As Antarctic Glaciers Flow Faster, an Iceberg Six Times Larger than Manhattan Drifts Toward the Open Sea
Last November, a gargantuan iceberg broke free from the Pine Island Glacier in Antarctica. Six times larger than Manhattan and possibly 1,600 feet thick, iceberg B31 is now drifting slowly toward the Southern Ocean.
The time-lapse video above, posted by NASA’s Earth Observatory this week, consists of images from the Terra and Aqua satellites. It starts in early November, just as the iceberg was calving from the glacier, and it continues through mid-March, tracking B31 as it moves through Pine Island Bay. In the video, watch as B31’s passage is made progressively easier as floating sea ice ahead of it breaks up and is blown out of the bay by strong katabatic winds.
Pine Island Glacier has accelerated dramatically in 40 years, according to a study published in March in the journal Geophysical Research Letters. It is one of six giant glaciers in West Antarctica that the study shows are moving considerably faster now, dumping increasing amounts of ice into the ocean and thereby causing global sea level to rise.
Nearly 10 percent of global sea level rise is coming from just these six glaciers, according to Jeremie Mouginot, a glaciologist at the University of California-Irvine who co-authored the paper. And their contribution is increasing.
Using satellite images from 1973 to 2013, Mouginot and his colleagues tracked the movement of cracks and other surface features in the glacial ice, thereby enabling them to determine how far the glaciers had traveled over time. Based on this analysis they found that the total ice discharge from the six glaciers had increased by 77 percent since 1973. Half of the increase occurred between 2003 and 2009.
The maps above, from the paper by Mouginot and his colleagues, depict what the research has revealed. The left panel shows the speed at which the ice in the glaciers was flowing toward the sea in 2011, in kilometers per year. The Pine Island Glacier is at upper left, marked A2.
The right panel shows the change in flow speed between 1996 and 2008.
These six glaciers drain the massive West Antarctic Ice Sheet. If the WAIS were to disintegrate, flow into the sea and melt, sea level would rise by an estimated 10 feet or more. This would inundate enough coastal territory to affect more than 100 million people.
It turns out that the West Antarctic Ice Sheet appears to be more at risk of this happening than its larger counterpart, the East Antarctic Ice Sheet. And the new research raises fears that the WAIS may indeed be headed toward such a collapse.
“This very large part of the Antarctic Ice Sheet may already be undergoing marine ice-sheet instability, which will inevitably accelerate sea-level rise,” Mouginot told me in an email message today.
He was referring to a long-hypothesized phenomenon in which acceleration of the glaciers that drain the West Antarctic Ice Sheet gains momentum and becomes self-reinforcing.
Here’s a video that does a pretty good job of visualizing the phenomenon:
In the visualization, based on data from radar instruments on European Space Agency satellites, the Pine Island Glacier is seen where it empties into Pine Island Bay. Past what’s known as the “grounding line,” where the glacier rests on bedrock, the ice floats and is part of a giant, permanent ice shelf that fringes the coast and tends to hold back the flow of the glaciers.
In the visualization, the ice shelf can be seen flexing up and down from tidal action. And as sea water, which has become warmer at least in part from human-caused global warming, circulates under the ice, it causes the shelf to thin. With less of a buttress to hold things back, the glacier speeds up. This, in turn, causes the grounding line to retreat.
And here’s the problem: Pine Island and the other five glaciers mostly rest on rock that’s significantly below the level of the sea. This is why the West Antarctic Ice Sheet is most vulnerable.
As the grounding lines of the glaciers retreat into areas of bedrock below sea level, the result should be accelerating iceberg calving and ice thinning, and a continued retreat of the grounding line — all in a process that feeds on itself and eventually leads to a runaway collapse of the ice sheet.
For an analogy, think of a dam developing a leak. As the water spits from the hole, it tears more and more material from the dam, causing progressive weakening, until eventually it just gives way and the water behind the dam rushes out in a massive collapse.
To be sure, this is an imperfect analogy, because the ice dynamics are actually very different. But here’s the similarity: Once the process begins — once the hole in the dam becomes big enough, and once a glacial grounding line slips into territory below sea level, the process gains momentum and can’t be stopped.
This is how the process is thought to work in theory. Of course, nature is a bit messier than that. So it should be noted that after speeding up by 75 percent, the ice flow at the grounding line of the Pine Island Glacier actually stabilized between 2009 and 2013. At the same time, however, ice velocity kept increasing farther inland — more quickly than predicted.
Also, calving from the nearby Thwaites Glacier during the past three years “has been much more extensive and way more impressive” than that from the Pine Island Glacier, Mouginot says. He and his colleagues found that following a decade of near-stability, the speed of ice across the glacier increased rapidly after 2006. (Click on the thumbnail at right to see an aerial photograph of the calving front of the Thwaites Glacier’s ice shelf.)
The result: a 33 percent increase in ice being discharged by the glacier into the sea, which was more than enough to make up for the slow down in the discharge of Pine Island Glacier.
When viewed over the long term, “all the glaciers in the ASE sector are losing an increasing amount of ice into the ocean every year,” Mouginot and his colleagues conclude in their paper. “These observations are a possible sign of the progressive collapse of this sector in response to the high melting of its buttressing ice shelves by the ocean.”
According to the latest report of the Intergovernmental Panel on Climate Change, the best available evidence indicates that continued warming of the climate could lead to a collapse of the West Antarctic Ice Sheet through the marine ice-shelf instability phenomenon.
But here is a big caveat: the IPCC report also notes that there is currently not enough information to say when it might start, exactly how much it might contribute to sea level rise, and over what period of time. The report cites one study that used a computer model to simulate a collapse of Antarctic ice sheets that occurred thousands of years ago. These simulations produced a rise in sea level of 7 meters (22 feet) over a time period ranging from 1,000 to 7,000 years. That’s a very long time — perhaps slow enough for societies to adapt should current climate change trigger a collapse of the West Antarctic Ice Sheet.
But here’s one last caveat: Neither this computer simulation, nor any others, “have indicated the possibility of self-accelerated ice discharge from these regions.” So should it occur, it sounds like scientists are in the dark about how fast things might progress.
To me, this is yet another argument for the precautionary principle. While we don’t have all the answers, we know enough to say that the consequences could be enormous. So better to be safe than sorry.
My opinion, at any rate.