A robotic float has achieved a groundbreaking feat by measuring temperature and salinity in previously unexplored parts of the ocean beneath East Antarctica’s massive floating ice shelves. Over two-and-a-half years, an Argo float equipped with oceanographic sensors collected nearly 200 profiles across a 300-kilometre journey under the Denman and Shackleton ice shelves.
During its mission, the float vanished beneath the ice, only to re-emerge with the first-ever ocean transect beneath an East Antarctic ice shelf. “We got lucky,” remarked Dr. Steve Rintoul, an oceanographer from CSIRO, Australia’s national science agency, which partnered with the Australian Antarctic Program Partnership at the University of Tasmania.
“Our intrepid float drifted beneath the ice and spent eight months under the Denman and Shackleton ice shelves, collecting profiles from the seafloor to the base of the ice every five days. These unprecedented observations provide new insights into the vulnerability of the ice shelves,” he added.
Unveiling Hidden Vulnerabilities
The measurements reveal that the Shackleton ice shelf, the most northerly in East Antarctica, is currently not exposed to warm water capable of melting it from below, rendering it less vulnerable for now. However, the Denman Glacier, with its potential 1.5-metre contribution to global sea level rise, is precariously positioned. Warm water is reaching underneath, and even minor changes in the thickness of the warm water layer could trigger higher melt rates, leading to an unstable retreat.
The transfer of heat from the ocean to the ice is influenced by the ocean conditions in the 10-metre thick ‘boundary layer’ immediately below the ice shelf. “A great advantage of floats is that they can measure the properties of the boundary layer that control the melt rate,” Dr. Rintoul explained.
“The float measurements will be used to improve how these processes are represented in computer models, reducing the uncertainty in projections of future sea level rise.”
Deploying more floats along the Antarctic continental shelf could revolutionize our understanding of the ice shelves’ vulnerability to oceanic changes, ultimately helping to reduce the largest uncertainty in future sea level rise estimates.
Scientific Collaboration and Future Implications
Leader of the Australian Antarctic Program Partnership, Prof. Delphine Lannuzel, who sampled the ocean near the ice shelves during the Denman Marine Voyage earlier this year, praised the float’s achievements. “Against the enormity of such a wild region, this is an amazing story of the little float that could,” she said. “Under incredibly testing conditions, a relatively tiny instrument has delivered us a wealth of invaluable information.”
The findings, published in Science Advances, are a collaborative effort by scientists from CSIRO, the Australian Antarctic Program Partnership, and the Institute for Marine and Antarctic Studies at the University of Tasmania. The research was supported by Australia’s Integrated Marine Observing System (IMOS), enabled by the National Collaborative Research Infrastructure Strategy (NCRIS).
Understanding the Broader Context
Sea level rise poses a significant threat to hundreds of millions of people living in coastal areas, including low-lying islands, deltas, and cities. The largest uncertainty in future projections is how much Antarctica will contribute to sea level rise. Part of the Antarctic Ice Sheet rests on bedrock below present-day sea level, making it potentially vulnerable to oceanic changes.
Most of the vulnerable ice is in East Antarctica, previously thought to be isolated from warm water and unlikely to melt. However, new observations indicate that large volumes of ice in East Antarctica are at risk. The stability of the Antarctic Ice Sheet depends on the floating ice shelves around its edge, which act as buttresses resisting the flow of ice from the continent to the ocean.
If these ice shelves weaken or collapse, more ice will flow into the ocean, causing sea levels to rise. The critical factor determining the fate of the Antarctic ice sheet—and consequently the rate of sea level rise—is how much ocean heat reaches the base of the floating ice shelves.
Challenges and Innovations in Observation
Observing the processes driving melt in ice shelf cavities is challenging due to the thickness of the ice shelves, which can be hundreds or thousands of metres thick. While drilling holes through the ice to lower oceanographic sensors is possible, it is expensive and rarely done, resulting in limited measurements in ice shelf cavities.
Floats that drift with ocean currents, periodically surfacing to collect temperature and salinity profiles, offer an alternative. “There was only one problem,” Dr. Rintoul noted. “As the ice prevented the float from reaching the sea surface, it was unable to communicate with satellites and get a GPS fix to tell us where it is.”
“We had to do some detective work to determine where the float measurements were made. Each time the float bumped its head on the ice, it provided a measurement of the depth of the ice shelf base, or ice draft. We could compare the ice draft measured by the float to satellite measurements of draft to work out the path of the float beneath the ice.”
This innovative approach to data collection under extreme conditions not only highlights the resilience of modern technology but also underscores the importance of continued research and collaboration in understanding and mitigating the impacts of climate change.
