Meltwater beneath Antarctic glaciers accelerates retreat

2023-10-30 12:22:06

A view of the ice tongue of the Denman Glacier in East Antarctica. – JAMIN S. GREENBAUM

MADRID, 30 Oct. (EUROPA PRESS) –

Meltwater flowing into the sea from beneath the Antarctic glaciers is causing them to lose ice more quickly.

It is the result of a new modeling study of the Antarctic ice sheet carried out by scientists from the Scripps Institute of Oceanography, University of California, San Diego.

Model simulations suggest that this effect is large enough to contribute significantly to sea level rise under high greenhouse gas emissions scenarios, the researchers report in the journal ‘Science Advances’.

Additional ice loss caused by meltwater flowing to the sea from beneath Antarctic glaciers is currently not accounted for in models that generate major sea level rise projections, such as those from the Intergovernmental Panel on Climate Change (IPCC). If this process turns out to be a major driver of ice loss across the Antarctic ice sheet, it could mean that current projections underestimate the pace of global sea level rise in the coming decades.

“Knowing when and how much sea level will rise is critical to the well-being of coastal communities,” says it’s a statement Tyler Pelle, lead author of the study and a postdoctoral researcher at Scripps. “Millions of people live in low-lying coastal areas and we cannot adequately prepare our communities without accurate projections of sea level rise.”

The study, funded by the National Science Foundation (NSF), NASA, and the Cecil H. and the Ida M. Green Foundation for Earth Sciences at the Scripps Institute for Geophysics and Planetary Physics, modeled the retreat of two East Antarctic glaciers until the year 2300 under different emissions scenarios and projected their contributions to sea level rise. Unlike previous models of the Antarctic ice sheet, this one included the influence of this flow of meltwater from beneath the glaciers towards the sea, what is known as subglacial discharge.

The two glaciers the study focused on, Denman and Scott, contain enough ice to cause sea level rise of nearly 1.5 meters. In a high emissions scenario (the IPCC SSP5-8.5 scenario, which assumes no new climate policy and a 20% increase in CO2 emissions by 2100), the model revealed that subglacial discharge increased the contribution of these glaciers to sea level rise by 15.7%, from 19 millimeters to 22 millimeters by the year 2300.

These glaciers, which are next to each other, sit on a continental trench more than three kilometers deep; Once its retreat reaches the steep slope of the trench, its contribution to sea level rise is expected to accelerate dramatically. With the added influence of subglacial discharge, the model found that glaciers retreated beyond this threshold about 25 years earlier than they did without it.

In addition to the understudied role of subglacial discharge in accelerating sea level rise, Greenbaum points to the importance of what humanity does in the coming decades to curb greenhouse gas emissions. Model runs in a low-emissions scenario did not show glaciers retreating into the trench. and avoided the resulting runaway contributions to sea level rise.

In Antarctica, subglacial meltwater is generated from melting that occurs where ice sits on continental bedrock. The main sources of heat that melt ice in contact with the ground are the friction of ice on bedrock and geothermal heat from the Earth’s interior that penetrates through the crust.

Previous research suggested that subglacial meltwater is a common feature of glaciers around the world and that it is present under several other massive Antarctic glaciers, including the infamous Thwaites Glacier in West Antarctica.

When subglacial discharge flows seaward, it is thought to accelerate the melting of the glacier’s ice shelf, a long tongue of floating ice that extends seaward beyond the last part of the glacier still in contact with land. (known as the ground line). Subglacial discharge is believed to accelerate ice formation.

According to Greenbaum, the idea that subglacial discharge causes further melting of ice shelves is widely accepted in the scientific community but has not been included in sea level rise projections because many researchers were unsure whether the effect of the process was large enough to raise sea level, mainly because its effects are located around the glacier’s ice shelf.

Once the researchers merged the three models into one, they made a series of projections to the year 2300 using a NASA supercomputer. The projections included three main scenarios: a control scenario without additional ocean warming, a low emissions scenario (SSP1-2.6) and a high emissions scenario (SSP5-8.5). For each scenario, the researchers created projections with and without the effect of current subglacial discharge levels.

Model simulations revealed that the addition of subglacial discharge reconciled the observed melt rates on the Denman and Scott glaciers. In the control and low-emissions models, the contributions to sea level rise were close to zero or even slightly negative, with or without subglacial discharge at 2300. But in a high-emissions scenario, the model found that the discharge subglacial increased the contribution of these glaciers to sea level rise from 19 millimeters to 22 millimeters in 2300.

In the high-emissions scenario that included subglacial discharge, the Denman and Scott glaciers retreated into the three-kilometer-deep trench beneath them in 2240, about 25 years earlier than they did in the model runs without subglacial discharge. Once the baselines of the Denman and Scott glaciers retreat beyond the edge of this trench, its annual contribution to sea level rise skyrockets, peaking at 0.33 millimeters per year, about half the current annual contribution to sea level rise from the entire Antarctic ice sheet.

“Subglacial meltwater has been inferred under most, if not all, Antarctic glaciers, including the Thwaites, Pine Island and Totten glaciers,” Pelle said. “All of these glaciers are receding and contributing to sea level rise. of the sea, and we are showing that subglacial discharge could be accelerating its retreat. “It is urgent that we model these other glaciers so that we can get an idea of ​​the magnitude of the effect that subglacial discharge is having.”

That’s what the researchers in this study are doing. According to Pelle, they are about to present a research proposal to extend their new model to the entire Antarctic ice sheet. “This also means that our results are probably a conservative estimate of the effect of subglacial discharge,” Greenbaum says. “That said, we still can’t say how much sea level rise will be accelerated by this process. Let’s hope it’s not too much“he concludes.