A wide view of the rift in Larsen C from the vantage point of NASA’s DC-8 research aircraft. NASA scientist John Sonntag snapped the photos on November 10, 2016, during an Operation IceBridge flight. Photo: NASA

One of The Largest Icebergs on Record Just Broke off Antarctica. Now What?

, senior climate scientist | July 13, 2017, 10:38 am EDT
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An iceberg, among the largest on record (since satellites started tracking in 1978), broke off the Larsen-C ice shelf along the Antarctic Peninsula.  The iceberg is greater than the area of Delaware and a volume twice that of Lake Erie.  What were the origins of this event, and now what?

Origins of the gigantic iceberg

Cryosphere shematic, terms, and contributions to global sea level rise. Source: IPCC 2013

Terms for cold regions of Earth. Contributions to global sea level rise as of the 2013 publication of the Intergovernmental Panel on Climate Change fifth assessment report (working group 1). Source: (to see enlarged graph) IPCC AR5 WG1 figure 4-25.

In order to understand the present and future implications, we can quickly run through some facts regarding the origins of this gargantuan iceberg.  As we do this, it’s helpful to get a refresher on terms and recent trends for sea level rise contributions from cold regions  (see figure from the IPCC AR5 WG1 Figure 4-25).

Glaciers outside of Greenland and Antarctica have been the largest ice source contribution to global sea level rise between 1993 and 2009.  Antarctica and Greenland have increased their contribution over the recent part of this period.

Now, a quick look at the iceberg, and how it formed:

What? An Iceberg, likely to be named A68, weighs more than a trillion tons.

Where?: This iceberg used to be part of the floating Larsen C Ice Shelf located along that part of Antarctica that looks like a skinny finger pointing toward South America.

When?:  The iceberg broke away sometime between July 10 and July 12, 2017 (uncertainty due to the gap between repeat passes by satellites).  Despite the current predominance of polar darkness in the region, several satellites detected this event with special instruments: NASA’s Aqua MODIS, NASA and NOAA’s Suomi VIIRS, European Space Agency Sentinel-1 satellites.

Why?:  It is natural for floating ice shelves to break off – or to “calve” – icebergs, as was captured in this unforgettable time lapse video clip from the film Chasing Ice.  The Larsen C ice shelf is a type that is fed by land-based ice – called glaciers – on the Antarctic Peninsula. The shelf size depends on the supply of ice from the glaciers and snow minus the loss of ice from calving and melting.

While calving is entirely natural, scientists are investigating other factors that could have played a role in the size and the timing of this event.  An ice shelf can melt and thin if the surface air temperature or ocean waters beneath an ice shelf warm above the freezing point.  The Antarctic Peninsula has experienced surface temperature warming over recent decades that is unprecedented over the last two millennia in the region.

Now what?

Break up of Larsen B Ice Shelf

Larsen B ice shelf demise (NASA MODIS image by Ted Scambos, National Snow and Ice Data Center, University of Colorado, Boulder). Source: https://nsidc.org/news/newsroom/larsen_B/index.html

Immediate Risks: Not much in terms of global sea level rise since the ice shelf was already floating.  Similar to the demonstration with floating ice cubes melting in a cup of water and the liquid water level remains the same. If iceberg A68 had instead suddenly calved from land-based ice, according to Gavin Schmidt (NASA), it would have contributed 2.8mm to global sea level.

The iceberg could pose a navigation hazard for ships.   Iceberg A68 can drift for years and, based on typical iceberg tracks for this region, it would likely move to lower latitudes where more ships would have to avoid navigating too close. For now, few ships would head that far south during the Antarctic winter and are likely to place greater risk on large waves when they pass through the seas surrounding Antarctica.  These have “unlimited fetch” where strong winds can generate some of the largest waves in the world with a well-earned reputation amongst seafarers embedded in the nautical terms referring to these hazardous southern latitudes: “roaring forties” and “furious fifties.”

Near-term risks:  Scientists will closely track developments to see if the Larsen C ice shelf rebounds or follows the fate of nearby and lower latitude ice shelves that have disintegrated (Larsen A and Larsen B) over the past two decades.

The data that will be tracked include processes observed during Larsen B disintegration such as meltwater ponding, changes to snow accumulation or loss, and meltwater penetrating deep into the ice shelf through cracks that can increase ice loss.

To better understand the risks, we also need critical information, currently difficult to obtain, regarding ocean temperatures underneath the Larsen C ice shelf.   Warmer ocean waters lapping at the new fresh edge of the Larsen C ice shelf and penetrating deeper underneath could increase the risks for Larsen C shelf thinning and potential disintegration.

Long-term risks: Ice shelves buttress glaciers. If ice shelves are no longer there to buttress the glaciers and “put the brakes on” the flow of ice from the land-based ice sources, these glaciers could accelerate ice flow rates and directly contribute to sea level rise.  Many studies document many times greater flow in glaciers after complete disintegration of the Larsen B ice shelf.

“Glacier-ice shelf interactions: In a stable glacier-ice shelf system, the glacier’s downhill movement is offset by the buoyant force of the water on the front of the shelf. Warmer temperatures destabilize this system by lubricating the glacier’s base and creating melt ponds that eventually carve through the shelf. Once the ice shelf retreats to the grounding line, the buoyant force that used to offset glacier flow becomes negligible, and the glacier picks up speed on its way to the sea.”  Image by Ted Scambos and Michon Scott, National Snow and Ice Data Center, University of Colorado, Boulder. Source: NSIDC

If a similar sequence of events were to occur with the Larsen C ice shelf, then coastal planners likely need to know the scale of the potential risk and how quickly it could happen.   The Larsen C ice shelf is fed by glaciers on the skinny Antarctic Peninsula which contains an estimated combined equivalent of 1 cm contribution potential to future global sea level.

The pace and timing are big questions for scientist to monitor and make projections based on models that incorporate the processes observed.  These could improve sea level rise projections in a world with the Paris Agreement fully implemented (i.e. with limits of no more than 2 degrees Celsius global temperature rise above pre-industrial) versus higher emissions scenarios.  A good resource on the current estimates for the timing of a threshold for chronic inundation for many U.S. coastal communities is the new UCS report released yesterday and the accompanying peer-reviewed publication –  Dahl et al., 2017.

IPCC 2013 WG 1 Figure 4-25
Ted Scambos NSIDC NASA

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