The average width of any one tree ring is dependant upon many variables. This can include tree species, tree age, availability of stored food, and many complex climatic factors (sunshine, precipitation, temperature, wind speed, humidity, etc.). The major problem facing dendroclimatologists (those who study climate through tree rings) is gleaning out growth changes due to climate change as compared with background noise, or factors independent of climate. Some examples of background noise includes tree disease, which affects their growth, and a local insect outbreak, which would defoliate tree leaves and decrease growth for that year.

The most common mode of measuring differences in climate is through tree ring width. In general, weather patterns that support growth such as warm, temperate, and moist conditions, induce a larger width. Cold and dry conditions tend to make rings thinner. Dendroclimatologists also use the density of each ring to help infer climate conditions. Finally, chemical analyses (i.e. isotopic variations within the xylem) help in reconstruction of past climate. In order to use markings precisely, scientists need a good idea of the age of the tree, and what year a respective ring represents.

Since temperature has been recorded by scientists since the 1850s, tree rings are generally used to interpret climatic conditions earlier than the 19th century. Beginning in the 1970s, paleoclimatologists used historical documents as well as tree rings to reconstruct the climate of the past 600 years. As more records become available, dendrochronologists can compare data with tree rings with actual recorded conditions. A summary of five intensive research studies of dendrochronology and historical documents has produced the table below (click on the image for a larger perspective).

Image Courtesy of NOAA

The five studies all seem to be indicating that the 20th century is the warmest on the 600 year record, with the most dramatic warming after 1920. Since there is good overall correlation between the studies, scientists can have greater confidence in the claims that dramatic global warming has occurred since the 19th century, and that the 1990s in particular are the warmest on record.