Tree Ring Chronology Selection Problem

This article is an attempt to explain why tree ring temperature reconstructions have no value at all as they are currently done. I am not trying to make the point that they have no potential value; only the point that current methods of chronology selection destroy that value. To begin, I want to point out the method in question by using a quote from an Osborn/Briffa paper – 06 – SI.

“We remove any series that was not positively correlated with it “local” temperature observations [taken from the nearest grid box of the HadCRUT2 temperature data set (S9)]. The series used by (S3) were already screened for positive correlations against their local annual temperatures, at the decadal time scale (Table S1). We removed series from (S1) that did not correlate positively with their local annual or summer temperatures (Table S1), or which did not extend into the period with instrumental temperature to allow a correlation to be calculated. The series from south-west Canada (named Athabasca) used by (S1) did not correlate positively with local temperature observations, but has been replaced by a new, better-replicated series (S10) that does correlate very highly with summer temperature (Table S1) and has also been RCS-processed to retain all time scales of variability"

Now here is a quote from Briffa responding to Steve McIntyre’s criticism of his Yamal chronology.

"Chronologies are constructed independently and are subsequently compared with climate data to measure the association and quantify the reliability of using the tree-ring data as a proxy for temperature variations."

Now, one would think that this would be a good selection method if we wanted to use tree ring reconstructions to tell us 20th century temperature. But actually, it doesn’t even do that. The chronology simply mimics the surface temperature record; right or wrong.

But the real purpose of a reconstruction is to give us an idea of the relative temperatures of different periods of history. Especially those were we had no thermometers. And for that purpose, the selection method described by Osborn and Briffa absolutely destroys the value of the reconstruction.

It is clear from the descriptions of dendrochronologists themselves, as well as from what we know about tree growth, that trees do not all grow at the same rate, and the same tree grows at different rates throughout its life. One of the factors effecting its growth is temperature. But there are many others – sunlight, nutrient, water, CO2 availability etc. At any given point in time a tree may be able to respond to increases in temperature, or it may have it’s growth limited by a problem with the other factors. So, over the course of the life of a tree it may change it’s growth rate because it’s roots have reached a new source of nutrition; because the flow of water on the hillside where it is growing has slightly changed giving it more or less of the stuff; because a nearby tree has died giving it more sunlight; because a nearby tree has grown giving it less sunlight; because a nearby tree has died and it’s carcass is feeding the tree; because some bears have decided that it’s a good place to relieve themselves, etc. So let’s say that at any point in time a tree has a 60% chance of responding properly to temperature variation because it has the other resources that it needs. The number could be 20% or 80% - for purposes of this example it doesn’t matter.

So now, when we are in a period of increasing temperature, we select the series or the chronologies, as the case may be, that reflect that temperature rise. This means that 100%, or nearly 100% of the trees in the 20th century portion of the chronology are responding to temperature. If the number is much smaller, then only a fraction of the temperature rise will be reflected. But what happens to other historical periods within that chronology. The selection process did absolutely nothing to insure that those same trees were responding to temperature in say the Medieval Warming Period. So the likelihood that they are responding correctly to temperature tends to move towards our theoretical 60% average for those later periods. This means that by using Briffa and Osborn’s selection process, you are introducing a huge bias for having the 20th century show much more warming than the MWP. In fact, if the 20th century turns out to have anywhere close to the warming of the MWP, it will produce a chart where the 20th Century shows up as being much warmer than the MWP. And Al Gore can claim “unprecedented warming” for the 20th century.

I don’t know if Osborn’s and Briffa’s selection methods are unique to them. I rather doubt it. Most likely they are used by all of the dendrochronologists. If this is the case, then any reconstructions that use tree rings will tend to be worthless.

The effect of the selection method in question can be seen quite clearly in this IPCC spaghetti graph of temperature reconstructions. Notice how the separate reconstructions come tightly together in 20th century because their chronologies have been selected based upon their matching the surface temperature record. But then moving to later times, they spread out dramatically because their sensitivity to temperature is no longer assured. The percentage of trees that are sensitive to temperature has fallen, and the number of trees that are sensitive to temperature in any given reconstruction can vary widely. How useful are they when there is a .8C variation between them at some points.