It has been shown that in certain 'signature' years tree-ring chronologies from living oaks throughout much of western Europe exhibit the same characteristic of either increasing or decreasing growth1. Remarkably, these chronologies, many of which are the product of the recent extension of the European tree-ring network2-4, are distributed over a latitude span of ∼10°and a longitude span of 20°. This phenomenon means that archaeological series from more distant areas can be compared, increasing the likelihood that gaps in regional dating chronologies can be bridged5,6. The question arises as to what circumstances could induce trees over such a large area to behave in unison so frequently. We have examined climate departures from modern signature years to determine whether or not there is a typical climate pattern associated with years of large-scale growth increase or decrease. We find that particular climatic conditions affecting the area from which the tree-ring chronologies are drawn can account for the signature years and suggest that this relationship may enable archaeological chronologies to be used in climate reconstruction.