Divergence_problem
The divergence problem is an anomaly from the field of dendroclimatology, the study of past climate through observations of old trees, primarily the properties of their annual growth rings. It is the disagreement between instrumental temperatures (measured by thermometers) and the temperatures reconstructed from latewood densities or, in some cases, from the widths of tree rings in far northern forests.
While the thermometer records indicate a substantial late 20th century warming trend, many tree rings from such sites do not display a corresponding change in their maximum latewood density. In some studies this issue has also been found with tree ring width.[2] A temperature trend extracted from tree rings alone would not show any substantial warming since the 1950s. The temperature graphs calculated in these two ways thus "diverge" from one another, which is the origin of the term.