Reconciling the glacial and dendrochronological records for the last millennium in the Canadian Rockies

Abstract

Reconciliation of the glacial and dendrochronological records of the last two millennia must recognise inherent differences in the nature of these records. The glacier record is a discontinuous, filtered signal which is abbreviated because the most extensive Holocene glacial events occurred in the 18th and 19th centuries. Summary “Little Ice Age” moraine chronologies for 60 glaciers are presented and reviewed. Recent investigations have revealed periods of earlier, less extensive glacier advances ca 3500–2800 B.P (4 glaciers, possibly two events), 1500–1900 BP (2 glaciers) and A.D. 1140–1370 (3 glaciers) in addition to the classical moraines of the 18th and 19th centuries. Fragmentary, less-precisely dated evidence of glacier events in the mid-16th and early 17th centuries occurs at some sites. The dendrochronological evidence to date is based on ringwidth chronology networks (24 Picea engelmannii, 12 Larix lyallii and 3 Pinus albicaulis sites) that routinely cover the last 3–400 years but include millennial chronologies in all 3 species. These records show strong regional suppression in ring-width series corresponding to the early 18th century, mid-19th and late 20th century glacial events plus periods in the mid 14th, mid-15th and late-16th/early 17th centuries for which little well dated glacial evidence is available.

These data provide evidence of multiple glacial events in the last 3000 years which, although poorly resolved prior to 1700 because of the nature of the evidence, generally indicate a pattern of progressively more extensive glacial advances during this period culminating in the 18th and 19th centuries. This trend is interpreted as the result of long term changes in summer radiation (due to orbital changes) superimposing an overall cooling trend on higher frequency (decade to century scale) climate fluctuations that control the periods of glacier advance and recession. The concept of a “Medieval Warm Period” and “Little Ice Age” as distinctive climatic intervals of several centuries duration is clearly a gross oversimplification as glacier advances and intervening warmer periods occurred throughout the last millennium. Nevertheless the term Little Ice Age should be retained as a collective term for the glacier advances of the last few centuries that produced a distinctive morphostratigraphic record.