11-13-2024

‘Lost’ seaglider reveals new details about the fate of Antarctic ice

Earth.com staff writer

Antarctic ice often holds secrets waiting to be discovered, and sometimes an unexpected event can reveal them.

This was exactly the case when an autonomous Seaglider, charmingly named Marlin and launched by the University of East Anglia’s School of Environmental Sciences, became unintentionally trapped beneath the vast expanse of the Ross Ice Shelf.

Unplanned path led to unseen depths

In December 2022, Marlin was launched out over the Ross Sea, programmed to venture north towards open waters, collecting significant data for climate studies on the way.

Contrary to the plan, Marlin ended up embarking on an unscheduled expedition under the Ross Ice Shelf, after being dragged there by a southward-flowing current.

The little voyager was under the ice for four days, during which it conducted 79 deep dives into the cavity beneath the ice shelf and collected water measurements up to 200 meters beneath the surface.

A surge of somewhat warm water

During its unintended sojourn, the sensors onboard Marlin detected a layer of relatively warmer water, about 50 meters thick, that had made its way into the shelf cavity from the open sea.

The temperature of this water intrusion ranged from -1.9°C to a marginally warmer -1.7°C under the ice.

Its warmth was capable of melting the ice shelf’s base, unlike the freezing-point waters that it probably replaced.

Long-term view on Antarctic ice melting

The surprise insights from Marlin’s stuck-under-the-ice saga led the researchers to re-examine all available measurements from the past four decades.

The result? Heat transported into the cavity has undoubtedly increased over the past 45 years, largely due to the effects of climate change that has warmed the Ross Sea.

“While the temperature increase – four thousandths of a degree a year – might not seem all that much, it could lead to around 20 to 80 cm of additional ice loss per year over the 45 years we look at,” explained Dr. Peter Sheehan, the lead author of the study.

“We found the waters of the intrusion were warm enough to melt the underside of the ice shelf, unlike the freezing-point waters they likely displaced.”

“What’s new here is that we can track the warm water pretty much from the open water of the Ross Sea at the ice front, back into the cavity. We have not seen one of these intrusions happening directly before.”

The role of winds and the rising concern

The research further suggested that certain wind patterns could drive warm surface water under the Ross Ice Shelf, leading to a southward flow in the surface ocean and directly into the ice shelf cavity.

These wind-driven, ocean-surface flows, known as Ekman currents, carry heat, effectively increasing the melting rate of the overlying ice.

“It appears reasonable to expect that the magnitude of the Ekman heat flux, and of the melting that it drives, will increase yet further as climate change drives continued ocean warming. This trend is a concern in itself,” said study co-author Professor Karen Heywood.

“The influence of surface-water intrusions, alongside the trends and variability in the Ekman dynamics that can drive these, must be incorporated into climate models, not least given continued uncertainty in the response of Antarctic land-based ice to climate change.”

This is particularly relevant as oceans absorb and redistribute a significant proportion of the Earth’s heat. This means that changes in this system could have significant impacts on global weather, sea levels, and temperature trends.

Continuous voyage into Antarctic ice dynamics

This is the first time such a comprehensive, multi-decadal data set has been used to study this process.

Earlier knowledge about surface-water intrusions came from studying hydrography in open water using observations from tagged seals, ship data, and ice moorings deployed inside a cavity.

As we prepare for the future, the chance encounter of Marlin with the Ross Ice Shelf highlights the importance of integrating the influence of surface-water intrusions and Ekman dynamics into climate models.

It also serves as a reminder of our enduring quest to understand the ever-changing dynamics of our precious blue planet.

The study is published in the journal Science Advances.