Pine Island Glacier, West Antarctica, shortly before losing an enormous iceberg. (NASA Earth Observatory)

If there is one story that, more than anything else, makes you wonder if global warming could cause very fast changes and hit planetary tipping points in our lifetimes, it was a moment in 2014.

That was when two separate research papers said there was reason to think a frozen sector of West Antarctica, called the Amundsen Sea region, may have been destabilized. West Antarctica as a whole contains enough ice to raise sea levels more than 3 meters (10 feet), and the Amundsen Sea’s ocean-front glaciers themselves account for about 1.2 meters (4 feet). Two of the largest are Pine Island Glacier, about 25 miles wide at its front that faces the ocean, and capable of someday driving about 1.7 feet of sea level rise, and Thwaites glacier, the true monster, which is 75 miles wide where it hits the ocean. It contains about 2 feet of potential sea level rise but also, it is feared, could destabilize the ice in all of West Antarctica if it goes.

On Thursday, the National Science Foundation and the U.K.’s Natural Environment Research Council made a joint announcement signaling how grave this really is — they will fund a multi-million dollar research initiative to the less-studied Thwaites, in order to determine just how much it is capable of contributing to sea level rise during our lifetimes, and by the end of the century.

It will take years of preparation for scientists to even get to the glacier, however. And in the meantime, a new study of Pine Island Glacier, just released in Geophysical Research Lettersreaffirms why this region of Antarctica is so worrisome. The study finds that as the ice melts, the glacier that remains has retreated so far backwards in the face of warm ocean temperatures, exposing so much additional thickness to the ocean in the process, that even a recent bout of cooler water temperatures did little to slow the pace of its ice loss. The work was co-authored by 20 separate scientists based at U.S., British, and Korean institutions, and the first author was Knut Christianson, a glaciologist at the University of Washington in Seattle.

The problem is that in this part of West Antarctica, you have everything you don’t want on a warming planet – a changing ocean up against glaciers that are both very wide and very deep. And scientists now know that warm ocean water is reaching these glaciers at depth, and melting them from below – causing them to shrink, leaving the remaining glacier to retreat backwards and inland. And as they retreat, the seafloor gets deeper the further back they go — what researchers refer to as a “retrograde” configuration. The deeper the water gets, the more ice that can be exposed to the ocean, and the more the glaciers are thereby capable of losing. So there is a fear that there is here something that is called a “marine ice sheet instability” in which, once you start this process, you can’t stop it — and that it has already been started.

While not as large and wide as Thwaites, Pine Island is among the fastest melting glaciers in Antarctica. A particular spot of ice atop the glacier will move 4 kilometers seaward per year, at the glacier’s fastest-moving sections, and the glacier as a whole dumps about 45 billion tons of ice into the ocean annually — a bigger contribution to sea-level rise than any other single glacier on Earth except for perhaps Thwaites, which is currently contributing roughly the same amount, according to Christianson. Pine Island Glacier is currently grounded in waters that are over a kilometer deep, sitting on a flat ridge at the moment, but with still deeper terrain behind it if it is to retreat further.

Pine Island is now in this situation because it has been in retreat for decades — and the retreat, over an area of 18 miles in length where the glacier first touches the seafloor, has been down a slope that gets deeper inland. Between 1974 and the year 2010, the new paper notes, the glacier’s ice increased its outward flow speed an extraordinary 75 percent even as the amount of ice that it lost annually increased by 750 percent.

Although it is extremely remote — even for Antarctica — scientists have ocean sensors in the waters around Pine Island Glacier, and were therefore able to detect an anomalous change in ocean temperatures between 2012 and 2014, which may have been spurred by a distant La Nina event. They also have instruments atop the glacier’s floating ice shelf.

These instruments allowed the researchers to identify the period of cooler ocean temperatures, a seemingly natural flux where waters between 450 and 770 meters depth cooled by about a degree Celsius. That’s enough of a change that you would expect Pine Island to slow down its melt.

And it did – somewhat. However, GPS devices atop the ice showed that the speed of flow only decreased a tiny bit — down to 3.85 kilometers per year, instead of the usual 4 kilometers per year. That’s not much change at all. And the speed quickly rebounded as the cool ocean period ended.

“It hasn’t finished reacting to the retreat that began some time ago, and so that means that small fluctuations like this are insufficient to stabilize the glacier system, and the strong mass loss and thinning that’s continuing in the basin is not going to be altered by this kind of ocean fluctuation,” said Christianson.

The gist is that the glacier has retreated too far already, and too much of it is exposed to the ocean, for a temporary cooling blip to matter. For Pine Island “or other drainages with retrograde bed slopes,” the authors conclude, “once unstable retreat has been triggered, reversing the long-term sea-level contribution likely would require significant multi-decadal shifts away from the climatic changes that initiated more rapid melting and ice flow.”

In other words, you’d need a much longer and sustained cold period — one that nobody thinks is coming.

“It’s a very large glacier, it’s losing mass very rapidly, there’s no reason to suspect that will change any time in the near future,” said Christianson.

And when the researchers mention “other drainages with retrograde bed slopes,” you have to think they’re referring not just to Pine Island, but to Thwaites.

Read more at Energy & Environment:

Greenland’s ice is melting from both above and below — and scientists say they’re connected

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