Larsen C project to monitor ice shelf's health before its eventual collapse
Posted October 3, 2008
Konrad Steffen is one of the world’s leading experts on climate change in Greenland, having traveled to the ice-covered island every year since 1990. He oversees a network of 22 weather stations that collect data about the ice sheet, and returns with his graduate students each year to the same field camp north of the Arctic Circle to study how the ice sheet and its glaciers are responding to a warming climate.
Greenland is the poster child for global warming, the canary in the coalmine, and every other cliché used to describe the phenomenal amount of ice loss under way — a discharge causing sea level to rise steadily, millimeter by millimeter, like grains of sand passing through an hourglass.
Far away, in another hemisphere, the West Antarctic Ice Sheet and its glaciers are pouring ice into the ocean just as quickly. And at the northern tip of the Antarctic Peninsula, which juts away from the continent like a rhinoceros horn, temperatures are rising faster than just about anywhere on the planet. The disintegration of large ice shelves, formerly thought a once-in-a-lifetime occurrence, has happened twice already in this decade.
This is where Steffen and NASA colleague Eric Rignot are turning their attention to a project to monitor the ice shelf most likely to fall next — Larsen C.
“Our focus is to measure the Larsen C from the glaciological viewpoint and climatological viewpoint … To find out what is happening in a warming climate,” explained Steffen during an interview in his office at the Cooperative Institute for Research in Environmental Sciences (CIRES) on the University of Colorado-Boulder campus, where he serves as director of the institute.
The Larsen Ice Shelf was a series of three ice shelves with distinct embayments in the northwest Weddell Sea. The Larsen A sloughed away in 1995, and the Larsen B followed in a spectacularly rapid fashion in February 2002, though a relatively small chunk remains. Farther south and larger than the states of New Hampshire and Vermont combined, the Larsen C may disintegrate within the next decade.
Like physicians visiting a terminally ill patient, Steffen and Rignot want to monitor the ice shelf before it disappears — an opportunity that has eluded scientists in the past. Earlier this year, the Wilkins Ice Shelf on the west side of the peninsula began its fitful breakup, a process that continued through the winter.
“We want to study the state of the health of the [Larsen] ice shelf and predict when it might collapse,” said Rignot by phone from his office at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., where he is a senior research scientist. “We want to gain insight into the collapse of an ice shelf.”
Ice shelves themselves don’t contribute to sea level rise, but their disappearance allows the glaciers that feed them to flow freely and more quickly into the ocean. Understanding the processes that contribute to destroying these floating blocks of ice, which are several hundred meters thick, will help the scientists develop better computer models for ultimately predicting how high the seas may rise in the future.
“The modeling is only as good as we understand the process. Currently, we know very little about the process of ice shelf disintegration,” Steffen explained.
What the scientists do know from satellite data and some past studies is that the ice shelf is definitely warming and changing. Rignot said Larsen C is thinning at the north but possibly thickening to the south. The region receives some of the heaviest precipitation on the entire continent, most of which is a polar desert.
Satellites have observed melting on the shelf surface, Steffen noted. “We can see these melt waves going across the entire Larsen C that did not happen earlier on, like 10 years ago,” he said.
On the ground
Steffen, his graduate student Dan McGrath, and Ala Khazendar, a post-doc from Rignot’s group, will head south in October for the field component of the project, which includes collaboration with Chilean and British scientists. The logistics of the operation — flying and landing on the ice shelf, and then establishing a camp and setting up weather stations — have proven to be challenging.
The U.S. Antarctic Program has limited assets in the region, with one small research base, Palmer Station, on the other side of the peninsula. The research vessel Laurence M. Gould supports the station and science out of Punta Arenas, Chile, across the often rough Drake Passage.
To jump to the other side of the thin but mountainous peninsula, Steffen and his team will need air support. Originally, the plan was to work with the Chileans to fly to the British Antarctic Survey (BAS) research station Rothera aboard a military plane, and then onward to the ice shelf on a Twin Otter aircraft. But contaminated jet fuel at the BAS station would have delayed the trip until January.
