Reconstruction of summer temperatures in interior Alaska from tree-ring proxies: Evidence for changing synoptic climate regimes

Maximum latewood density and delta(13)C discrimination of Interior Alaska white spruce were used to reconstruct summer (May through August) temperature at Fairbanks for the period 18001996, one of the first high-resolution reconstructions for this region. This combination of latewood density and delta(13)C discrimination explains 59.9%a of the variance in summer temperature during the period of record 1906-1996. The 200-yr. reconstruction is characterized by 7 decadal-scale regimes. Regime changes are indicated at 1816, 1834, 1879, 1916, 1937, and 1974, are abrupt, and appear to be the result of synoptic scale climate changes. The mean of summer temperature for the period of reconstruction (1800-1996) was 13.49degreesC. During the period of instrument record (1903-1996) the mean of summer temperature was 13.31degreesC for both the reconstruction and the recorded data. The coldest interval was 1916-1937 (12.62degreesC) and the warmest was 1974-1996 (14.23degreesC) for the recorded data. The reconstruction differs from records of northern hemisphere temperatures over this period, especially because of Interior Alaska warm periods reconstructed from 1834 to 1851 (14.24degreesC) and from 1862 to 1879 (14.19degreesC) and because of the cool period in the early part of the 20th century (1917-1974). We show additional tree ring data that support our reconstruction of these warm periods. Alternate hypotheses involving autogenic effect of tree growth on the site, altered tree sensitivity, or novel combinations of temperature and precipitation were explored and while they cannot be ruled out as contributors to the anomalously warm 19th century reconstruction, they were not supported by available data. White spruce radial growth is highly correlated with reconstructed summer temperature, and temperature appears to be a reliable index of carbon uptake in this system.