Just as ponds and lakes in northern states develop a layer of ice on their surfaces during cold winters, the surface of the Arctic Ocean also freezes, forming sea ice.
Seawater has a lower freezing point than fresh water, but once it is chilled to around -2°C (about 28°F), the salty liquid begins to solidify. Ice crystals appear on the sea’s surface, and if the air is cold enough, the crystals expand to form a slushy mix, then a solid covering of ice that can thicken over time. In the Arctic Ocean, the area covered by sea ice, which scientists call the sea ice extent, grows and shrinks over the course of the year. Each fall, as less sunlight reaches the Arctic and air temperatures begin to drop, additional sea ice forms. The total area covered by ice increases through the winter, usually reaching its maximum extent in early March. Once spring arrives with more sunlight and higher temperatures, the ice begins to melt back, shrinking to its minimum extent each September.
Early observations of sea ice coverage in the Arctic come to us both from the oral histories of native populations and from the records of early European mariners who were seeking a “Northwest Passage” to the lucrative markets in Asia. For as long as humans have been keeping track, large areas of the Arctic Ocean have remained covered by sea ice throughout the year. However, over the past 30 years, the area covered by ice has shown a dramatic decrease. Since satellite-based measurements began in the late 1970s, data show a trend of more ice melting away during summers and less new ice forming during winters. Minimum sea ice extent observed each September has decreased by an average of 12 percent per decade compared to the 1979-2000 average.
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This graph shows the average area covered by sea ice during September each year. Minimum sea ice extent has decreased 12% per decade since 1979.
Now, for the first time in recorded human history, the Northwest Passage may become a useful shipping route during Arctic summer. In 2011, a route through the straits of the Canadian Arctic Archipelago opened for the fifth year in a row. Other routes across the Arctic may also open, providing ships with viable alternatives to traveling through the Panama Canal or around the southern tip of South America. This new reality will have impacts not only on the environment, but also on the world economy and national security, as nations compete to gain rights to shipping lanes and newly accessible resources in the Arctic.
Climate scientists are particularly concerned about the decrease of sea ice because its white surface reflects up to 80 percent of incoming sunlight, deflecting additional energy away from the planet. With less ice present, the dark surface of ocean water absorbs considerably more sunlight energy, leading to further warming of the atmosphere and more melting of ice, which leads to further warming… Scientists are actively studying the effects of this positive feedback loop to help them understand and predict how the observed decrease in Arctic sea ice will affect the global climate system.
Arctic Meltdown, Economic and Security Implications of Global Warming. Foreign Affairs, Scott G. Borgerson. Accessed March 17, 2009.
All About Sea Ice, National Snow and Ice Data Center. Accessed March 17, 2009.
Intergovernmental Panel on Climate Change. (2007). Climate Change 2007: The Physical Science Basis Summary for Policymakers, A Report of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.
September Sea Ice Extent, National Snow and Ice Data Center.