Questo sito web utilizza i cookie per potervi offrire la migliore esperienza d'uso possibile. Le informazioni contenute nei cookie vengono memorizzate nel browser dell'utente e svolgono funzioni quali il riconoscimento dell'utente quando torna sul nostro sito web e l'aiuto al nostro team per capire quali sezioni del sito web sono più interessanti e utili per l'utente.
June 12, 2023 Alessandro Silvano, Andrew Meijers, Shenjie Zhou
Our new research in the Antarctic suggests that the vital layer of cold water on the sea bed, which circulates the globe and influences the ocean’s ability to continue absorbing much of the rise in atmospheric heat and greenhouse gas emissions, is heating up and shrinking.
Much of this is a result of human-made climate change, which is melting Antarctic ice shelves and disrupting the complex system that controls this circulation. But it appears, as far as the past 30 years are concerned, a natural cycle may have been partly responsible for the changes observed.
The ocean has absorbed more than 90% of the excess heat and around 30% of the extra carbon dioxide humans have generated since the start of the industrial age. This has greatly reduced the impact of climate change at the Earth’s surface where we live.
Most of this exchange of gases and heat between the atmosphere and the ocean happens in the Southern Ocean around Antarctica through the complex vertical movement of water. One of the biggest drivers of this vertical movement is the production of what oceanographers call Antarctic bottom water.
Around the Antarctic coastline, seawater near freezing point contacts the much colder air and freezes into sea ice, expelling salt and consuming freshwater to leave cold, salty and dense water.
The vast majority of this dense water is produced at only a few locations around Antarctica. In these places, wind blowing off the frigid continent continually pushes newly formed sea ice away from the surrounding ice shelves to create areas of open water known as polynyas.
These polynya ice factories produce great volumes of cold and salty water which cascade down Antarctica’s continental slope like a submarine waterfall to the ocean bottom. Once there, Antarctic bottom water, the world’s deepest and densest water mass and the biggest of its kind spreads around the globe, storing carbon from the atmosphere for hundreds or even thousands of years.
As Antarctic bottom water moves north along the sea floor it drives the great ocean conveyor, also known as the overturning circulation: currents that redistribute heat, carbon and nutrients around ocean basins and regulate the global climate.
Our new research used observations from ships and satellites to reveal that the bottom water volume in the Weddell Sea, the Atlantic sector of the Southern Ocean and one of the biggest producers of this water mass, has decreased by more than 20% over the past 30 years, causing the deep Weddell Sea to warm four times faster than the global average.
Our evidence suggests weakening offshore winds in the region are to blame for polynyas shrinking and making less of the cold, dense, salty water which feeds Antarctic bottom water and drives the global ocean conveyor. This could slow down the deep overturning circulation, with profound implications for the climate system.
Previous studies have linked the slowing global ocean conveyor with less cold, dense water forming in the Southern Ocean due to increasing meltwater from ice shelves. While man-made climate change is significant, our new research suggests that natural variability in wind and sea ice are also important.
Publication DOI: https://doi.org/10.1038/s41558-023-01695-4