14 DAY TRIAL // There are two types of sea ice covers in The Arctic Ocean. One is thick perennial ice that resists thaw year-round and the other is thinner seasonal ice that melts during the summer and freezes again in the winter.
Global warming started provoking a withdrawal on both types. In 2005 and 2006, the extent of winter perennial ice was about 6% smaller than the average amount over the past 26 years. The diminution is also larger than the long-term decrease of 1.5 - 2 % in winter ice cover, observed per decade, over the same time period.
Summer warmer temperatures have been causing more and more ice to melt during summer, with the last four summers registering record lows in ice cover. Josefino Comiso of NASA's Cryospheric Sciences Branch in Greenbelt, Maryland, US, has used satellite data stretching back to 1979 to show that less of the melt water is refreezing in the wintertime. "It is the strongest evidence yet in the Arctic of global warming" Comiso said.
Mark Serreze, a senior research scientist for the US National Snow and Ice Data Center in Boulder, Colorado agrees. "There is a lot of natural climate variability - it is a complex science - but the best explanation of what we are seeing is the emerging signs of greenhouse warming,"
Comiso found that from 1979 to 2004, the extent of winter ice in the Arctic remained virtually the same - despite reductions in the summer cover. He believes the recent decline is due to a reduction in the length of the Arctic ice season and unusually warm wintertime temperatures in the region. "If the winter ice retreat continues, the effect could be very profound - especially for marine mammals," Comiso says.
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). 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. "Recent changes in Arctic sea ice are rapid and dramatic," said Nghiem. "If the seasonal ice in the East Arctic Ocean were to be removed by summer melt, a vast ice-free area would open up. Such an ice-free area would have profound impacts on the environment, as well as on marine transportation and commerce." The loss of perennial ice in the East Arctic Ocean, above Europe and Asia, neared 50 percent during that time, as some of the ice moved to the West Arctic Ocean, above North America. Data from the National Centers for Environmental Prediction in Boulder, Colorado, suggest that winds pushed perennial ice from the East to the West Arctic Ocean and moved ice through the Fram Strait, a deep passage between Greenland and Spitsbergen, Norway. This movement of ice out of the Arctic is a different mechanism for ice shrinkage than the melting of Arctic sea ice, but it produces the same result--a reduction in the amount of perennial Arctic sea ice.
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. 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.
The decrease in perennial ice raises the possibility that Arctic sea ice to retreat to another record low extent this year. This follows four summers of very low ice-cover, as observed by active and passive microwave instruments.
Polar bears, which rely on drifting ice to hunt seals, are believed to be hit especially hard by the diminishing icepack. In Canada's Hudson Bay, the bears population has dropped 21%, from an estimated 1200 individuals in 1989 to 950 in 2004, according to Claire Parkinson, also of NASA's Cryospheric Sciences Branch.
Progressively earlier breakup of the Arctic sea ice, stimulated by climate warming, shortens the spring hunting season for female polar bears in Hudson Bay and is likely responsible for the continuing fall in the average weight of these bears. As females become lighter, their ability to reproduce and the survival of their cubs. "In 1980 the average weight of adult females in western Hudson Bay was 650 pounds. Their average weight in 2004 was just 507 pounds -- a 143-pound reduction," said Ian Stirling, a senior scientist with Canadian Wildlife Service, Edmonton, Alberta. A 1992 study in the Canadian Journal of Zoology indicated that no females weighing less than 416 pounds gave birth the following spring. It is likely that in 20-30 years polar bear reproduction in western Hudson Bay will be significantly limited. Similar events may eventually happen in other areas included in the study.
Also, as the bears become thinner, they are more likely to push into human settlements for food, giving the false impression that the population is increasing. Claire Parkinson, a scientist at NASA's Goddard Space Flight Center, Greenbelt, Md., and Stirling showed the reduction in sea ice cover in several specific areas where there are known polar bear populations, from 1979 till now, using NASA's satellite data. In most of the areas studied, they found that ice break-up in these areas has been progressively occurring earlier. "Our research strongly suggests that climate warming is having a significant and negative effect on a primary species reliant on the sea-ice cover for survival," said Parkinson. The researchers studied the sea ice in regions that are home to five different polar bear populations: western Hudson Bay, eastern Hudson Bay, Foxe Basin, Baffin Bay and Davis Strait-Labrador Sea. "Polar bears live much of their lives on the sea ice, which is fundamental for their survival, at least in terms of their traditional lifestyles," said Parkinson. "It's the sea ice surface that provides them a platform from which to hunt seals and other marine mammals for food."
Sea ice is most scarce during the summer months, causing the bears to retreat to land and fast on their stored fat reserves until sea ice comes back in the fall. "Our concern is that if the length of the sea ice season continues to decrease, then the polar bears will have shorter periods on the ice, when they can feed, and longer periods on the land, during the open-water season in summer and early fall," she said. "Their stored fat from life on the ice will likely not provide enough nourishment for the fasting period on land, posing a clear danger to their health and, in the long term, possibly to their species."
Sea-ice cover in these regions has decreased since at least 1978, the beginning of consistent satellite monitoring. The researchers used 26 years of satellite data using data from NASA's Nimbus 7 satellite and the Defense Meteorological Satellite Program's Special Sensor Microwave Imager.
"By reviewing satellite data, we found that sea-ice cover break-up in western Hudson Bay took place about seven to eight days earlier per decade," said Stirling. "An extra month of fasting resulting from this phenomenon over four decades can significantly impact the polar bears' eating habits and survival."