Long-term perspectives on northern Rockies climatic variability from tree rings in Glacier National Park, Montana

Abstract

Instrumental climate records reveal fluctuations in summer moisture anomalies and winter snowpack in Glacier National Park, Montana, on decadal and multidecadal timescales. However, because climate records for the region are limited to the 20th century, studies on the impacts of long-duration variations in climate on physical and ecosystem processes were limited. Therefore, a reconstruction of summer moisture variability (June - August) spanning A.D. 1540-2000 was created from a multi-species network of tree-ring chronologies sampled in Glacier National Park. The reconstruction shows decadal-scale shifts between drought and pluvial events with a pronounced cool/wet period spanning the end of the Little Ice Age (A.D. 1770-1840). The single most exceptional drought event occurred over the 20th century (A.D. 1917-1941) and was associated with the most spatially consistent drought regime throughout the northern Rockies and Pacific Northwest over the past ~500 yrs. Among a wider spatial network of hydroclimatic reconstructions arrayed along a north-south Rocky Mountain transect, trends at Glacier National Park were found to be most similar to those in the Canadian Rockies and the Pacific Northwest.

Also, many decadal-scale drought/pluvial events were consistent among all sites along the north-south transect - although magnitude, intensity, and time of onset varied. To investigate climatic drivers related to the Little Ice Age glacial maximum and rapid 20th-century retreat, I explored the impact of north Pacific Basin sea-surface temperature anomalies on low-frequency variations in winter snowpack for the park. Temperature anomalies in the north Pacific basin exhibit tight linkages to variations in snowpack; therefore, I used a tree-ring based reconstruction of north Pacific temperature variability and summer drought as proxies for winter glacial accumulation and summer ablation, respectively, over the past three centuries (A.D. 1700-2000). These records show that the 1850's glacial maximum was likely produced by ~70 yrs of cool/wet summers coupled with high snowpack. Glacial retreat coincided with an extended period (>50 yr) of summer drought and low snowpack culminating in the exceptional events of 1917-1941 when retreat rates exceeded 100 m/yr. This research highlights the difficulty in detecting regional expression of global climate change when 'natural' decadal-scale variations in climate are regionally common.