The United States holds thousands of glaciers spread across Alaska and the mountainous West, but the vast majority of them are shrinking. A recent inventory of the contiguous 48 states counted 1,331 glaciers covering roughly 367 square kilometers, along with more than a thousand smaller perennial snowfields. Alaska, not included in that tally, dwarfs the lower 48 with glaciers covering an area larger than the state of West Virginia. Together, these ice masses shape river flows, support fisheries, feed marine ecosystems, and contribute measurably to global sea-level rise.
Where the Glaciers Are
When most people think of US glaciers, Alaska comes to mind first, and for good reason. The state contains the overwhelming majority of American glacier ice, with massive systems like the Bering Glacier (the largest glacier in North America), the Malaspina piedmont glacier, and hundreds of tidewater glaciers that calve directly into the ocean. But glaciers also survive across much of the mountain West in the lower 48, from Washington’s Cascade Range down through Oregon, California’s Sierra Nevada, Montana’s northern Rockies, Wyoming’s Wind River Range, and even a handful of tiny ice bodies in Colorado and Nevada.
The most thorough modern count of glaciers in the contiguous United States identified 1,331 glaciers and 1,176 perennial snowfields, plus 35 buried-ice deposits hidden under rock debris. The glaciers alone cover about 367 square kilometers, while the snowfields add roughly another 31 square kilometers. Washington State dominates the lower-48 inventory, with Mount Rainier alone hosting more than two dozen named glaciers. Montana, Wyoming, and California round out the list of states with significant glaciated area.1Earth System Science Data. Inventory of glaciers and perennial snowfields of the conterminous USA
The distinction between a glacier and a perennial snowfield matters more than it might seem. A glacier moves under its own weight; a perennial snowfield persists year-round but lacks that internal flow. As glaciers shrink, many are reclassified as snowfields or simply disappear from inventories altogether. That reclassification can make it look like glaciers vanished overnight when in reality they spent years slowly crossing the threshold from one category to the other.
Alaska’s Outsized Role in Sea-Level Rise
Alaska’s glaciers are not just the most numerous in the US; they are among the fastest-melting large ice masses on Earth. Research measuring glacier thinning across the state found that from the mid-1990s to around 2000, Alaskan glaciers were losing ice at an average rate of about 1.8 meters of thinning per year. Extrapolated across the state, that worked out to roughly 96 cubic kilometers of water lost annually, enough to raise global sea levels by about 0.27 millimeters per year. At the time of that measurement, the losses from Alaska’s glaciers alone were nearly double the estimated annual losses from the entire Greenland Ice Sheet during the same period.2PubMed. Rapid wastage of Alaska glaciers and their contribution to rising sea level
Those numbers have continued to shift as warming accelerates. Alaska’s contribution to sea-level rise is disproportionate compared to its latitude because many of its glaciers sit at relatively low elevations and extend into maritime climates where warm ocean water and rain eat away at ice from below and above. Tidewater glaciers, which terminate in the sea, are especially vulnerable. Their centuries-long behavior follows a natural cycle of slow advance, brief stability, and rapid retreat, but warming ocean temperatures can accelerate the retreat phase dramatically.3Nature Communications. Tidewater cycle drives alpine glacial sediment plume geochemistry
Glacier National Park and the Rocky Mountain Losses
No place in the US tells the glacier-loss story more visually than Glacier National Park in Montana. The park’s glaciers shrank from about 21.6 square kilometers in 1850 to 7.4 square kilometers by 1979, a roughly two-thirds reduction during a period when global temperatures had risen by less than half a degree Celsius. Modeling under a carbon-dioxide-driven warming scenario projected that all glaciers in the park’s Blackfoot-Jackson basin would disappear by 2030.4BioScience. Modeled Climate-Induced Glacier Change in Glacier National Park, 1850–2100
That projection has attracted a lot of public attention. The National Park Service famously installed signs stating the glaciers would be gone by 2020, then had to revise them when small remnants persisted longer than expected. The glaciers are still there in some form, but several have lost so much mass that they no longer meet the minimum-area threshold to be classified as glaciers. Whether the park retains any features technically called glaciers by mid-century depends on how quickly warming proceeds and whether occasional heavy snow years can slow the decline. Either way, what visitors see today bears little resemblance to the ice fields that gave the park its name.
