Glacial Death Watch: Why an Ice Shelf Snapped in 2002 and What's Coming Next

Collapse of the Larsen-B Ice Shelf on January 31, 2002. Source: MODIS/NASA

Like Cape Fear or Starvation Lake, a place called Scar Inlet might be a good setting for … the perfect murder.

Unfortunately for aspiring homicidal maniacs, the population of Scar Inlet is zero, so this 900-square-mile ice cube on the northernmost finger of the Antarctic Peninsula will have to serve as the site of another kind of vanishing.

Scar Inlet used to sit next to what was once a 1,250-square-mile ice shelf -- basically a sheet of ice above water, not land -- that shattered unexpectedly and spectacularly in early 2002. The inlet is next.

No one is likely to be hurt by the collapse of Scar Inlet, just as everyone survived so-called Larsen-B in 2002. But the disappearance of an ice shelf is basically like popping a cork. It clears the way for land-based glaciers to slide toward the ocean. That's the genius of global warming. It's so subtle that even though we're responsible, nobody calls it a crime.

Extensive study of the Larsen-B ice shelf's demise has shown that surface warming -- which we know is driven by smokestacks large and small -- was most likely the guilty party, and a new study out today provides further evidence.

Here's the case so far.

The summer of 2002 was a hot one at the bottom of the world, particularly West Antarctica, which saw about 20 percent more warmth than normal. The area has warmed about 4.3 degrees Fahrenheit (2.4 Celsius) since the late 1950s.

On non-freezing summer days, melting ice typically pools on the surface. More than 2,750 "meltwater lakes" had formed by January 2002 on the Larsen-B ice shelf. All that water puts enormous pressure on the ice beneath it, causing it to crack in places or "hydrofracture," as the ice scientists put it. Coincidentally, that's the very same word that's abbreviated to "fracking" in the context of shale gas drilling. 'Twas natural fracking that killed the shelf.

Trickles grew into a gush. Lakes flowed one into another. Meltwater poured into the ice, triggering other lakes to drain in a chain reaction that carved vertical cavities across the white expanse.

The surface couldn't withstand any more melted ice, "and then -- boom! -- it just broke up," says Eugene Domack, an oceanography professor at the University of South Florida and a co-author of the study released today about the Larsen-B collapse. "It's a perfect example of a tipping point."

It was apparent in 2002, in real time, that surface warming had a role in ice shelf disintegration. For years since then, scientists have also wondered if warmer ocean water was eating away at the underside of Larsen-B, or if some other disruption from below destabilized it.

The new study by Domack and several colleagues, published today in the journal Science, helps rule that out. The work was funded by the U.S. National Science Foundation and the Italian National Program for Antarctic Research.

The researchers analyzed the part of the sea floor now accessible because the Rhode Island-size Larsen-B ice is no longer covering it. They found what they understood to be the submerged, old extent of land-based ice, which previously reached farther from the continent. A view of the seafloor gave them a new opportunity to assess whether the bottom of the Larsen-B deteriorated in recent decades or over a longer period of time.

If the Larsen-B had been resting on ground before the last century or so, and melted from the bottom-up, then the seafloor would show evidence of recent disturbance or sediment older than 2002. But the sediment found there told a different story: Carbon-dating revealed it to be 12,000 years old. The Larsen-B had been floating atop the sea since the days when humans were inventing agriculture and moving into permanent settlements.

What that means is that surface warming, which is driven by combustion of fossil fuels, was the culprit that set the Larsen-B's demise in motion. That piece of information -- the ice faced death from the sky, not from the deep -- should help scientists better understand how Antarctic Peninsula ice shelves melt. Because they’re melting.

Evidence like this is always needed, even when the crime itself doesn't come as a surprise. "One of the warning signs that a dangerous warming trend is under way in Antarctica," wrote Ohio State polar scientist John Mercer, in 1978, "will be the breakup of ice shelves on both coasts of the Antarctic Peninsula."

Scar Inlet is up next, ripe for collapse during the next hot Southern summer. Domack says that may come as soon as the next year or two. Once Scar Inlet is gone, glaciers behind it will flow out faster, just as they did after the Larsen-B failed, turning more ice to water, and raising global ocean levels. The same forces will then go to work on the much larger Larsen-C shelf.

The Vanishing of Scar Inlet has yet to take place, but it will be well-chronicled when it does by both satellite and Earth-bound sensors. As Ted Scambos, a senior research scientist at the National Snow and Ice Data Center and a co-author of the new Science paper, puts it: "We want to catch this in the act."

More by Eric Roston (@eroston on Twitter):

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