Growth rings in tree trunks are among the best records of local climate variation that there are: they provide an annual “stratigraphy”.  So intricate are the records that it has proved possible to match ring sequences in ancient but still growing trees to those found in logs of even greater antiquity, thereby building up a “dendrochronology” that extends back into history.  Tree rings help historians link human affairs to a background of changing conditions for life.  Henri Grissino-Mayer of the University of Tennessee has brought together a wealth of dendrochronological information in his Ultimate Tree Ring Pages at web.utk.edu/%7Egrissino/default.html.