Fennoscandian summers from AD 500: Temperature changes on short and long timescales

Quantitative estimates of 1480 years of summer temperatures in northern Fennoscandia have previously been derived from continuous treering records from northern Sweden. Here we show the results of spectral analyses of these data. Only a few peaks in the spectra are consistently significant when the data are analyzed over a number of sub-periods. Relatively timestable peaks are apparent at periods of 2.1, 2.5, 3.1, 3.6, 4.8, ~ 32-33 and for a range between ~ 55-100 years. These results offer no strong evidence for solar-related forcing of summer temperatures in these regions. Our previously published reconstruction was limited in its ability to represent long-timescale temperature change because of the method used to standardize the original tree-ring data. Here we employ an alternative standardization technique which enables us to capture temperature change on longer timescales. Considerable variance is now reconstructed on timescales of several centuries. In comparison with modern normals (1951-70) generally extended periods when cool conditions prevailed, prior to the start of the instrumental record, include 500-700, 790-870, 1110-1150, 1190-1360, 1570-1750 (A.D.) with the most significant cold troughs centred on about 660, 800, 1140, 1580-1620 and 1640. Predominantly warm conditions occurred in 720-790, 870-1110 and 1360-1570 with peaks of warmth around 750, 930, 990, 1060, 1090, 1160, 1410, 1430, 1760 and 1820.