Scientists from the University of Oregon recently found that the underwater section of a glacier in southeast Alaska is melting at rates up to two orders of magnitude greater than those predicted by theory. The results, published in the journal Science, challenge the current models used to predict the melting of tidewater glaciers worldwide.
Tidewater glaciers play an important role in maintaining glacier stability, and their melting is accelerating overall ice loss in Greenland and in Antarctica. According to the study, no one has yet directly measured the melting of the underwater portion of a tidewater glacier. Instead, scientists have relied on untested theoretical models.
Glaciers that terminate in the ocean come in two forms: Ice shelves, which are horizontal slabs of ice that extend into the ocean, and tidewater glaciers, which end in relatively vertical ice faces, Rebecca Jackson, one of the study’s authors, told GlacierHub.
Jackson explained that observing tidewater glaciers from below is difficult. Ice shelves retreat horizontally, and this can be seen by satellites and other remote sensing techniques. But the horizontal retreat of a tidewater glacier’s vertical face is too slight to be detected with remote sensing.
With funding from the National Science Foundation, the University of Oregon scientists used a new method to directly observe tidewater glacier melt: creating and comparing sonar images of the glacier over time. They studied the melting of Alaska’s LeConte Glacier, mainly because of its accessibility, Dave Sutherland, the lead author of the study, told GlacierHub. The physics of the interactions between glacier and ocean at LeConte are the same as in other systems around the world, including tidewater glaciers in Greenland, Patagonia, and the west Antarctic, he said.
They made observations at the glacier six times from 2016 to 2017, Sutherland told GlacierHub.
The results were striking. “We have direct observations that show melt rates are much higher than we we expected,” Jackson told GlacierHub.
This doesn’t change the amount of glacier ice currently being lost to the ocean, she explained. “Right now we know, to a pretty good degree, how much glaciers are losing ice and raising sea level,” Jackson said. “That’s a pretty well-documented quantity and our results don’t change that.”
Instead, the study illuminates what portion of the glacier ice being lost to the ocean is the result of underwater ice melting as opposed to calving—the process by which chunks of the glacier break off and float into the ocean as icebergs. “The sub-marine melt rates are higher than we expect, which means that the amount calving off is slightly less,” said Jackson.
That means that the ocean is playing a larger role than expected in the loss of water-terminating glacial ice, Sutherland told GlacierHub.
With a warming ocean, this news suggests that tidewater glaciers could disappear quicker in response to climate change than previously thought, Jackson explained. “There’s a hypothesis that ocean warming can increase submarine melting and then that triggers a glacier acceleration that deposits more ice overall into the ocean,” she said.
In other words, if the portion of glacier submerged in ocean water melts quicker, then the rate at which the glacier flows toward the ocean will increase, and the rate of calving will increase as well.
“You could imagine that if sub-marine melting was depth-dependent, you could undercut the glacier and destabilize it, leading to increased calving,” Sutherland told GlacierHub. And indeed their study found that the melt rates of the glacier were depth-dependent.
“Ultimately what we want to be able to do is start answering questions like, if the ocean warms by one or two degrees, how will that affect the glacier?” said Jackson. And in order to do that, the faulty model must be replaced by a more accurate theory. The same team of scientists is currently working on that new model, Jackson said.
Although the Science study does not address why the previous theory might be incorrect, the scientists involved have a hypothesis, Jackson said. The velocity of the water touching a tidewater glacier face affects its melting rate: A higher velocity means a higher melt rate, just as pouring hot coffee over an ice cube melts the ice cube faster than placing it into the hot coffee. The current theory takes into account how freshwater that seeps out from the bottom of the glacier increases the water velocity as it rises to the top of ocean along the surface of the ice. However, it doesn’t take into account other drivers of ocean currents near the glacier, including wind, tide, and waves. “Those can also enhance the velocity along the ocean ice boundary, and that can also enhance melt rate,” Jackson told GlacierHub.
Collecting data from glaciers with different fjord conditions and glacier characteristics will provide the scientists with the information needed to model tidewater glacier melt as a function of the physical properties of the glacier and adjoining ocean. Since ocean conditions vary from season to season, the team is continuing to collect data at LeConte Glacier throughout the year, with the same goal of discovering how oceanic properties affect glacier melt. “One thing we’re excited about is what we present is the new method for directly measuring sub-marine melting that hopefully can be used at many other glaciers around the globe,” said Jackson.
In the mean time, estimates of sea level rise might need adjustment.