Background
The divergence problem, to quote the authors, is "an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings," which problem "has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century."

What was done
D'Arrigo et al. "review the current literature published on the divergence problem to date, and assess its possible causes and implications."

What was learned
With respect to the cause or causes of the divergence problem, the four researchers list the following possibilities: (1) moisture stress, (2) non-linear or threshold responses to warming, (3) local pollution, (4) delayed snowmelt, (5) changes in seasonality, (6) differential responses to maximum and minimum temperatures, (7) global dimming, (8) methodological issues related to "end effects," (9) biases in instrumental target data, (10) the modeling of such data, (11) declining stratospheric ozone concentrations, (12) increased UV-B radiation at ground level, and (unlucky number 13) "an upward bias in surface thermometer temperature measurements in recent years related to heat island effects."

What it means
One of the major difficulties resulting from the existence of the divergence problem, in the words of D'Arrigo et al., is that "reconstructions based on northern tree-ring data impacted by divergence cannot be used to directly compare past natural warm periods (notably, the Medieval Warm Period) with recent 20th century warming, making it more difficult to state unequivocally that the recent warming is unprecedented," although climate alarmists such as Al Gore do it regularly and with great ease.

With respect to the resolution of the issue, D'Arrigo et al. say their review "did not yield any consistent pattern that could shed light on whether one possible cause of divergence might be more likely than others," leading them to conclude that "a combination of reasons may be involved that vary with location, species or other factors, and that clear identification of a sole cause for the divergence is probably unlikely." As a result one can validly conclude there are a number of reasons why it is premature to be claiming that recent warming is unprecedented over the past millennium or more, particularly on the basis of tree-ring data.