Fueling Arctic warming: Seasonal ice absorbs more solar energy
Let’s call it the polar express.
The multi-year ice floes that once dominated the Arctic Ocean have been shrinking fast over the past decade, replaced by pans that form anew each winter and melt away each spring. The increase in the extent of this seasonal ice has the potential to dramatically accelerate the warming of the planet’s far northern sea, according to a new four-year study published this week in Geophysical Research Letters.
It’s all hinges on how much sunshine gets absorbed and how much is bounced back into space over the course of a season, a physical process that scientists call albedo.
“My connection is really to answer a simple question — ‘Where does all the sunlight go?’” is how lead researcher Donald Perovich explained it in a You Tube video about the research near Barrow. “Or, to put it more scientifically, how is solar radiation partitioned among reflection, absorption and the ice in transmission to the ocean?”
The dazzling white and rugged surface of old, thick ice has the potential to reflect gobs of sunlight all summer. But the more fragile and flatter seasonal floes behave differently as the weeks pass — morphing from melting snow to standing ponds to the roiling green-gray of actual open water. As the ocean surface dominated by seasonal ice becomes less reflective, it absorbs more and more energy.
Self-fulfilling process
Spread the result over thousands of square miles season after season, and the Arctic Ocean will grow warmer at a much faster rate, boosting some forms of sea life while advancing the destruction of the seasonal ice cap, according to the findings in “Albedo evolution of seasonal Arctic sea ice.” In a sense, it’s becoming a self-fulfilling process, where the meltback of sea ice leads to an increasingly warmer upper ocean that leads to even greater ice loss in coming seasons.
“Once surface ice melt begins, seasonal ice albedos are consistently less than albedos for multi-year ice, resulting in more solar heat absorbed in the ice and transmitted to the ocean," wrote Perovich and co-author Christopher Polashenski, both with the U.S. Army’s Cold Regions Research and Engineering Laboratory in New Hampshire. This "shift from multi-year to seasonal ice cover has significant implications for the heat and mass budget of the ice and for primary productivity in the upper ocean.”
Tracking the dynamics of Arctic ice has never been more urgent. The extent and volume of the polar ocean’s ice cap has been shrinking fast over the past decade, climaxing each September with minimums at record and near record levels. (This spring, the extent of Arctic ice was greater than any late winter since the early 2000s, but still far below the long-term average for the time of year. See the May update from the National Snow & Ice Data Center.) Some climate models predict the Arctic will lose most of its multi-year ice by the end of the century if trends don’t change, with most of the ocean becoming ice-free by the end of every summer.
Theats to polar bears, walruses
An extensive polar ice cap has long played an essential role in stabilizing the world’s climate. It also provides essential habitat for healthy populations of polar bears, walruses and seals, and its disappearance during summer may threaten their survival. There are expensive indirect consequences too. The summer loss of ice has been creating fetches of hundreds of miles along the Chukchi and Beaufort sea coasts that now often expose Alaskan villages to catastrophic storms and erosion when fall weather starts bearing down.
To measure how the albedo — or reflectiveness — transforms over the course of a melt season on seasonal and old ice, Perovich and his team measured the absorption of solar energy along a 200-meter line on seasonal ice near Barrow over the course of four seasons. (The research gets a detailed and lively treatment in two 2008 videos — part one and part two.) They compared the results to data gathered during a study in 1997-1998 on multiyear ice far to the north — an area that now, ironically, has since become dominated by seasonal ice.
What they found suggests that the flat pans of seasonal ice undergo a summer meltdown that’s different — and darker — than what occurs on the mutli-year floes.
“The albedos for snow covered seasonal and multiyear ice are the same (but) … when melt begins, the thinner snow cover on seasonal ice melts in less time, resulting in more rapid transition to bare ice and a smaller albedo,” the scientists wrote.
More ponds
Melting on both types of ice floes generate ponds, which decrease reflectiveness and absorb more solar energy. But ponds on seasonal ice end up covering a much bigger area than ponds on the old floes.
“Level, undeformed seasonal ice can reach pond fractions greater than (about 70 percent) as meltwater,” they explained in the paper. “In contrast, multiyear ice, with its undulating topography, typically has peak pond fractions of only (about 30 to 40 percent) because meltwater is collected into deeper ponds with less area.”
The ice morphed through seven distinct stages of melt — including periods with melting snow, the formation of ponds on top of the ice, a rapid drainage into the ocean and some weeks of open water — each with a different degree of reflectiveness or absorption of sunshine. At almost every stage during the meltdown, the multi-year floes reflected much more solar energy than the seasonal ice. Its ponds were smaller, and it didn’t shrink into oblivion and expose the dark sea to the sky. Check out this chart comparing the albedoes over course of the season.
“During the melt season the albedo of seasonal ice is consistently smaller than multiyear ice,” Perovich and Polashenski concluded here. “Thus the ongoing shift from multiyear ice to seasonal ice will increase the total solar heat input to the ice cover, enhance summer melting, and increase the amount of sunlight transmitted through the ice into the upper ocean.”
Contact Doug O'Harra at doug(at)alaskadispatch.com
http://www.alaskadispatch.com/article/fueling-arctic-warming-seasonal-ice-absorbs-more-solar-energy
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