Retreat of Glaciers in Glacier National Park

Map of the glaciers in Glacier NP.

In Glacier National Park (GNP) some effects of climate change are strikingly clear. Glaciers are melting, and many glaciers have already disappeared. The rapid 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, around the end of the Little Ice Age and most glaciers were still present in 1910 when the park was established. In 2015, measurements of glacier area indicate that there were 26 remaining glaciers larger than 25 acres, the size criteria used by USGS researchers to define a glacier.

WHAT IS A GLACIER? A glacier is a body of snow and ice of sufficient size and mass to move under its own weight. Glacier movement may be detected by the presence of crevasses, cracks that form in the ice as the glacier moves. In Glacier National Park (GNP), USGS scientists define glaciers according to the commonly accepted guideline in which a body of ice has an area of at least 0 .1 km2 (100,000 m2), or about 25 acres. Below this size, the ice is generally stagnant and does not move, unless it is on a steep slope. Glaciers are dynamic , changing in response to temperature and precipitation. A glacier forms when winter snowfall exceeds summer melting. Glaciers retreat when melting outpaces accumulation of new snow.  

While the glaciers that carved GNP’s majestic peaks were part of a glaciation that ended about 12,000 years ago, the relatively small alpine glaciers that cling to mountainsides today 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 AD and reached their maximum size at the end of the LIA around 1850 AD. Their maximum sizes can be inferred from the mounds of rock and soil left behind, known as moraines.  Aerial photography reveals moraines of over 150 glaciers that existed in Glacier National park at the end of the LIA. In 2015, only 26 active named glaciers remain.

Early park visitors and scientists noted that glaciers were retreating as early as 1914.  The climate was already warming and glaciers were responding, but the industrial revolution added more carbon dioxide to the atmosphere, furthering glacier retreat.  In Glacier National Park, as well as around the globe, glacier mass and extent has decreased during the 20th century in response to warmer temperatures.

WHAT IS THE STATUS OF GLACIERS AT GLACIER NATIONAL PARK?

In 2017, the USGS published a time series analysis of the glacier margins of the named glaciers of Glacier National Park . The areas measured are from 1966, 1998, 2005 and 2015/2016, marking approximately 50 years of change in glacier area. Scientists used aerial photography and satellite imagery to measure the perimeters of the glaciers in late summer when seasonal snow had melted to reveal the extent of the glacial ice. The data table shows that all glaciers have been reduced in area since 1966 with some glaciers having been reduced by as much as 85% by 2015.  The average area reduction over the approximately 50-year period is 39%.  Currently, only 26 glaciers are larger than 0.1 square kilometers (25 acres) which is used as a guideline for deciding if bodies of ice are large enough to be considered glaciers.

Glacier Times Series Data Table – area measurements of the named glaciers of GNP

Glacier Time Series Info

WHY ARE GLACIERS IMPORTANT?

Alpine glaciers, such as the glaciers in Glacier National Park, have been landscape features for thousands of years.  They play integral roles in the ecology of the region where they exist.  They are essentially frozen reservoirs of water which release cold water in late summer when streams might otherwise have low flows or no flows.  Adding water to streams and cooling streams is a function that several known species of aquatic insects, such as the meltwater and western glacier stoneflies, have come to rely upon.  Native fish species, such as cutthroat trout, have evolved in the presence of sustained cold water inputs from glaciers and are threatened by hybridization facilitated by warmer stream temperatures .

In some places, glacier meltwater provides drinking water for communities.  Glacier meltwater also contributes to agricultural practices and recreational uses like boating and fishing.  Local economies and livelihoods are connected to glacial input in these ways.   As glaciers disappear, there will be a reduction in water input at the same time the demand is going up.  Wildfire, and it’s multitude of devastating impacts ecologically and economically, is more likely when the landscape has less water.    

WHY ARE THEY MELTING?

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. A glacier forms when winter snowfall exceeds summer melting. Glaciers retreat when melting outpaces accumulation of new snow.  Despite occasional big winters or frigid weeks that occur, the glaciers of GNP, like most glaciers worldwide, are melting as long term average temperatures increase. 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. 2011), 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.

In conjunction with the past century’s long-term temperature increase, ocean-driven climate trends (Pacific Decadal Oscillation of PDO) 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.  Glacier National Park, located in the mountainous region of northwest Montana, is experiencing effects of climate change related to changing snowmelt and precipitation patterns, exacerbating ecological and socioeconomic issues tied to water supply.  

Large moraines at the base of Logan Glacier (2016) mark the extent of the glacier before it began its retreat around 1850 AD.(Public domain.)

WHAT ARE THE EFFECTS OF LOSING GLACIERS? The loss of glaciers in GNP will have significant consequences for park ecosystems as well as impacting landscape aesthetics valued by park visitors. While winter weather will still deposit snow in the mountains, this seasonal snow does 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 stream temperatures will increase and may cause the local extinction of temperature sensitive aquatic insects, disrupting the basis of the aquatic food chain. Such changes in stream habitat may also have adverse effects for the threatened native bull trout and increase hybridization of native cutthroat trout, leaving the native population at risk. Loss of a sustained water source may also impact vegetation communities and contribute to a dryer landscape overall.

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.
• Loss of alpine meadows will put some high-elevation species at risk as habitats become greatly diminished or eliminated.
• 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.
• Fire frequency and burned area may be increased as fire season expands with earlier snowpack melt out and increasing number of hot days.  Large fires may greatly impact regional air quality, increase risk to people and property, and negatively affect tourism.

This image shows the perimeter of Chaney Glacier in Glacier National Park in 1966, 1998, 2005, and 2015.(Public domain.)

OTHER LINKS OF INTEREST

Repeat Photography

• USGS Benchmark Glacier Research
• USGS Repeat Photography Project
• US Global Change Research Program
• National Climate Assessment
• FAQs on climate change
• GlacierHub
• Find us on Twitter @USGSglaciology

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