Slowing of the ocean’s deep breath

The deeper half of the world ocean, below 2 km depth, is renewed by sinking of cold and dense waters in localized gravitational plumes near Antarctica and Greenland¹. Antarctic sinking plumes carry the densest waters, and constitute the primary ventilation pathway — or ‘respiratory route’ — connecting the abyssal ocean (deeper than 3 km) to the atmosphere². By transferring mass, oxygen, nutrients and carbon from the surface to large depths, this pathway influences major ocean circulation cells^2,3, Antarctic glaciers⁴, marine ecosystems and global climate⁵. An ongoing slowdown of this ventilation pathway has been suspected based on observed widespread warming of bottom waters⁶. However, trends in Antarctic sinking and causes of abyssal warming have remained elusive due to a lack of direct observations of ocean flows. Now, writing in Nature Climate Change, Kathryn Gunn et al.⁷ show observational evidence that transport of bottom waters in two important plumes feeding the abyssal Australian–Antarctic basin has dwindled since the early 1990s. The transport trends mirror trends in the salinity of source waters on the Antarctic shelf, suggesting that projected mass losses of Antarctic ice⁸ may further stifle abyssal ventilation by freshening and lightening shelf waters.