Hidden ocean heat is creeping toward Antarctica’s fragile ice shelves
A major new study drawing on decades of ocean data has found clear evidence that heat from the deep ocean is shifting toward Antarctica. This change poses a growing threat to the continent's delicate ice shelves, which line its coast and help stabilize the ice sheet. Researchers led by the University of Cambridge, working with colleagues from the University of California, analyzed long-term measurements collected by research ships and robotic ocean instruments. Their findings show that a large body of relatively warm water, known as 'circumpolar deep water,' has expanded and gradually moved closer to the Antarctic continental shelf over the past two decades. First Clear Evidence of a Long-Predicted Shift Until now, scientists lacked enough continuous data to confirm this warming trend. "It's concerning, because this warm water can flow beneath Antarctic ice shelves, melting them from below and destabilizing them," said Joshua Lanham, lead author of the study at Cambridge Earth Sciences. Ice shelves act as barriers that hold back Antarctica's inland glaciers and ice sheets. Together, these frozen reserves contain enough water to raise global sea levels by about 58 meters. According to Lanham, this is the first time researchers have directly observed deep-ocean heat shifting across the Southern Ocean. "It's something that had been predicted by climate models due to global warming, but we hadn't seen it in data." Combining Ship Records and Floating Sensors Historically, scientists relied on ship-based surveys of the Southern Ocean, which circle Antarctica. These surveys were conducted about once every ten years and provided detailed snapshots of temperature, salinity, and nutrient levels. However, gaps between measurements made it difficult to track long-term changes. To improve the picture, the research team combined ship records with data from a global network of autonomous floats, known as Argo floats, which continuously measure conditions in the upper ocean. Using machine learning, the team merged float data with patterns from ship measurements, reconstructing a detailed, month-by-month record of ocean conditions over the past 40 years, revealing the steady advance of warmer waters. Antarctica's Cold Buffer Is Weakening "In the past, the ice sheets were protected by a bath of cold water, preventing them from melting. Now it looks like the ocean's circulation has changed, and it's almost like someone turned on the hot tap and now the bath is getting warmer!" said Prof. Sarah Purkey, a senior author from Scripps Institution of Oceanography. Purkey noted this expansion of warm water aligns with what scientists expect in a warming world. More than 90 percent of excess heat from global warming is absorbed by the oceans, with the Southern Ocean taking in a large share. Global Climate Implications Beyond Antarctica The consequences extend beyond melting ice. "The Southern Ocean plays a key role in regulating global heat and carbon storage, so changes in heat distribution here have wider implications for the global climate system," said Prof. Ali Mashayek, another senior author from Cambridge Earth Sciences. Near the poles, extremely cold and dense water forms and sinks into the deep ocean, pulling heat, carbon, and nutrients downward, driving a global system of currents. This system includes the Atlantic Meridional Overturning Circulation (AMOC), which moves water throughout the Atlantic. Climate models suggest rising air temperatures and increased freshwater from melting ice are reducing dense water formation in the North Atlantic, potentially weakening the AMOC. Similar shifts are now expected in the Southern Ocean. Models indicate less cold, dense water will form around Antarctica, allowing warmer circumpolar deep water to move closer to the continent. "We can now see this scenario is emerging in observations," said Lanham. "This isn't just a possible future scenario; it's happening now, affecting carbon, nutrients, and heat cycling in the global ocean."
Source: ScienceDaily