Global warming could be causing glaciers to melt underwater 100 times faster than previously estimated
Glaciers beneath the ocean’s surface are melting 100 times faster than previously forecasted by scientists. The finding is significant as ice loss from the world’s glaciers and ice sheets — mostly due to global warming — contributes to a rise in sea levels, influences ocean circulation, and can impact ecosystem productivity. The results, according to University of Oregon oceanographer Dave Sutherland, highlight an urgent need to reevaluate existing models of tidewater glacier ice loss.
The question — how fast does warm ocean water melt glaciers that end in the sea — is central to understanding how fast ice sheets may lose mass, and thus how fast sea level will rise, in response to global warming. However, there is not much data on the subject.
Led by Dr. Sutherland, the research team studied the subsurface melting of the LeConte Glacier (see below video made from time-lapse photos by Jason Amundson, University of Alaska Southeast), which flows into LeConte Bay south of Juneau, Alaska. They made the first direct measurement of the submarine melt rate of the tidewater glacier in Alaska and concluded that theoretical models are underestimating the glacial melt by up to two orders of magnitude. The team observed melt rates up to a hundred times larger than those predicted by theory. The study, which could lead to improved forecasting of climate-driven sea-level rise, has been published in Science.
"Many glaciers around the globe – in Greenland, Alaska, and elsewhere – are losing mass and causing sea-level rise. There is a hypothesis that changes in submarine melting might be triggering the acceleration, thinning, and retreat of glaciers, which contributes to ice loss. The idea behind this hypothesis is that changes in the submarine melt rate might cause more icebergs to calve into the ocean and generally destabilize the glacier. This means changes in submarine melting might trigger changes in how fast the glaciers flow into the ocean," Dr. Sutherland told MEA WorldWide (MEAWW).
"This hypothesis (of the ocean being a trigger for glacier changes) is well supported by evidence from Antarctic glaciers that terminate in ice shelves. But this hypothesis is really an open question for tidewater glaciers like the one we have studied," he added.
"We think the ocean might be playing a role in driving the retreat of tidewater glacier retreat around Greenland, Alaska, and elsewhere, but this hypothesis has been difficult to test because there have been no direct measurements of the melt rates, let alone how they might vary in time. In this study, we have directly measured the melt rate at LeConte Glacier with novel methods, and the melt rates are much higher than expected," Dr. Sutherland pointed out.
He added, "We have made a critical step towards understanding the interaction between the ocean and glacier by directly measuring the melt rates at present and showing that our current theory for melting is inaccurate."
What are tidewater glaciers?
Tidewater glaciers are valley glaciers that flow down to the ocean. Like vast frozen rivers, tidewater glaciers flow from the land and into the sea, forming a partially submerged ice-ocean boundary. However, unlike terrestrial glaciers, which are often sequestered to high-altitudes, tidewater glaciers can be far more dynamic and subject to ongoing changes driven by underwater melting and iceberg calving where the ocean meets the ice.
"Tidewater, or marine-terminating, glaciers are the literal ice-ocean boundary in high-latitude environments, connecting the oceans to the continental ice sheets that cover Greenland and Antarctica, as well as smaller ice caps and ice fields such as those found in Alaska. The dynamics of these glaciers influence the rates of ice mass loss and global sea-level rise," stated the study.
However, though it is widely recognized that ice loss from these glaciers influences both the rate of sea-level rise and potentially global ocean circulation — a primary driver of global climate — the understanding of the dynamics of tidewater glacier melt, particularly as a response to accelerated warming, is largely forecasted based on very little data, indirect inferences and an unconstrained theoretical model of subsurface melting, said the team.
"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 this theory had never been directly tested," they added.
What technique did the research team use?
To address the lack of direct measurements, the research team comprising oceanographers and glaciologists used a multibeam 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 enabled them to image and profile large swaths of the underwater ice, where the glacier drains from the Stikine Icefield. Data on the temperature, salinity, and velocity of the water downstream from the glacier was collected, which allowed the researchers to estimate the meltwater flow.
The researchers also looked for changes in melt patterns that happened during the study period. "The sonar images were used to document and create a time-variable, three-dimensional record of changes in the glacier face that could be linked to melting and calving patterns," stated the study.
The team discovered seasonally increasing submarine melt across the glacier face and at rates far greater than theory-based predictions. They also found that submarine melt rates were high across the glacier's face over both the seasons surveyed and that the melt rate increased from spring to summer.
"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," said Dr. Rebecca Jackson, assistant professor, Rutgers Department of Marine and Coastal Sciences.
Earlier models ignore a critical aspect, say researchers
Current theoretical models, according to the researchers, have been ignoring a critical factor called "ambient" melt. "There are two main categories of glacial melt: discharge-driven and ambient melt. Subglacial discharge occurs 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," stated the finding.
The researchers explained that most previous studies on ice-ocean interactions focused on these discharge plumes. The plumes, however, typically affect only a narrow area of the glacier face, while ambient melt instead covers the rest of the glacier face. "Predictions have estimated ambient melt to be 10-100 times less than the discharge melt, and, as such, it is often disregarded as insignificant," elaborated Dr. Sutherland.
According to the team, while the study focused on one marine-terminating glacier, the new approach should be useful to researchers who are studying melt rates of other glaciers, and it would help to improve projections of global sea-level rise. "Ultimately we want to understand why the melt rates are so much higher than expected, and if we can understand the processes that control the melt rates — that is, how melting depends on ocean conditions, glacier plumes, etc. — then we can work towards understanding how melt rates will change in time (for example, with ocean warming) and how that might affect glaciers and sea-level rise," he shared with MEAWW.
Ice loss is accelerating from Antarctica, Greenland, and mountain glaciers around the globe. Changes in glaciers and icecaps, and glacial melt from the Greenland and Antarctica Ice Sheets have been identified as significant factors that contribute to sea-level rise. Future sea-level rise is primarily determined by how much ice is stored in these ice sheets. Experts further said that it is critical to calculate the correct amount of melt to plan for sea-level rise.
The study’s results, the team said, suggest that some glaciers may be in "hotter water" than previously thought. Accordingly, if existing theories are grossly underestimating underwater melting, it could lead to devastating consequences. "The fact that we show the existing theory is wildly inaccurate at one glacier — the only glacier where we can make a robust comparison between theory and observations — should lead us to be very skeptical of its current use in studying any tidewater glacier," added Dr. Jackson.