Meltwater flowing beneath Antarctic glaciers is making them lose ice faster – and could lead to even higher sea level rises than predicted, warns a new study.
American researchers using a new technique to map Antarctic ice sheets say meltwater flowing underneath glaciers is making them lose ice faster.
Climate scientists warn that this meltwater was not accounted for in current models predicting major sea-level rises – meaning already worrying projections could be underestimating how much levels will rise.
The authors of the study, published in the journal Science Advances, warn that the two glaciers focused on could singlehandedly contribute to rises of 1.5 meters (5 feet) by the year 2300.
The researchers also warned the climate community that their study served as a ‘wake-up call’ for the modeling community, which could be underestimating sea-level rises which could destroy coastal communities in the coming decades.
The study, from climate scientists at the University of California (UC) San Diego’s Scripps Institution of Oceanography, modeled the retreat of two glaciers in eastern Antarctica through to the year 2300 under different emissions scenarios; projecting their contributions to sea-level rises.
Unlike previous Antarctic ice sheet models, this latest study included the influence of the flow of meltwater from beneath glaciers out to sea, known as subglacial discharge.
The two highlighted glaciers – named Denman and Scott – together hold enough ice to cause a near 1.5-meter rise in global sea levels.
In a high emissions scenario – such as the Intergovernmental Panel on Climate Change’s (IPCC) SSP5-8.5 scenario, which assumes no new climate policy and features 20 percent higher CO2 emissions by 2100 – the researchers’ model found glacial meltwater increased sea-level rise contribution of these glaciers by 15.7 percent, from 19 millimeters (0.74 inches) to 22 millimeters (0.86 inches) by the year 2300.
The neighboring glaciers sit atop a continental trench more than two miles deep, and, once their retreat reaches the steep slope of the trench, the researchers expect their contribution to sea-level rises to accelerate dramatically.
Subglacial meltwater is generated from melting which occurs where the ice sits on its continental bedrock.
The main sources of heat melting the glacial ice are friction from the ice grinding across the bedrock and geothermal heat from the Earth’s core permeating through the crust.
With the added influence of the meltwater, the researchers’ model found the glaciers retreated past this threshold about 25 years earlier than they did without.
Dr. Jamin Greenbaum, co-author of the study and a researcher at Scripps’ Institute of Geophysics and Planetary Physics, warned: “I think this paper is a wake-up call for the modeling community.
“It shows you can’t accurately model these systems without taking this process into account.”
Other than the understudied – and understated – role of subglacial discharge in accelerating sea-level rise, another key takeaway from the study is the importance of what we do in the coming decades to rein in greenhouse gas emissions, Dr. Greenbaum says.
When running the low emissions scenarios of the model, the glaciers were not found to retreat all the way into the trench and avoided the resulting runaway contributions to sea-level rise.
“If there is a doomsday story here,” Dr. Greenbaum explained, “it isn’t subglacial discharge.
“The real doomsday story is still emissions, and humanity is still the one with its finger on the button.”
Study lead author Dr. Tyler Pelle, a postdoctoral researcher at Scripps, explained that accurate modeling was also crucial to warning the millions of people living in coastal areas.
“Knowing when and how much global sea levels will rise is critical to the welfare of coastal communities,” Dr. Pelle said.
“Millions of people live in low-lying coastal zones and we can’t adequately prepare our communities without accurate sea-level rise projections.”
Previous studies have suggested subglacial meltwater is a common feature of glaciers around the world, present under several other massive Antarctic glaciers including the huge Thwaites Glacier in West Antarctica – which is around 14 times the size of Great Britain, or the size of India.
When glacial meltwater flows out to sea, it is believed to accelerate the melting of the glacier’s ice shelf – a long, floating tongue of ice that extends out to the sea beyond the last part of the glacier still in contact with solid ground, known as the ‘grounding line’.
It speeds up ice shelf melting and glacial retreat, the researchers say, by causing ocean mixing that stirs in additional ocean heat within the cavity beneath a glacier’s floating ice shelf.
This enhanced melting then causes the upstream glacier to accelerate, driving rises in sea levels.
Dr. Greenbaum says the belief that subglacial discharge causes additional ice shelf melting is one widely accepted across the scientific community.
However, it has thus far been omitted from sea-level rise projections, as many scientists were unsure if the effect of the process was large enough to cause sea levels to rise.
