Arctic Sea Ice

Climate Change Indicators in the United States: Arctic Sea Ice
www.epa.gov/climatechange/indicators - Updated June 2015

Arctic Sea Ice

This indicator tracks the extent and age of sea ice in the Arctic Ocean.

Background

Sea ice is an integral part of the Arctic Ocean. During the dark winter months, sea ice essentially covers
the entire Arctic Ocean. In summer, some of this ice melts because of warmer temperatures and long
hours of sunlight. Sea ice typically reaches its minimum thickness and extent in mid-September, when
the area covered by ice is roughly half the size of the winter maximum. The ice then begins expanding
again.

The extent of area covered by Arctic sea ice is an important indicator of changes in global climate
because warmer air and water temperatures are reducing the amount of sea ice present. Because sea
ice is more reflective than liquid water, it plays a significant role in the Earth's energy balance and
keeping polar regions cool. (For more information on the effects of surface color on reflecting sunlight,
see the Snow Cover indicator) Sea ice also keeps the air cool by forming a barrier between the cold air
above and the warmer water below. As the amount of sea ice decreases, the Arctic region's ability to
stabilize the Earth's climate is reduced, potentially leading to a "feedback loop" of more absorption of
solar energy, higher air temperatures, and even greater loss of sea ice.

The age of sea ice is also an important indicator of Arctic conditions, because older ice is generally
thicker and stronger than younger ice. A loss of older ice suggests that the Arctic is losing ice faster than
it is accumulating it.

Changes in sea ice can directly affect the health of Arctic ecosystems. Mammals such as polar bears and
walruses rely on the presence of sea ice for hunting, breeding, and migrating. These animals face the
threat of declining birth rates and restricted access to food sources because of reduced sea ice coverage
and thickness. Impacts on Arctic wildlife, as well as the loss of ice itself, are already restricting the
traditional subsistence hunting lifestyle of indigenous Arctic populations such as the Yup'ik, Inupiat, and
Inuit.

While diminished sea ice can have negative ecological effects, it can also present commercial
opportunities. For instance, reduced sea ice opens shipping lanes and increases access to natural
resources in the Arctic region.

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Climate Change Indicators in the United States: Arctic Sea Ice

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

Dwindling Arctic Sea Ice

September 1979

Source: NASA, 20151

About the Indicator

Figure 1 presents trends in Arctic sea ice extent from 1979, when extensive measurements started, to
spring 2015. Sea ice extent is defined as the area of ocean where at least 15 percent of the surface is
frozen. This threshold was chosen because scientists have found that it gives the best approximation of
the edge of the ice. Data are collected throughout the year, but for comparison, this indicator focuses on

2

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Climate Change Indicators in the United States: Arctic Sea Ice
www.epa.gov/climatechange/indicators - Updated June 2015

the average sea ice extent during peak freezing and peak melting months. September is typically when
the sea ice extent reaches its annual minimum after melting during the spring and summer. Conversely,
March is typically when sea ice reaches its maximum extent after winter freezing. Data for this indicator
were gathered by the National Snow and Ice Data Center using satellite imaging technology.

Figure 2 examines the age of the ice that is present in the Arctic during the week in September only with
the smallest extent of ice. By combining daily satellite images, wind measurements, and data from
surface buoys that move with the ice, scientists can track specific parcels of ice as they move over time.
This tracking enables them to calculate the age of the ice in different parts of the Arctic. Although
satellites started collecting data in 1979, Figure 2 only shows trends back to 1983 because it is not
possible to know the full age distribution until the ice has been tracked for at least five years.

• September 2012 had the lowest sea ice extent on record, 49 percent below the 1979-2000
average for that month.

• The September 2014 sea ice extent was nearly 700,000 square miles less than the historical
1979-2000 average for that month—a difference more than twice the size of Texas (see Figure
1). March sea ice extent reached a new low in 2015—about 9 percent less than the 1979-2000
average.

• All months have shown a decreasing trend in sea ice extent over the past several decades. The
largest year-to-year decreases have occurred in the summer and fall months.2,3

• Evidence of the age of Arctic sea ice suggests an overall loss of multi-year ice. The proportion of
sea ice five years or older has declined dramatically over the recorded time period, from more
than 30 percent of September ice in the 1980s to 8 percent in 2014. A growing percentage of
Arctic sea ice is only one or two years old. This thinning of Arctic ice makes it more vulnerable to
further melting.

Key Points

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Climate Change Indicators in the United States: Arctic Sea Ice
www.epa.gov/climatechange/indicators - Updated June 2015

Figure 1. March and September Monthly Average Arctic Sea Ice Extent,
1979-2015

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Climate Change Indicators in the United States: Arctic Sea Ice
www.epa.gov/climatechange/indicators - Updated June 2015

Figure 2. Age of Arctic Sea Ice at Minimum September Week, 1983-2014

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1935 1990 1995 2000

Year

2005

2010

This figure shows the distribution of Arctic sea ice extent by age group during the week in September
with the smallest extent of ice for each year. The total extent in Figure 2 differs from the extent in Figure
1 because Figure 1 shows a monthly average, while Figure 2 shows conditions during a single week.

Data source: NSIDC, 2014s

Indicator Notes

Increasing temperatures associated with climate change are not the only factor contributing to
reductions in sea ice. Other conditions that may be affected by climate change, such as fluctuations in
oceanic and atmospheric circulation and typical annual and decadal variability, also affect the extent of
sea ice. Determining the age of ice is an imperfect science, as there are cases where a small amount of
older ice might exist within an area classified as younger, or vice-versa.

Data Sources

The data for this indicator were provided by the National Snow and Ice Data Center. Data for Figure 1
are also available online at: http://nsidc.org/data/seaice index/archives.html, while Figure 2 is based on
an analysis by the University of Colorado and a map published

at: http://nsidc.org/arcticseaicenews/2014/10/. The National Snow and Ice Data Center produces a
variety of reports and a seasonal newsletter analyzing Arctic sea ice data.

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Climate Change Indicators in the United States: Arctic Sea Ice
www.epa.gov/climatechange/indicators - Updated June 2015

1 NASA (National Aeronautics and Space Administration). 2015. Annual Arctic sea ice minimum 1979-2014 with
area graph. NASA/Goddard Space Flight Center Scientific Visualization Studio, http://svs.gsfc.nasa.gov/cgi-
bin/details.cgi?aid=4301.

2 NSIDC (National Snow and Ice Data Center). 2012. Arctic sea ice 101. http://nsidc.org/icelights/arctic-sea-ice.

3 Comiso, J. 2012. Large decadal decline of the Arctic multiyear ice cover. J. Climate 25(4):1176-1193.

4 NSIDC (National Snow and Ice Data Center). 2015. Sea ice data and image archive. Accessed May 2015.
http://nsidc.org/data/seaice index/archives.html.

5 NSIDC (National Snow and Ice Data Center). 2014. Arctic sea ice news and analysis. October 7, 2014.
http://nsidc.org/arcticseaicenews/2014/10.

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