U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3046

June 2009

Glaciers respond to climate changes 

by thickening and advancing down-valley 

towards warmer lower altitudes or by 

thinning and retreating up-valley to higher 

altitudes. Glaciers average changes in 

climate over space and time and provide 

a picture of climate trends in remote 

mountainous regions. A qualitative method 

for observing these changes is through 

repeat photography—taking photographs 

from the same position through time (fig. 

2). The most direct way to quantitatively 

observe changes in a glacier is to measure 

its mass balance: the difference between the 

amount of snowfall, or accumulation, on 

the glacier, and the amount of snow and ice 

that melts and runs off or is lost as icebergs 

or water vapor, collectively termed ablation

(fig. 3). Positive mass balance occurs when 

accumulation is greater than ablation and 

if maintained over long periods results 

in glacier growth. Conversely, sustained 

periods of negative mass balance, where 

accumulation is less than ablation, results in 

glacier shrinkage. A shrinking glacier thins 

faster near the terminus (the lowest part of 

the glacier) than near the head (the highest 

1928

1979

1959

2003

Pacific Ocean

United States

Canada

Alaska

Wolverine

Gulkana

South Cascade

Fifty-Year Record of Glacier Change Reveals Shifting 

Climate in the Pacific Northwest and Alaska, USA

Fifty years of U.S. Geological Survey (USGS) research on glacier 

change shows recent dramatic shrinkage of glaciers in three climatic 

regions of the United States. These long periods of record provide 

clues to the climate shifts that may be driving glacier change. 

The USGS Benchmark Glacier Program began in 1957 as a result 

of research efforts during the International Geophysical Year 

(Meier and others, 1971). Annual data collection occurs at three 

glaciers that represent three climatic regions in the United States: 

South Cascade Glacier in the Cascade Mountains of Washington 

State; Wolverine Glacier on the Kenai Peninsula near Anchorage, 

Alaska; and Gulkana Glacier in the interior of Alaska (fig. 1). 

 Figure 1. The Benchmark Glaciers.

Figure 2. Retreat of South Cascade Glacier, Washington, during the 20th Century 

and the beginning of the 21st Century.
Cumulative Net Balance

The cumulative net balance is the total loss or gain of 

glacier mass since the beginning of the study period and is 

presented as spatially averaged thickness change (fig. 5). South 

Cascade Glacier lost mass at an almost constant rate during 

the late 1950s to the late 1960s, then gained mass until 1976, 

and has been losing mass at a fairly constant and rapid rate 

since 1977. At the same time that South Cascade Glacier was 

gaining mass in the early 1970s, Wolverine Glacier was losing 

mass. The roles reversed in 1977, as South Cascade Glacier 

began to lose mass once again while Wolverine Glacier began 

to gain mass and continued to do so until 1989. The opposite 

trends for these two maritime glaciers were the result of a 

shift in the winter storm tracks coming from the northeast 

Pacific Ocean. This shift most likely was due to a shift in the 

phase of the Pacific Decadal Oscillation (PDO), which is an 

ocean-climate oscillation (similar to the El Niño Southern 

Figure 4. Setting a measurement stake at South Cascade Glacier, Washington.

Figure 3. Diagram of a glacier showing components of 

mass balance.

part of the glacier), which is why the terminus retreats up-valley 

while the glacier head remains in place (fig. 2). The net balance

is the average mass balance over the entire glacier for one 

glaciological year, the time between the end of the summer 

ablation season from one year to the next.

The USGS measures accumulation and ablation using a 

network of stakes distributed over the glacier (fig. 4). USGS 

scientists visit each site in spring and autumn (sometimes more 

often) to keep a running tally of accumulation and ablation 

for each stake. These stake measurements, along with local 

temperature and precipitation data, allow USGS scientists to 

estimate the glacier net balance, as well as the seasonal winter

(total accumulation) and summer (total ablation) balances that 

sum to the net balance. 

Regional climate exhibits strong control on glacier mass 

balance, and over decadal time scales mass balances correlate 

with regional climate change. Glaciers in coastal, maritime 

locations (such as Wolverine Glacier) exist primarily because of 

high snowfall rates, despite warm low-elevation temperatures. 

