300 glaciers have disappeared from the American West since the mid-20th century – the survivors have a few things in common

Glaciers in the Rocky Mountain West appear as small white patches nestled in rugged mountain landscapes when viewed from an airplane. These glaciers are much smaller than those found in Alaska or Greenland. In the mid-20th century, over 1,450 glaciers existed in the contiguous United States, covering 217 square miles (562 square kilometers) across six western states. Today, a recent study published in the Proceedings of the National Academy of Sciences shows that the total area of these glaciers has shrunk by 35% and they continue to shrink, possibly at an accelerating rate. The study, led by a team of researchers including the author, analyzed glacier changes over time to reach these conclusions.

Since the mid-20th century, more than 300 glaciers have disappeared from the American West. This number is higher than initially expected by the researchers. The disappearance of these glaciers reflects their increasing vulnerability as global temperatures rise. Most of the remaining glaciers in the Rockies and Sierra Nevada are small and located in topographically protected areas, such as shaded niches in mountains. These niches provide insulation from direct sunlight and additional snow accumulation from avalanches, which helps the glaciers survive longer than they would otherwise. Without these protective features, many of these glaciers would likely have vanished already.

The average size of glaciers in the American West is about one-tenth of a square mile (0.25 square kilometers). The largest glacier, located on Mount Rainier in Washington, covers about 4.3 square miles (11.24 square kilometers). Glaciers in the Rocky Mountains, including those in Colorado, Montana, and Wyoming, as well as in California’s Sierra Nevada, survive due to their unique positions in shaded areas of the landscape. However, glaciers in the Beartooth-Absaroka Mountains of Montana are thinning the fastest, losing about 1.5 feet (0.5 meters) of ice thickness per year. In contrast, glaciers on Mount Rainier and Mount Baker in Washington, which are larger and receive more snowfall, are thinning at a slower rate of about half a foot (0.15 meters) per year.

Glacier loss varies significantly across the American West. In Washington, higher elevations tend to have less glacier loss, as cooler temperatures at higher altitudes slow the melting process. However, in the Rocky Mountain region, there is no similar trend, suggesting that the surviving glaciers depend on local topographic benefits like shade and snow accumulation from avalanches. The study highlights that glaciers in Washington, particularly on Mount Rainier and Mount Baker, are likely to survive past 2100, though their long-term future remains uncertain. In contrast, glaciers on Washington’s Olympic Peninsula are projected to disappear by 2070 under current climate trends.

Glaciers play a crucial role in providing water to high alpine ecosystems, especially in late summer when seasonal snow has melted and rainfall is scarce. The meltwater from glaciers helps sustain cold-adapted fish and other aquatic life in streams. In Washington, glaciers also contribute to hydropower production, making them economically significant. However, the shrinking of glaciers poses risks, including the potential for debris flows. As glaciers retreat, they leave behind steep-sided piles of rocks called moraines. These moraines can become unstable and, when undercut by water from rain or melting ice, may collapse, triggering landslides and debris flows that can damage infrastructure and close highways.