Sunday, October 12, 2014

Underwater landslide may have doubled 2011 Japanese tsunami

An underwater landslide the size of Paris may have triggered the worst of the tsunami that struck Japan on 11 March 2011, a new study claims.

Most of the destruction that day was caused by a 10-meter surge that overwhelmed coastal defenses from south of Fukushima to the northern tip of Honshu island. But along a 100-km mountainous stretch called Sanriku, indented with bays and small harbors, the incoming waves rose to a monstrous 40 meters. About a quarter of the tsunami’s 18,000 victims died in those ports, yet experts have struggled to find a satisfactory explanation for the exceptional inundation that killed them.

Seismologist Kenji Satake of the University of Tokyo’s Earthquake Research Institute, one of the world’s leading authorities on tsunamis, thinks a second earthquake was responsible. This temblor, he says, occurred north of the main submarine thrust, involved a thin sliver of crust, and left no trace in the seismic record of the day.

But Stephan Grilli, an oceanographer at the University of Rhode Island, Narragansett Bay,wasn’t convinced. Movements along Earth’s faults, he says, don’t jolt the sea surface in the right way to focus a band of waves on just a hundred kilometers of coastline, as happened in 2011.

In the new study, Grilli and colleagues worked back from details of the ocean surface motion recorded by gauges along the Japanese shore on the day of the earthquake. Much as sound waves can help the ear pinpoint the source of a gunshot and whether a small pistol or a large cannon fired it, tsunami waves carry the imprint of the ocean floor disturbance that created them.

The team concludes that during the earthquake a slab of sediment 20 km by 40 km and up to 2 km thick slid about 300 meters down the steep slope of Japan Trench, “acting like a piston.” Grilli’s calculations also identified where the slump must have happened: near the northern end of the 2011 rupture, 170 km from the Japan shore, and under 4.5 km of water. And when marine geologist and co-author David Tappin of the British Geological Survey compared Japanese maps from before and after the earthquake, he identified just the right kind of slump in the target area. The team’s paper is in press at Marine Geology.

The authors make a good case but are far from proving it, says Costas Synolakis, a tsunami expert at the University of Southern California in Los Angeles. Synolakis collaborated with Tappin and Grilli on previous investigations establishing that a similar slump caused the deadly 1998 tsunami off Papua New Guinea. This time, however, he worries the researchers are too fixated on details of the tsunami modeling at the expense of the big picture. “Anyone who thinks you can model the behavior of a tsunami to better than a factor of two is crazy!” he says.

A close up geophysical survey to test the evidence would settle the case one way or the other, Synolakis adds. “The key point is that 4 years on, we still don’t know anything about the mechanism” of the tsunami, he says. “The observed run-up is not compatible with anything the seismologists know about.”

Satake, however, maintains that his two-quake explanation is adequate and that the existing seafloor mapping reveals nothing.

If submarine landslides are responsible, “then it’s a game-changer,” says team member Robert Geller, a seismologist at the University of Tokyo. Geller, who has lived and worked in Tokyo for a quarter of a century, has long criticized the Japanese program of earthquake forecasting, which he says produces hazard maps of little worth based on doubtful science. If towering tsunamis can also be produced by collapses along the Japan Trench, he says, the chance of anticipating the next one is nearly impossible.

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Tuesday, September 23, 2014

The World Is Warming

   So Why Is Antarctic Sea Ice Hitting Record Highs?

For four days in a row this week, sea ice surrounding Antarctica has broken all-time records. But—get this—the ice is expanding, not shrinking.

It’s the third consecutive year that the icebergs that surround the continent have expanded into unseen territory. What the heck is going on? Didn’t Antarctica get the climate change memo? Isn’t polar ice supposed to be melting?

First, let’s distinguish between sea ice and land ice. Since sea ice floats in the ocean, its growth or melt doesn’t affect global sea levels. Antarctic land ice, on the other hand, does contribute to sea level rise, and it’s losing volume at a record pace. In fact, a frightening study earlier this year found that a key glacier in West Antarctica has entered an inevitable, slow-motion collapse phase, with dire consequences for the world’s coastal cities. A follow-up study last month for the first time put an upper bounds on the impacts of melting Antarctic glaciers in our children’s lifetimes.

The Antarctic (a cold continent surrounded by a warm ocean) is the geographic opposite of the Arctic (a warming ocean surrounded by cold continents). That difference, along with the fact that our economy is based on planet-warming fossil fuels, has allowed Arctic sea ice to dramatically decline in recent years, at a rate of about 4 percent per decade. At the other end of the world, Antarctic sea ice has been increasing at a fraction of that pace, less than 1 percent per decade.

It seems weird, but like this winter’s epic polar vortex outbreaks in North America, this week’s record-breaking Antarctic sea ice could be a further sign of global warming. Even though there's been more ice, the Southern Ocean is warming, not cooling. One theory says that warmer ocean waters are more effective at melting the tongues of Antarctic land ice glaciers that stick out into the sea. The resulting excess of freshwater raises the freezing point of the surrounding salt water, allowing more ice to form.

Not only is the ocean around Antarctica warming, it’s warming at a rate faster than the rest of the world’s oceans. So why is there a record amount of ice?

Another theory is that the winds that encircle Antarctica are growing stronger,  in part due to the hole in the ozone layer, and pushing ice farther and farther away from the continent, allowing additional ice to take its place closer to Antarctica’s frozen shores. This theory is favored by the British Antarctic Survey, and a number recent papers have backed it up.

A third theory is that since warmer air can hold more water vapor, it’s likely that there’s more rain and snow falling over the Southern Ocean. That too could decrease the ocean’s salinity near the surface, boosting sea ice levels.

The reality is probably some combination of the above. After all, there’s still a lot of legitimate debate among scientists on this topic. The Intergovernmental Panel on Climate Change, the leading global authority on climate change science, admitted as much  in its latest report, which was released last year.

New evidence shows the Antarctic sea ice trend itself may have been overestimated because of a statistical fluke.

Eventually, scientists expect the sheer temperature increase from global warming to swamp whatever complex combination of atmospheric and oceanographic physics that’s producing the counterintuitive ice growth, and Antarctic sea ice will begin to decline as Arctic ice already has.

By the end of the week, Antarctic sea ice will probably set yet another record. Just remember, it’s definitely not because Antarctica (or any other large swath of the planet) is getting colder.

 As long as humans have been keeping track, there’s never been this much sea ice surrounding Antarctica. What gives?

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