More than 300 glaciers have disappeared from the western United States since the mid 20th century while the total area covered by the remaining glaciers has declined by 35 per cent according to a new study published in the journal Proceedings of the National Academy of Sciences (PNAS).
The study reviewed glaciers across six western states and found that their combined area has continued to shrink with some regions experiencing particularly rapid ice loss. Researchers warned that rising global temperatures are increasing pressure on these mountain glaciers, many of which survive only because of their location in shaded and snow protected areas.
In the middle of the 20th century, more than 1,450 glaciers covered approximately 217 square miles (562 square kilometres) across the contiguous United States. Today, the number has fallen by more than 300, and the remaining glaciers occupy a substantially smaller area.
Unlike the enormous glaciers found in Alaska and Greenland most glaciers in the lower 48 US states are relatively small. The study found that the average glacier covers around one-tenth of a square mile, or approximately a quarter of a square kilometre. The largest glacier in the region, located on Mount Rainier in Washington, covers about 4.3 square miles (11.24 square kilometres).
Shaded mountain areas help glaciers survive
Researchers found that many surviving glaciers in the Rocky Mountains, including those in Colorado, Montana and Wyoming, as well as the Sierra Nevada in California, occupy sheltered locations that protect them from direct sunlight.
These glaciers often receive additional snow from avalanches and benefit from the surrounding terrain, which helps them retain ice despite rising temperatures. Without these natural advantages, many of the remaining glaciers might already have disappeared.
The study identified the Beartooth-Absaroka Mountains in southern Montana as the region experiencing the fastest glacier thinning. Glaciers there are losing approximately 1.5 feet, or around half a metre of ice thickness each year.
Conditions are different in Washington, where glaciers on Mount Rainier and Mount Baker are larger, experience cooler summers and receive heavier snowfall. Although these glaciers are also shrinking, their ice thickness is declining at a slower rate of around half a foot per year.
The total glacier area on Mount Rainier alone exceeds the combined glacier area of all the other states covered by the study.
Researchers also observed differences in how elevation affects glacier survival. In Washington, glaciers generally experience less ice loss at higher elevations because temperatures are cooler. No similar pattern was found across the Rocky Mountain region, suggesting that local landscape features play a particularly important role in determining which glaciers survive.
Many glaciers could disappear in coming decades
The findings raise concerns about the future of glaciers across the American West. Previous modelling by researchers examining glaciers on Washington’s Olympic Peninsula suggested that most could disappear by 2070 if current climate trends continue.
These glaciers are particularly vulnerable because they are located at relatively low elevations near the Pacific Ocean. By comparison, some glaciers on Mount Rainier and Mount Baker could survive beyond 2100, although their long term future remains uncertain.
Scientists warn that the continued loss of glaciers could have serious consequences for ecosystems, water availability and communities across the region.
Glacier loss threatens ecosystems and water supplies
Glaciers play an important role in sustaining high-altitude ecosystems by releasing meltwater during late summer. This provides cool water for cold-adapted fish and other aquatic organisms, particularly after seasonal snow has disappeared and rainfall is limited.
In Washington, glacier meltwater also contributes to water supplies used for hydropower production. The loss of glaciers could therefore affect both natural ecosystems and human activities that depend on reliable water flows.
Another concern is the risk of landslides and debris flows in mountainous areas, particularly around the volcanic peaks of the Cascade Range, which stretches from California to Washington.
As glaciers retreat they leave behind steep deposits of rock and debris known as moraines. These formations can become unstable when heavy rainfall or meltwater erodes their bases, potentially triggering destructive flows of water, rocks and sediment down mountain slopes.
Similar events occurred on Mount Hood and Mount Rainier in 2006 resulting in the closure of several highways. Researchers warn that such incidents could become more frequent as glaciers continue to retreat and the surrounding terrain becomes increasingly exposed.
The study highlights the vulnerability of the remaining glaciers in the western United States. Scientists say their continued decline could reshape mountain ecosystems affect waterways relied upon by wildlife and people and increase certain geological hazards in the decades ahead.