The divergence problem is the disagreement between the temperatures measured by the thermometers (instrumental temperatures) on one side and the temperatures reconstructed from the widths of tree rings on the other side, in the northern forests.
While the thermometer records indicate a substantial warming trend, many tree rings do not display a corresponding change in their width. A temperature trend extracted from tree rings alone would not show any substantial warming. The temperature graphs calculated in these two ways thus "diverge" from one another since the 1950s, which is the origin of the term."
Something to remember when the AGW cultists tell you the tree rings prove something about the climate in the past.
source of quote: en.wikipedia.org/wiki/Dendroclimatology