Dr. Pelle says subglacial discharge first entered his radar in 2021, when he and his colleagues observed that East Antarctica’s Denman Glacier’s ice shelf was melting faster than expected, given local ocean temperatures.
Bizarrely, Denman’s neighbor Scott Glacier’s ice shelf was melting much slower, despite virtually identical ocean conditions.
To test whether glacial melting could reconcile the melt rates seen at the Denman and Scott ice shelves – as well as whether subglacial meltwater might accelerate sea-level rise – the research team combined models for three different environments: the ice sheet, the space between the ice sheet and bedrock, and the ocean.
Once married, the researchers ran a series of projections up to the end of the 23rd Century using a NASA supercomputer.
The projections featured three main scenarios: a control that featured no additional ocean warming, a low emissions pathway (SSP1-2.6), and a high emissions pathway (SSP5-8.5).
For each scenario, the researchers created projections with and without the effect of present-day levels of glacial melting.
The model’s simulations revealed that adding in subglacial discharge reconciled the melt rates seen at Denman and Scott Glaciers.
As for why Scott Glacier was melting so much slower than Denman, Dr. Pelle explained “[The model showed] a strong subglacial discharge channel drained across the Denman Glacier grounding line, while a weaker discharge channel drained across the Scott Glacier grounding line.”
The strength of the discharge channel at Denman, he added, was behind its speedy melt.
For the control and low-emissions model runs, the contributions to sea-level rise were close to zero or even slightly negative with or without subglacial discharge at 2300.
However, in a high emissions scenario, the model found subglacial discharge increased the sea-level rise contribution of these glaciers from 19 millimeters (0.74 inches) to 22 millimeters (0.86 inches) in 2300.
In the high emissions scenario that included subglacial discharge, the two glaciers retreated into the two-mile-deep trench beneath them by 2240 – around 25 years earlier than they did in the model runs without subglacial discharge.
Once the grounding lines of the Denman and Scott Glaciers retreat past the lip of the trench, their annual sea-level rise contribution exploded, reaching a peak of 0.33 millimeters (0.01 inches) per year – around half of the present-day annual sea-level rise contribution of the entire Antarctic ice sheet.
The scientists said the trench’s steep slope is behind this explosive increase in sea-level rise contribution: as the glacier retreats downslope, its ice shelf begins losing thicker and thicker slabs of ice from its leading edge.
The process of ice loss quickly outpaces ice accumulation at the ice sheet’s interior, causing further glacial retreat.
Researchers refer to topography such as the trench beneath Denman and Scott Glaciers as a ‘retrograde slope’, and worry that it creates a positive feedback loop by which glacial retreat begets more retreat.
Large areas of the West Antarctic Ice Sheet, such as the gigantic Thwaites Glacier, also have retrograde slopes that, whilst not as dramatic as the Denman-Scott trench, contribute to fears of broader ice sheet instability.
“Subglacial meltwater has been inferred beneath most if not all Antarctic glaciers, including Thwaites, Pine Island, and Totten glaciers,” Dr. Pelle said.
“All these glaciers are retreating and contributing to sea-level rise and we are showing that subglacial discharge could be accelerating their retreat.
“It’s urgent that we model these other glaciers so we can get a handle on the magnitude of the effect subglacial discharge is having.”
Practicing what they preach, the UC San Diego team is currently in the process of submitting a research proposal to extend their new model to the entire Antarctic ice sheet.
Future versions of the model may also attempt to couple the subglacial environment with the ice sheet and ocean models so that the amount of subglacial meltwater dynamically responds to these other factors.
Greenbaum says the current version of their model kept the amount of subglacial meltwater constant throughout the model runs, and that making it respond dynamically to the surrounding environment would likely make the model more true to life.
“This also means that our results are probably a conservative estimate of the effect of subglacial discharge,” Dr. Greenbaum said.
“That said, we can’t yet say how much sea-level rise will be accelerated by this process – hopefully it’s not too much.”
Part of Dr. Greenbaum’s upcoming fieldwork in Antarctica – supported by the National Science Foundation (NSF) and NASA – aims to directly investigate the impacts of subglacial meltwater in both the East and West Antarctic ice sheets.
In collaboration with the Australian Antarctic Division and the Korea Polar Research Institute, Dr. Greenbaum and his collaborators will be visiting the ice shelves of Denman and Thwaites Glaciers in East and West Antarctica, respectively, looking for direct evidence that subglacial freshwater is discharging into the ocean beneath the glaciers’ ice shelves and contributing to warming.
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