Arctic sea ice 101

The Arctic Ocean is blanketed by sea ice that expands during the frigid Arctic winter, reaching a maximum extent in March. Sea ice retreats during the summer, reaching its minimum extent in September. The Arctic ice cover plays an important role in maintaining the Earth’s temperature—the shiny white ice reflects light and heat that the ocean would otherwise absorb, keeping the Northern Hemisphere cool.

Arctic sea ice extent in September, 2010, was the third-lowest in the satellite record. Credit: NSIDC/NASA Earth Observatory

Arctic sea ice is declining at an increasing rate in all months of the year, with a stronger decline in summer months. Researchers who study climate and sea ice expect that at some point, the Arctic Ocean will lose its ice cover completely in late summer. A variety of evidence suggests that Arctic sea ice is declining because of climate warming resulting from increased greenhouse gases in the atmosphere. Because greenhouse gases persist and are expected to increase, scientists see no reversal to the downward trend in ice extent.

Despite year-to-year variations, satellite data show a decline of more than 11 percent per decade in September ice extent, since the satellite record started in 1979. Before 1979, the data are less comprehensive, but shipping records and other evidence show that the ice extent has been in a continued state of decline for at least the last hundred years. Climate models predicted that Arctic sea ice loss would accompany warming temperatures in the Arctic. But the ice loss has happened faster than any models predicted, and researchers now expect that the Arctic Ocean will be ice-free in summer well before the end of the century.

Impacts of ice loss
Scientists are concerned that the loss of Arctic sea ice could set up cycles that increase future climate warming. As ice cover retreats, areas of open water absorb heat that the ice normally reflects. The water warms up, and before ice can form again in the fall, the ocean must release that heat to the atmosphere. Scientists call this cycle “Arctic amplification.” Recent research suggests that it is already occurring.

Sea ice also covers the ocean near Antarctica, but the ice is thinner and the climate very different. Credit: Ted Scambos, NSIDC

In addition to changes in the Arctic climate, the loss of sea ice on the Arctic Ocean could open up new regions to shipping, tourism, and oil extraction. Researchers do not yet know how those activities will affect the region.

NSIDC scientists also monitor the extent of Antarctic sea ice. In contrast to Arctic sea ice, the sea ice cover that surrounds the Antarctic continent has actually grown slightly since the start of the satellite record. However, this trend is smaller (1 to 4 percent in different months), and the Antarctic sea ice cover is more variable than the Arctic. Also, unlike Arctic sea ice, a majority of Antarctic sea ice already melts away each summer, and has done so since at least 1979. Even if wintertime Antarctic sea ice were to increase or decrease significantly in the future, it would not have a huge impact on the climate system. During the Antarctic winter, energy from the sun is at its weakest point; its ability or inability to reflect the sun’s energy back into space has little effect on regulating the planet’s temperature.