The Ross Ice Shelf in eastern Antarctica is the largest ice shelf in the world. Here, too, researchers fear increased melting, as on other Antarctic ice shelves. Using new probes both in the water and from the air, an international team of scientists has now discovered details that, in addition to local conditions, warmer, less saline seawater from the neighboring Amundsen Sea creates conditions that could accelerate the melting process, releasing even more freshwater.
Located in the Ross Sea, the 480,000 square kilometer ice shelf is the largest ice shelf in the world and is fed by several glaciers in East Antarctica. With its mass, the shelf acts like a cork in a champagne bottle, preventing uncontrolled outflow of glaciers, which in turn would be responsible for a significant rise in sea level. Therefore, the ice shelf is of striking importance. But the ice shelf, like most in Antarctica, is endangered and thinning. The culprit is warmer surface water that washes around the edges and bottom of the ice and melts it. Unlike in large parts of Antarctica, however, mainly local water masses are involved in the process. According to Dave Porter of Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study, “The changing melt rates on the Ross Ice Shelf are primarily caused by the local buildup of heat at the water surface. The question now is what dictates how much heat is built up in the summer? And the answer is that it’s caused primarily by local weather processes along the ice shelf front.”
Porter and the team had to collect a new way to obtain the necessary data (temperature, salinity, depth, current) for these results. Usually, measurement cruises are only undertaken in summer when the pack ice has disappeared; on the other hand, measurement probes are anchored at depths of 200 meters and more to be protected from drifting icebergs. To plug the holes in this type of data collection, Porter and his team used so-called Airborne Autonomous Micro Observers (ALAMO) dropped by parachutes from an aircraft over the ocean and near the ice front, in addition to moored measurement probes. These probes then sink to the seafloor, measuring the required parameters as they do so, and then rise back up and measure those parameters again. At the surface, they radio the data to the researchers via satellite before sinking again. This cycle is repeated daily. To prevent a collision with pack ice or icebergs, the probes have a special program built into their software. If sensors detect ice, the probes will remain underwater until the situation is better. To prevent deep currents from driving the probes away, they were also equipped with a “parking function” that keeps them at certain depths and positions. The team used a total of 13 probes (7 moored, 6 airborne) that provided data between 2013 and 2017.
The results of the work showed that sunlight rapidly warms the water surface after the pack ice disappears. In addition, large amounts of fresh water were found from the ice shelves of the adjacent Amundsen Sea. This water changes the mixing of heat at the ice front from the surface to the base of the shelf. This finding is new and has not been considered in models before. “This new approach to data collection from remote Antarctic continental shelves provides a new way to test the reliability of numerical models that allow us to understand how Antarctica’s ice sheet will respond to future changes in the oceans surrounding the continent,” explains co-author Scott Springer. Incorporating local conditions as small-scale processes and looking at areas not as obviously affected by climatic changes into global models is also evident from this new study. “A lot of ongoing field work focuses on parts of Antarctica that we know are changing. But we also need to collect data in regions that haven’t changed (yet) to understand how the ice sheet works as a whole,” explains Helen Amanda Fricker of the University of California SCRIPPS and co-author. “This is critical as projections of Antarctica’s contribution to sea level in future climates continue to vary widely,” she concludes. As summers get longer and pack ice melts earlier in the future as the climate warms, these amounts of heat in the water may prove devastating to shelf stability, scientists predict.
Quelle: Earth Institute, Columbia University / Porter et al. (2019) J Geophys Res Oceans EPub …