Summer temperatures in the Canadian Rockies during the last millennium: a revised record

Abstract

We present a significant update to a millennial summer temperature reconstruction (1073-1983) that was originally published in 1997. Utilising new tree-ring data (predominantly Piceaengelmannii), the reconstruc- tion is not only better replicated, but has been extended (950-1994) and is now more regionally representative. Calibration and verification statistics were improved, with the new model explaining 53% of May-August maximum temperature variation compared to the ori- ginal (39% of April-August mean temperatures). The maximum latewood density data, which are weighted more strongly in the regression model than ringwidth, were processed using regional curve standardisation to capture potential centennial to millennial scale vari- ability. The reconstruction shows warm intervals, com- parable to twentieth century values, for the first half of the eleventh century, the late 1300s and early 1400s. The bulk of the record, however, is below the 1901-1980 normals, with prolonged cool periods from 1200 to 1350 and from 1450 to the late 19th century. The most ex- treme cool period is observed to be in the 1690s. These reconstructed cool periods compare well with known regional records of glacier advances between 1150 and the 1300s, possibly in the early 1500s, early 1700s and 1800s. Evidence is also presented of the influence of solar activity and volcanic events on summer temperature in the Canadian Rockies over the last 1,000 years. Although this reconstruction is regional in scope, it compares well at multi-decadal to centennial scales with Northern Hemisphere temperature proxies and at mil- lennial scales with reconstructions that were also pro- cessed to capture longer timescale variability. This coherence suggests that this series is globally important for the assessment of natural temperature variability over the last 1,000 years.