A SwiftHack Topic: Tree Rings and Climatology

As I am about to launch into a series of posts dealing with Shifthack, aka 'ClimateGate', I first want to cover some scientific issues central to that smear campaign which are not widely understood. Whenever I write about tree rings, this is the post I will link back to, just to bring people up to speed.

Yes, tree rings. I am writing a post about tree rings, and this is important if you want to understand certain key things about Swifthack, so wake up class, and pay attention.


One of the many lines of research in Global Warming theory is that of paleoclimatology. This is the reconstruction of historical climate change over the entire history of the Earth. There are various techniques used, and for them all, please visit this overview Wikiedia article. I'm sticking to some modest discussion of Dendroclimatology, the retrieval of climate information from tree rings. From the dendrocimatology Wikipedia article:

Dendroclimatology is the science of determining past climates from trees (primarily properties of the annual tree rings). Tree rings are wider when conditions favor growth, narrower when times are difficult. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies (sometimes with other climate proxy records), scientists have estimated past regional and global climates (see Temperature record of the past 1000 years).

One of the things you can determine from tree rings in an approximation of temperature. Scientists bore into living trees, and also take cross sections from dead undecayed trees (pre-fossil). From research they know that trees growing on the upper elevation tree line have growth lines strongly influenced by temperature, and can thus determine a yearly temperature average by analyzing each tree ring found. This is known as getting temperature by proxy, thus you will see the phrase 'tree ring proxies' and 'tree ring temperature proxies' literred throughout certain discussions concerning Global Warming and temperature.

Another, more confusing phrase and related acronym floating around is maximum latewood density (MXD). This refers to a close look at the level of thickening of the cells walls towards to end of the growing season, and is understood to add more certainty to the temperature proxy.

This does not, of course, give you day-to-day temperature. It only gives a reasonable approximation of each year's average temperature.

This is not a substitute for a thermometer. When confronted with tree ring data and thermometer data, the thermometer will always be more accurate.

Feeling adventurous? Try reading this for more information (warning: pdf).

Tree ring temperature proxies are known to be reasonably reliable because temperature data derived from tree rings have been compared to known temperatures taken by thermometer. So what is learned by that comparison can be applied backwards in time to determined average yearly temperatures for ages which we lack temperature records from.

The famous and much-overhyped* 'Mann Hockey Stick' was formed in part using temperature data derived from tree rings proxies.

The Divergence Issue

This is a known issue, reported over 14 years ago (pdf to the paper), in which the temperature determined from the tree rings ceases to substantially agree for the years 1961+ with the more reliable temperature data taken from instruments. Why this is so is still not fully understood, but common practice is to ignore the data after that year -- 'hide the decline' if you will.

This applies to trees in northern areas, not southern areas.

Skeptical Science explains this issue well, in context of the SwitftHack / 'ClimateGate' emails.

You can read more about the divergence issue in these papers, if you are brave...

Briffa, K., Schweingruber, F., Jones, P., Osborn, T., 1998a. Reduced sensitivity of recent tree growth to temperature at high northern latitudes. Nature 391, 678–682. (Abstract)

Briffa, K., Schweingruber, F., Jones, P., Osborn, T., Harris, I., Shiyatov, S., Vaganov, A., Grudd, H., 1998b. Trees tell of past climates: but are they speaking less clearly today? Philos. Trans. R. Soc. Lond., B 353, 65–73. (pdf)

* And I mean overhyped by the denialists. For years they have pushed the false narrative that defeating Mann's Hockey Stick would sink all of Global Warming theory. The theory stands independent of the work of Mann et al.

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Update December 16, 2009:

The Christian Science Monitor  did a full piece on this issue at http://www.csmonitor.com/Environment/Bright-Green/2009/1215/Climategate-global-warming-and-the-tree-rings-divergence-problem