Ice in Motion

Jason Box spent the summer of 2009 waiting for Greenland’s Petermann Glacier to break apart. Everything signaled the glacier was ready to go. Melt ponds were pooling on its surface, and massive cracks were opening on the icy tongue that stretched offshore into Baffin Bay. Box, a glaciologist at Ohio State University in Columbus, spent two months on a ship, his cameras trained on the volatile edge where ice meets ocean. He was determined to catch Petermann in the act.

As luck would have it, the glacier held out one more year. When Box wasn’t looking, on August 4, 2010, a piece of ice four times the size of Manhattan broke off. It was the largest iceberg to calve in the Arctic since 1962.

The world’s frozen places are full of glaciers like Petermann, slow-moving rivers of ice flowing into the ocean and poised on the edge between stability and collapse. In recent years, many of these volatile glaciers have spit out more and more chunks of ice to float away as icebergs and then melt. The more icebergs a glacier discharges, the faster it tends to move, thin and retreat away from the coast.

Scientists say that understanding the processes that control ice’s march to the sea, known as “ice dynamics,” is crucial for understanding the future of the planet’s great ice sheets atop Greenland and Antarctica. Were it all to melt, there’s more than enough ice there to raise sea level by 67 meters.

A complete meltdown isn’t likely, even centuries from now. But many researchers worry that Greenland and parts of Antarctica could soon contribute more to rising sea level, as temperatures are increasing fastest in those regions. And a small difference — say a foot of sea level rise, versus a meter — could mean the difference between much of Miami staying above water or going under in the next century or two.

As ice melts, water trickling through the remaining ice affects how quickly a glacier moves and breaks apart. To better understand and forecast such effects, researchers are targeting a few key outlet glaciers, which funnel ice from the high frozen interior into the oceans. By peppering these glaciers with instruments to measure their flow, cameras to photograph their every move, and even underwater submersibles to test the surrounding oceans, scientists hope to learn what drives the ice.

“We know that outlet glaciers are complex and dynamic and important,” says Leigh Stearns, a glaciologist at the University of Kansas in Lawrence. “We need to pick a few locations and understand them there.”

In Greenland, for instance, research is showing how warm ocean waters can affect the terminus, or end, of a glacier at the coastline. In Antarctica, scientists have found that the 2002 breakup of a major floating ice shelf — Petermann, supersized — caused the glaciers flowing into it to speed up. Together, such studies are revealing the mercurial nature of Earth’s icy realms.

Slip sliding away

Because of ice dynamics, Greenland loses as much ice each year as is contained in the entire Alps. Within Greenland, perhaps no glacier is more responsible for that ice loss than Jakobshavn Isbrae, on the island’s west coast. As glaciers go, it’s one of the world’s fastest, with ice flowing up to 14 kilometers per year — fast enough that you can see it move.

Jakobshavn is one of Greenland’s biggest outlet glaciers and has been spitting ice into the ocean for a long time; scientists consider it the likely source of the iceberg that sank the Titanic in 1912. But after 1997 something funny happened. The glacier began thinning rapidly and accelerating. The faster it moved, the more ice it coughed out, causing its terminus to retreat back farther onto land. “That thing is in a death spiral,” says Ian Howat, another glaciologist at Ohio State.

In part, that’s because the once-large ice shelf that disintegrated at Jakobshavn’s terminus removed the physical support that had held the glacier back like a buttress. But that isn’t the whole story, researchers are finding. “You really can’t explain all of the speedup by a loss of ice shelf,” says Ian Joughin, a remote-sensing expert at the University of Washington in Seattle.

The glacier may also be undergoing changes in internal stress upstream and may be slipping more across the bedrock at its base. Studies in recent summers have shown how meltwater forms ponds on the surfaces of glaciers, then percolates down through cracks to help lubricate the ice’s mighty grind toward the ocean. In 2008 Alberto Behar of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., even dropped a flock of 90 rubber ducks into watery tunnels atop Jakobshavn to see if the toys would track the flow all the way through the glacier and out into the ocean. (So far, no luck.)

In a step up in instrument sophistication, Joughin and colleagues have been using radar from airplane surveys and satellites to track Jakobshavn’s speed. The glacier speeds up and slows down seasonally, but between 2004 and 2010 it accelerated some 2 to 5 percent, Joughin reported in December in San Francisco at a meeting of the American Geophysical Union. At that rate, he said, the glacier should back away from the ocean entirely and end on land in roughly a century.

On the other side of Greenland, its eastern coast, researchers are studding instruments all over another accelerating glacier and the waters into which it flows. Helheim Glacier, also one of Greenland’s largest, began speeding up along with others in the southeastern part of the island in the early 2000s. Helheim has since slowed down and, in fact, gained mass over the last decade — about 26 billion metric tons, says Howat, who presented the data in Maryland in January at a planning meeting for Greenland researchers. Compare that with Jakobshavn, which lost nearly 300 billion tons over the same period, and it’s clear that Greenland’s glaciers can behave in different ways.

Helheim continues to belch ice into a fjord called Sermilik, now the target of a major scientific push. The goal is to understand how Helheim’s snout melts when it hits the water. Such ice-ocean interactions are one of the hottest topics in glaciology today. “We’d like to know how much ice is discharging from the glacier high up, versus how much is melting actively at the front,” says Stearns.

For the past three summers, Fiammetta Straneo of the Woods Hole Oceanographic Institution in Massachusetts and her colleagues have been dodging icebergs to drop long instrument-laden cables into Sermilik’s waters. The scientists also charter helicopters to fly above the glacier’s snout and let instruments fall atop the ever-moving ice. Each year the researchers yank up the cables, pick up any remaining instruments and deploy new ones.

