Explainer: Retreat of Glaciers in Glacier National Park

US Geological Survey


In Glacier National Park (GNP), MT some effects of global climate change are strikingly clear. Glacier recession is underway, and many glaciers have already disappeared. The retreat of these small alpine glaciers reflects changes in recent climate as glaciers respond to altered temperature and precipitation. It has been estimated that there were approximately 150 glaciers present in 1850, and most glaciers were still present in 1910 when the park was established. In 2010, we consider there to be only 25 glaciers larger than 25 acres remaining in GNP. A computer-based climate model predicts that some of the park’s largest glaciers will vanish by 2030 (Hall and Fagre, 2003). This is only one model prediction but, if true, then the park’s glaciers could disappear in the next several decades. However, glacier disappearance may occur even earlier, as many of the glaciers are retreating faster than their predicted rates.


A glacier is a body of snow and ice that moves. Glacier movement is detected by the presence of crevasses, cracks that form in the ice as the glacier moves. Glaciers are dynamic – changing in response to temperature and precipitation. A glacier forms when winter snowfall exceeds summer melting. It retreats when melting outpaces accumulation of new snow. A commonly accepted guideline for glacier activity and movement is that a glacier must be 0 .1 km2 (100,000 m2), or about 25 acres in size. Below this size, the ice is generally stagnant and does not move, unless it is on a steep slope.

While the glaciers that carved GNP’s majestic peaks were part of a glaciation that ended about 12,000 years ago, current glaciers are considered geologically new, having formed about ~7,000 thousand years ago. These glaciers grew substantially during the Little Ice Age (LIA) that began around 1400 A.D and reached their maximum size at the end of the LIA around A.D.1850. Their maximum sizes can be inferred from the mounds of rock and soil left behind by glaciers, known as moraines (Key, 2002), which provide a scientific baseline for comparison to current glacial extent.


Glaciers, by their dynamic nature, respond to climate variation and reveal the big picture of climate change. Unable to adapt, like living creatures, GNP’s relatively small alpine glaciers are good indicators of climate, the long-term average of daily weather conditions. While occasional big winters or frigid weeks may occur, the glaciers of GNP, like most worldwide, are melting as long term mean temperatures increase. Glaciers are like a visual checking account of the status of the cold part of the ecosystem. Analysis of weather data from western Montana shows an increase in summer temperatures and a reduction in the winter snowpack that forms and maintains the glaciers. Since 1900 the mean annual temperature for GNP and the surrounding region has increased 1.33°C (Pederson et al. 2010), which is 1.8 times the global mean increase. Spring and summer minimum temperatures have also increased (Pederson et al. 2010), possibly influencing earlier melt during summer. Additionally, rain, rather than snow, has been the dominant form of increased annual precipitation in the past century (Selkowitz et al. 2002). Despite variations in annual snowpack, glaciers have continued to shrink, indicating that the snowpack is not adequate to counteract the temperature changes.

The simulation below reflects the predicted exponential rise in atmospheric CO2 concentrations, a 2xCO2 "global warming" scenario, with a concurrent warming of 2-3 degrees centigrade (4-5 degrees Fahrenheit) by the year 2050. In addition it assumes that precipitation, primarily in the form of rain, will increase over the same time period about 10 percent (based on the research of Dr. Steven Running, University of Montana). The animation view of the Blackfoot-Jackson basin along the Continental Divide, includes Gunsight Lake in the foreground and a portion of Lake Ellen Wilson visible over Gunsight Pass. 

In conjunction with the past century’s long-term temperature increase, ocean-driven climate trends (Pacific Decadal Oscillation) influence GNP’s regional climate. Tree-ring based climate records reveal PDO effects that have resulted in 20-30 year periods of hot, dry summers coupled with decreased winter snowpack (Pederson et al. 2004). These periods have induced rapid recession, as high as 100 m/yr between 1917-1941, and influence the current rate of recession. Even during cooler phases of the PDO cycle, glaciers have continued to shrink, albeit at a slower rate.


The loss of glaciers in GNP will have significant consequences for park ecosystems as well as impacting landscape aesthetics valued by park visitors. While winters will still deposit snow in the mountains, this seasonal snow will not function the same as glacial ice since it melts early in the summer season. Glaciers act as a “bank” of water (stored as ice) whose continual melt helps regulate stream temperatures and maintains streamflow during late summer and drought periods when other sources are depleted. Without glacial melt water, summer water temperatures will increase and may cause the local extinction of temperature sensitive aquatic species, disrupting the basis of the aquatic food chain (Pepin and Hauer 2002). Such changes in stream habitat may also have adverse effects for the threatened native bull trout (Salvelinus confluentus) and other keystone Salmonid species (Pederson et al. 2010).

Other impacts of climate change in GNP:

  • Mountain snowpacks hold less water and have begun to melt at least two weeks earlier in the spring. This impacts regional water supplies, wildlife, agriculture, and fire management (Mote, 2005).
  • Loss of alpine meadows will put some high-elevation species at risk as habitats become greatly diminished or eliminated (Peterson, 1998).
  • Mountain pine beetle infestation will likely spread further, causing areas of forests to die which will impact wildlife and stream habitat, wildfire risk, and recreation use (Western Forestry Leadership Council, 2009).
  • Fire frequency and burned area may be increased as fire season expands with earlier snowpack melt out and increasing number of hot days (Westerling et al. 2006).
    Large fires may greatly impact regional air quality, increase risk to people and property, and negatively affect tourism.