If Antarctica's ice sheets melted, the worlds oceans would rise by 200 - 210 feet, everywhere. Antarctica holds 6,957,470 cubic miles of ice. That represents over 90% of all the ice on the planet and between 60 to 70% of the worlds fresh water. The sea ice begins to expand at the beginning of winter, and eventually advances by 40,000 square miles per day, surrounding the continent and doubling the size of the ice in Antarctica. The snow that falls takes about 100,000 years to flow to the coast of Antarctica before it breaks off the marine extension of glaciers, or ice shelves, and becomes an iceberg. Once an iceberg breaks off the ice platform, any resistance that it provided to hold back the land ice is lost, and the continental ice can accelerate it’s flow to the southern ocean.
The three dimensional upward spiral of North Atlantic Deep Water through the Southern Ocean. (Photo: courtesy of the authors). Study from MITThe Conversation notes that for the first time in 7,000 years a phenomenon known as upwelling (the upward flow of warm ocean water to the surface) is rotting the ice shelves from below. It is thought that the upwelling has caused the many ice shelf collapses and ice thinning that we have seen over the past decades.
The ocean surrounding Antarctica is extremely cold, but water over 300m deep, Circumpolar Deep Water (CDW), is about 3⁰C above the melting point of ice. Normally, the very cold water above keeps this away from ice shelves. But in some areas, CDW is spilling onto the shallow Antarctic continental shelf, causing the ice to thin.
Ice shelf thinning has accelerated in recent decades, but the picture is not the same everywhere. While the east of the Antarctic has shown modest gains in ice thickness, the west has outstripped this with significant ice loss – up to 18% in vulnerable areas like the Amundsen and Bellingshausen Seas.
The pattern of ice loss and other observations indicate that warmer water upwelling beneath these ice shelves is driving it. But what has caused this upwelling? Is it related to human activity? And how concerned should we be?
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This coincidence of timing with the onset of industrialisation shows it is possible that human made greenhouse gasses, thought to cause atmospheric warming, are having an impact on the position of the winds, the increase in warm water reaching the surface, and ultimately the melting of more ice in the Antarctic.
Irrespective of the causes of past changes in SHWW positions, the link between winds and ocean upwelling is cause for concern, as future projected global warming may shift SHWW belts and promote further upwelling and melting. More research is now needed to fully understand the link between CDW and past climate, and to estimate the strength of upwelling since the 1940s compared to upwelling before 7,000 years ago. But the emerging picture is one of the potentially increased vulnerability of West Antarctic ice sheets, and possible future sea level rise.
See NASA’s Jet Propulsion Laboratory news release from September of 2017 for an expanded review.
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