Climate Change Indicators in the United States
Data source: NSIDC, 2015 5
Data source: NSIDC, 2015 6
- September 2012 had the lowest sea ice extent on record, 49 percent below the 1979–2000 average for that month.
- The September 2015 sea ice extent was more than 900,000 square miles less than the historical 1979–2000 average for that month—a difference more than three times 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. 3,4
- 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 9 percent in 2015. 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.
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. Evidence also suggests that the melt season has become longer; the ice is starting to melt earlier in the year and freeze later than it used
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, Iñupiat, 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.
About the Indicator
Figure 1 presents trends in Arctic sea ice extent from 1979, when extensive measurements started, to 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 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.
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.
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/
archives.html, while Figure 2 is based on an analysis by the University of Colorado and a map published at: http://nsidc.org/
10/2015-melt-season-in-review. 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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