Glaciers in continental or interior climates (for example, 

Gulkana Glacier) exist primarily because cold temperatures 

result in low ablation. During climate changes, coastal glaciers 

are more sensitive to changes in precipitation (or the storm 

tracks that bring precipitation), whereas interior glaciers are 

more sensitive to changes in temperature.

The 50-year mass balance records for these three USGS 

Benchmark Glaciers show annual and interannual fluctuations 

that reflect the controlling climatic conditions. Mass-balance 

data are presented here in three ways to show different aspects 

of the response of glaciers to climate: (1) cumulative net 

balance, (2) seasonal mass balance, and (3) mass turnover.
summer balance (red bars), in accordance with its location in a 

coastal climate. Conversely, the net balance of Gulkana Glacier 

is best correlated with summer balance as expected because of 

the drier, continental climate of interior Alaska.

For all three glaciers, the summer balance records show 

that consistently more ablation took place during the period 

from 1987 to 2004 than did prior to 1987. This change, when 

coupled with a reduction in accumulation (thin snowpacks), 

has resulted in very negative net balances. This is exemplified 

by 2004, when strongly negative summer balances (fig. 6) 

resulting from record high temperatures and low precipitation, 

also set the stage for large, intense wildfires that burned in 

Alaska. Despite drought conditions, the warm weather caused 

excessive melting of glaciers in Alaska and consequent flooding 

of many glacier-fed rivers. One interpretation of the trend 

towards increasing summer melt at the Benchmark Glaciers is 

that climate changes that are larger in scale and intensity than 

periodic shifts in ocean conditions (PDO) are underway.

 Despite increasingly negative summer balances in the 

1980s and 1990s, South Cascade Glacier did have a few years 

of positive mass balance; the most significant occurred in 1999, 

following the record 1998–99 winter snowfall. Snowy winters 

would have to occur much more frequently in the future to 

reduce or reverse mass loss from the glacier. 

Figure 6. Time series of winter balance (net accumulation—blue), 

summer balance (net ablation—red), and net balance (yellow) for 

the three benchmark glaciers (Josberger and others, 2007). 

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

-4

-2

0

2

4

6

-4

-2

0

2

BALANCE (METERS OF WATER EQUIVALENT)

YEAR

Wolverine

Gulkana

Gain

Gain

-4

-2

0

2

4 South Cascade

Gain

Loss

Loss

Loss

Oscillation) in which the pattern of ocean surface temperatures 

and atmospheric pressures in the North Pacific Ocean oscillates 

on about a multi-decadal timescale. In its warm phase, the 

PDO tends to be associated with more northerly storm tracks, 

which in the past provided moisture for the growth of Wolverine 

Glacier at the expense of South Cascade Glacier. The PDO 

cold phase tends to be associated with more southerly storm 

tracks directed at the Washington coast; South Cascade Glacier 

has tended to gain mass from this moisture at the expense of 

Wolverine Glacier. Gulkana Glacier is inland and relatively 

isolated from maritime influences; therefore, the glacier has not 

responded distinctively to shifts in the PDO and to positions of 

winter storm tracks. 

Since 1989, the cumulative net balances of all three 

glaciers show trends of rapid and sustained mass loss. This 

synchronous decline could be the result of recent climate 

changes that are stronger than the PDO-induced variations of 

the earlier periods of record. 

Seasonal Mass Balance

The cumulative net mass balance describes long-term 

trends in the glacier shape and size, that is, whether the 

glacier is growing or shrinking. On the other hand, seasonal 

balances (fig. 6) can reveal whether variations in temperature 

or precipitation are primary factors for observed changes. For 

example, several years of unusually high snowfall during the 

late 1970s and early 1980s caused the subsequent growth of 

Wolverine Glacier. Figure 6 shows the strong influence of 

regional climate. The net balance of Wolverine Glacier (yellow 

bars) tracks the winter balance (blue bars) more closely than the 

Figure 5. Cumulative net balance of South Cascade, Wolverine, and 

Gulkana Glaciers (Josberger and others, 2007). Densities of snow and 

ice differ considerably and before glacier-average thickness changes 

in each material can be summed to the net balance, the changes must 

be converted to a common basis. By custom, the common basis is 

“meters water equivalent” (MWEQ), which is the thickness of water 

that would result from melting a given thickness of snow or ice.