Farther south in the Rockies, Wyoming’s Wind River Range hosts some of the most-studied glaciers in the lower 48. Ice cores from the Upper Fremont Glacier in that range have served as climate archives stretching back 300 years, revealing shifts in dust sources as the landscape around the glacier changed over time. The dust record shows a transition from long-range mineral dust transport to increasingly local sources, consistent with drying and land-use changes in the surrounding region.5Quaternary Science Reviews. Ice core record of dust sources in the western United States over the last 300 years
The Sierra Nevada’s Small but Persistent Ice
California’s Sierra Nevada is not the first range most people associate with glaciers, but it holds dozens of small glaciers and ice patches, mainly on north-facing cirque walls above 3,500 meters. These remnants are the latest in a series of glacial advances and retreats recorded in the range’s moraine deposits. Research on moraines in the central Sierra Nevada has identified at least three distinct Holocene advances: the Recess Peak advance, the Matthes advance, and the modern remnants that survive today. Each advance corresponded to relatively modest climate shifts, with summer temperature drops of roughly 1 to 3 degrees Celsius and small increases in winter snowfall compared to present conditions.6Quaternary Science Reviews. Holocene glaciation of the central Sierra Nevada, California
That sensitivity cuts both ways. Because Sierra Nevada glaciers formed and survived under conditions only slightly cooler and snowier than today, they are extremely vulnerable to even small warming trends. Most are already tiny, and several have all but vanished in recent decades. Their practical significance lies less in water supply, given how small they are relative to the Sierra’s snowpack, and more in what their presence or absence tells us about climate thresholds. When the last Sierra glacier melts, it signals that regional summer temperatures have crossed a line that held for thousands of years.
How Glaciers Feed Rivers in the Pacific Northwest
In Washington and Oregon, glaciers do something that matters beyond scenery: they release meltwater into rivers during the hottest, driest weeks of summer, exactly when snowpack has already melted and rain is scarce. This function makes glacier-fed rivers behave differently from purely snowmelt-driven systems. In the Skagit River watershed in Washington, glaciers still contribute roughly 6 to 12 percent of total summer streamflow, and that share doubles to about 12 to 24 percent during August and September, the months when water demand from agriculture, cities, and salmon habitat is highest.7Northwest Science. Impact of Recent Glacial Recession on Summer Streamflow in the Skagit River
But the buffer is eroding. The Skagit study found that glacier recession over 50 years had already cut glacier meltwater contributions by about a quarter compared to the early 1960s. A similar pattern shows up in the Skykomish River basin, where glacier area shrank by 45 percent between 1958 and 2009, driving a 38 percent drop in glacier runoff. Summer streamflow for the whole river fell 26 percent when comparing the 1950–1985 period to 1985–2009. The glacier runoff decline hit hardest during critically low-flow periods in August and September, exactly when ecosystems and water users can least afford the loss.8Hydrological Processes. Skykomish River, Washington: Impact of ongoing glacier retreat on streamflow
The broader trajectory for Pacific Northwest rivers follows a pattern that researchers sometimes describe as “peak water.” In high-elevation basins, glacier melt can temporarily offset declining snowpack, actually propping up summer flows for a while as warmer temperatures expose more ice to melting. But once the glaciers shrink past a tipping point, that buffer vanishes, and summer flows drop sharply. Lower-elevation basins with smaller glaciers are already past peak water. Higher basins may not cross that threshold until later in the century, but the direction is the same.9Water Resources Research. Glacier Recession and the Response of Summer Streamflow in the Pacific Northwest United States, 1960–2099
Surging Glaciers and Other Unusual Behaviors
