Arctic Sea Ice
Most Arctic sea ice occupies an ocean basin largely enclosed by land. Because there is no landmass at the North Pole, sea ice extends all the way to the pole, making the ice subject to the most extreme oscillations between wintertime darkness and summertime sunlight. Likewise, because the ocean basin is surrounded by land, ice has less freedom of movement to drift into lower latitudes and melt. Sea ice also forms in areas south of the Arctic Ocean in winter, including the Sea of Okhotsk, the Bering Sea, Baffin Bay, Hudson Bay, the Greenland Sea, and the Labrador Sea.
Arctic sea ice reaches its maximum extent each March and its minimum extent each September. This ice has historically ranged from roughly 16 million square kilometers (about 6 million square miles) each March to roughly 7 million square kilometers (about 2.7 million square miles) each September.
On time scales of years to decades, the dominant cause of atmospheric variability in the northern polar region is the Arctic Oscillation (AO). (There is still debate among scientists whether the North Atlantic Oscillation and the Arctic Oscillation are the same phenomenon or different but related patterns.) The Arctic Oscillation is an atmospheric seesaw in which atmospheric mass shifts between the polar regions and the mid-latitudes. The shifting can intensify, weaken, or shift the location of semi-permanent low and high-pressure systems. These changes influence the strength of the prevailing westerly winds and the track that storms tend to follow.
During the “positive” phase of the Arctic Oscillation, winds intensify, which increases the size of leads in the ice pack. The thin, young ice that forms in these leads is more likely to melt in the summer. The strong winds also tend to flush ice out of the Arctic through the Fram Strait. During “negative” phases of the oscillation, winds are weaker. Multiyear ice is less likely to be swept out of the Arctic basin and into the warmer waters of the Atlantic. The Arctic Oscillation was in a strong positive phase between 1989 and 1995, but since the late 1990s, it has been in a neutral state.
Current Status and Trends
In September 2008, Arctic sea ice dropped to its second-lowest extent since satellite records began in 1979: 4.67 million square kilometers (1.8 million square miles). Between 1979 and 2006, the annual average decline was 45,100 square kilometers per year, which is about 3.7 percent per decade. But the September minimum ice extent dropped by an average of nearly 57,000 square kilometers per year, which is just over 7.5 percent per decade. In every geographic area, in every month, and every season, current ice extent is lower today than it was during the 1980s and 1990s.
Natural variability and rising temperatures linked to global warming both appear to have played a role in this decline. The Arctic Oscillation’s strongly positive mode through the mid-1990s flushed thicker, older ice out of the Arctic, replacing multiyear ice with first-year ice that is more prone to melting. After the mid-1990s, the AO assumed a more neutral phase, but sea ice failed to recover. Instead, a pattern of steep Arctic sea ice decline began in 2002. The AO likely triggered a phase of accelerated melt that continued into the next decade thanks to unusually warm Arctic air temperatures.
|Year||Average Minimum Extent (million square kilometers)||Compared to 1979-2000 Average (million square kilometers)||Compared to 1979-2000 Average (percent)|
The sea ice minimum was especially dramatic in 2007, when Arctic sea ice extent broke all previous records by mid-August, more than a month before the end of melt season. Both the southern and northern routes through the Northwest Passage opened in mid-September. Ice also became particularly prone to melting in the Beaufort Gyre that summer. The Beaufort Gyre is a clockwise-moving ocean and ice circulation pattern in the Beaufort Sea, and starting in the late 1990s, ice began to melt in the southernmost stretch of the gyre. In the summer of 2007, sea ice retreat was especially pronounced in the region encompassing the Beaufort, Chukchi, East Siberian, Laptev, and Kara Seas.
Many global climate models predict that the Arctic will be ice free for at least part of the year before the end of the century. Some models predict an ice-free Arctic by mid-century, and some even sooner. Depending on how much Arctic sea ice continues to melt, the ice could become extremely vulnerable to natural variability. In the future, the ice might respond even more dramatically than it has in the past to natural cycles such as the Arctic Oscillation.
Impacts of Arctic Sea Ice Loss
Projected effects of declining sea ice include loss of habitat for seals and polar bears, as well as movement of polar bears onto land where bear-human encounters may increase. Indigenous peoples in the Arctic who rely on Arctic animals for food have already described changes in the health and numbers of polar bears.
As sea ice retreats from coastlines, wind-driven waves—combined with permafrost thaw—can lead to rapid coastal erosion. Alaskan and Siberian coastlines have already experienced coastal erosion.
Other potential impacts of Arctic sea ice loss include changed weather patterns: less precipitation in the American West, a weaker storm track that has shifted south over the Atlantic, or (according to one simulation) an intensified storm track.
Some researchers have hypothesized that melting sea ice could interfere with ocean circulation. In the Arctic, ocean circulation is driven by the sinking of dense, salty water. A cap of freshwater resulting from rapid, extensive sea ice melt could interfere with ocean circulation at high latitudes. Although a study published in 2005 suggested that the Atlantic meridional (north-south) overturning circulation had slowed by about 30 percent between 1957 and 2004, that conclusion was not based on comprehensive measurements. Subsequent modeling analyses indicated that the freshwater from melting sea ice was not likely to affect ocean circulation for decades.