Subglacial meltwater flowing beneath Antarctic glaciers could accelerate retreat: Study

Meltwater flowing out to sea from beneath Antarctic glaciers, or subglacial discharge, could be making them lose ice faster, researchers have found in a new study. Modelling the influence of this subglacial discharge on the retreat of two glaciers in East Antarctica, the researchers found that it raised the glaciers' contribution to sea-level rise by 15.7 per cent -- from 19 millimetres to 22 millimetres -- by the year 2300.

The findings, relevant in a high emissions scenario featuring 20 per cent higher CO2 emissions by 2100, suggested that the subglacial discharge's influence was large enough to make a meaningful contribution to global sea-level rise, the researchers at the University of California San Diego's Scripps Institution of Oceanography in the US said.

The East Antarctic glaciers, named Denman and Scott, together hold enough ice to cause nearly 1.5 metres, or 5 feet, of sea-level rise, they said in their study published in the journal Science Advances. Current models making major sea-level rise projections, including those of the Intergovernmental Panel on Climate Change (IPCC), do not take into account this mechanism of subglacial discharge, the researchers said.

Thus, it could mean that current projections underestimate the pace of global sea-level rise in decades to come, they said. "Knowing when and how much global sea-level will rise is critical to the welfare of coastal communities," said Tyler Pelle, the study's lead author and a postdoctoral researcher at Scripps.

"Millions of people live in low-lying coastal zones and we can't adequately prepare our communities without accurate sea-level rise projections," said Pelle. In Antarctica, subglacial meltwater is generated from melting occurring where the ice sits on continental bedrock.

When subglacial discharge flows out to sea it is thought to accelerate melting of the glacier's ice shelf, which is attributed to ocean mixing that stirs in additional ocean heat within the cavity beneath a glacier's floating ice shelf. The resulting glacial retreat can then contribute to and drive sea level rise.

The reason why this mechanism of subglacial discharge is not currently considered in sea-level rise projections is because many researchers weren't sure if its localised effects were sufficiently large to increase sea-level rise globally, according to Jamin Greenbaum, co-author of the study and a researcher at Scripps Institute of Geophysics and Planetary Physics.

A key takeaway of the study, however, is the importance of what humanity does in the coming decades to rein in greenhouse gas emissions, said Greenbaum. This is because their model found that in a low emissions scenario, the glaciers did not retreat all the way into the trench and thus did not result in making runaway contributions to sea-level rise.

"If there is a doomsday story here it isn't subglacial discharge," said Greenbaum. "The real doomsday story is still emissions and humanity is still the one with its finger on the button."