Case Study: Revealing the Complex Dynamics of Antarctic Ice Shelf Melting with Sea-Bird Scientific’s SBE 49 FastCAT CTD + PDIM

“I’m a physical oceanographer – so I’m interested in understanding how the ocean moves. And in this particular project, I’m interested in understanding how the ocean melts the ice from below.”

Specifically, Davis and his team are assessing the basal melt rate of the ice shelves of Thwaites Glacier, one part of the West Antarctic ice sheet that is incredibly vulnerable to warm ocean currents. According to Davis, Thwaites is uniquely poised to affect sea level rise more than any other ice shelf over the next 100 years.  

Using numerical models, scientists were able to assess the physical processes that drive glacial melting in Antarctica. The grounding line, the region where the ice shelf first begins floating as it extends out over the ocean from a land mass, is critical in understanding future sea level rise. Melting in this region can accelerate glacial retreat, therefore threatening the stability of the ice sheet.  

Based on these models, scientists across the globe understood the criticality of studying the processes that drove melting at the grounding line, leading to the creation of the MELT project. 

International Collaboration

The International Thwaites Glacier Collaboration (ITGC) is a landmark scientific program led by the United Kingdom and the United States, alongside other contributions from countries like Sweden, South Korea, and Germany, who each brought in their own area of expertise in contribution to the mission to multiply efforts and scientific outcomes from the ITGC. Thwaites Glacier, in particular, is hundreds of kilometers from any permanent research station. In order to gain access to it to conduct research, logistical and scientific expertise from all these critical partners was necessary to effectively execute the research plans.  

The MELT team melted large amounts of snow to create the hot water to drill through Thwaites Glacier to reach the ‘grounding line’. Photo credit: Peter Davis

MELT is one of eight scientific projects nested within the ITGC. The objectives of these projects spanned all areas of observation and modeling of the ice shelf and glacier. The MELT project contained both modeling and observational components, which were focused on understanding how the melting is occurring in the grounding zone where the ice shelf first begins to float. The observational component of the MELT team was made up of researchers and scientists from institutions such as the British Antarctic Survey, Cornell University, New York University, Oregon State University, and the University of Portland. 

Methodology & Instrumentation

Dr. Davis goes on to describe their methodology to gain access to sub-shelf ice melt: hot water drilling.  

“My role is to use a technique known as hot water drilling to make an access hole through these ice shelves, maybe through – 500-800 meters of ice, and then deploy ocean instruments underneath to observe how the ocean is moving, how much heat there is in the ocean, and how the ocean is driving melting of these ice shelves.”

The location of the access point determines how deep the team drills. Once the borehole is complete, the first instrument that is lowered into the icy depths is Sea-Bird Scientific’s SBE 49 FastCAT CTD alongside the PDIM (power data interface module). 

Ice base shot from inside borehole. Photo credit: Peter Davis

Sea-Bird Scientific’s SBE 49 FastCAT CTD  was chosen as the leading profiling CTD because of its ability to effectively operate in deep water environments. This particular SBE 49 FastCAT CTD was outfitted with a pressure sensor rated at up to 2,000 meters. When asked about the performance of the SBE 49 in difficult environments like Antarctic Ocean water, Dr. Davis notes,  

“We’ve always found the instruments to be very stable.  It’s a particularly harsh environment to be put through, but [the SBE 49 FastCAT CTD and PDIM system] performs very well.”

SBE 49 FastCAT

Insights gained from the SBE 49 FastCAT CTD and PDIM profiling system give the team ideas of where they should deploy moored instruments long-term.  

Results: What the Thwaites Glacier Ice Melt Tells Us

Through this research from the MELT project, the team was able to discern, as expected, that the temperature of the water at the grounding zone is about a degree and a half above freezing. This heat is what is driving basal melting in this region.  

