From an airplane, most of the glaciers of the Rocky Mountain West look like small patches of white nestled into rugged landscape where the topography folds. The white is the snow cover, and lower down you can see the gray-blue bare ice.
In the middle of the 20th century, over 1,450 glaciers could be found in the contiguous U.S. Together, they covered 217 square miles (562 square kilometers) in six western states. Today, a comprehensive review my colleagues and I just published in the Proceedings of the National Academy of Sciences shows their total area has shrunk by 35% and they are still shrinking, perhaps faster.
A little more than 300 glaciers have disappeared, way more than I had anticipated.
Those that survive reflect just how vulnerable mountain glaciers have become as global temperatures rise. Where glaciers once covered large expanses of the western mountains thousands of years ago, most of the remaining glaciers in the Rockies and Sierra Nevada are small and found in topographically protected places.
Where US glaciers are disappearing fastest
When people think of glaciers, they often picture huge ice streams with icebergs calving off the front, like you’ll find in Alaska or Greenland. In the Lower 48 states, with the exception of a few large glaciers in Washington, they’re much smaller.
We found that the average glacier area is about one-tenth of a square mile (about a quarter of a square kilometer) in size. The area of the largest glacier, found on Washington’s Mount Rainier, is about 4.3 square miles (11.24 square kilometers).
I was surprised to find that glaciers in the Rocky Mountains – those in Colorado, Montana and Wyoming and in the Sierra Nevada of California – subsist because of their unique position on the landscape, which somewhat insulates them from the otherwise less-favorable regional climate.
The surviving Rocky Mountain glaciers inhabit shaded niches less exposed to direct sunshine and gain additional snow from avalanches. If these glaciers were exposed to more sun and less snow accumulation, they likely wouldn’t exit at all.
We found the fastest-thinning glaciers are in the Beartooth-Absaroka Mountains of the Rockies in southern Montana, which are losing about 1.5 feet (about half a meter) of ice thickness per year.
It’s a different story in Washington, where the glaciers on Mount Rainier and Mount Baker are much larger, have cooler summers and get a lot more snowfall. Although they too are shrinking, they do so at a much slower rate, about half a foot of ice thickness per year. The glacier area on Mount Rainier alone is greater than the total sum in all the other states.
In Washington, generally speaking, the higher you go, the less glacier loss you find. That makes sense; it’s cooler at higher elevations. But in the Rocky Mountain region there is no similar trend, suggesting the survivors are niche glaciers that depend on local topographic benefits.
What’s ahead
In a study a few years ago, colleagues and I modeled the glaciers on Washington’s Olympic Peninsula and found most would disappear by 2070 under current climate trends. That indicates that glaciers elsewhere are probably not far behind.
The Olympics are vulnerable because they’re close to the Pacific Ocean at somewhat lower elevation. The other Washington glaciers, particularly on Mount Rainier and Mount Baker, will likely survive past 2100, but how much longer is an open question.
That raises a few concerns.
The principal importance of the glaciers is to provide water to high alpine ecosystems. The glaciers partially melt in late summer, providing cool water for cold-adapted fish and other stream life. They provide a water supply that’s otherwise absent in the dry late summer, after the seasonal snow disappears, when the region doesn’t get a lot of rain.
While the glaciers of the Rockies are primarily important for ecosystems at this point, those in Washington also contribute to hydropower production.
An important hazard is also associated with the glaciers on the steep-sided volcanoes of the Cascade Range, which stretches from California to Washington. The moraines formed by these glaciers are large, partly due to the highly erodible volcanic rock beneath the ice. Moving ice deposits eroded rocks, forming aprons at their lower ends. As the glaciers retreat, they leave the accumulated rocks behind as steep-sided moraines that are no longer buttressed by the ice.
High water flows from rain or glacier melt can undercut the moraines, triggering landslides and creating debris flows that go rocketing down the mountains. This happened in 2006 on Mount Hood and Mount Rainier and closed a number of highways. We’ll see that happen more often as the glaciers retreat.
The shrinking and eventual loss of all of these glaciers will affect ecosystems across the West, as well as waterways humans and wildlife rely on.
Andrew Fountain, Professor of Geography, Portland State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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