There has been considerable discussion on the divergence in recent years of temperature trends derived from tree ring data and from surface air temperature measurements. I have discussed this in two past posts on my weblog:
In the first post, the abstract of the paper includes the text
“An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings.”
In the second post, I wrote
“Dear Drs. Wilson and D’Arrigo
Thank you for your announcement and invitation for this very important
session. While I will not be able to attend the AGU Conference this
December, I did want to e-mail to encourage you to add another topic to
your list of questions. This is
How accurately does the in-situ (station data), when used to construct the
regional temperature trends, compare with the tree-ring data that are used
represent the actual temperature environment in which the trees grow?
Also, is the statistical relationship improved when the comparison with
the tree ring derived data is compared with maximum and minimum
temperatures, as well as different temperature measures of the growing
season, such as first and last date below selected threshold temperatures.
For the growing set of documentation of the USHCN sites, the siting of the
in-situ temperature measurement sites is a major problem (see
http://www.surfacestations.org and http://www.climateaudit.org). A
presentation of photographs for the surface temperature stations that are
used as part of the calculation of the temperature trends for each region
might be very insightful. Satellite derived surface temperatures (e.g. see
Comiso, 2006: Weather. pages 70-76) can be very helpful also in this
assessment, but the interpretation to the heights that the tree responds
to is also a challenge, as well as that the satellite is not sampling on
The testing of the robustness of the air temperature data trends would be
quite informative, and the availability of these photographs would be
With respect to the science of the issue raised in the otherwise excellent Der Spiegel article I (and others) disagree with their statement that
“….Tree-ring data indicates no global warming since the mid-20th century, and therefore contradicts the temperature measurements. The clearly erroneous tree data was thus corrected by the so-called “trick” with the temperature graphs.”
The reason that the tree ring data differs from the surface air temperature data in recent years has not been answered, despite the above statement from Der Speigel. Possible (speculative) explanations (besides the issues with the relationship of the surface air temperature data to the tree ring proxy data that I reported on above) include the effect of the increase of atmospheric concentrations of carbon dioxide and/or nitrogen deposition from human emissions on tree growth. The increased concentrations of carbon dioxide and/or the addition of nitrogen to the soil in which the trees grow could be altering their relationship to temperature from what it was in previous years.
Since the microclimate of the trees that were sampled are quite different from the microclimate where the surface air temperature data has been collected, this is also a possible explanation that needs to be examined. Photographs of the locations where the tree ring and surface air temperature data are collected should be a priority.
The tree ring proxy temperature data is not necessarily erroneous, but it is has diverged from the in-situ measured air temperature trend analysis. The reason for this difference needs further exploration.