Biological evidence of multiple temporal and spatial scales of hydrological variation in the western interior of Canada

Biological records of past variations in moisture balance are available fro m a number of sites in the western interior of Canada. Such evidence includ es fossil pollen, plant macrofossils, peatland records, paleolimnological i ndices, and tree rings. From these records, a temporal and spatial hierarch y of changes in moisture balance can be recognized. At the largest temporal and spatial scales there is a late Holocene increase in moisture that is a pparent in fossil pollen and peatland initiation records from throughout th e western interior. This increase in moisture likely reflects decreasing su mmer insolation and associated changes in atmospheric circulation. At the s ubregional scale, asynchronous changes in moisture balance that persist for centuries to millennia can be recognized in a number of fossil pollen and paleolimnological records. The explanation for such changes remains specula tive. At the smallest scale, annually resolved tree-ring records and instru mental climate data indicate that annual to multi-year droughts of varying severity and geographic extent have been a regular feature of the western i nterior climate. Such variations in central North America have been attribu ted to changes in North Pacific sea surface temperatures, solar-magnetic cy cles and lunar nodal tidal cycles. Some of the pre-20th century droughts ap pear to have been more severe and more extensive than any recorded in the i nstrumental record. (C) 2000 Elsevier Science Ltd and INQUA. All rights res erved.