Earth Institute News
posted 02/02/00 10:OO A.M. EST
Tree-Ring Evidence Confirms
Reconstruction of mean summer (May, June, July, August) temperatures for northern Alaska.
The temperature data is the average of University Experiment Station in Fairbanks, Fort Yukon and McKinley Park stations extending from 1906 to 1990. Four density series were used based on tree sampling sites at 412, Arrigetch, Savage River and Twelve Mile Summit. A principal component regression was performed using all four tree-ring density series. The regression explains 41% of the variance in the summer temperatures after adjusting for loss of degrees of freedom due to the regression.
The 7.2° C estimated summer temperature for 1783 is over 5 standard deviations below the mean of 11.3° C. Given that May and June are not described as being unusual in the oral history; July and August could have been mostly below freezing as described by the history.
Although the reconstruction extends only to 1680, some tree-ring samples are older. The density series from site 412 extends to 1580 and no earlier values are as low as 1783.