The growth‐promoting effects of the historical increase in the air's CO2 content are not yet evident in tree‐ring records where yearly biomass additions are apportioned among all plant parts. When almost all new biomass goes into cambial enlargement, however, a growth increase of 60% or more is observed over the past two centuries. As a result, calibration of tree‐ring records of this nature with instrumental climate records may not be feasible because of such growth changes. However, climate signals prior to about the mid‐19th century may yet be discovered by calibrating such tree‐ring series with independently derived proxy climate records for those times.