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Ring widths from trees on certain sites reflect climatic variation. Therefore, long time series derived from replicated and precisely dated ring -width chronologies may be utilized to extend climatic records into prehistoric times. Multivariate analyses of tree -ring chronologies from western North America are used to derive response functions from which one can ascertain what climatic information each ring -width chronology contains.In addition, multivariate analyses are utilized to calibrate a large number of ring -width chronologies of diverse response functions and from widely dispersed sites with a large number of regional climatic variables. A series of transfer functions are derived which allow estimates of anomalous climatic variation from tree -ring records.Reconstructions of anomalous variation in atmospheric circulation for portions of the northern hemisphere back to A.D. 1700 are obtained by applying the transfer functions to tree -ring data for time periods when ring data are available but climatic data are not.

A response of plant growth to increased atmospheric carbon dioxide, which has been anticipated from laboratory data, may now have been detected in the annual rings of subalpine conifers growing in the western United States. Experimental evidence shows that carbon dioxide can be an important limiting factor in the growth of plants in this high-altitude environment. The greatly increased tree growth rates observed since the mid-l9th century exceed those expected from climatic trends but are consistent in magnitude with global trends in carbon dioxide, especially in recent decades. If correctly interpreted, these findings have important implications for climate studies involving tree ring observations and for models of the global carbon dioxide budget.