...Whereas our results on orbitally forced climate trends in a 2,000-year MXD chronology seem to be in line with coarseresolution Holocene proxies2,3 and are supported by CGCM evidence7,8, little attention has been paid to the lack of these trends in long-term tree-ring records and implications thereof.The JJA temperature reconstruction presented here closes this gap, a finding that largely stems from the exceptionally strong and temporally stable climate signal, and the unprecedented length and replication of the new N-scan MXD chronology. The ability of MXD data to retain millennial scale temperature trends seems to result from a number of properties, including a reduced age trend24 and biological persistence25 resulting in less distortion of retained trends through regional curve standardization26 (RCS), the ability of tree populations to develop cell walls of continuously changing thickness over millennia and the non-plastic response of the termination of cell-wall lignification with respect to the integrated heat over the high and late summer seasons27. It is the combination of these properties that seems to enable the retention of a millennial scale trend in the MXD record and the lack of this lowest frequency variance in existing TRW records. These findings together with the trends revealed in long-term CGCM runs suggest that large-scale summer temperatures were some tenths of a degree Celsius warmer during Roman times than previously thought.
It has been demonstrated4 that prominent, but shorter term climatic episodes, including the Medieval Warm Period and subsequent Little Ice Age, were influenced by solar output and (grouped) volcanic activity changes, and that the extent of warmth during medieval times varies considerably in space.