Annual climate variability in the Holocene: interpreting the message of ancient trees

Over vast areas of the world's landmasses, where climate beats out a strong seasonal rhythm, tree growth keeps unerring time. In their rings, trees re cord many climate melodies, played in different places and different eras. Recent years have seen a consolidation and expansion of tree-ring sample co llections across the traditional research areas of North America and Europe , and the start of major developments in many new areas of Eurasia, South A merica and Australasia. From such collections are produced networks of prec isely dated chronologies; records of various aspects of tree growth, regist ered continuously, year by year across many centuries. Their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensio ns of time and space. With their extensive coverage, high temporal resoluti on and rigid dating control, dendroclimatic reconstructions contribute sign ificantly to our knowledge of late Holocene climates, most importantly on t imescales ranging from 1 to 100 years. In special areas of the world, where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved, work building chronologies covering many millenni a continues apace. Very recently, trees have provided important new informa tion about major modes of general circulation dynamics linked to the El Nin o/Southern Oscillation and the North Atlantic Oscillation, and about the ef fect of large volcanic eruptions. As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, su pports the notion that the last 100 years have been unusually warm, at leas t within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impactin g on the 'natural' growth of trees in different ways, making the task of is olating a clear climate message subtly difficult. (C) 1999 Elsevier Science Ltd. All rights reserved.