Environment: Trends

Not only do the Arctic and Antarctic experience opposite seasons (summer in one hemisphere and winter in the other), but they also have roughly opposite land-ocean configurations. The Arctic is an ocean basin largely surrounded by land, so sea ice extends northward to the pole, and the ice's growth is constrained on its outermost edge by land. In contrast, Antarctica is a continent surrounded by a vast ocean. Because it is starting out farther from the pole and closer to the equator, Antarctic sea ice melts back to a smaller extent in the summer, and yet its wintertime ice has more room to grow.

In late winter, Arctic sea ice covers roughly 15.5 million square kilometers (6 million square miles), and Antarctic sea ice covers about 18 million square kilometers (7 million square miles). In late summer, Arctic sea ice melts back to roughly 6.5 million square kilometers (2.5 million square miles), and Antarctic sea ice shrinks to about 2.5 million square kilometers (1 million square miles).

Satellites have shown the fluctuation in sea ice from year to year since 1972, and have provided nearly continuous monitoring of sea ice conditions since November 1978. From November 1978 through 2019, Arctic sea ice extent declined in all months, with the smallest decline in April (-2.6 ± 0.5 percent per decade), and the largest decline in September (-12.9 ± 2.2 percent per decade). Over the same period, Antarctic sea ice showed an overall slight positive trend in all months except November (-0.1 ± 0.8 percent per decade), with the greatest increase in March (1.9 ± 3.7 percent per decade). Arctic sea ice set its record-lowest summer minimum extent in 2012. Summer minimum extents in 2007, 2016, and 2019 statistically tied for second lowest in the satellite record.

sea ice extent departures from monthly means for the Northern Hemisphere
Northern Hemisphere sea ice extent anomalies over the course of the continuous satellite record, from November 1978 to 2019.
Credit: Sea Ice Index, National Snow and Ice Data Center, University of Colorado, Boulder, CO.
sea ice extent departures from monthly means for the Northern Hemisphere
Southern Hemisphere sea ice extent anomalies over the course of the continuous satellite record, from November 1978 to 2019.
Credit: Sea Ice Index, National Snow and Ice Data Center, University of Colorado, Boulder, CO.

Scientists theorize that the gradual reduction in sea ice results from a combination of shifting atmospheric winds that naturally break up ice cover, as well as higher temperatures, which may be linked to the greenhouse effect.

NSIDC provides a more in-depth analysis of sea ice trends for a scientific audience, called the Sea Ice Index. The Index provides information such as images and graphs of average ice extent trends and anomalies (the difference between the average ice edge and that of a given month).

Last updated: 3 April 2020