Tuesday, 26 April 2016

Ice melt extends into East Antarctica

Melt Expanding into East Antarctica as Nansen Ice Shelf Crack Produces 20 Kilometer Long Iceberg

25 April, 2016

Ever since 1999 a gigantic crack has been growing in the Nansen Ice Shelf in East Antarctica. By 2014, expansion of the crack accelerated. As of early 2016, the crevice had grown to 40 kilometers in length. Flooded by melt along the Ice Shelf’s warming surface and weakened by the heating of ocean waters from below, on April 7th, according to ESA reports, this East Antarctic Ice Shelf produced an immense 20 kilometer long iceberg. A towering block of ice covering an area larger than Manhattan floating on out toward the world’s shipping lanes.

Nansen Ice Shelf Fracture
(Surface melt water flooding into a great crack along the Nansen Ice Shelf. Large volumes of melt water flooding into ice shelf cracks forces them to widen even as they dive toward union with the warming waters below. Image source. ESA.)

The Nansen Ice Shelf, before this most recent very large iceberg calving event, was a 10 mile wide and 30 mile long ice shelf that buttressed the Presley and Reeve Glaciers of East Antarctica. It abuts the north side of the Drygalski Ice Tongue, and runs out from Mount Nanson just inland of the coast of Victoria Land, Antarctica. And it’s yet another large shelf of ice that appears to be facing severe weakening as global average temperatures are driven above 1 C warmer than those experienced during the late 19th Century by an ongoing and reckless fossil fuel emission.

Nansen occupies a region of the world that has come under increasingly intense observation due to a number of scientific studies highlighting its accelerating rates of melt and a related risk of rapidly rising global sea levels.  Human-forced heating of the world’s ocean has caused waters warm enough to accelerate glacial melt to encroach upon Antarctica from the Southern Ocean. These warmer waters are drawn along beneath the floating ice shelves as fresh melt water flooding out along the ocean surface generates a landward-moving bottom current. These warmer waters eventually push beneath the ice shelves — eating away at their undersides.
Nansen Ice Shelf Fracture
(Massive ice berg breaks away from the Nansen Ice Shelf on April 7th in this ESA satellite shot.)

In West Antarctica, glaciers are seen as especially vulnerable. One region containing the Pine Island and Thwaites glaciers — alone capable of increasing global sea levels by 3-6 feet — is experiencing 3-4 meters of melt along the undersides of ice shelves each year. Meanwhile, the Pine Island Glacier’s grounding line — the point upon which the floating ice shelf is anchored — has retreated more than 31 kilometers further into Antarctica.

As ice shelves and glaciers melt from below, more of the glacial mass is floated away from the main ice mass. And since West Antarctica has a retrograde slope, more warm ocean water comes flooding in beneath the glaciers. As more ice calves off the glacial faces, ice fronts along the ocean become taller. The brittle ice cannot retain structural integrity for long and ever taller ice faces produce swifter and swifter rates of collapse into the ocean. On the surface, warmer conditions cause rivers of melt water to flow into cracks. The extreme weight of the water flooding these cracks serves as a kind of wedge — widening the cracks and shoving sea-facing sections along the cracks into the ocean.

Antarctic glacial melt
(Though the most rapid rates of glacial melt dominate the Antarctic Peninsula and the region near the Pine Island and Thwaites glaciers, increasing rates of volume loss from Antarctic ice shelves have been creeping into a section of East Antarctica near the Nansen Ice Shelf. With global average temperatures now exceeding 1 C above pre-industrial, we can expect melt and net volume loss to expand along the Antarctic coastline. Image source:Science.)

East Antarctica, a region occupied by Nansen, has been seen as less vulnerable to melt from human-forced warming than West Antarctica and the Antarctic Peninsula. That said, increasing melt rates have progressed on into much of the Ross Sea coastal region. Though the current very large calving event at Nansen appears to have taken place within the expected time-frame (once every 30 years), a context of melt is beginning to encroach. For almost all glaciers along this section of East Antarctica show increasing rates of ice loss.

Hat Tip to Colorado Bob

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