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.