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

-25

-20

-15

-10

-5

0

5

CUMULATIVE NET BALANCE (MWEQ)

YEAR

Wolverine

Gulkana

South Cascade
0

2

1

3

4

6

5

TURNOVER (MWEQ)

YEAR

South Cascade

Gulkana

Wolverine

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Mass Turnover

A third way to explore the mass balance history of these 

three glaciers is by examining mass turnover, which describes 

the amount of water moving through the glacier system and 

the intensity of the local hydrologic cycle. Mass turnover is 

estimated by averaging the absolute values of winter and 

summer balances. Large mass turnover is typical of coastal, 

maritime glaciers where winter precipitation and snow 

accumulation are large. These glaciers can extend far downvalley to warm, low elevation regions (sometimes to sea level) 

where high ablation rates offset abundant maritime snow. In 

drier climates, such as the Rocky Mountains or interior Alaska, 

glaciers typically have relatively small mass turnover. With low 

total snowfall, glaciers in these areas melt before they extend far 

down valley. In accordance with these concepts, figure 7 shows 

that higher turnover rates occur at South Cascade and Wolverine 

Glaciers than at Gulkana Glacier.

The USGS Benchmark Glaciers each exhibit a longterm trend of increasing mass turnover, which have become 

more well-defined and stronger from 1985 to 2005. Increased 

intensity of hydrologic cycling provides the simplest 

interpretation for the trend.

Conclusions

The net mass balance of a glacier is the difference between 

the mass gain through accumulation of snow and the mass 

loss through ablation. A glacier that is in balance with the 

current climate will have zero net balance and a steady size. A 

glacier that is out of balance with the current climate will grow 

(advance) or shrink (retreat) depending on how the climate is 

changing (colder or wetter climate will cause advance, warmer 

or drier climate will cause retreat). 

The USGS has been monitoring the mass balance of three 

glaciers in the Pacific Northwest and in Alaska for nearly 50 

years. This mass balance record is now sufficient to interpret 

glacier responses to short- and long-term climate changes in the 

Pacific Northwest, the Alaska Coast, and the Alaska interior. 

What have we learned so far?

• All three glaciers have lost mass since USGS monitoring 

began more than four decades ago.

• Mass loss has accelerated during the last 15 years, coincident 

with the highest melt years on record.

• Mass balance of the coastal South Cascade and Wolverine 

Glaciers correlate well with the PDO during the first few 

decades of the period of record, showing the effects of oceancondition periodicity on glacier health.

• This correlation has weakened during the last two decades, as 

global average temperature has increased.

• Mass turnover has increased throughout monitoring and the 

trend of increase has become stronger during the last two 

decades. 

The accelerating loss of mass, the weakening correlation 

with the PDO, and increasing mass turnover likely are the 

result of changes to warmer and (or) drier climate conditions 

that are affecting all three regions. The climate changes could 

be overwhelming the previously observed responses of the 

maritime glaciers to periodic shifts in ocean conditions, such as 

are represented by the PDO. 

References Cited

Josberger, E.G., Bidlake, W.R., March, R.S., and Kennedy, B.W., 2007, 

Glacier mass-balance fluctuations in the Pacific Northwest and 

Alaska, USA: Annals of Glaciology, v. 46, p. 291-296.

Meier, M.F., Tangborn, W.V., Mayo, L.R., and Post, A., 1971, 

Combined ice and water balances of Gulkana and Wolverine 

Glaciers, Alaska, and South Cascade Glacier, Washington, 1965 and 

1966 Hydrologic Years: U.S. Geological Survey Professional Paper 

715-A, 23 p., 4 pl.

For more information contact:

Edward Josberger, Research Oceanographer (ejosberg@usgs.gov)

William Bidlake, Hydrologist (wbidlake@usgs.gov)

U.S. Geological Survey, Washington Water Science Center

934 Broadway, Suite 300

Tacoma, WA 98402

Rod March, Hydrologist (rsmarch@usgs.gov)

U.S. Geological Survey, Alaska Science Center 

3400 Shell Street

Fairbanks, AK 99701 

Shad O’Neel, Glaciologist (soneel@usgs.gov)

U.S. Geological Survey, Alaska Science Center

4210 University Drive

Anchorage, AK 99508

Website

http://ak.water.usgs.gov/glaciology

Figure 7. Mass turnover of the three benchmark glaciers. (5-year 

moving averages are plotted as lines). (Josberger and others, 2007).