The surface area of sea ice in the Arctic Ocean shrank this winter to one of its lowest levels in decades -- more bad news for polar bears, which depend on the ice to survive.
Since the National Snow and Ice Data Center in Boulder, Colo., began tracking sea ice three decades ago, only in 2006 was there as little ice during a Northern Hemisphere winter -- 5.65 million square miles.
That's nearly 8 percent less than the average of 6.12 million square miles recorded from 1979 to 2000.
The expanse of polar ice reached its maximum on March 7, according to the center. The date of maximum ice in the studies has ranged from Feb. 18 to March 31. As of March 22, the ice had declined for five consecutive days, leading scientists to conclude it will only shrink further. However, scientists noted, sea ice responds rapidly to winds and temperature this time of year and could expand again.
Steven Armstrup, senior scientist for the nonprofit Polar Bears International, noted that 2010 was one of the warmest years on record and that last autumn's ocean circulation patterns led to late and weak ice formation throughout much of the Arctic.
"This combination may (mean) early ice melt this year and large retreats of sea ice from polar basin coastlines," Armstrup said.
Eight months ago, Armstrup retired after 30 years as project leader for Polar Bear Research at the U.S. Geological Survey's Alaska Science Center in Anchorage. He has conducted research on Beaufort Sea polar bears since 1980 and is a past chairman of the International Union for Conservation of Nature.
"Because polar bears depend on the surface of the sea ice to catch seals, those things are not favorable for polar bear survival. So, if you are a polar bear, this could be a tough year -- with increased starvation especially among the young and very old."
But Armstrup is far less concerned with what happens in 2011 or any individual year than he is with rising average temperatures in the Arctic.
"The world will continue to warm as greenhouse gas concentrations rise," he said. "There will continue to be fluctuations in temperatures ... just as there always have been. Temperatures, however, will fluctuate over a rising baseline."
Natural ups and downs in the Earth's climate may mask the larger trend in any given year, he said.
"If we are lucky, we will have a series of cold years with circulation patterns that conserve sea ice. Ultimately, however, the greenhouse gases signal will clearly emerge. After that, all of the years will be bad for polar bears ... and polar bears ultimately will disappear."
Julienne Stroeve and her colleagues at the National Snow and Ice Data Center are studying the ripple effects of the shrinking of what Stroeve calls "the air conditioner of the Northern Hemisphere." She believes the Arctic may be ice-free during the summer within three decades.
"When I first started out studying sea ice, or even just climate in general in the Arctic, I didn't really think that we were in the midst of this global warming phenomena yet.
"But then, these last few years when we just continued to see these record ice losses, I started to change my way of thinking and realize that we are having a huge impact on our climate and we're actually causing the ice cover to pretty much disappear. And, yeah, it's been alarming -- because we don't really fully understand the implications of this, and I think that's the biggest fear, is that we really don't know what we are doing."
Stroeve said she's less concerned with the extent of the ice coverage than with the thickness of the ice.
"Generally, older ice is thicker ice -- and thickness is the key," she said.
The 2009-10 winter, for instance, showed average ice coverage. "But some of it is very thin ice and it will melt very quickly when it warms up, she said.
Reach reporter Mike Campbell at email@example.com or 257-4329.