First complete census of lower-48 glaciers finds a quarter already gone

Overview

Updated Oct 18

For the first time, researchers have counted and measured every glacier in the lower 48 states. The census, published in the Proceedings of the National Academy of Sciences (PNAS), shows 398 glaciers have vanished since the mid-20th century.

The 1,331 remaining glaciers hold about a third less area and a third less ice than in the 1950s. Glacier ice is a frozen reservoir: it melts during hot, dry periods and slows during cool, wet ones, smoothing streamflow when it matters most. Most survivors are small and tucked into shaded, avalanche-fed niches; Mount Rainier alone holds more ice than every other lower-48 state combined.

Why it matters

Western watersheds lose their natural drought buffer as glaciers that release water during hot, dry periods keep shrinking.

Questions about this story

Free account needed to ask — your question is kept and asked for you right after sign-up. Answers are public.

I'm confused. If it was the first time that they've done this census, how could they know that it declined? That would imply that they had a baseline. What am I not understanding?

It was the first complete modern census, not the first ever—the baseline is the USGS topographic maps from the mid-1900s, which already showed these glaciers.

That baseline turns a suspicion into a hard number: a quarter of lower-48 glaciers are already gone.

4 details4 sources

USGS 1:24,000 topographic maps from the mid-20th century listed glaciers across the western U.S. and were the reference for glacier change for about 60 years.
For the new census, researchers first built a database of every glacier on those old maps, then used aerial and satellite imagery—mostly from 2015—to redraw their current outlines and compare.
The comparison found about a quarter of the glaciers have disappeared; overall glacier area and volume are down about one-third.
The new inventory of 1,331 glaciers, published in 2023, is an update of that older baseline, not a measurement with nothing to compare against.

0 AI answers with web search. May be wrong.

Key Indicators

~0.5 m/yr

Fastest thinning rate (Beartooth-Absaroka, MT)

Ice in southern Montana's Beartooth-Absaroka range loses about half a meter of thickness a year; Washington's largest glaciers thin roughly a third as fast.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Andrew Fountain

Professor emeritus of geology, Portland State University

Led the glacier inventory that underpins the PNAS study

Organizations Involved

Portland State University

Public research university

Compiled the comprehensive glacier inventory

The university's geology department produced the first complete glacier inventory of the lower 48.

United States Geological Survey

Federal science agency

Produced the baseline maps and co-authored the loss analysis

The USGS compiled the original mid-1900s glacier maps and hosts current glacier monitoring research.

Proceedings of the National Academy of Sciences (PNAS)

Peer-reviewed journal

Published the comprehensive glacier loss study

One of the most widely cited scientific journals, it published the first comprehensive comparison of lower-48 glacier change.

Timeline

2015 mid-190

6 events Latest: October 17th, 2026 · 4 weeks ago

Tap a bar to jump to that date

mid-190
USGS maps establish glacier baseline
Upcoming Baseline survey

USGS 1:24,000 topographic maps inventory glaciers across the western US, becoming the reference for glacier change for 60 years.
mid-1900s
October 2026
Comprehensive glacier loss analysis published in PNAS
Latest Study

First complete census paired with the 1950s baseline finds 398 glaciers gone and area and volume down a third.
October 17th, 2026
2025
Mass loss hits record pace
Study

USGS finds glaciers in western Canada and the conterminous US lost mass at twice the 2010-2020 rate during 2021-2024.
2025
2023
Full inventory published in ESSD
Publication

Earth System Science Data publishes the census: 1,331 glaciers, 1,176 perennial snowfields, 35 buried-ice features.
2023
2022
Updated glacier inventory released
Data release

Portland State University releases the updated inventory of glaciers and perennial snowfields.
2022
2015
Aerial campaign photographs US glaciers
Data collection

National Agricultural Imagery Program aerial photos capture over 70% of the glacier outlines used in the new census.
2015

Scenarios

Glacier loss accelerates after record 2021-2024 period

Likely Resolves by End of 2028

Discussed by: USGS 2025 study documenting doubled mass loss; PBS NewsHour reporting on 2030-2080 disappearance estimates

The 2021-2024 period saw mass loss at twice the prior decade's rate, driven by low winter snowpack, early heat waves, and wildfire debris darkening ice. If that acceleration holds, the next comprehensive survey would show area and volume declining faster than the historical 8% per decade. Researchers quoted by PBS put complete disappearance of lower-48 glaciers between 2030 and 2080 depending on winter conditions.

Shaded high-elevation glaciers stabilize in some locations

Possible Resolves by End of 2032

Discussed by: USGS analysis noting lower-48 glaciers are largely maintained by local topographic influences

Many surviving glaciers are small and sit in shaded cirques or high elevations where local conditions favor ice preservation. Avalanche snow, north-facing slopes, and cool canyon air could keep some ice alive even as regional cover declines. The next inventory would show a subset of glaciers in Washington, Montana, or Wyoming maintaining or gaining area.

Western water managers build storage to replace lost glacier buffering

Possible Resolves by End of 2028

Discussed by: Portland State researchers noting watersheds become more susceptible to drought as glacier buffering fades

Glaciers release water during hot, dry periods, smoothing seasonal streamflow. As that capacity fades, states in glacier-fed basins may pursue reservoirs, groundwater recharge, or other storage to replace the lost natural regulation. A major project approved or funded in the Columbia system, the Sierra Nevada, or the northern Rockies would mark this adaptation.

Retreating Cascade glaciers trigger debris flows

Possible Resolves by End of 2036

Discussed by: Andrew Fountain, writing in The Conversation

Glaciers on the steep-sided volcanoes of the Cascade Range leave behind steep moraines of easily eroded volcanic rock as they recede. Rain or meltwater can undercut these ridges, triggering debris flows down the mountain — a 2006 event on Mount Hood and Mount Rainier closed a number of highways, and Fountain expects it to happen more often as the glaciers retreat.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

1950s-1960s

USGS topographic glacier maps (mid-1900s)

USGS field crews produced 1:24,000 scale topographic maps showing glaciers across the western US. These maps became the standard reference for glacier distribution and extent, even as the ice retreated over the following decades.

Then

The maps provided a detailed baseline of mid-20th century glacier extent across the lower 48.

Now

For 60 years they remained the only comprehensive glacier inventory of the conterminous US, growing increasingly outdated as glaciers retreated.

Why this matters now

The 2026 PNAS study compares today's glaciers against this mid-century baseline, using modern aerial imagery to quantify the change for the first time.

2021-2024

Unprecedented western North American glacier mass loss (2021-2024)

Glaciers in western Canada and the conterminous US lost mass at 22.2 gigatons per year on average over 2021-2024, about double the 2010-2020 rate, according to a USGS study. Low winter snowfall, early-season heat waves, and wildfire debris darkening the ice all contributed.

Then

Total ice volume in the region fell about 12% between 2020 and 2024.

Now

The acceleration suggests the long-term shrinkage documented by the new census could speed up, pulling forward estimates for when lower-48 glaciers disappear.

Why this matters now

This recent acceleration is the freshest data point on the same glaciers the census measured, showing the decline is not steady but accelerating.