Climate Change Indicators in the United States: Antarctic Sea Ice
www.epa.gov/climate-indicators - August 2016
Antarctic Sea Ice
This indicator tracks the extent of sea ice around Antarctica.
Background
The Southern Ocean around Antarctica freezes to form sea ice every year. This sea ice reaches its
maximum extent in September or early October, around the end of winter in the Southern Hemisphere.
At that point, virtually the entire continent of Antarctica is surrounded by a ring of sea ice (see images
below). The ice melts in the summer months (December to February) because of warmer temperatures
and long hours of sunlight. Unlike the Arctic, where a large area of sea ice lasts year-round (see the
Arctic Sea Ice indicator), the sea ice around Antarctica is thinner and nearly all of it melts in a typical
summer. This happens because no landmass is present to stop Antarctic sea ice from breaking up and
drifting northward into warmer waters, where it eventually melts.
Like Arctic sea ice, Antarctic sea ice affects global climate, regional climate, and ecosystems. Because ice
is more reflective than liquid water, it plays a significant role in maintaining the Earth's energy balance
and helping to keep polar regions cool. (For more information on how the color of the Earth's surface
affects the absorption of 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. Antarctic sea ice is an
important part of the habitat for penguins, seals, and other animals.
Warmer air and ocean temperatures are generally expected to reduce the amount of sea ice present
worldwide. While warming has already driven a noticeable decline in sea ice in the Arctic, extent in the
Antarctic as a whole has not yet declined (and has actually increased slightly), which may reflect
influences of wind patterns, ocean currents, and precipitation around the continent.1
oEPA
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Climate Change Indicators in the United States: Antarctic Sea Ice
www.epa.gov/climate-indicators - August 2016
Antarctic Sea Ice
Source: NASA, 20162
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Climate Change Indicators in the United States: Antarctic Sea Ice
www.epa.gov/climate-indicators - August 2016
About the Indicator
Figure 1 shows Antarctic sea ice extent from 1979, when routine monitoring by satellites started, to
early 2016. 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
the months when sea ice typically reaches its minimum and maximum extent. February is typically when
the sea ice extent reaches its annual minimum after melting during the spring and summer. The ice
typically reaches its maximum extent in late September or early October after winter freezing, with the
largest monthly average extent occurring in September. Data for this indicator were gathered by the
National Snow and Ice Data Center using satellite imaging technology and data processing methods
developed by the National Aeronautics and Space Administration.
Unlike EPA's Arctic Sea Ice indicator, this indicator does not show the age of Antarctic sea ice. This is
because nearly all of the Antarctic ice melts every summer, unlike the Arctic, where a large portion of ice
survives for multiple years.3
• Antarctic sea ice extent in September and February has increased somewhat over time,
although the most recent year was below average. The September maximum extent reached the
highest level on record in 2014—about 7 percent larger than the 1981-2010 average—but in
2015 it was slightly below the 1981-2010 average. As for February extent, 2013, 2014, and 2015
were three of the six largest years on record, but extent in 2016 was about 9 percent below the
1981-2010 average (see Figure 1).
• Slight increases in Antarctic sea ice are outweighed by the loss of sea ice in the Arctic during the
same time period (see the Arctic Sea Ice indicator). Overall, the Earth has lost sea ice at an
average rate of 13,500 square miles per year since 1979—equivalent to losing an area larger
than the state of Maryland every year.4 This decrease affects the Earth's energy balance.
Key Points
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Climate Change Indicators in the United States: Antarctic Sea Ice
www.epa.gov/climate-indicators - August 2016
Figure 1. February and September Monthly Average Antarctic Sea Ice Extent, 1979-2016
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Climate Change Indicators in the United States: Antarctic Sea Ice
www.epa.gov/climate-indicators - August 2016
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 NASA (National Aeronautics and Space Administration). 2016. NASA's Goddard Space Flight Center Scientific
Visualization Studio, https://svs.gsfc.nasa.gov.
3 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.
4 Parkinson, C.L. 2014. Global sea ice coverage from satellite data: Annual cycle and 35-yr trends. J. Climate
27(24):9377.
5 NSIDC (National Snow and Ice Data Center). 2016. Sea ice data and image archive. Accessed July 2016.
http://nsidc.org/data/seaice index/archives.html.
6 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.
7 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.
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