.
Roughly 80 percent of the Earth’s biomass is bacteria. They fart CO2 and fart more when it’s warm. If people truly believe CO2 is driving global warming, we should be spraying antibiotics on a planetary scale.
However, suspended H2O (clouds) is the most influential “greenhouse gas,” and for some reason, nobody wants to regulate cloud formations either.
“Global Warming” is a proxy itself. It’s a proxy for people who want a neo-Marxist/Stalinist “planned environment” and the first order of business is to put a noose on the neck of people who use energy. Like, for instance, people. Gack.
Schiller – the most amazing list of total furphies – you obviously are immune from any updating of knowledge.