In a surprise, Straneo’s team found that warm waters penetrated into the fjord after diverting from their usual path about 150 kilometers offshore. “The waters outside of Sermilik are some of the warmest ocean waters you will find around Greenland,” Straneo says.

These waters come all the way from the subtropics, her team reported in 2010 in Nature Geoscience, and measure about 4° Celsius. Compared to ice, that’s warm. “It’s like putting an ice cube in a relatively warm bathtub,” says Straneo.

The studies are showing just how important water circulation is to the behavior of glaciers that flow into that water. But many mysteries remain. For instance, warmer temperatures in the fjord occur in the winter, says Stearns, but the glacier still goes faster in the summer.

Ice down under

Warm water is also being fingered as a suspect in a case of meltdown much farther south: the Pine Island Glacier, which helps drain Antarctica’s frozen interior.

Parts of the Antarctic ice sheet have been gaining mass in recent years, as more snow falls there and compresses down to ice. But the U.S.-German GRACE gravity-measuring satellites, among others, have shown that overall Antarctica is losing mass. And Pine Island is the biggest loser on the continent. Its narrow floating ice shelf has been thinning and retreating rapidly over the last few decades.

Two years ago, the British Antarctic Survey sent an unmanned submersible called Autosub3 swimming under the Pine Island Glacier’s floating tongue. There the robot discovered an underwater ridge running along the seafloor perpendicular to the glacier. Until recently, the glacier was grounded on this ridge, which stabilized it, researchers reported last year in Nature Geoscience. But now the glacier has retreated behind the ridge, where the bottom of the ice is exposed to much deeper water circulation that can erode it further.

Also last year, scientists flying as part of NASA’s IceBridge mission — a series of plane flights over the polar regions to gather data — discovered an underwater channel that diverts water around one end of the same ridge. This channel could allow warm water from offshore, pushed by Antarctica’s counterclockwise winds, to flow up under the glacier, the researchers reported at the geophysics meeting in San Francisco. “It really melts the ice at high rates,” says glaciologist Robert Bindschadler of the University of Maryland Baltimore County.

Bindschadler and his colleagues have been schlepping research equipment and supplies closer to Pine Island in preparation for a full-out push on the glacier late this year. The scientists plan to take the best measurements yet of the shape of its tongue of floating ice and the water underneath, including by drilling through the ice to get to the water below. If Pine Island were to disintegrate further, it might release pressure holding back much of Antarctica’s ice and allow even more to flow out, like popping the cork on a bottle of champagne.

“This is something we are really concerned about,” says Michael Studinger, IceBridge’s chief scientist and another researcher at the University of Maryland Baltimore County. “There is tremendous potential for rapid drainage of large parts of the West Antarctic ice sheet.”

“Potential” remains a key disclaimer, however. One recent study suggests that despite its recent acceleration, Pine Island may have stabilized its speed for now. “Our model indicates substantial, but not catastrophic, loss from Pine Island over the next century,” says Joughin, whose paper appeared in October in Geophysical Research Letters.

After Pine Island, the next biggest loser in Antarctica is the Antarctic Peninsula, a narrow spit of land that stretches up toward Chile and has warmed 2.5 degrees Celsius in the last half-century. Along the eastern side of the peninsula, a major ice shelf known as Larsen A disintegrated in 1995. An adjacent one to its south, called Larsen B, went in 2002, removing an area of ice the size of Rhode Island in a single summer. Now researchers are closely watching the southern fringe of what’s left, the still-standing and largest of all, Larsen C.

Scientists are also watching what the removal of Larsen A and Larsen B did to the glaciers that fed them. New calculations show that between 2001 and 2006 those glaciers were responsible for about one-third of the ice lost from the Antarctic Peninsula, says Ted Scambos, a glaciologist at the National Snow and Ice Data Center in Boulder, Colo. “It tells us Antarctica is vulnerable,” he says.

Ice shelves disintegrate when crevasses form on their tops and then widen into cracks that are too big to reseal themselves during winter. In summer, as the ice surface melts, water begins ponding and filling these cracks, and the water pressure serves to widen the crack and keep it propagating. “Some shelves go for years with water on the surface, but then in one particular year you get that disintegration,” says Scambos.

Hoping to catch one in the act, Scambos and his colleagues have targeted a part of Larsen C known as Scar Inlet. The team put a sophisticated measuring station, bristling with cameras and other instruments, on the floating ice of Scar Inlet itself, as well as another on the glacier feeding it. If the Antarctic Peninsula gets a particularly warm summer in the next couple of years, the team expects to be able to watch Scar Inlet disintegrate. If so, the glaciers feeding the Larsen area may speed up even more, thus accelerating the rate at which ice is dumped into the ocean.

One thing is certain about glacier studies in a warming world: There’s plenty of job security. “The events in the ’90s and this past decade suddenly showed us lots of things to focus on,” says Scambos. Events like Greenland glaciers accelerating and the Larsen ice shelves disintegrating “all told us what the important precursors were before you saw a great big change,” he says. Now scientists have the data to start understanding how those precursors lead to change.

“We’re definitely moving from the realm of observation to the realm of prediction, and that’s the goal,” says Howat. What matters in the end is not the details of a particular glacier in Antarctica, but what that means for people living along coasts. From a practical perspective, he says, “we don’t really care what the ice sheet’s doing now. We want to know what it’ll be doing in 100 years.”