Alaska's LeConte Glacier is melting underwater at rates nearly a hundred times greater than what is currently estimated, according to a detailed sonar survey of the glacier's submerged surfaces.
The newly observed melt rates are up to two orders of magnitude greater than those calculated by some current predictive models. The findings, published in the July 26 issue of Science, are the first based on direct subsurface measurements of a tidewater glacier and suggest that similar glaciers worldwide may be in far "hotter water" than previously known.
In Earth's Arctic region, vast rivers of ice flow slowly from the ice sheets and ice fields that blanket frozen high-latitude landscapes. Some glaciers are terrestrially bound and often sequestered among high-altitude mountain peaks from the alpine valleys below. Those that meet the sea, however, are called tidewater glaciers. As these bodies of ice push outward into the ocean, their precipitous semi-submerged fronts crumble, or calve, to release a near-constant stream of icebergs into the ocean's currents beyond.
Tidewater glaciers are most common in polar regions, like Greenland, Antarctica and coastal Alaska, where they form the literal boundary between the ocean and the planet's immense reservoirs of frozen freshwater. The nexus between ice and sea makes tidewater glaciers especially influential on the rates of overall ice mass loss and global sea level rise. Furthermore, the large quantities of fresh glacial meltwater discharged into the salty sea have the potential to destabilize or alter global ocean circulation, which is a primary driver of global climate regimes.
Like all of Earth's ice, tidewater glaciers are melting at unprecedented rates, but exact rates are a challenge to ascertain. Unlike alpine glaciers, tidewater glaciers are highly dynamic and can undergo rapid advances and retreats independent of climate changes. Much of the melt attributable to overall ice loss is difficult to measure and occurs either as increased calving or beneath the water. What is known about the dynamics of tidewater glacier melt is predicated upon sparse data and indirect inferences that inform theoretical models of subsurface melt.
Understanding the unique dynamics of tidewater glaciers, particularly as a response to the accelerated warming occurring in high-latitude environments, requires direct observations. However before the survey no direct measurements of underwater melting at tidewater glacier fronts have been made, said David Sutherland, lead author of the study and University of Oregon oceanographer
To address the need for direct submarine melt observations, Sutherland and a team of researchers conducted comprehensive field observations at LeConte Glacier in Southeast Alaska. LeConte, the southernmost tidewater glacier in the Northern Hemisphere, flows from the Stikine Icefield in Southeast Alaska for 21 miles before spilling into a narrow fjord at the edge of the Pacific Ocean. In spite of its size, LeConte's ice flows quite quickly — at a clip of nearly 25 meters per day, or three feet an hour.
"Working in front of a calving ice front is extremely challenging, as you have to balance safety of personnel and expensive instruments with collecting the data you need," said Sutherland.
"It turns out it's really difficult to get a ship in the right place along with all the other instruments you need," he said, which helps explain why the study's results are the first of their kind.
By boat and amid crashing chunks of constantly calving ice, Sutherland and the team of researchers tracked back and forth across the fjord while conducting repeat multibeam sonar surveys of the submerged vertical face of LeConte Glacier. Their goal was to see if the method could be used to directly observe submarine ice melt through changes in the shape of the glacier's front over time.
Multibeam sonar is typically used to survey depths and map the ocean floor from above, however, the researchers instead tipped the instrument on its side and pointed it at the face of the glacier to image the usually hidden surface of the submerged ice in unprecedented detail.
Constantly shifting ice and ocean conditions necessitated many overlapping and carefully coordinated measurements from both land- and ship-based teams and included a variety of other ocean, ice and atmospheric observations.
Once the data were combined, Sutherland and colleagues created a comprehensive time-variable and three-dimensional record of melting and calving patterns based on direct observations at LeConte in August 2016 and May 2017.
"Beyond proving that [the method] is doable, we found that melt rates over most of the glacier were extremely high compared to those predicted by theory," said Sutherland, and that they changed seasonally, increasing from spring to summer. For example, melt rates based on these direct observations suggest melt rates of more than eight meters per day in August, whereas theoretical models predict a single meter of daily change.
Based on the physics of ice melt along the ice-ocean interface, Sutherland is confident that similar submarine melt is occurring elsewhere, particularly at the far larger tidewater glaciers in Greenland or west Antarctic Peninsula.
"The hope is these observations provide new constraints on melt rates at glacier termini that will ultimately improve our predictions of sea level rise and the rate in which it's likely to happen," said Sutherland.
Sutherland noted that the findings would have been impossible if it wasn't for the Alaskan collaborators, which included researchers from the University of Alaska as well as members of the local community of Petersburg.
According to Sutherland, the University of Alaska is a world leader in Arctic oceanography and glaciology; however, the state's "draconian" 41% cut to the University system's total funding puts future Arctic research at risk in Alaska.
"What our research at LeConte shows is the tragedy that would be," said Sutherland. "Alaska has all these amazing regions in its backyard where we can study globally important physical processes."