Storm ‘echoes’ could break up ice shelves

Slow tsunami-like waves are rolling into the waters off Antarctica. Generated by storms churning as near as the Patagonia coast and as far away as the Gulf of Alaska, these waves jostle the continent's giant floating ice shelves.

According to a new study appearing in the journal Geophysical Research Letters, the rumbling may account for some of the decade's most dramatic ice breakups, which may only get worse as the planet's climate changes.

In 2002 the Larsen B ice shelf, a lid of ice the size of Rhode Island, disintegrated, a shocking example of the environmental change under way in the waters around Antarctica. As if to underscore that this phenomenon was no fluke, a small portion of the giant Wilkins shelf collapsed in March 2008 off the west coast of the Antarctic peninsula.

Warming ocean waters play a major role in these dramatic events. But a new study has found that what scientists call "infragravity waves" could be the triggers behind the breakups, rumbling in underneath shelves and lifting them up to an inch (2.54 centimeters) or so with each swell.

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That may not sound like much, but ice shelves can be well over 1,000 feet (300 meters) thick. The ice shelves are essentially immune to the effects of ocean waves. Infragravity waves are another animal, though. They form when waves from a large oceangoing storm crash into shallow waters. Energy from the waves is warped, elongated and cast back out to sea, and can echo for thousands of miles.

"Regular sea swell chips off little icebergs from the edges," Peter Bromirski of the Scripps Institute of Oceanography said. "Infragravity waves could be affecting a much greater part of the ice shelf."

Bromirski led a team of researchers who examined seismic rumblings on Antarctica's biggest shelf, the France-sized Ross Ice Shelf during the southern summer of 2005-2006.

The Ross Ice Shelf is stable, but the team found that winter storms in the north Pacific Ocean sent infragravity waves all the way to Antarctica. The ice rattled noticeably as each wave rolled underneath.

"The key thing is we are not at a position yet to say, 'Oh my God, infragravity waves are the proximal cause of ice shelf breakup,'" team member Douglas MacAyeal of the University of Chicago cautioned.

However, a large storm did pound the Patagonia coast just before the Wilkins shelf shattered in 2008.

If it's more than just a coincidence and infragravity waves from the storm did indeed ruffle the Wilkins enough to break it, the implications are sobering.

Predictions for future climate suggest increased storm activity over much of the globe.

"If you have more storms, you have more waves, and you get more impact," Bromirski said. "It's a direct connection."