To forecast sea level rise, a flotilla of robot subs must map the unseen bottom of a melting ice shelf—if they are not sunk by it
Deep below the bright, weather-smoothed surface of Antarctica’s ice shelves there is a dark landscape unlike any other on Earth. Fed by ice sheets on land, these giant shelves float on the frigid waters of the Southern Ocean. In their undersides melting water has carved out great inverted canyons and caves reaching up hundreds of meters, with terraces and ledges that step upward into the gloom. Sharp crevasses have been created where tidal pulls cracked the ice. And where the shelf is thickest above, ridges of ice below reach down and snag on the seabed, causing further stresses and cracks.
This month a fleet of seven underwater robots developed by the University of Washington (U.W.) in Seattle is heading into this world on a risky yearlong mission. Their goal: help forecast sea level rises by observing the melting process in this hidden topsy-turvy world, where layers of warm and cool water mix at the shelf. Because the complex physics in this unique region are poorly understood, scientists have been unable to make good predictions about the ice shelves’ future on a scale of tens of years, which is what will affect humans living near the ocean today.
“We’ve known for about 40 years that ice shelves are intrinsically unstable,” says Knut Christianson, a glaciologist on the mission and a leader of U.W.’s Future of Ice initiative. “But we don’t really understand the variability of these systems, let alone how they react to a significant external forcing like warming sea temperatures.”
The robots have been toughened up for their long mission but were originally designed for use in open water rather than under a hard, frozen shoreline, surrounded by unknown hazards and with no way to communicate if they have problems. “There is a real risk that some of the instruments will not come back,” says Jason Gobat, an oceanographer at U.W.’s Applied Physics Laboratory who will be traveling to Antarctica to deploy the bots.
The drones will be sent under the Pine Island Ice Shelf, where two of Antarctica’s largest glaciers go to die. Ice from these glaciers is on the move, traveling downhill and then 30 miles or more into the ocean, where it raises the sea level and eventually melts. The flow of ice from Pine Island Glacier has sped up by nearly 75 percent over the past 40 years and its shelf is holding back enough ice to flood every coastal city on Earth. The only way to take key temperature, pressure, water chemistry and turbulence measurements is to put instruments directly into the sea below it.
Previous efforts to do this have involved scientists drilling through ice shelves in one or two locations or sending robotic submarines on short trips beneath them. But these have been restricted to small spots and brief periods of time—snapshots that do not necessarily reflect the full behavior of the ice-and-water system, Christianson says. That is the reason he and his colleagues are deploying their fleet of autonomous submersibles for such a long time and to cover more than 50 square miles.
The robotic explorers consist of three self-propelled drones called Seagliders accompanied by four drifting floats. Each of the $100,000 Seagliders will follow a route under, around and back from the ice shelves, lasting a few weeks. The drones swim by adjusting their buoyancy and the pitch of their wings to glide slowly in the direction they are programmed to go. And they can locate themselves by triangulating their position from three sonic buoys the researchers will moor outside the shelf—a bit like an acoustic GPS system. Once each robot is clear of the ice shelf, it will attempt to surface and upload its precious data to an orbiting satellite, which can also beam down new instructions for the next dive.
The $30,000 floats, by contrast, are at the mercy of ocean currents, able only to regulate their buoyancy to rise or sink. Oceanographers believe warm, salty water flows toward the base of the ice shelf near the seabed, with cooler, fresher water flowing out above it. So the floats have been programmed to travel low and inward for a couple of weeks before rising up to be—if everything works—swept out again.
If a robot rises into a crevasse or gets trapped under one of the terraces, however, it has no escape plan and cannot even call back for help. “It’s a rather dramatic environment under there and not simple to navigate in or explore, especially as we’d really like to know what’s happening right at the boundary between the ice and the ocean,” Christianson says.
The mission is so perilous that the National Science Foundation passed on funding it, leaving its future uncertain until the Paul G. Allen Philanthropies foundation stepped in with a donation of nearly $2 million. “It’s a very risky prospect,” says Mick West, an engineer at the Georgia Tech Research Institute who dropped a tethered robot through Antarctica’s Ross Ice Shelf in 2016. “Losing a vehicle under the ice is an expensive proposition for government agencies. We ended up using a tether on our robot even though it is totally autonomous, because losing a close to $1-million vehicle would have been devastating to our research.”
U.W. scientists will return early to Antarctica in 2019 to collect drones that survive. If the robots cannot be found, they still have enough battery life to operate for another year—and if they come up, they might be snagged by other researchers in the Southern Ocean. But long before that Christianson anticipates using the data the robots will upload to improve models for the planet’s rising seas. “Hopefully, we can start to do the long-term measurements that are so important for our understanding both of Pine Island’s current state, how quickly it reacts to inputs and how long that reaction lasts,” he says.