An underwater glacier in Alaska is melting 100 times faster than previously predicted because experts had been overlooking the importance of ambient melt.
Current theoretical models had mainly focused on discharge-driven melt, in which plumes of meltwater cause localised melting along the glacier face.
Researchers identified the unexpectedly high melting rate using a new sonar technique which enabled them to study the submerged edge of a glacier directly.
An underwater glacier in Alaska is melting 100 times faster than previously predicted because experts had been overlooking the importance of ambient melt
Oceanographer Dave Sutherland of the University of Oregon and colleagues studied the underwater melting of the LeConte Glacier, which lies south of Juneau in Alaska.
To map the submerged edge of the glacier, the team used a multibeam sonar scan deployed from a fishing vessel six times in August 2016 and five times in May 2017.
Researchers also collected data on the temperature, salinity and velocity of the water downstream from the glacier, in order to estimate the meltwater discharge.
From this, the team were able to look for changes in melt patterns between the August and May measurement periods.
‘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,’ said paper author and oceanographer Rebecca Professor Jackson.
‘These two sets of measurements show that melt rates are significantly, sometimes up to a factor of 100, higher than existing theory would predict.’
Melt rates were high in both seasons, but increased from spring to summer, the researchers reported.
Current theoretical models had mainly focused on discharge-driven melt, in which plumes of meltwater cause localised melting along the glacier face
The advantage of the sonar technique is that it enables analysis of vertical-faced glaciers that terminate at the ocean, which cannot be studied using the conventional technique of boring through ice shelves to directly reach the ocean-ice interface.
‘We don’t have that platform to be able to access the ice in this way,’ said Professor Sutherland.
‘Tidewater glaciers are always calving and moving very rapidly, and you don’t want to take a boat up there too closely.’
Furthermore, previous research into underwater glacier melting had relied on measuring conditions near glaciers and then using this to predict melt rates based on existing theory that had never been directly tested.
‘This theory is used widely in our field,’ said Professor Jackson.
‘It’s used in glacier models to study questions like: how will the glacier respond if the ocean warms by one or two degrees?’
Researchers identified the unexpectedly high melting rate using a new sonar technique which enabled them to study the submerged edge of a glacier directly
According to the researchers, the reason that the theoretical predictions are underestimating the rate of glacial melt stems from the fact that one form of glacial melt is being largely overlooked.
Models are typically focused on discharge-driven melt, in which large volumes — or plumes — of buoyant meltwater are released from below the glacier.
This plume combines with the surrounding water, picking up speed and volume as it rises up against the glacial face, helping to eat away at the ice before it slowly diffuses away into the surrounding water.
However, Professor Sutherland argued, these discharged plume typically impacts only a small portion of the glacier’s face.
In contrast, ambient melt — although conventionally thought to be 10–100 times weaker and therefore insignificant — in reality works against the whole glacial face.
According to Professor Jackson, the sonar-based measurement approach can be applied to other glaciers in the future and would allow projections of future sea level rise to be made more accurate.
‘Future sea level rise is primarily determined by how much ice is stored in these ice sheets,’ Professor Sutherland said.
‘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 modelling, we have to know more about where melting occurs and the feedbacks involved.’
The full findings of the study were published in the journal Science.
Oceanographer Dave Sutherland of the University of Oregon and colleagues studied the underwater melting of the LeConte Glacier, which lies south of Juneau in Alaska