“Failing to account for subglacial water means global sea-level rise projections are underestimated by up to two metres by 2300,” Dr Chen Zhao, an ARC DECRA Senior Research Fellow at the University of Tasmania, says in a paper just published. This proclamation reflects a nascent worry among climate scientists, specifically the elephant in the room impact on Antarctica’s ice sheet instability. Water tucked under you as you stand kilometers deep in ice, and that, more than anyone had thought before, underpins the ebb and flow of ice, and, under future warming, when the ocean around it has its temperature cranked up, could run into it three times faster than ice can, washing three times as much of it away.
The Antarctic Ice Sheet, covering an area of ~14 million km2 and the largest reservoir of freshwater ice on Earth, has long been identified as an important player in global sea-level change. But hidden underneath its frigid exterior is an intricate network of subglacial lakes and streams that has tremendous implications for how the ice moves. These water systems, which are generated by geothermal heat and melting due to friction, act as lubricants that aid the ice in sliding more quickly toward the ocean. The implications are staggering: If those subglacial water dynamics are overlooked in predictive models, tipping points for irreversible ice loss could come decades sooner than scientists had hoped.
The new experimental and modeling studies combine new hydrology and ice sheet models to demonstrate that a variable subglacial water pressure serves to promote basal sliding and induce grounding-line retreat. One of the significant features in ice sheet dynamics, the grounding line (the boundary where floating ice meets grounded ice in contact with the bedrock or land) may also be particularly sensitive to perturbations in effective pressure (a combination of the ice pressure and the water pressure) [36] because grounding lines can be in a state of mechanical instability [37]. As Zhao et al. (1) study and determine: “Including the effects of evolving subglacial water in ice sheet models can triple the amount of ice flowing to the ocean.” Their findings, published in Nature Communications, imply that these dynamics could contribute an extra two meters of sea-level rise by 2300 figures that are substantially higher than what most previous research has concluded.
One quantity that matters critically is the effective pressure the pressure due to the overlying ice minus the pressure of subglacial water. Using simulations over a wide variety of sliding relations, we numerically demonstrate that an effective pressure RC law produces higher rates of ice discharge than does the classical linear Weertman (LW) law. For RC models (which assume implicit effective pressure and height-above-flotation scaling (or HAF)), and under a high-emissions scenario at 2300, grounding-line fluxes were predicted to approach > 8000 gigatons per year an almost fourfold escalation of that seen in LW models. This kind of cumulative rise in ice discharge was precisely what Zhao was describing when he darkly said: ”Failing to account for subglacial water means global sea-level rise projections are underestimated.”
One region of special concern is West Antarctica, where floating marine ice sheets such as the Pine Island and Thwaites Glaciers are subject to loss of stability. Thus, GD were vulnerable to, Marine Ice Sheet Instability (MISI) (Goodman et al., 2023; 2024), and so to retrograde, below-sealevel grounded glaciers (MISI). As grounding lines retreat into jowly, deep basins, ice accumulates and hence acts as a positive feedback, increasing melting leads to increased retreat. For example, the RC_iN_HAF simulations share a conservative prediction with only 60 (20-120) kilometers of retreat by 2300 while the Thwaites Glacier grounding line retreat beyond 300 kilometers inland (thermodynamic fluxes will be released through the flow models) models show less. The Pritchard et al. of paper (currently in press) have also shown that the rapid ice loss across Hawaiian glaciers takes place before 2095, under RC models, and several decades earlier than similar standard current LW-force models.
The ramifications extend well beyond West Antarctica. For the Wilkes Subglacial Basin in East Antarctica, effective pressure tipping points are therefore forecast to occur (as opposed to model known-transdicious- the rubbish drop to that constant pressure) nearly a century early However, while subglacial hydrology remains less responsive in this case, the grounding line fluxes in the Aurora Subglacial Basin (where more and deeper ice is present) are higher with Davidson glacier in an intact subglacial hydrology. These outcomes are in accord with independent modelling indicating ice-hydrology interactions yield sea-level contributions 30% higher than ice-only models (Williams et al., 2012).
Better have predictions conditional on the near-grounding line functional pressure spatial structure. Subglacial water is released to the ocean where low-pressure density “bubbles” of turbulent ice flow can assume geostrophic balance. In these low-basal-resistance Coulomb regimes, grounding lines migrate landward, increasing mass loss during ice sheet retreat. This lack of constraint presents intuitively the possibility that the global extent of such low-drag areas (which could be scaled by the limit of height-above-flotation, hT) is underdetermined in nature and thus caused concerns about the reliability of existing modelling approaches.
But these results highlight the need to explicitly represent subglacial water dynamics in ice sheet models there are observational hurdles. Ice-penetrating radar and laser altimetry provide tantalizing insights, but many places in Antarctica remain mysteries. “More observations are needed to improve our models, particularly from remote regions such as East Antarctica.” Zhao says.
The stakes are high. A rise in sea levels of two meters would put coastal cities in peril, meaning the involuntary relocation of many tens of millions of people and economic damage in the tens of trillions of dollars. And the time frame of tipping points some expected to cross around 2050 implies irreversible iceberg loss could occur within the lifetime of people alive now. “Understanding this hidden threat is crucial as the world grapples with the consequences of global warming especially rising seas.” Zhao concludes.
This further challenges those working in climate science, glaciology, and environmental research to refine projections and the underlying assumptions about subglacial water systems. As the Antarctic Ice Sheet teeters on the edge of instability, the need to shed light on its secret mechanics becomes all the more urgent.