Not all glacier retreat is steady and predictable. Alaska is home to some of the world’s most dramatic surging glaciers, ice masses that alternate between long quiet periods and sudden bursts of rapid motion. Variegated Glacier’s 1982–1983 surge remains one of the best-documented examples. During the surge, the glacier sped up roughly a hundredfold. Research showed the cause was a buildup of high water pressure beneath the glacier, enabled by a fundamental shift in the geometry of the basal drainage system. Essentially, the plumbing under the glacier reorganized in a way that trapped water and lubricated the bed, letting the ice slide forward dramatically.10PubMed. Glacier surge mechanism: 1982-1983 surge of variegated glacier, alaska
Bering Glacier, the continent’s largest, surged more recently between 2008 and 2011. Monitoring revealed that small-scale acceleration events during the quiet phase gradually built up internal stresses by as much as 70 percent. When the surge finally kicked off, acceleration spread synchronously across much of the glacier’s length. The surge propagated downstream in a pattern closely tied to how driving stress evolved, while upstream acceleration followed a different and somewhat independent mechanism.11The Cryosphere. Surge dynamics on Bering Glacier, Alaska, in 2008–2011
Surges are not caused by climate warming directly; they are an internal instability in glacier dynamics. But warming can influence how frequently glaciers build up enough mass to surge, and retreating glaciers may eventually become too thin to generate surges at all. The practical concern with surges is their unpredictability. A surge can dam rivers, create sudden lakes, and alter sediment loads downstream in ways that catch communities off guard.
Outburst Floods from Ice-Dammed Lakes
When glaciers block valleys, they create lakes that can drain catastrophically when the ice dam weakens. These glacial lake outburst floods, often called GLOFs or by their Icelandic name jökulhlaups, have historically been among the most dangerous glacier-related hazards in Alaska. What makes the current picture somewhat counterintuitive is that despite rapid glacier thinning and retreat across the state, the frequency of these drainage events has held roughly steady over the past 35 years. About half of ice-dammed lakes in Alaska drain in any given year, with individual lakes draining on average every two years or so.12Nature Communications. Unchanged frequency and decreasing magnitude of outbursts from ice-dammed lakes in Alaska
The good news is that the magnitude of these floods has been decreasing. As glaciers thin, the lakes they impound tend to be smaller, which means less water released during a drainage event. So while outburst floods keep happening at roughly the same rate, the peak discharge of each event has been dropping. The overall hazard from ice-dammed lake floods is declining regionally, though individual lakes can still behave dangerously depending on their specific geometry and the condition of their ice dam.
What Glacier Loss Means for Ecosystems
Glaciers do not exist in isolation. The cold, sediment-rich meltwater they produce creates habitat conditions that certain species depend on entirely. In Glacier National Park, two alpine stonefly species illustrate the point. The meltwater stonefly, Lednia tumana, was found to occur preferentially in cold streams close to glaciers and permanent snowfields, with densities declining as distance from the stream source increased. A related species, Zapada glacier, turned up in only 10 streams across its entire known range. Both species are restricted to short stretches of cold alpine streams, many of which flow from glaciers projected to disappear within two decades.13PubMed. Climate-induced glacier and snow loss imperils alpine stream insects
These stoneflies became something of a poster species for glacier-related extinction risk. Lednia tumana was listed as threatened under the Endangered Species Act in 2019, the first species to receive federal protection primarily because of glacier loss. The insects cannot simply migrate to higher or colder waters because there is nothing above them. They already live at the upper limit of stream habitat, and when the glacier feeding their stream is gone, the thermal and hydrological conditions they need vanish with it.
