History of Glaciers in Glacier National Park
The history of glaciation within current Glacier National Park boundaries spans centuries of glacial growth and recession, carving the features we see today. Glaciers were present within current Glacier National Park boundaries as early as 7,000 years ago but may have survived an early Holocene warm period (Carrara, 1989), making them much older. These modest glaciers varied in size, tracking climatic changes, but did not grow to their Holocene maximum size until the end of the Little Ice Age (LIA) around A.D. 1850. While they may not have formed in their entirety during the LIA, their maximum perimeters can be documented through mapping of lateral and terminal moraines. (Key, 2002) The extent and mass of these glaciers, as well as glaciers around the globe, has clearly decreased during the 20th century in response to warmer temperatures.
Climate reconstructions representative of the Glacier National Park region extend back multiple centuries and show numerous long-duration drought and wet periods that influenced the mass balance of glaciers (Pederson et al. 2004). Of particular note was an 80-year period (~1770-1840) of cool, wet summers and above-average winter snowfall that led to a rapid growth of glaciers just prior to the end of the LIA. Thus, in the context of the entire Holocene, the size of glaciers at the end of the LIA was an anomaly of sorts. In fact, the large extent of ice coverage removed most of the evidence of earlier glacier positions by overriding terminal and lateral moraines.
Tree-ring based climate records and historic photographs indicate the initiation of frontal recession and ice mass thinning between A.D. 1860 and 1880. The alignment of decadal-scale climate anomalies over the early 20th century produced a period of glacial recession somewhat analogous to conditions experienced over the past few decades. The coupling of hot, dry summers with substantial decreases in winter snowpack (~30% of normal) produced dramatic recession rates as high as 100 m/yr from A.D. 1917-1941 (Pederson et al. 2004). These multidecadal episodes have substantially impacted the mass balance of glaciers since A.D. 1900.
The decline of glacial ice may be linked to increases in mean summer temperature and/or a reduction in the winter snowpack that forms and maintains glaciers. Instrumental weather data from western Montana indicate a trend of increasing annual average temperature for the period of record (1900 – present). A larger regional temperature analysis that includes the northern part of the Crown of the Continent Ecosystem indicates that spring and summer minimum temperatures have increased more than other temperatures (Watson et al. 2008). Warmer spring temperatures, in particular, mean that the glaciers are not as cold and that melting is more readily initiated by the onset of summer. Despite the fluctuations in temperature over the past century, glaciers continued to shrink even during the cooler periods.
Glaciers could also be shrinking if less snow arrives during the winter. Annual precipitation has actually increased 10% during the past century (Selkowitz et al. 2002) but no similar long-term trend in snowpack exists in the vicinity of the glaciers. Snowpacks varied in a cyclical fashion, but glaciers continued to shrink throughout the period, indicating that the snowpack was not adequate to counteract the temperature changes. The overall threshold for maintaining glaciers was probably exceeded sometime between 1850 and the 1920s (Selkowitz et al. 2002). This recession trend corresponds with the data on glaciers elsewhere on earth – almost all mountain glaciers are receding as global temperatures increase. The USGS Climate Change in Mountain Ecosystems Program is studying glacial recession in Glacier National Park as part of a long-term monitoring program that utilizes several index glaciers to monitor recession. A detailed study of the mass balance of Sperry Glacier is also underway in an effort to determine how directly the dramatic changes in glacier area reflect climate trends in this region.
Carrara, P. E. 1989. Late quaternary glacial and vegetative history of the Glacier National Park Region, Montana. United States Government Printing Office. Denver, CO, USA. 64 p.
Pederson, G. T., D. B. Fagre, S. T. Gray, and L. J. Graumlich. 2004. Decadal-scale climate drivers for glacial dynamics in Glacier National Park, Montana, USA. Geophysical Research Letters. 31(L12203, doi:10.1029/2004GL0197770) .
Selkowitz, D. J., D. B. Fagre, and B. A. Reardon. 2002. Interannual variations in snowpack in the crown of the continent ecosystem. Hydrological Processes. 16:3651-3665.
Watson, E., G.T. Pederson, B.H. Luckman, and D.B. Fagre. 2008. Glacier mass balance in the northern U.S. and Canadian Rockies: paleo-perspectives and 20th century change. Pages 141-153 In Ben Orlove, Ellen Wiegandt and Brian Luckman (Eds.), Darkening Peaks: Glacier Retreat, Science, and Society, University of California Press, Berkeley, CA. USA. 280pp.
Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana
Glaciers and Glacial Features of Glacier National Park