Tree Rings: The growth of trees (and the width of the annually created tree rings within them) is said to be positively correlated with temperature; in other words, tree ring widths are a "proxy" for temperature. Hence, if one could examine tree ring widths of very old trees, one could approximately know the temperature annually back until the time the tree started growing. Almost all multiproxy studies that use tree rings as temperature proxies have included bristlecone pine (BCP) trees.
There are many problems with using BCPs as temperature proxies, namely:
-Precipitation, in addition to carbon dioxide (CO2) and Nitrogen (N) fertilization, increase the growth rate of trees and confound the temperature signal 1. It is not possible to deduce the levels of these other factors in the past, nor is it possible to do this in the present to some extent. To account for these factors would require building a model that used accurate measurements of precipitation, CO2, Nitrogen, and temperature during the calibration period (see Statistical Analysis (multiproxy studies)) to account for tree ring growth. Furthermore, there would have to be accurate proxies for the non-temperature variables during the past millenium, and these proxies would need to be close in location to the sampled trees. This is the only way one could remove their effects on tree ring growth (before recordings of precipitation, CO2, and N existed). One may claim that the evidence for CO2 and N fertilization being significant factors in tree growth is shakey, but precipitation is certainly not and is widely regarded to be a far more powerful tree growth factor than temperature [REFERENCE].
-A possible inverted U-shape correlation of tree ring widths [REFERENCE]...
- strip-barked trees...
- intra and inter-tree variability...
- failed verification statistics... [move to "Statistics" page]
- suspicion over proxy updates or lack thereof... [refer to specific non-updates, expand on using as a holdout sample in the "Statistics" page]
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Steve and a couple other CA participants have taken their own tree ring samples to "update the proxies" in Almagre.
![[WWW]](http://wikispot.org/wiki/eggheadbeta/img/sycamore-www.png){kind=small}
Almagre - Crowley Style - A post comparing extended Almagre data with a smoothed version from Crowley and Lowery (2000). The extended data ends at near the long term median. Almagre Chronologies
More posts about Almagre are on the blog, but haven't been entered here yet.
Relevant threads (move to appropriate bullet): http://www.climateaudit.org/?p=2214
References:
1) http://www.sciencemag.org/cgi/content/abstract/225/4666/1019
