Wednesday, 25 January 2017

Antarctic ice melt: Did Föhn Winds Just Melt 2 Miles of Antarctic Surface Ice in Just One Day?

Abnormal Antarctic Heat, Surface Melt, Giant Cracks in Ice Shelves — More Troubling Signs of a World Tipping Toward Climate Chaos

23 January, 2017

Around its edge zone, and from glacier top to ice shelf bottom, Antarctica is melting. Above-freezing surface temperatures during the austral summer of 2016-2017 have resulted in the formation of numerous surface-melt ponds around the Antarctic perimeter. Large cracks grow through Antarctic ice shelves as warmer ocean currents melt the towering glaciers from below. The overall picture is of a critical frozen region undergoing rapid change due to the human-forced heating of our world — a warming that has brought Antarctica to a tipping point, for such fundamental alterations to Antarctic ice are now likely to bring about a quickening rate of sea-level rise the world over.
Surface Melt Visible From Satellite

During 2016-2017, Antarctic surface temperatures ranged between 0.5 and 1 degree Celsius above the already warmer-than-normal 1979 to 2000 average for most of Southern Hemisphere summer. While these departures for this enormous frozen continent may not sound like much at face value, they’ve translated into periods of local temperatures up to 20 C above average. As a result, measures around Antarctica along and near the coastal zone have risen above the freezing mark on numerous occasions. These periods of much-warmer-than-normal weather have in turn precipitated widespread episodes of surface melt.

(This Antarctic volume-change melt map, which tracks thinning along various coastal ice shelves from 1994-2012, provides a good geographical reference for ice shelves experiencing surface melt or severe rifting. The Amery Ice Shelf [AME], King Baudouin Ice Shelf [BAU], and the Lazarev Ice Shelf [LAZ], stable through 2012, all showed extensive surface melt this summer. Meanwhile the Larsen C Ice Shelf [LAC] and Brunt Ice Shelf [BRU] both feature large rifts that threaten destabilization. Image source: Volume Loss from Antarctica’s Ice Shelves is Accelerating/

This year, one region in particular has seen temperatures hitting above 0 C consistently: the valley into which the Lambert, Mellor, and Fisher glaciers flow into the Amery Ice Shelf. There, warming has resulted in the formation of multiple large surface-melt ponds. The below image is a January 22nd NASA satellite shot of an approximate 100-by-40-mile section of this glacial outflow zone. The blue areas are melt ponds, some as large as 3 miles wide and 20 miles long.

The Amery Ice Shelf is one of East Antarctica’s largest. Like many of Antarctica’s ice shelves, Amery is melting, with about 46 billion tons of ice lost from this shelf alone each year. As with other Antarctic ice shelves, Amery’s melt is mostly below the surface, caused by warming ocean waters. However, in recent years, considerable surface melt on Amery’s feeder glaciers likely also contributed to significant volume losses in the self.

(Large melt ponds up to 20 miles long cover glaciers flowing into the Amery Ice Shelf on January 22, 2017. Image source: LANCE MODIS.)

Surface melt for Amery has become an increasingly prevalent feature since 2013, with 2017 melt for January 22 the most widespread for any of the past five years in this region. East Antarctica rarely saw large surface melt events prior to the 2000s, and this year’s warming and large melt ponds are a considerable feature. While basal warming is often the cause of the greatest mass losses, surface melt can act like a giant wedge driven into ice shelves, helping to break them up. Melt wedging in glaciers can also increase their forward rate of movement as heat content rises and as the points at which glaciers contact the ground become lubricated.
Moving north toward Dronning Maud Land along the East Antarctic coast, we find another region of surface melt ponding on the King Baudouin Ice Shelf. Nearly as widespread and extensive as the melt on the Amery Shelf’s glaciers, the King Baudouin melt is no less impressive and concerning.

