Tree Rings Don't Indicate Past Temperatures

Tree Rings Don't Indicate Past Temperatures

Telegraph, November 23, 2007

HYPERLINK "http://my.telegraph.co.uk/reasonmclucus/november_2007/tree_rings_don_t_indicate_past_temperatures.htm" http://my.telegraph.co.uk/reasonmclucus/november_2007/tree_rings_don_t_indicate_past_temperatures.htm

By Reason McLucus

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.

HYPERLINK "http://www.wolfram.com/services/training/loehle.html" Craig Loehle has published a study in [ HYPERLINK "http://www.ncasi.org/publications/Detail.aspx?id=3025" Energy & Environment] 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.

Loehle cites a satellite survey covering 1982 to 2003 that indicated in tundra areas photosynthetic activity increased, but such activity decreased in forested areas, possibly because of the density of trees. As density increases individual trees may receive less sunlight and have less energy available for growth.

Loehle doesn't examine the fact that trees reduce heating of the air by converting solar radiation into the chemical bonds of the carbon molecules deposited in the trunk. The greater the tree growth the lower the temperature if solar radiation remains constant.