Pine Island Glacier, one of the most vulnerable in West Antarctica, is being melted from below by seawater.

MARTIN JAKOBSSON

辽宁35选7走势图:Rising bedrock below West Antarctica could delay catastrophic ice sheet collapse

The news last week out of Antarctica was sobering. According to a consensus estimate published in Nature, the continent has lost 3 trillion tons of ice in the past 25 years—most of it from the vulnerable West Antarctic Ice Sheet, where the loss rate tripled over the study period. Although West Antarctica contributed just 6 millimeters of sea level rise in that time, scientists say ice-sheet collapse there could raise global sea levels by 3 meters in the coming centuries. The accelerating loss could be a sign that the catastrophe has already been set in motion.

But a study in this week's Science offers a glimmer of hope, documenting a process that could slow the collapse. As ice melts and the load on the crust lightens, the bedrock beneath West Antarctica is rising rapidly. In places it could rise 8 meters over the coming century—potentially protecting the ice from the warm seawater that is melting it from below. "It may just buy the world a few extra decades," says Rick Aster, a seismologist at Colorado State University in Fort Collins and an author of the new study.

The West Antarctic Ice Sheet is vulnerable because its bed lies far below sea level, forming a giant basin that slopes inland to a depth of more than a kilometer. Glaciers—"rivers" of ice—shed ice into the ocean. For the moment, some are snagged on ridges in the sea floor, slowing their flow. But as the warming ocean erodes them from below, they could retreat behind the ridges. Seawater would then pour into the basin, lifting ice off the bedrock and melting it in a runaway process. "It's a very unstable situation," says Natalya Gomez, a geophysicist at McGill University in Montreal, Canada.

The bowl under West Antarctica was created during the last ice age, when the weight of the ice, much thicker at the time, pressed down on the bedrock. But the rock was ready to spring back. "The earth acts like a memory foam mattress," says Valentina Barletta, a geophysicist at the Technical University of Denmark in Kongens Lyngby who led the new study. Some rebound occurs immediately, as soon as ice melts. But some takes place more slowly, as the gummy rock of the deeper mantle gradually readjusts to the lighter burden.

Gomez models this process and had found that rising bedrock might slow ice retreat, by raising the bowl and the ridges that stabilize the glaciers. But just how much help crustal rebound can offer depends on how fast it takes place, which reflects how hot and gooey the underlying mantle is.

To measure the rebound, Barletta and her colleagues tracked tiny changes in elevation using six GPS sensors that they fixed to ice-free bedrock in locations around the Amundsen Sea—the epicenter of West Antarctic ice loss that includes the rapidly receding Thwaites and Pine Island glaciers. Soon after the sensors were deployed, between 2010 and 2012, the team noticed they were rising fast, says Terry Wilson, a polar geologist at The Ohio State University in Columbus who led the deployment. But it took 2 or 3 years before the team realized, "Oh geez, this is real," she says.

They found that, in some places, the bedrock was rising more than 4 centimeters per year—one of the fastest rebound rates in the world. "That's huge," says Robin Bell, a geophysicist at Columbia University who wasn't involved in the study. "It shows that the earth is gooey there, a lot gooier than we thought." Bell says another finding, reported in Nature last week, shows that the process happened in the past. The study found that the West Antarctic Ice Sheet shrank at the end of the last ice age 12,000 years ago, but began growing again as the rebound effect took hold. "It shows you how dynamic the planet can be and how quickly it can respond to moving ice," Bell says.

The mantle is so gooey under West Antarctica that it is readjusting to ice lost decades or centuries ago, not the retreat of ice age glaciers thousands of years ago, the more standard time frame for mantle rebound, Barletta says. And the uplift will accelerate as the region sheds more ice weight. Barletta predicts that, in a century, it will be three times faster. By then, she says, the bedrock in some locations will have rebounded by about 8 meters, lifting the ice sheets along with it and lowering water levels where the vulnerable glaciers meet the ocean.

But some scientists say no amount of rebound will prevent ice sheet collapse in the long term, given the pace of carbon emissions and global warming. "It's not a get out of jail free card," says Ted Scambos, a glaciologist at the National Snow and Ice Data Center in Boulder, Colorado. "It's more of a refinement on the pace of [ice sheet] collapse," he says, especially if we continue "stomping on the climate gas pedal." Ingo Sasgen, a geophysicist at the Alfred Wegener Institute in Bremerhaven, Germany, agrees. "It's still a rather slow process compared to melting," he says. "If you have a very strong warming from the ocean, the ice sheet will disintegrate whatever the solid earth does."