(King Baudouin Ice Shelf shows extensive melt ponding along a 40-mile swath of its southwestern corner in January 2017. Image source: LANCE MODIS.)
The largest melt zone shows nearly continuous ponding along a 40-mile-wide diagonal near the ice shelf’s southwestern contact point with East Antarctica’s mainland. A smaller section of melt appears as light blue splotches about 60 miles to the west of the larger melt zone in the image above (for reference, bottom edge of frame represents about 250 miles).
Unlike glacial surface ponding near Amery, melt on King Baudouin occurs directly over the floating ice shelf. This form of melt adds greater stresses as the heavy pools of water can act as wedges that drive gaps in the ice apart. Past instances of widespread surface ponding have occurred in conjunction with the rapid break-up of Larsen ice shelves along the Antarctic Peninsula. Taking a look at past years in the satellite record, we find that this region of King Baudouin has been susceptible to melt since at least 2013. However, the extent of 2017 melt is the greatest in the record for this time of year.
The next ice shelf to the west of King Baudouin, the Lazarev Ice Shelf, shows extensive melt along what appear to be various rifting features streaming out from an open ocean gap where the ice shelf contacts land:
(Ten-mile-long melt ponds visible on the surface of the Lazarev Ice Shelf. Image source:LANCE MODIS.)

Over recent years, the ocean gap — visible as a dark section in center-bottom frame of the image above — has slowly grown larger. There, open ocean water has gradually taken up a larger and larger section of Lazarev’s land-contact point. Meanwhile, from 2013 to 2017, melt ponds have tended to radiate out from this open gap region along rifts in the ice shelf structure during summer as air temperatures have risen above freezing.
This year, melt appears to be quite extensive with two parallel 10-mile-long melt ponds filling in rift features with many smaller melt ponds interspersed. The open ocean gap combined with rifts filling with what is now seasonal melt water gives the overall impression of a rather weak structure.
Ice Shelves Cracking Up

Though regions on or near the Amery, King Baudouin and Lazarev Ice Shelves show the most obvious surface melt features, large melt ponds also formed near the Fimbul Ice Shelf. Ponds also formed during a Föhn wind event near the Drygalski Ice Tongue. Even as such instances of surface melt became a more obvious feature across Antarctica, at least two large ice shelves were run through by growing rifts that threatened their stability.

One such rapidly-expanding rift forced the British Halley VI research team to evacuate their base of operations on the floating Brunt Ice Shelf. This rift, which had until late 2016 been growing only gradually, doubled in length in less than three months. Its gaping chasm threatened to cut the expedition off from the Antarctic mainland and set it adrift at sea —forcing an early evacuation as a precaution.

(Drone footage of Brunt Ice Shelf’s rapidly growing crack. From October through early January, the crack doubled in size from 22 kilometers in length to 44 kilometers. Video source: Antarctic Survey.)

Meanwhile, a large crack that will soon result in a 2,000-square-mile iceberg breaking from the Larsen C Ice Shelf recently grew by another six miles to 100 miles long. The Connecticut-sized ice chunk now only hangs by a 15 to 20 mile thread. With the loss of this very large segment of ice, researchers are concerned that Larsen C may destabilize and ultimately succumb to the fate of Larsen A and Larsen B — breaking into thousands of separate icebergs and floating away into the Southern Ocean.

Signs of Melt, Destabilization as More Above-Freezing Temperatures are on the Way

With so many large melt ponds and melt-related rifts forming in Antarctica’s ice shelves, it’s worth considering that these shelves serve as a kind of door jam holding large glaciers back from flooding into the ocean. And as more ice shelves melt and destabilize, the faster these glaciers will move and the faster the world’s oceans will rise.
So much widespread melt and rifting of Antarctica’s ice shelves is a clear warning sign. And if enough of the ice shelves go, then rates of sea-level rise could hit multiple meters this century.
(Many locations along the coast of Antarctica will see 5-15 C above-average surface temperatures this week, a continuation of a strong surface melt pressure for the austral summer of 2016-2017. Image source: Climate Reanalyzer.)

This week, another spate of near- or above-freezing temperatures will run along the coastal regions of both east and west Antarctica, so the amazing atmospheric melt pressure that we are now seeing should continue to remain in play at least for the next seven days as austral summer continues. As for the melt pressure coming from the warming ocean beneath the ice shelves — that is now a year-round feature for many locations.

Hat tip to Shawn Redmond
Hat tip to Jeremy in Wales
Hat tip to Colorado Bob

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