A new decades-long study based on oceanographic data offers the first evidence that deep ocean heat is approaching Antarctica, threatening the fragile ice shelves that rim the continent.
The study has been led by the University of Cambridge (United Kingdom) with collaborators from the University of California (United States) and is published in the journal ‘Communications Earth&Environment’.
The work compiles long-term ocean measurements collected by ships and robotic floating devices to show that A warm mass called ‘circumpolar deep water’ has expanded and moved towards the Antarctic continental shelf in the last 20 years.
Previously, scientists did not have enough ocean observations to detect the warming trend. “It is worrying because this warm water can seep beneath the Antarctic ice shelves, melting them from below. and destabilizing them,” according to Joshua Lanham of Cambridge Earth Sciences.
The ice shelves play an important role in containing the ice caps and glaciers in Antarctica’s interior, which together hold enough fresh water to raise sea levels by about 58 metres.
It’s the first time scientists have observed the change in deep ocean temperature across the entire Southern Ocean, according to Lanham, who adds: “It’s something that climate models had predicted due to global warming, but we hadn’t seen it in the data.”
Uncertainty about the spread of heat
Previous observations of the Antarctic were limited to transects recorded by ships once a decade. This information, collected as part of a long-running international program, provided detailed data on 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 gaps in the record, researchers, including scientists at the Scripps Institution of Oceanography and the University of California, Los Angeles, supplemented the ship’s measurements with publicly available data collected by a global network of autonomous buoys drifting across the ocean surface.
These buoys provide continuous snapshots of the ocean, but the program has not been in operation as long as ships that compile detailed hydrographic sections.
Using machine learning, the researchers took the data from the buoys and combined it with long-term patterns extracted from ship measurements to build a new record that captures detailed monthly snapshots over the past four decades, allowing them to discover the change in warm waters.
“Before, the ice sheets were protected by a mass of cold water that prevented them from melting. Now it seems that The ocean circulation has changed and it is as if someone turned on the hot water tap and now the water is getting warmer.“says Sarah Purkey of the Scripps Institution of Oceanography.
More than 90% of the excess heat from global warming is stored in the ocean and the Antarctic absorbs most of the anthropogenic heat. The findings not only have implications for Antarctic melting and sea level rise.
“The Southern Ocean plays a critical role in regulating global heat and carbon storage, so changes in heat distribution in this region have broader implications for the global climate system,” said Ali Mashayek, from the School of Earth Sciences at the University of Cambridge.
Atlantic current
In the icy waters surrounding the poles, extremely cold and dense water forms and sinks to the depths of the ocean. As water sinks, it absorbs heat, carbon and nutrients, setting in motion a global network of currents, including the Atlantic Meridional Overturning Circulation (AMOC), which transports water throughout the Atlantic.
Climate models indicate that The increase in air temperatures and the contribution of fresh water from melting ice are reducing the formation of this dense water mass in the North Atlanticwhich could lead to a weakening of the AMOC.
Similar changes have recently been predicted for the Antarctic. Climate models suggest that production of cold, dense water will decrease in Antarctica, causing warmer circumpolar deep water to move toward the continent to fill the space left by the decline in cold water.
“We can now see that this scenario is already playing out in observations. This is not just a possible future scenario suggested by models; it is something that is happening right now, with broader implications for how carbon, nutrients and heat circulate through the global ocean,” concludes Lanham.