A new decades-long study of oceanographic data provides the first evidence that deep-ocean heat has moved closer to Antarctica, threatening the fragile ice shelves that fringe the continent.
The study, led by the University of Cambridge with collaborators from the University of California, compiled long-term ocean measurements collected by ships and robotic floating devices to show that a warm mass called 'circumpolar deep water' has expanded and shifted toward the Antarctic continental shelf over the past 20 years.
Previously, scientists hadn't had enough ocean observations to detect the 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 play an important role in holding back Antarctica's inland ice sheets and glaciers, which collectively hold enough freshwater to raise sea level by about 58 metres.
It's the first time that scientists have observed the shift in deep-ocean heat throughout the Southern Ocean, said Lanham. "It's something that had been predicted by climate models due to global warming, but we hadn't seen it in data."
Previous observations of the Southern Ocean, which encircles Antarctica, were limited to transects recorded by ships roughly once a decade. This information, collected as part of a long-running international programme, provided detailed snapshots of temperature, salinity and nutrients throughout the water column but, without continuous data, scientists were more uncertain about long term changes in heat distribution.
To fill the gaps in the record, the researchers, including scientists from the Scripps Institution of Oceanography and UCLA, supplemented the ship measurements with
publicly available data collected by a global array of autonomous floats, which drift through the upper ocean. These so-called Argo floats provide continuous snapshots of the ocean, but the programme hasn't been running as long as ships have been collecting detailed hydrographic sections.
Using machine learning, the researchers took the Argo float data and combined it with long-term patterns drawn from ships measurements to build a new record capturing detailed monthly snapshots over the last four decades, allowing them to uncover the shift in warm waters.
"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, one of the senior authors of the study from Scripps Institution of Oceanography. It makes sense that this pool of warm water is expanding, said Purkey. More than 90 percent of excess heat from global warming is stored in the ocean, with the Southern Ocean absorbing most of the anthropogenic heat.
The findings not only have implications for Antarctic ice melt and sea level rise, said Prof. Ali Mashayek, one of the senior authors of the study from Cambridge Earth Sciences. "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."
In the frigid waters around the poles, extremely cold, dense water forms and sinks to the deep ocean. As the water sinks, it draws down heat, carbon and nutrients, setting in motion a global 'conveyor belt' of currents, including the Atlantic Meridional Overturning Circulation (AMOC), which shuttles water around the Atlantic.
Climate models, including those used by the IPCC, indicate that warmer air temperatures and added freshwater from ice melt are reducing the formation of this dense water in the North Atlantic, potentially leading to a weakening of the AMOC.
Similar changes have recently been forecast for the Southern Ocean. Climate models have suggested that the production of cold, dense water will decline in Antarctica, causing the warmer circumpolar deep water to draw toward the continent to occupy the space left by the shrinking cold water.
"We can now see this scenario is already emerging in the observations," said Lanham. "This isn't just a possible future scenario suggested by models; it's something that is happening now, bringing wider implications for how carbon, nutrients and heat are cycled through the global ocean."
