Underwater glacial melting is happening far faster than previously forecast, according to a pioneering new study.
Researchers developed a new method to allow for the first direct measurement of the submarine melt rate of a tidewater glacier.
And they found that current theoretical models may be dramatically underestimating glacial melt rates.
A team of American scientists, led by oceanographer Professor Dave Sutherland, studied the subsurface melting of the LeConte Glacier, which flows into LeConte Bay south of Juneau in Alaska.
Direct melting measurements previously have been made on ice shelves in Antarctica by boring through to the ice-ocean interface beneath.
But in the case of vertical-face glaciers terminating at the ocean, those techniques are not available.
Prof Sutherland, of the University of Oregon, said: “We don’t have that platform to be able to access the ice in this way.
“Tidewater glaciers are always calving and moving very rapidly, and you don’t want to take a boat up there too closely.”
He explained that most previous research on the underwater melting of glaciers relied on theoretical modeling, measuring conditions near the glaciers and then applying theory to predict melt rates.
But that theory had never been directly tested.
Study co-author Dr Rebecca Jackson, an oceanographer at Rutgers University, said: “This theory is used widely in our field.
“It’s used in glacier models to study questions like: how will the glacier respond if the ocean warms by one or two degrees?”
To test the models in the field, the research team deployed a multi-beam sonar to scan the glacier’s ocean-ice interface from a fishing vessel six times in August 2016 and five times in May 2017.
The sonar allowed the team to image and profile large swathes of the underwater ice, where the glacier drains from the Stikine Icefield.
They also gathered data on the temperature, salinity and velocity of the water downstream from the glacier, which allowed them to estimate the meltwater flow.
The research team then looked for changes in melt patterns that occurred between the August and May measurements.
Dr Jackson said: “We measured both the ocean properties in front of the glacier and the melt rates, and we found that they are not related in the way we expected.
“These two sets of measurements show that melt rates are significantly, sometimes up to a factor of 100, higher than existing theory would predict.”
She said that there are two main categories of glacial melt: discharge-driven and ambient melt.
Subglacial discharge happens when large volumes, or plumes, of buoyant meltwater are released below the glacier. The plume combines with surrounding water to pick up speed and volume as it rises up swiftly against the glacial face.
The current steadily eats away from the glacier face, undercutting the glacier before eventually diffusing into the surrounding waters.
Most previous studies of ice-ocean interactions have focused on discharge plumes.
But the plumes typically affect only a narrow area of the glacier face, while ambient melt instead covers the rest of the glacier face.
Prof Sutherland said that predictions have estimated ambient melt to be 10 to 100 times less than the discharge melt, and, as such, it is often disregarded as insignificant.
The research team found that submarine melt rates were high across the glacier’s face over both of the seasons surveyed, and that the melt rate increases from spring to summer.
While the study focused on one marine-terminating glacier, Dr Jackson said that the new approach should be useful to any researchers who are studying melt rates at other glaciers which would help to improve projections of global sea level rise.
Prof Sutherland added: “Future sea level rise is primarily determined by how much ice is stored in these ice sheets.
“We are focusing on the ocean-ice interfaces, because that’s where the extra melt and ice is coming from that controls how fast ice is lost.
“To improve the modeling, we have to know more about where melting occurs and the feedbacks involved.”
The findings were published in the journal Science.