Thursday, November 3, 2016

New Study Shows Staggering Losses of Antarctic Ice - And Probable Devastating Future Sea Level Rise




Two 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. 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 farther 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.

Now, a new study by a team led by Ala Khazendar, a geophysicist and polar expert at NASA's Jet Propulsion Laboratory, shows 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. That translates to a total of half a kilometer of ice lost during the study period.

The study team attributed the extreme, radar-detected melting to an influx of warmer water from the Amundsen Sea over the time period. Some specialists, not part of the Khazendar team, have suggested that changes in wind patterns and atmospheric circulation transported more warm water from other parts of the ocean but the effect has tapered off near the end of the decade. Hence, over the interval 2009-14 ice loss slowed significantly and the Pope, Kohler glaciers seemed to stabilize.  However, the Smith Glacier continued its retreat, behavior attributed to differences in topography.

The new research points explicitly to global warming that's heating up the oceans, and puts the kibosh on the deniers' trope that Antarctic ice is net "growing". . Especially unnerving is the evident retreat of the Thwates and Pine Island glaciers which melting holds some of the greatest potential for sea level increases. To be specific, these sea -backing glaciers have the potential to cause about 4 feet of sea level rise while the ice contained in all of West Antarctica could trigger a ten foot sea level rise if it melted.

Previous research, e.g. by Rignot et al suggested these glaciers experienced unusual rapid retreat in the 2000s, according to Khazendar.   The new study has taken a closer look at what is happening to the glaciers below the surface of the water. This definitely gets a better handle on the physical processes at work.

The most alarming aspect is that the rapid ice loss appears to be a runaway process. The more the bottom of the shelves melt, the more ice is exposed to warm water that accelerates further rapid melt.  One hopes that climate change deniers take note of this new study as well as the previous ones cited, and for once put their delusions behind them.

But one might as well hope that Trump graciously bows out of this election before November 8th and spare the nation the aftermath of what is already a debacle.


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