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Antarctic sea ice extent has reached record lows in recent years<\/p>\n
Sebnem Coskun\/Anadolu Agency via Getty Images<\/p>\n<\/p>\n
Scientists have been debating why Antarctic sea ice, which once seemed impervious to climate change, has shrunk dramatically in the past decade. Now research suggests stronger winds have churned up warming water from the deep ocean, breaking through upper water layers that were protecting the ice from melt.<\/p>\n
While Arctic sea ice has declined about 40 per cent over four decades, until recently the sea ice around Antarctica was slightly expanding, confounding most climate models. Then after 2015 ice extent fell from a record high to several record lows, losing an area<\/a> the size of Greenland.<\/p>\n Some research<\/a> has suggested the sea ice may be melting largely due to air temperatures, which have been so high in recent years that Antarctic researchers have posed for photos<\/a> in swimwear. Two new studies make the case that ocean warming played a bigger role in this \u201cregime shift\u201d.<\/p>\n \u201cPlenty of people will say\u2026 that it was atmospheric warming which melted the sea ice from above,\u201d says Simon Josey<\/a> at the National Oceanography Centre in Southampton, UK, who wasn\u2019t involved in the research. \u201cNow these scientists have done the thorough analysis and have got a plausible chain of events, which says that the ocean is the key player in that 2016 melt. Nobody\u2019s put that argument together before.\u201d<\/p>\n As part of global ocean circulation, a mass of warm, salty water called circumpolar deep water flows southward from the tropics and circles Antarctica at depths below 200 metres. But it\u2019s increasingly coming to the surface where it can melt sea ice, two decades of temperature and salinity measurements from several hundred drifting buoys suggest.<\/p>\n Antarctica is surrounded by a belt of strong winds and storms in the latitudes of the \u201croaring forties\u201d, \u201cfurious fifties\u201d and \u201cscreaming sixties\u201d. Climate change has shifted this storm track southward, bringing more precipitation into the sea ice zone, according to a study by Earle Wilson<\/a> at Stanford University and his colleagues. Initially, the precipitation created a layer of fresh surface water that better insulated the bottom of the sea ice from warm deep water, allowing it to expand to its 2014 record extent.<\/p>\n But the southward-shifted storm track also delivers stronger winds that blow surface water and ice forward. Due to the spinning of the Earth, water moves 90 degrees to the left of the wind direction, generating spirals like the Weddell Sea gyre. As surface water is flung to the edges, deep water wells up from below to fill the void at the centre.<\/p>\n Between 2014 and 2016, this wind-driven upwelling began to win the \u201ctug-of-war\u201d against the protective layer of increased precipitation, and the sea ice began to melt away in the Weddell Sea. When the researchers plugged the observed changes in temperature and salinity into a simple computer model, it projected sea ice would expand and then contract, as it did in the real world.<\/p>\n \u201cMost signs point to a persistent and sustained decline in sea ice, because even with the precipitation potentially suppressing the deep ocean heat\u2026 the heat is still there,\u201d Wilson says. \u201cAll it would take is a sudden reversal of conditions for that heat to come back up.\u201d<\/p>\n