They also document temperature and cloud cover as they impact tree growth as well.
Each ring marks a complete cycle of seasons, or one year, in the tree's life.
Each year trees add growth rings, which can indicate what sort of growing season the tree experienced.
Interestingly these rings are more than a temperature indicator, they also tell the researcher about moisture and cloudiness as well.
Dendrochronology is the study of climate change as recorded by tree growth rings.
Each year, trees add a layer of growth between the older wood and the bark.
See the cal BP discussion for additional information about radiocarbon calibration.