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Post by marchesarosa on May 30, 2011 20:29:52 GMT 1
If you want to understand Michael Mann's proxy work read this. It's short and sweet. I almost died of pleasure and thought I was in heaven! Explained with all Willis Eschenbach's usual clarity. wattsupwiththat.com/2011/05/30/kill-it-with-fire/#more-40782Here's just one of the pictures which shows the proxies which give rise to the hockeystick shape. It's some of the treerings and the upsidedown Tiljander lake sediments, of course! None of the others.  |
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Post by StuartG on May 30, 2011 20:43:09 GMT 1
Clarified the situation nicely MM, I thought a 'Dendrogram' was a message written on a stick, You know, like you poke in your eye. "I almost died of pleasure and thought I was in heaven!" Steady old gal, sounds a bit 'orgasmic'.
StuartG
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Post by marchesarosa on May 31, 2011 3:43:10 GMT 1
Isn't a dendrogram one of those messages that intrepid explorers sent back to civilization stuck in a cleft stick carried by barefoot aboriginal runners? Well, stu, an old lady does get a bit orgasmic enjoying such an exquisite exposure of a piece of pure crap. It's all in the mind, you know!  |
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Post by nickrr on May 31, 2011 20:10:12 GMT 1
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Post by marchesarosa on May 31, 2011 21:14:37 GMT 1
You are wrong and I don't even need to look at your link to be sure of this.
All the proxy temperature reconstructions which show a hockeystick shape do so because they contain the bristlecone pine proxies. Even though he was told by the North Committee and the Wegman Committee in 2006 they were unsuitable as proxies for temperature Mann and Co continued to use them because this was the ONLY way they could get the required uptick.
You can see from the dendrogram that the other proxies Mann used in combination with the bristlecone treerings do not indicate a late 20th century rise. Only the bristlecones and upsidedown Tiljander lake sediment do, the latter for obvious reasons.
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Post by marchesarosa on Mar 21, 2012 2:49:48 GMT 1
A whole new biasMar 20, 2012 on Bishop Hill 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. www.agu.org/pubs/crossref/2012/2011GB004143.shtmlTree 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.
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Post by marchesarosa on Mar 21, 2012 2:58:29 GMT 1
Comments at Bishop Hill Presumably, this new source of bias applies just as much to tree ring studies where the increase in growth is ascribed to temperature ------ And if CO2 produces the same results, how does one tell them apart? ------- And if increased precipitation produces the same results, how does one tell them apart? -------- You don’t need to know anything about dendrochronology to see that the possibilities of sample bias are numerous. The fact that trees are sampled at their “Limits to Growth” i.e. on tops of mountains or at the northernmost limit of their occurence means that you’re losing any trees which were too young to survive a particularly harsh winter. Since you need a certain minimum sample size, you’re almost certainly going to sample where trees are sufficiently numerous, and miss trees which were to young to survive that year, and therefore underestimate short term temperature variability. It’s like doing a survey of obesity on the top deck of a bus. A lot of your potential sample just won’t make it up the stairs. The scandal is that this paper wasn’t written before dendrochronoloy was used to overthrow history. bishophill.squarespace.com/blog/2012/3/20/a-whole-new-bias.html#comments |
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