Tree rings,a record of seasonal variations in past climate

The width and other characteristics of the growth layers in trees provide valuable information on past variations in climate. Not only can past climate be deduced from past growth, but computers can be used to calibrate the tree growth with climate and to obtain quantitative estimates or reconstructions of the climatic variables in the past from the measurements of ring widths. Large arrays of many tree-ring chronologies exhibit large-scale spatial anomalies in growth. These anomalies can be converted by means of the calibration equations into large-scale reconstructions of spatial anomalies in climate. In this way a map is produced of climatic conditions for individual seasons within each year as far back in time as tree-ring data are available for the sites used for calibration. Some of the biological and physical phenomena that influence the climatic records are described, and the unique opportunities provided by tree-ring analysis of climate are summarized.