Many of you have probably read about the disputes over temeprature histories like Mann’s hockey stick chart. I thought you might be interested in how some of these 1000-year long proxies are generated. There are several different approaches, but one that Mann relied a great deal on is measuring tree rings in bristle cone pine trees. Here is a picture of a researcher taking a core from a very old tree that is then sent to a lab to have it’s ring widths measured.
In theory, these ring widths are directly proportional to annual temepratures, but there are a lot of questions about whether this is really true. Other factors, like changing precipitation patterns, might also affect ring widths, and there may be reasons why the scale could change over time. Remember, we only have a few decades, at most, of good temperature data to scale growth in a tree that goes back over a thousand years. In fact, scientists are finding that, more recently, tree ring proxy data for current growth is diverging from surface temperature data, meaning either that surface temperature data is flawed or that they don’t really understand how to scale tree ring data yet. Interestingly, and as a sign of the health of climate science, researchers have reacted to this problem by … not updating tree ring proxy databases for recent years. That’s one way to handle data that threatens your hypothesis — just refuse to collect it. Much more on proxy histories here.