"Tree Rings Don't Indicate Past Temperatures"

Excerpt from this link:

One of the reasons I doubt that the climatologists who talk of global warming are scientists is the claim that tree rings provide precise evidence of past average temperatures that can be compared to thermometer measured temperatures.

Craig Loehle has published a study in National Council for Air and Stream Improvement indicating that trees rings are not reliable for determining past temperatures. Loehle focused on the Medieval Warm Period (MWP) (1000 - 1400) which is not shown in tree rings, but is apparent using other so-called proxies.

Loehle notes that tree ring width respond to temperature in an inverse parabolic manner to temperature with growth increasing up to some optimal temperature and then decreasing with further temperature increases. In other words narrower rings could actually indicate higher temperature rather than lower temperature. Loehle suggests that higher evaporation rates at the higher temperatures may slow growth. Tree growth responds to changes in the amount of CO2 in the atmosphere by increasing growth as CO2 increases.