Interestingly, data collected while conducting profiles with the SBE 49 FastCAT CTD revealed that there is a layer of fresh water right at the ice base, which creates a density barrier. This density barrier, by and large, prevents much of the heat that’s in the ocean from reaching the ice.   

Based on the amount of heat at the grounding zone, they expected that the melt rate would be in excess of 10-20 meters per year; however, because of the newly discovered density barrier, they only observed melting at a rate of 3-5 meters per year, which was significantly lower than what Dr. Davis and his team were expecting.  

Dr. Davis and his team thought they would see that rapid grounding line retreat implies rapid melting – when in fact, what they actually discovered is that even a small amount of basal melting can still drive rapid retreat. 

While low melt rates may sound optimistic, Dr. Davis emphasizes, 

“…the point is, the ice shelf is still retreating quickly. The grounding line is still retreating”  

The team also discovered that topographically, the ice base was not as smooth as they were expecting. It has many crevasses, channels, terraces, steps, and numerous scale variations in the ice thickness. These variations in the base of the ice shelf correlate with different rates of melting. For example, on flat, horizontal surfaces, the melt rate was extremely suppressed, while on the vertical faces, the rate was much higher. These findings highlight how complex melting processes and ocean-ice dynamics are. 

Implications for the Future of Antarctic Research

Dr. Davis underscores the need to incorporate what the team unearthed about melting into our models of future sea level rise. The findings from the MELT project are unilaterally applicable when conducting modeling to make assessments about basal melt rates around the globe. Dr. Davis highlights, 

“…you really have to worry about how melt is varying on a kilometer scale to get the average melt rate right. The implications are that we need to spend a lot more time improving the models to get these detailed complex physics factored in, in order to be absolutely certain about projections of future sea level rise.” 

While Dr. Davis does not believe the projections of future sea level rise currently publicized are unreliable, he emphasizes the importance of bringing down the uncertainty in projections using the findings from the MELT project so that policymakers can plan for future adaptation and mitigation in response to sea level rise using the best available figures.  

“…ice melt beneath Antarctica is complex – and we need to understand those details so that we can get the melting correct around the whole of the continent.”  

Once these findings are applied unilaterally, it enables more accurate simulations of future sea level rise with lower uncertainties. Therefore, global leaders can more accurately plan for future mitigation and policy measures by utilizing these models.  

Dr. Davis emphasizes that this research is extremely iterative.  Each time they have gone and observed an ice shelf, they’ve witnessed a different melting process – a different way the ocean is driving ice melt. Therefore, he notes the importance of continual monitoring and assessment to ensure they are capturing the full picture. For example, assessing different areas of the Antarctic ice sheet where they know different melting processes could exist would be really helpful. These critical findings unearthed by the MELT team help characterize the complexity of ice melt in the Antarctic.  

Acknowledgements

The MELT project, a component of the International Thwaites Glacier Collaboration (ITGC) was supported by the National Science Foundation (NSF, grant no. 1739003) and the Natural Environment Research Council (NERC, grant no. NE/S006656/1). Antarctic logistics were provided by NSF U.S. Antarctic Program and NERC British Antarctic Survey.  

References

PED Davis, KW Nicholls, DM Holland, BE Schmidt, P Washam, et al. (2023). Suppressed basal melting in the eastern Thwaites Glacier grounding zone. Nature 614 (7948), 479-485. DOI: 10.1038/s41586-022-05586-0 

BE Schmidt, P Washam, PED Davis, KW Nicholls, DM Holland, et al. (2023). Heterogeneous melting near the Thwaites Glacier grounding line. Nature 614 (7948), 471-478. DOI: 10.1038/s41586-022-05691-0 

PED Davis, KW Nicholls, DM Holland, BE Schmidt, P Washam, et al. (2025). Lateral Fluxes Drive Basal Melting Beneath Thwaites Eastern Ice Shelf, West Antarctica. Geophysical Research Letters 52 (3), e2024GL111873. DOI: 10.1029/2024GL111873