Variations in the stable isotope content of the wood of tree rings are known to reflect variations in the sea’s surface temperature. The sea’s surface supplies the water for continental rainfall and the yearly uptake of water by trees. The isotope ratios in the wood thus provide a record of past ocean temperatures. Variations in the sun’s output of heat may cause the changes in the temperatures of the seas. Analysis of the oxygen isotope content of Japanese cedar tree rings over 1,800 years provides evidence for cycles of 156 years, 124 years, 97 years, 88 years, 70 years, and 55 years. Other species of trees and other isotopes have similar lengths to their cycles. The tree-ring data closely parallel the mercury thermometer records that have been available since the seventeenth century. The records of cherry blossoming and lake thaws were also employed to validate the tree-ring data. Based on her work, Libby thinks that we are entering a cooling period, but that the warming impact of the greenhouse effect may modify this cycle projection.
Libby, Leona Marshall. “Evaluation of Historic Climate and Prediction of Near-Fuiure Climate from Stable-Isotope Variations in Tree Rings.” In Climate: History, Periodicity, and Predictability. Edited by Michael R. Rampino, John E. Sanders, Walter S. Newman, and L. K. Konigsson. New York: Van Nostrand Reinhold, 1987.
