Wednesday 26 August 2020

Heat from below - the story that is hardly told

Growing underwater heat blob 

speeds demise of Arctic sea ice

In March, soon after arriving aboard the Polarstern, a German icebreaker frozen into Arctic sea ice, Jennifer Hutchings watched as ice broke up around the ship, weeks earlier than expected. Even as scientists on the research cruise scrambled to keep field instruments from plunging into the ocean, Hutchings, who studies ice deformation at Oregon State University, Corvallis, couldn’t suppress a thrill at seeing the crack up, as if she had spotted a rare bird. “I got to observe firsthand what I studied,” she says.

Arctic sea ice is itself an endangered species. Next month its extent will reach its annual minimum, which is poised to be among the lowest on record. The trend is clear: Summer ice covers half the area it did in the 1980s, and because it is thinner, its volume is down 75%. With the Arctic warming three times faster than the global average, most scientists grimly acknowledge the inevitability of ice-free summers, perhaps as soon as 2035. “It’s definitely a when, not an if,” says Alek Petty, a polar scientist at NASA’s Goddard Space Flight Center.

Now, he and others are learning that a warming atmosphere is far from the only factor speeding up the ice loss. Strengthening currents and waves are pulverizing the ice. And a study published last week suggests deep heat in the Arctic Ocean has risen and is now melting the ice from below.

Ice has kept its grip on the Arctic with the help of an unusual temperature inversion in the underlying waters. Unlike the Atlantic or Pacific oceans, the Arctic gets warmer as it gets deeper. Bitter winters and chilly, buoyant freshwater from Eurasian rivers cool its surface layers, which helps preserve the underside of the ice. But at greater depths sits a warm blob of salty Atlantic water, thought to be safely separated from the sea ice.

As the reflective ice melts, however, it is replaced by darker water, which absorbs more of the Sun’s energy and warms. Those warming surface waters are likely migrating down into the blob, which robotic temperature probes, moorings, and oceanographic surveys show is steadily warming and growing. With enough heat to melt the Arctic’s ice three to four times over, the blob could devour the ice from below if the barrier of the cold surface layers ever dissipates.

Measurements from the eastern Arctic Ocean, published last week in the Journal of Climate, show the blob, usually found 150 meters below or deeper, has recently moved up to within 80 meters of the surface. Increased turbulence means some of that heat is now melting ice, says Igor Polyakov, an oceanographer at the University of Alaska, Fairbanks. “This heat has become, regionally, the key forcing for sea ice decay.”

The process, called “Atlantification,” is already well underway in the Barents Sea, north of Norway, where fingers of warm Atlantic water have spread north and risen, melting sea ice even in winter months. The invasion shows no sign of stopping, says Helene Asbjørnsen, an oceanographer at the University of Bergen who has helped chart this migration. “Ultimately we expect it to extend into the Arctic more.”

The $134 million Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC), based on the Polarstern, is exploring another ice-destroying feedback. The ship froze itself into a floe in October 2019, to give the team a chance to observe the floe for one full year as the summer melt season shifted back into freezing. But the project ran into challenges. First came the COVID-19 pandemic, which made planned personnel rotations difficult. Then the ice drifted too far south too quickly. In late July, the day after the team pulled up its remaining instruments, the floe broke up and melted. “To me that is a big loss, and I’m pretty bummed about it,” says Matthew Shupe, a climate scientist at the University of Colorado, Boulder, who helped lead U.S. contributions to the cruise. But, he added, there was a bonus: “We never planned to be around for that ‘death of an ice floe’ process.”

The Polarstern’s floe is not an isolated case. Remote sensing satellites show that over the past 20 years, ice has been drifting faster, potentially sweeping it into warmer waters, says Sinéad Farrell, a sea ice scientist at the University of Maryland, College Park. One reason for the change in pace could be faster currents in the Arctic Ocean, as ice melt exposes more water to the push of the wind, says Arild Sundfjord, a physical oceanographer at the Norwegian Polar Institute. “We think we see signs of that.”

Another factor could be an increase in the roughness of the sea ice, which allows wind to catch and propel it. MOSAiC scientists deployed GPS stations across the floe’s melange of first-year and thicker multiyear ice to monitor its speed and deformation. They suspect that as the ice becomes thinner and weaker, it is more prone to the crunch and crumble that builds up wind-catching ridges, Hutchings says, but they’re still resolving whether that is true. The turmoil took a heavy toll on the expedition, crushing some instruments like aluminum cans and destroying snow sampling sites. It was frustrating, Shupe says. “We don’t really control anything here,” he says. “The Arctic is telling us its story and we just need to be clever enough to document it.”

ICESat-2, a laser altimeter launched by NASA in 2018, will help extrapolate findings from MOSAiC to the rest of the Arctic. Unlike previous satellites, ICESat-2 can distinguish between ice floe cracks and melt ponds on top, and it is already showing stark differences between multiyear and first-year ice, Farrell says. In a surprise, the ICESat-2 team is finding that the multiyear ice overall is twice as rough as first-year ice. “It’s kind of like aging skin,” she says. “They get more wrinkly over time.” The satellite also seems to be capable of capturing waves amid the ice, and linking them to nearby storms, Petty says. It’s another worrying mechanism that could speed up ice loss, he says. “As waves break the ice apart, it gets more exposed to heat—and melts further.”

The retreat of the ice bodes ill for global climate, but it is making the Arctic easier to study. This month saw the start of the Synoptic Arctic Survey, which will knit together more than a dozen national Arctic cruises by ice breakers and other research ships. The survey will cover the Arctic’s entirety, providing a near-simultaneous picture of currents, life, and water conditions and chemistry, rather than a collection of regional snapshots over time. The pandemic delayed all but two of the cruises, which were planned for this summer: those of Japan’s Mirai and South Korea’s Aron. But once completed, the survey could answer basic questions, such as whether the Arctic is a net source or sink of carbon dioxide.

And it could not have been done in the ice-bound Arctic of old. “Now,” Sundfjord says, “we can go wherever, and whenever, we want.”

From 2012

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