Advancing Alaskan Glacier Holds Clues to Global Sea Level Rise: Scientific American.
ICY BAY, Alaska—The icebergs looked impenetrable. Roman Motyka needed a route through.
“If you see an opening anywhere, let me know,” said the University of Alaska Fairbanks glaciologist, at the wheel of a small skiff puttering through the ice-choked bay off the Gulf of Alaska.
Beyond the iceberg maze loomed the nose of a glacier that, contrary to a warming climate, is advancing into the sea. Motyka and his team were here – in one of the most ice-covered regions on the planet – to find out why.
“Just a hair to the left,” said Tim Bartholomaus, doctoral student at UAF, poised at the bow with fishing spear in hand to fend off encroaching ice.
The boat thunked against a truck-sized ice cube and redirected through a small opening between bergs. Ice scraped the aluminum hull like frozen fingernails on chalkboard.
The source of the ice – the Yahtse Glacier – is one of five glaciers that empty into Icy Bay beneath the towering Saint Elias Mountains. The Yahtse’s rogue advance is one stage in what glaciologists call the “tidewater glacier cycle” – a drama of growth and retreat that unfolds over centuries.
This process of glacial equilibrium can be sparked by changes in climate but then assume a life of its own. It can lead to runaway glacier retreats seen elsewhere in Alaska and Greenland – big contributors to a rising sea.
But as the Yahtse advances, it is also thinning, underscoring the mystery behind exactly how these glaciers change over time. Recent research has fingered the ocean as a trigger for tidewater glacier retreats. Now Bartholomaus and his team are investigating what happens when this advancing glacier meets the ocean in a region where about 54 glaciers empty into the sea.
By anchoring underwater instruments in front of Yahtse Glacier, they hope to see how it melts beneath the surface. Seismometers measure ice quakes; airborne laser surveys show thinning; global-positioning systems and time-lapse photography reveal movement. Never before have so many tools been used on one glacier for a single project. By blending scientific disciplines, the team will paint a picture of the Yahtse’s dynamic forces.
With so many instruments on the Yahtse, researchers have a unique opportunity to monitor changes along the length of the glacier and discover how, for example, local changes in ocean temperature and currents relate to movement further up the glacier.
Understanding the ocean’s influence could help reveal how glaciers around the world are feeding the rising seas.
Motyka, the man at the skiff’s helm, first linked glacier melting and ocean dynamics on Alaska’s LeConte Glacier in 2003. He found that underwater melting was responsible for over half of the ice lost at the terminus – more than the thunderous calving of ice from the glacier’s face. Motyka more recently studied the same effect on Greenland’s Jakobshavn Glacier.
Earlier this summer the Jakobshavn shed a 2.7-square-mile chunk of ice – about twice the size of New York’s Central Park – after a mild winter that saw no ice form in the surrounding bay.
“What we learned [in Alaska] with tidewater glaciers to some degree we can apply to Greenland and vice versa,” said Motyka.
Increasingly, it appears rising ocean temperatures are driving tidewater glacier retreats in Greenland and Antarctica. “I would call that the smoking gun,” Motyka said. “This could indeed be the oceanic process by which [Jakobshavn] became unstable.”
The Gulf of Alaska warmed one degree Celsius between the late 1970s and the mid-2000s, but so far no one has studied its impact. A more recent cooling trend was interrupted by last year’s El Nino.
“It’s a big jump to say that represents everything in the Gulf of Alaska,” Motyka said. “But if it did, that degree warming would be affecting the [tidewater] glaciers regionally.”
The team now hopes to discover how submarine melting is influencing Yahtse Glacier.
“The water end of the glacier can be very influential on how glaciers move, whether they speed up or slow down or advance or retreat,” explained Bartholomaus.
The tidewater glacier cycle describes this state of flux. Yahtse Glacier is now in the advancing stage of the cycle. Typically, this will continue slowly until the glacier finds equilibrium with several factors: its own mass, the shape of the fjord, and lastly, the climate. Once in equilibrium, the glacier will enter a “stable-extended” stage when it becomes more sensitive to climatic changes. If the climate continues to warm, it could trigger a retreat stage, knocking the glacier back from its protective shoal of sediment pushed forward during advance. This allows warmer ocean water to rush in and melt the entire submarine face of the glacier, greatly increasing calving icebergs and potentially resulting in a runaway retreat. This unfolding drama can last for centuries.
Such a retreat created Icy Bay itself. Just 100 years ago, the bay didn’t even exist. All five of its glaciers were merged together into one giant glacier that stretched all the way to the Gulf of Alaska; the little skiff would be buried under hundreds of feet of ice.
When the climate warmed in the late 1800s, it triggered the retreat phase of the tidewater glacier cycle as warm ocean water melted the ice. Since then, the ice has retreated over 25 miles – one of the largest historic retreats in Alaska.
“Once these things start retreating, you just can’t fix it,” explained Chris Larsen, UAF glaciologist and the project’s lead scientist, from his office in Fairbanks. A retreat often creates its own feedback loop, obliterating the climate signal. Even if the climate cooled, a retreat, once started, might not stop anytime soon.
Like the Yahtse, the other Icy Bay glaciers are still in flux, undergoing different phases of the tidewater glacier cycle. The Guyot Glacier is retreating rapidly, losing about 150 vertical feet of surface ice from its terminus every year. Across the bay, the Tyndall Glacier is comparatively stable. With such diverse dynamics in close proximity, it is clear that factors other than climate are at play.
As for the advancing Yahtse, Bartholomaus thinks it is still seeking equilibrium after the ruinous retreat over the last century. The glacier is basically top-heavy, with too much weight up high where heavy snows keep pushing it down and out into the bay. Eventually, it will find balance. At that point, climate will again project more of an influence.
While Yahtse Glacier may be ignoring climate signals for now, Larsen and his colleagues aren’t about to discount the impact of recent warming trends entirely. “Having said all that,” said Larsen, “the current climate could slow down the advance of Yahtse or it could stop it a lot sooner than it would if we didn’t have this warming trend going on right now.”
Out on the bay the next day, the icebergs parted. In a bigger boat, the team motored closer than ever to Yahtse Glacier’s 250-foot-tall terminus face. Falling icebergs thundered into the bay with explosive force. High on the glacier, seismometers recorded the rumble of another ice quake. Seals lounging on icebergs barely seemed to notice.
After six months of preparation, Bartholomaus was ready to set loose the instruments that will record temperature and current changes in the fjord for a full year. Flashing sensors adorned a 200-foot rope with orange buoys on top and a rusty, cast iron woodstove from Icy Bay Lodge as a budget anchor.
Bartholomaus and two others heaved the stove off the side of the boat. It splashed and then plummeted into the dark depths, pulling the blue nylon rope and expensive equipment down with it. Soon the only sign of the mooring was a small, yellow buoy, looking alien in the icy-blue fjord.
His work finally done, Bartholomaus let out a victorious yelp that echoed across the bay. Now he just has to wait while the sensors do the work, and hope he can find the little buoy next year in this ever-changing, ice-age environment.
“I’ll save the champagne for the recovery,” he said.