“We cannot wait that long because it’s beginning to melt on the ice shelf,” Steffen said. “You can’t land your Twin Otter on an ice shelf that starts to get wet; you get too much friction and can’t take off.”
Instead, just a month ago, BAS volunteered to coordinate the entire operation, taking Steffen’s team and two British groups to the ice shelf, where they will share the same field camp and make complementary measurements. “In Greenland, I’m used to organizing my own logistics. … I just charter a plane and go where I need to go. In Antarctica, it’s not that easy.”
The big task in this first of three field seasons will be the deployment of several 10-meter-tall automatic weather towers like the ones in Greenland, which record climate data such as precipitation and temperature. A GPS will measure the horizontal and vertical motions of the shelf. “We want to measure the tidal motion, so we go to millimeter accuracy,” Steffen said.
In addition, the scientists will scout the underbelly of the ice shelf using ground-penetrating radar. The thought is that warmer ambient temperature — about 2.5 degrees centigrade in the last 50 years — alone isn’t breaking up the ice shelf. Steffen and other glaciologists believe a slightly warmer ocean is assaulting the ice from underneath, carving out huge cavities and channels that thin and weaken the shelf.
“We have a warming climate, but the temperature doesn’t really penetrate all the way through an ice shelf,” Steffen explained. He has already observed the phenomenon in Greenland’s glaciers and floating ice tongues, similar to ice shelves but bounded by mountains. There he observed the ocean whittle away a half-kilometer-thick glacier down to 100 meters in places. In addition, melt pools form on top, changing the structure of the ice as it seeps down and refreezes.
“If you do that over several seasons, you can warm up the ice body to a temperature not where it’s breaking but where the water can penetrate farther down. There must be weakening from underneath and from the top,” Steffen explained. “You only need a very small temperature change in the ocean, or a current change underneath the ice shelf, that’s able to carve out these bigger channels.”
In the air
NASA is funding the airborne component of the project, according to Rignot. He and Robert Thomas of EG&G Services at NASA’s Wallops Flight Facility in Virginia are leading that part of the project in collaboration with Chilean colleagues from Centro de Estudios Cientificos.
A P3 aircraft from the Chilean Navy will carry a radar sounder from the Applied Physics Laboratory and NASA’s laser altimeter system, which is flown every year in Greenland. The radar can determine the thickness of the ice in some areas. The laser altimeter with centimeter accuracy will map the topography.
NASA-funded airborne studies made similar radar passes in the region in 2002 and 2004, the latter part of a larger three-week expedition that included sites in Central and South America. Still, there are little data on the ice thickness, particularly of the 10 to 15 glaciers that feed into Larsen C. That’s because they’re deeply entrenched in narrow valleys, where the ice contains more cracks and possibly many water inclusions that absorb the radar signal
“It makes it difficult when you want to model a system like this, and you’re limited from the start by the definition of its basic geometry,” Rignot said.
NASA scientists will use the results of the project to develop models for the processes they measure. Rignot said he could not speculate as to when they might be able to create reliable models that will predict ice shelf breakup, glacier speedup and the subsequent rise in sea level.
“I think we’re at the stage where the uncertainty is larger than before, because we realize that the existing models are not able to explain the current evolution of ice shelves and ice sheets. They are too simplistic,” he explained. “Hopefully, the Larsen C project will provide critical observation of that particular ice shelf.”
Still, Rignot remains optimistic about filling in some vital data gaps over the next few years before Larsen C cracks apart and slips away into a thousand icebergs.
“It’s going to be an exciting experiment because we have a lot of tools in hand to make some major advances in our understanding of what’s happening at Larsen C right now,” Rignot observed. “Hopefully, this will help us refine how these ice shelves are collapsing with time. Once they collapse, there is no return; our natural laboratory is gone.”
NSF-funded research in this story: Konrad Steffen, CIRES; Eric Rignot, UCI and NASA Jet Propulsion Laboratory; Award no. 0732946. This award from NSF supports an International Polar Year project. For more about Steffen’s work in Greenland, see the recent article "Greenland Melting" in Rolling Stone magazine.
About the Sun