A pair of new studies shows that winter sea ice in the Arctic has shrunk dramatically in the past two years and that perennial ice in particular is disappearing.
Two types of sea ice cover the Arctic Ocean: thick perennial ice that resists thaw year-round and thinner seasonal ice that melts during the summer and freezes again in the winter. Both types are experiencing decline, according to analyses of microwave satellite data.
Researchers led by Joey Comiso of NASA's Goddard Space Flight Center in Greenbelt, Md., found that the amount of ice covering the Arctic has declined by 6 percent over each of the last two winters, compared to a loss of merely 1.5 percent per decade since 1979.
Comiso's team did not distinguish between perennial and seasonal ice, but he told LiveScience most of the loss was likely seasonal ice.
"This amount of Arctic sea ice reduction the past two consecutive winters has not taken place before during the 27 years satellite data has been available," Comiso said.
The researchers said that warming temperatures and a shorter winter-ice season are likely to blame.
"In the past, sea-ice reduction in winter was significantly lower per decade compared to summer sea ice retreat," Comiso said. "What's remarkable is that we've witnessed sea ice reduction at 6 percent per year over just the last two winters, most likely a result of warming due to greenhouse gases."
Losing perennial ice
Another study led by Son Nghiem of NASA's Jet Propulsion Laboratory measured the extent and distribution of perennial sea ice in the Arctic using NASA’s QuikScat satellite.
In addition to finding a loss of the ice cover extent, the team found a stark change in ice distribution. The perennial ice shrunk abruptly by 14 percent between 2004 and 2005, with an overall decrease of 280,000 square miles (725,200 square kilometers) — an area the size of Texas.
That's a much faster rate than NASA's earlier data, which found that between 1979 and 2003 Arctic perennial sea ice had been decreasing at a rate of 9 percent per decade, with the most significant loss in 2001, 2002 and 2003.
While perennial ice can reach a thickness of more than 10 feet (3 meters), seasonal ice thickness ranges from 1 to 7 feet (0.3 to 2 meters).
The team is still trying to figure out the reason for the shrinking ice cover. Typically, a loss of sea ice results from an increase in temperatures, which causes the ice to melt. However, Nghiem suggested that,in this case, strong winds pushed the thicker sea ice from the East to the West Arctic Ocean, sending giant chunks of ice along the eastern coast of Greenland toward warmer climes. That means this once melt-resistant ice could melt.
More seasonal ice floating atop the Arctic Ocean could have dire consequences for the surrounding water. Seasonal ice "can absorb more sunlight during the summer, because it has a lower albedo,” Nghiem told LiveScience.
Albedo is a measure of how much light a surface reflects.
The thick ice, which has increased in thickness as layer upon layer of snow melts and freezes on top of it, contains loads of air bubbles. “These bubbles scatter the sunlight out of the ice so less solar energy can be absorbed,” Nghiem said. The seasonal ice doesn’t contain these sun-scattering bubbles, and thus absorbs more sunlight.
If the perennial sea ice cover continues to decline and be replaced by thinner ice, the surrounding ocean could get warmer, further accelerating summer ice melts and impeding fall freeze-ups, the scientists said.