Climate Change Indicators in the United States: Glaciers
http://www.epa.gov/climatechange/indicators - Updated June 2015

Glaciers

This indicator examines the balance between snow accumulation and melting in glaciers¦, and it
describes how glaciers in the United States and around the world have changed over time.

Background

A glacier is a large mass of snow and ice that has accumulated over many years and is present year-
round. In the United States, glaciers can be found in the Rocky Mountains, the Sierra Nevada, the
Cascades, and throughout Alaska. A glacier flows naturally like a river, only much more slowly. At higher
elevations, glaciers accumulate snow, which eventually becomes compressed into ice. At lower
elevations, the "river" of ice naturally loses mass because of melting and ice breaking off and floating
away (iceberg calving) if the glacier ends in a lake or the ocean. When melting and calving are exactly
balanced by new snow accumulation, a glacier is in equilibrium and its mass will neither increase nor
decrease.

In many areas, glaciers provide communities and ecosystems with a reliable source of streamflow and
drinking water, particularly in times of extended drought and late in the summer, when seasonal
snowpack has melted away. Freshwater runoff from glaciers also influences ocean ecosystems. Glaciers
are important as an indicator of climate change because physical changes in glaciers—whether they are
growing or shrinking, advancing or receding—provide visible evidence of changes in temperature and
precipitation. If glaciers lose more ice than they can accumulate through new snowfall, they ultimately
add more water to the oceans, leading to a rise in sea level (see the Sea Level indicator). The same kinds
of changes occur on a much larger scale within the giant ice sheets that cover Greenland and Antarctica,
potentially leading to even bigger implications for sea level. Small glaciers tend to respond more quickly
to climate change than the giant ice sheets. Altogether, the world's small glaciers are adding roughly the
same amount of water to the oceans per year as the ice sheets of Greenland and Antarctica. During the
last two decades, they added more water overall to the oceans than the ice sheets did.1

About the Indicator

This indicator is based on long-term monitoring data collected at selected glaciers around the world.
Scientists collect detailed measurements to determine glacier mass balance, which is the net gain or loss
of snow and ice over the course of the year. A negative mass balance indicates that a glacier has lost ice
or snow. If cumulative mass balance becomes more negative over time, it means glaciers are losing mass
more quickly than they can accumulate new snow.

Figure 1 shows trends in mass balance for a set of 37 reference glaciers around the world that have been
measured consistently since the 1970s, including a few that have been measured since the 1940s. Data
from these reference glaciers have been averaged together to depict changes over time. Figure 2 shows
trends for three "benchmark" glaciers: South Cascade Glacier in Washington state, Wolverine Glacier
near Alaska's southern coast, and Gulkana Glacier in Alaska's interior. These three glaciers were chosen

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Climate Change Indicators in the United States: Glaciers
http://www.epa.gov/climatechange/indicators - Updated June 2015

because they have been studied extensively by the U.S. Geological Survey for many years and because
they are thought to be representative of other glaciers nearby.

This indicator describes the change in glacier mass balance, which is measured as the average change in
thickness across the surface of a glacier. The change in ice or snow has been converted to the equivalent
amount of liquid water.

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Climate Change Indicators in the United States: Glaciers

http://www.epa.gov/climatechange/indicators - Updated June 2015

Sources: Post, 1958;2 Nolan, 20033

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Climate Change Indicators in the United States: Glaciers
http://www.epa.gov/climatechange/indicators - Updated June 2015

Key Points

•	On average, glaciers worldwide have been losing mass since at least the 1970s (see Figure 1),
which in turn has contributed to observed changes in sea level (see the Sea Level indicator). A
longer measurement record from a smaller number of glaciers suggests that they have been
shrinking since the 1940s. The rate at which glaciers are losing mass appears to have accelerated
over roughly the last decade.

•	All three U.S. benchmark glaciers have shown an overall decline in mass balance since the 1950s
and 1960s and an accelerated rate of decline in recent years (see Figure 2). Year-to-year trends
vary, with some glaciers gaining mass in certain years (for example, Wolverine Glacier during the
1980s). However, the measurements clearly indicate a loss of glacier mass over time.

•	Trends for the three benchmark glaciers are consistent with the retreat of glaciers observed
throughout the western United States, Alaska, and other parts of the world.4 Observations of
glaciers losing mass are also consistent with warming trends in U.S. and global temperatures
during this time period (see the U.S. and Global Temperature indicator).

