Giant
Waves Quickly Destroy Arctic Ocean Ice and Ecosystems
The
biggest waves seen in northern sea ice show how this vital cover can
be crushed much faster than expected
21
April, 2015
The
chance encounter of a Norwegian research vessel with the largest
waves ever recorded amid floating packs of Arctic ice shows how such
rollers could reroute shipping, damage oil platforms and threaten
coastal communities with erosion. In a March report in Geophysical
Research Letters scientists at the U.S. Naval Research Laboratory
(NRL) describe how large waves can penetrate more deeply into ice
cover and break it up faster and more completely than anyone had
suspected.
Less
ice means more open water to generate large waves—creating a
feedback loop that could doom the ice cap. (This dangerous cycle is
illustrated in “Waves of Destruction” in the May issue of
Scientific American.)
Every
year Aleksey Marchenko of The University Center in Svalbard, a
Norwegian archipelago north of Scandinavia, leads students across the
chilly waters of the Barents Sea to study the seasonal ice pack. Near
its edges pack ice is composed of pieces loosely drifting on the
water. Farther inside the pack there are kilometers-wide chunks that
have been blown together into a nearly solid mass. Toughened ships
like Marchenko’s converted Arctic fishing vessel, the RV Lance, can
usually pick their way slowly through it. When the Lance left port in
May 2010, Marchenko was expecting two or three days of leisurely
fieldwork. In previous years the group had even camped out on large
floes.
The
Lance sailed east and around 80 kilometers from the small island of
Hopen moored next to a large expanse of pack ice on May 2. Marchenko
prepared to lead his class out onto the floe. “We were ready to go
but when I went out, I discovered many cracks around,” he
remembers. He decided to move the Lance deeper into the pack for
safety. As he did so, the ship encountered small waves that grew in
size over time—a surprise as even a little ice near the pack edges
usually damps out waves. These waves then rapidly broke up the ice
around the ship into thousands of smaller pieces.
Within
an hour there was a four-meter swell. The Lance’s navigation system
ultimately recorded occasional waves more than six meters in height,
the largest ever measured amid Arctic ice. “And we could see even
bigger waves higher than the deck of the ship—30 feet [nine meters]
or more,” Marchenko says.
This
incident marks the first time that scientists had recorded any waves
over three meters high amid Arctic pack ice. Marchenko later gave his
measurements to Clarence Collins and his colleagues at the NRL in
Mississippi, who analyzed the interaction of the waves and ice. It
turned out that although the ice damped incoming waves, it also
contributed to its own destruction.
Ice
near the outer edge of the pack absorbed some energy from arriving
waves but also focused the remaining energy into pulses that could
strike deeper into the pack, lifting it as the waves rolled beneath.
The rise and fall strained ice to the breaking point. Once broken,
the smaller ice chunks allowed the largest waves to pass almost
unhindered and attack solid ice farther in. The ice went from
blocking almost all the wave energy to none at all within just one
hour. The process happened so fast, in fact, that Collins calculated
waves were destroying the pack at a rate of over 16 kilometers of ice
an hour.
Scientists
had never imagined that Arctic waves could break up pack ice so
quickly. Historically, the region’s extensive ice cover left no
large expanses of open water needed by storms to whip up really big
rollers. But climate change has brought milder winters, warmer sea
temperatures and bigger storms, which create a vicious cycle that
promises less sea ice and more wind and open water to generate
ice-crushing waves.
The
waves’ unexpected speed and ferocity makes them impossible to
predict with current low-resolution computer models, based on ice
observations that are typically updated only daily. That could spell
disaster for mariners, oil companies and native communities who are
unprepared for large waves or rely on sea ice to protect them. And
that is to say nothing of wildlife like polar bears and walruses that
rely on abundant sea ice to survive.
Collins
does not expect the record waves of 2010 to stand for long. As the
ice-breaking feedback loop accelerates in years to come, more and
more towering waves are likely to batter the shrinking ice cap. For
the Arctic Ocean, there are stormy times ahead.
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