Climate Change
Climate Change Indicators in the United States
This figure shows Arctic sea ice extent from 1979 through 2009 using data from September of each year, which is when the minimum extent typically occurs.
Data source: NSIDC, 2009 3
Source: NASA, 2009 1
Key Points
- The lowest sea ice extent on record occurred in September 2007. Compared with the previous minimum set in September 2005, the 2007 total reflected a loss of 490,000 square miles of sea ice—an area larger than Texas and California combined (see Figure 1).
- Compared with the 1979 to 2000 average, the extent of Arctic sea ice in 2007 was lower by 1 million square miles—an area approximately the size of Alaska and Texas combined (see Figure 1).
- Although September 2009 saw an increase in sea ice extent compared with 2007 and 2008, the 2009 sea ice extent was still 24 percent below the 1979 to 2000 historical average.
- Although the annual minimum of sea ice extent typically occurs in September, all months have shown a decreasing trend in sea ice extent over time. 2
Background
Sea ice is a key feature in the Arctic Ocean. During the dark winter months, sea ice covers nearly 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 extent in mid-September, then begins expanding again through the winter.
The extent of area covered by Arctic sea ice is considered a sensitive indicator of global climate because a warmer climate will reduce the amount of sea ice present. Because sea ice is more reflective than liquid water, it also plays a role in regulating global climate by keeping polar regions cool. (For more information on the effects of surface color on reflecting sunlight, see the Snow Cover indicator.) Thus, as the amount of sea ice decreases because of increased air temperatures, 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.
Arctic mammals, such as polar bears and walruses, rely on the presence of sea ice to preserve their hunting, breeding, and migrating habits. These animals might become threatened if birth rates decline or access to food sources is restricted because of diminished sea ice. Impacts on Arctic wildlife, as well as the loss of ice itself, threaten the traditional lifestyle of indigenous Arctic populations such as the Yup'ik, Iñupiat, and Inuit. In addition to reducing the number of animals available to hunt, diminished sea ice extent and earlier melting can severely limit hunting seasons and access to hunting grounds, making traditional subsistence hunting more difficult. While diminished sea ice can have negative ecological effects, it can also present positive commercial opportunities. For instance, reduced sea ice opens shipping lanes and increases access to natural resources in the Arctic region.
About the Indicator
This indicator reviews trends in Arctic sea ice extent from 1979 to 2009. Sea ice extent is defined as the area of ocean where at least 15 percent of the surface is frozen. Data are collected throughout the year, but for comparison, this indicator focuses on sea ice extent data for September of each year. This is because September is typically when the sea ice extent reaches its annual minimum after melting during the summer months. Data for this indicator were gathered by the National Snow and Ice Data Center using satellite imaging technology.
Indicator Confidence
Increasing temperatures associated with climate change are not the only factor contributing to reductions in sea ice. Other conditions, such as fluctuations in oceanic and atmospheric circulation and typical annual and decadal variability, also affect the extent of sea ice. Additionally, changes in the age and thickness of sea ice—a trend toward younger and thinner ice—might also increase the rate at which the ice melts in summer, making year-to-year comparisons more complex.
Data Sources
The data for this indicator were provided by the National Snow and Ice Data Center and are available online at: http://nsidc.org/data/seaice_index/ archives/index.html. The National Snow and Ice Data Center also produces a variety of reports and a seasonal newsletter analyzing Arctic sea ice data.
Indicator Documentation
- Download related technical information PDF (6 pp, 46K)