Low-frequency temperature variations from a northern tree ring density network

We describe new reconstructions of northern extratropical summer temperatur es for nine subcontinental-scale regions and a composite series representin g quasi "Northern Hemi sphere" temperature change over the last 600 years. These series are based on tree ring density data that have been processed u sing a novel statistical technique (age band decomposition) designed to pre serve greater long-timescale variability than in previous analyses. We prov ide time-dependent and timescale-dependent uncertainty estimates for all of the reconstructions. The new regional estimates are generally cooler in al most all precalibration periods, compared to estimates obtained using earli er processing methods, particularly during the 17th century. One exception is the reconstruction for northern Siberia, where 15th century summers are now estimated to be warmer than those observed in the 20th century. In prod ucing a new Northern Hemisphere series we demonstrate the sensitivity of th e results to the methodology used once the number of regions with data, and the reliability of each regional series, begins to decrease. We compare ou r new hemisphere series to other published large-regional temperature histo ries, most of which lie within the lo confidence band of our estimates over most of the last 600 years. The 20th century is clearly shown by all of th e palaeoseries composites to be the warmest during this period.