Thursday, July 5, 2012

Tree Rings As Climate Proxies

Watts Up With That? discusses a paper recently published that addresses what might seem a rather obscure question: how well tree rings work as proxies for temperature.

First of all, what's a proxy?  Something that stands in for something that is missing.  We don't have regular weather reports for the last few thousand years, so when scientists attempt to understand how climate has changed, they have to look for evidence that stands in for that data.  One such proxy has traditionally been the width of tree rings.  The assumption has been that warm or wet weather (which may not be the same thing) shows up as wider rings; dry or cold weather should show up as narrower rings.  (I understand that extraordinarily warm or wet weather can sometimes produce two rings.)

But this is just an assumption.  This paper compares two different proxies: tree rings, and O16/O18 ratio in lake sediments--and finds that they don't agree as perfectly as you would hope.  Either tree rings aren't so good, or the oxygen isotope rates aren't so good.  At a minimum, those who claim that they can tell us temperatures over the last couple thousand years should be a bit more humble.


  1. Is there any survey of tree rings from the time period that we do have climate measurements from?

    Or is establishing a baseline not quite that simple?

  2. Either tree rings aren't so good, or the oxygen isotope rates aren't so good.

    Or both aren't so good.

    Not that it matters much to this part of the scientific community when they allow major findings to be based on exquisite cherry picking of a very few sets of trees.

  3. Karrde, it is my understanding that even attempting to establish a baseline can be rather complicated. Temperature is only one factor in determining the size of a given ring; available water, weather, disease, and other factors can affect the sizes of rings.

    Thus, using rings as proxies for temperature is a difficult task in and of itself (especially since we don't have records for these events for the exact same reason we don't have records for temperature).

    To confound things even further, it is my understanding that the typical study only focuses on trees in two or three regions; that is a far cry from global temperatures!

  4. Yes, but if you have a political result in mind, using tree rings or oxygen isotope rates as temperature proxies is way better than using the records of monks in their cloisters complaining about the ink freezing as proxies to say there was a sudden and severe decline in temperatures in Europe circa 400-1000 A.D.