In the Arctic, sea ice extent fluctuates with the seasons. It reaches its peak extent in March, near the end of Northern Hemisphere winter, and its minimum extent in September, at the end of the summer thaw. In September 2007, Arctic sea ice extent was the smallest area on record since satellites began collecting measurements about 30 years ago.
Although a cold winter allowed sea ice to re-cover much of the Arctic in the months that followed, this pair of images reveals that conditions were far from normal. The February 2008 ice pack (right) contained much more young ice than the long-term average (left). In the past, more ice survived the summer melt season and had the chance to thicken over the following winter. This perennial ice generally gets thicker each winter, which makes it more likely to survive the next summer.
The area and thickness of sea ice that survives the summer has been declining over the past decade. Whereas perennial ice used to cover 50-60 percent of the Arctic, it covered less than 30 percent in 2008—down 10 percent from 2007. The ice that remains is also getting younger. In the mid- to late 1980s, over 20 percent of Arctic sea ice was at least six years old; in February 2008, just 6 percent of the ice was six years old or older.
Independent of human-caused global warming, the extent and thickness of Arctic sea ice can vary as a result of natural ocean and atmospheric cycles. One important cycle is the Arctic Oscillation, which seesaws between a positive and negative phase over three-to-seven-year periods. During the positive phase, persistent lower than normal atmospheric pressure over the polar latitudes steers winds and storms away from the Arctic; winds tend to flush sea ice out of the Arctic basin to lower latitudes, where it melts. In the negative phase of the oscillation, persistent higher than normal pressure over the pole brings winds and storms toward the Arctic basin, and keeps sea ice circulating within the Arctic, allowing the ice to build up.
According to polar scientist Walt Meier of the National Snow and Ice Data Center, sea ice extent did go up and down with the phases of the Arctic Oscillation throughout much of the 1970s and through the mid-1980s. Since the 1990s, however, human-caused global warming appears to be driving the losses of perennial sea ice. For the past ten years or so, the Arctic Oscillation has been mostly neutral or negative, says Meier, but sea ice has continued on a downward slide.