The ecological reach of glacier melt extends all the way to the ocean in Alaska. Research tracing biogeochemical signatures in coastal marine food webs found that organic matter derived from glacier runoff and surrounding watersheds contributed an estimated 12 to 44 percent of the nutritional base for upper-level marine species including fish and seabirds. The young organic matter released from glacier ecosystems forms a meaningful subsidy to nearshore food webs, which means changes in freshwater runoff driven by glacier retreat have the potential to ripple through coastal marine ecosystems in ways that are only beginning to be understood.14PubMed. Tracing biogeochemical subsidies from glacier runoff into Alaska’s coastal marine food webs
Projections for the Coming Decades
Modeling of nine well-studied North American glaciers under different emission scenarios paints a stark picture, especially for small glaciers. Even under a low-emission scenario, two glaciers in the study, West Gulkana and Blue Glacier, are projected to vanish entirely by around 2030 and 2040, respectively. West Gulkana is a south-facing valley glacier subjected to high summer temperatures with limited precipitation; Blue Glacier receives enormous amounts of precipitation, but much of it falls as rain rather than snow, which cannot sustain ice mass. Under a medium-emission scenario, a third glacier (Worthington) disappears by 2100, and another is reduced to less than a tenth of its 1957 volume. Under high emissions, all but two of the nine glaciers studied are gone by the end of the century.15Scientific Reports. Sixty years of observations and future projections of nine declining North American glaciers
The pattern across these projections is that small glaciers at lower elevations or with southern exposure are effectively doomed regardless of future emissions. Larger glaciers at higher elevations and in wetter climates have more staying power, but even the most optimistic scenarios project substantial volume losses. For the lower 48 states, where most glaciers are already small, the math is grimmer than for Alaska. Many features currently classified as glaciers in Washington, Montana, and Wyoming will likely cross the threshold into perennial snowfields or bare rock within a generation.
Algae and the Feedback Loop That Speeds Melting
One of the less obvious accelerants of glacier loss is biological. Glacier algae, microscopic organisms that thrive on ice surfaces during the melt season, darken the glacier surface and reduce its ability to reflect sunlight. Research on European Alpine glaciers demonstrated that algal presence increases the absorption of solar radiation, creating a feedback loop: warmer conditions promote algal growth, the algae darken the surface, the darker surface absorbs more heat, and melting accelerates.16Scientific Reports. Glacier algae foster ice-albedo feedback in the European Alps
While that particular study focused on European glaciers, the same biological process operates on ice surfaces worldwide, including in Alaska and the Pacific Northwest. The darkening effect compounds other surface-albedo changes like dust deposition and soot from wildfires. In the American West, where wildfire seasons have been growing longer and more intense, the combination of soot fallout and biological darkening may be pushing some glaciers past thresholds they would not yet cross from air temperature alone. Quantifying the algal contribution specifically for US glaciers remains an active area of research, but the mechanism is well established and adds another layer to the already unfavorable outlook for American ice.
The Deep Past Behind Today’s Remnants
The glaciers visible in the US today are tiny vestiges of ice sheets that once buried much of the continent. During the last glacial maximum, roughly 21,000 years ago, the Laurentide Ice Sheet covered most of Canada and extended south into what is now the northern United States, reaching as far as present-day New York, Ohio, and Montana. A separate Cordilleran Ice Sheet blanketed the northern Rockies and Pacific Northwest mountains. Reconstructions of these ice sheets trace a buildup that began around 115,000 years ago, with ice extent and flow patterns shifting through multiple advances and retreats before reaching maximum coverage.17Quaternary Science Reviews. North American Ice Sheet build-up during the last glacial cycle, 115–21 kyr
The landscapes those ice sheets left behind are everywhere. The Great Lakes are glacier-carved basins. Long Island and Cape Cod are terminal moraines, piles of debris pushed to the ice sheet’s southern edge. The fertile soils of the upper Midwest were deposited by glacial meltwater. Even the current glaciers of the Cascades and Rockies owe their existence to the high-elevation cirques carved by their much larger ancestors. Understanding the deep glacial history helps put the current situation in perspective: the ice we are watching disappear is a thin remnant of a system that once shaped the entire continent, and its loss, while small in absolute terms compared to those ancient ice sheets, marks the end of a geological inheritance that stretches back tens of thousands of years.