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Climate Change Indicators in the United States: Glaciers
http://www.epa.gov/climatechange/indicators - Updated June 2015

Figure 1. Average Cumulative Mass Balance of "Reference" Glaciers
Worldwide, 1945-2014

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This figure shows the cumulative change in mass balance of a set of "reference" glaciers worldwide
beginning in 1945. The line on the graph represents the average of all the glaciers that were measured.
Negative values indicate a net loss of ice and snow compared with the base year of 1945. For
consistency, measurements are in meters of water equivalent, which represent changes in the average
thickness of a glacier. The small chart below shows how many glaciers were measured in each year.
Some glacier measurements have not yet been finalized for the last few years, hence the smaller number
of sites.

Data sources: WGMS, 2015"

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Climate Change Indicators in the United States: Glaciers

http://www.epa.gov/climatechange/indicators - Updated June 2015

Figure 2. Cumulative Mass Balance of Three U.S. Glaciers, 1958-2014

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This figure shows the cumulative mass balance of the three U.S. Geological Survey "benchmark" glaciers
since measurements began in the 1950s or 1960s. For each glacier, the mass balance is set at zero for
the base year of 1965. Negative values indicate a net loss of ice and snow compared with the base year.
For consistency, measurements are in meters of water equivalent, which represent changes in the
average thickness of a glacier.

Data sources: O'Neel et al., 2014;6 USGS, 20157

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Indicator Notes

The relationship between climate change and glacier mass balance is complex, and the observed
changes at specific reference or benchmark glaciers might reflect a combination of global and local
variations in temperature and precipitation. Individual glaciers also vary in their structure, flow, and

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Climate Change Indicators in the United States: Glaciers
http://www.epa.gov/climatechange/indicators - Updated June 2015

response to climate. Slightly different measurement and analysis methods have been used at different
glaciers, but overall trends appear to be similar.

Long-term measurements are available for only a relatively small percentage of the world's glaciers. This
indicator does not include the Greenland and Antarctic ice sheets, although two decades of satellite
data suggest that these ice sheets are also experiencing a net loss of ice.8 Continued satellite data
collection will allow scientists to evaluate long-term trends in the future.

Data Sources

The World Glacier Monitoring Service compiled data for Figure 1, based on measurements collected by a
variety of organizations around the world. The U.S. Geological Survey Benchmark Glacier Program
provided the data for Figure 2. Historical data, as well as periodic reports and measurements of the
benchmark glaciers, are available on the program's website at: http://ak.water.usgs.gov/glaciology.

1	IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis.
Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge
University Press, www.ipcc.ch/report/ar5/wgl.

2	Post, A. 1958. McCall Glacier. Glacier photograph collection. Boulder, Colorado: National Snow and Ice Data
Center/World Data Center for Glaciology. http://nsidc.org/data/gQ0472.html.

3	Nolan, M. 2003. McCall Glacier. Glacier photograph collection. Boulder, Colorado: National Snow and Ice Data
Center/World Data Center for Glaciology. http://nsidc.org/data/gQ0472.html.

4	IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis.
Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge
University Press, www.ipcc.ch/report/ar5/wgl.

5	WGMS (World Glacier Monitoring Service). 2015 update to data originally published in: WGMS. 2013. Glacier
mass balance bulletin no. 12 (2010-2011). Zemp, M., S.U. Nussbaumer, K. Naegeli, I. Gartner-Roer, F. Paul, M.
Hoelzle, and W. Haeberli (eds.). ICSU (WDS)/IUGG (IACS)/UNEP/UNESCO/WMO. Zurich, Switzerland: World
Glacier Monitoring Service, www.wgms.ch/mbb/mbbl2/wgms 2013 gmbbl2.pdf.

6	O'Neel, S., E. Hood, A. Arendt, and L. Sass. 2014. Assessing streamflow sensitivity to variations in glacier mass
balance. Climatic Change 123(2):329-341.

7	USGS (U.S. Geological Survey). 2015. Water resources of Alaska—glacier and snow program, benchmark glaciers.
http://ak.water.usgs.gov/glaciology.

8	Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third
National Climate Assessment. U.S. Global Change Research Program, http://nca2014.globalchange.gov.

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