Friday, June 15, 2018

New Multi-Input Study Shows West Antarctic Ice Shelf Melt Accelerating

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Four  years ago, I cited a study showing that West Antarctica's unstoppable ice shelf collapse had likely begun. (Physics Today, July, 2014, p. 10). A  team led by Eric Rignot at UC-Irvine and NASA had documented accelerated glacial retreat in the red-shaded region of Fig. 1.. Most importantly, the researchers pointed out the lack of any geological features (e.g. bedrock 'bumps')  that might re-stabilize the ice.

The fragility of the West Antarctic ice shelf can be traced to several factors which physics can shed light upon. These are illustrated in Fig. 2, below (ibid.):
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First, as shown, the ice rests on a bed that lies below sea level. Second, note that the bed slopes backward, actually falling deeper below sea level further inland. It is this confluence of conditions that gives rise to marine ice-sheet instability since currents of warmer water eat away at the ice from below. When the 'grounding line' (see Fig. 2) gets pushed back by warmer-than-usual water then the rate of melt discharge increases and the glacier retreats further.

Rignot and colleagues used 20 years of interferometric synthetic aperture radar (Insar) data to track the retreat of grounding lines in 4 West Antarctic glaciers, including the two shown in Fig. 1. Their data showed that for these two areas the grounding lines had retreated between 10 and 35 km over the 20 year time and the rates are now speeding up. 

Then in 2016, a study by a team led by Ala Khazendar, a geophysicist and polar expert at NASA's Jet Propulsion Laboratory, showed some of the most rapid ice losses ever observed in the same region.  This new research published Tuesday in journal Nature Communications, focuses on the Smith, Pope and Kohler glaciers which are buttressed by the Dotson and Crosson ice shelves, not far from the Thwaites Glacier.  The Khazendar team analyzed radar survey data collected by NASA research aircraft at various intervals between 2002 and 2014 which provided direct measurements of ice loss below the surface of the ocean. The team found that between 2002 and 2009 the affected glaciers experienced some of the fastest ice loss in decades. This was particularly so for the Smith Glacier, for which it was found that the ice shelf thinned below the surface by 40m - 70m a year. 

Now a study published Wednesday in the journal Nature shows that the melting is speeding up even faster, indeed accelerating.  The rate at which Antarctica is losing ice has tripled since 2007, according to the latest available data. The continent is now melting so fast, scientists say, that it will contribute six inches (15 centimeters) to sea-level rise by 2100. This is at the upper end of what the Intergovernmental Panel on Climate Change has estimated Antarctica alone could contribute to sea level rise this century.

Between 1992 and 2017, Antarctica shed three trillion tons of ice. This has led to an increase in sea levels of roughly three-tenths of an inch, which doesn’t seem like much. But 40 percent of that increase came from the last five years of the study period, from 2012 to 2017.

Graphic showing increased melt levels in Antarctica.

Most noteworthy is how outside experts have praised this study as authoritative. Unlike single measurement studies, this investigation examined ice loss in 24 different ways including the use of 10-15 different satellites, as well as ground and air measurements and computer simulations. This is according to lead author Andrew Shepherd of University of Leeds.  

Prof. Shepherd also noted:  "Under natural conditions we don't expect any loss of ice at all. There are no other plausible signals doing this other than climate change."  

University of Colorado ice specialist Waleed Abdalati put a final emphasis on this, observing:  "The forces that are driving these changes aren't going to get any better".  
And to quote yet another ice specialist, Twila Moon, 

"Ice speaking, this situation is dire."  

Again, let's reiterate that in Antarctica it is mostly warmer water causing the massive melting - eroding the floating edges of the ice as depicted in Fig. 2. Readers interested in the relevant forces would do well to study this graphic closely.

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