Science likes to surprise us. That’s the extraordinary, mind-opening thing about it.

It’s possible that is now happening with one of the most stunning stories yet in the climate change saga — the finding that the enormous glaciers of West Antarctica appear to be retreating in an “unstoppable” way. It’s a process which, if it continues, could ultimately turn the West Antarctic ice sheet into an area of wide open ocean and raise global sea levels by 10 feet.

It has long been assumed that this destabilization of West Antarctica was caused by human-induced climate change. However, a new study published in the journal Nature Wednesday may have just made that story considerably more complicated.

The new research, led by researchers with the British Antarctic Survey but with accompaniment from scientists at U.S., German, Dutch, Swiss, and British universities, focuses on Pine Island Glacier, one of the largest and most threatening in West Antarctica. It is dumping nearly 50 gigatons (or billion tons) of ice into the oceans each year right now – more than any other glacier on the globe except for its next door neighbor, Thwaites — and could ultimately raise ocean levels by close to two feet all on its own.

This is happening because the glacier has been retreating backwards and downhill — the marine-based glacier rests in very deep waters, and the terrain behind where it currently touches the ocean gets even deeper inland. It’s an unstable configuration, and scientists have long suspected that warm ocean waters created the problem by effectively un-grounding the glacier from a roughly 800 meter deep undersea ridge, upon which it was resting in a more stable alignment.

The surprise from the new study, though, is the suggestion that the un-grounding may have started all the way back in the early to mid-1940s — while the entire world was at war and we didn’t have satellite images of Antarctica. It was a period that saw an early and distinct pulse of planetary warming, but things were not as hot as they are today.

The early 1940s were hot for a very particular reason — a strong and long-lasting Pacific El Nino event spanning from 1939 to 1942. This mega-El Nino, a precursor to the massive El Ninos we’ve since seen in 1997-1998 and 2015-2016, affected the circulation of the atmosphere all the way down in Antarctica, where stronger winds in the Amundsen sea region can allow warmer deep waters, called “circumpolar deep water,” to move in towards the glaciers. There’s general agreement that these waters are responsible for West Antarctic retreat.

The new study required scientists to set up camp atop the now floating section of Pine Island glacier and drill through the thick ice all the way down to the 800 meter deep ridge beneath it, where the ice once rested. There, they took several seafloor samples, or cores, from different parts of the ridge.

The researchers were able to date the sediments in the cores, and to discern key clues from them. For instance, when the ice sat on the ridge it “bulldozed” (in the scientists’ words) the seafloor down and buried areas on the ocean side of the ridge in new sediments. Whereas once the ice lifted from the ridge and shifted backwards, the sediment layering became different, finer and more characteristic of the influence of the ocean and warm circumpolar deep waters.

Thus, the researchers could date when the ice began to lift from the ridge and when it had completed the process of doing so. And they conclude that a little after the El Niño of 1939 to 1942, an “ocean cavity” opened up behind the ridge, one that warm waters could get into — a first sign of destabilization. However, the ice did not lift fully off the ridge until around 1970, the researchers believe.

The unavoidable question is what this sequence of events says about our own responsibility for destabilizing Pine Island (and, perhaps, other West Antarctic glaciers). The world was less warm in the 1940s, after all, and the role of human-caused global warming on El Nino events remains debated.

For their part, the authors insist they’re staying neutral on the human role, known in the science world as anthropogenic — they’re just reporting new, extremely hard-to-get observations.

“Teasing out whether the processes, the mechanisms that we’re talking about were related to anthropogenic forcing was never really the focus of this paper,” says James Smith, a glaciologist with the British Antarctic Survey who’s the first author of the study. “The 1940s is potentially a few years before the really big spike in anthropogenic-forced warming but it’s certainly within the realms of human-induced change.”

Smith also noted that while other glaciers in the region have also been retreating — including the very dangerous one at Thwaites, which is even bigger than the one at Pine Island — the study couldn’t say whether they, too, were in effect pushed by the same factors in the 1940s. And yet it’s hard not to wonder if whatever affected Pine Island during that era also affected its neighbors.

The study drew a range of comments from other experts not involved in the work — all of whom praised it, but who came to varied interpretations of its broader significance when it comes to the key question of human causation.

“Pine Island Glacier and probably others in the Amundsen Sea were destabilized in the 1940s, and have been retreating ever since, with a few brief periods of stability,” said Eric Steig, an Antarctic expert at the University of Washington-Seattle who has previously published on how El Nino events warm West Antarctica. “It will almost certainly continue this way. We don’t know if we caused it. It’s frustrating to not be able to say anything definitive on that last point, but I really don’t think we can.”

However, Richard Alley, a noted glaciologist at Penn State University, had a different interpretation. He suggested the possibility that what happened in the 1940s may have been the last in a long chain of El Nino and La Nina-linked wobbles back and forth for Pine Island, before more decisive human influences came in and destabilized it for good.

“The data collected probably would not be able to see earlier ungroundings on the upglacier side of the ridge, because later groundings would erase the record,” Alley continued. “If this model is correct, then the ‘real’ event that is most important is the main retreat in the 1970s, which is after human forcing had become more important.”

“In terms of the human influence on the Antarctic, I think you have to be very cautious about over-interpreting the results of the paper, that’s the bottom line,” added Eric Rignot, a polar researcher with NASA and the University of California-Irvine who published a blockbuster study on the destabilization of the Amundsen Sea in 2014.

Rignot praised the new observations, but said that recent changes in the Amundsen Sea region are strong and synchronous, and appear to be something different from what may have happened in the 1940s.

“Pine Island is not the only one, you have Thwaites, you have Smith-Kohler, they all sort of retreat at the same rate, so to blame things on just one little ridge, on one glacier, is probably a little bit risky,” he said. “The whole picture is there’s a common forcing to all of those, and there’s no way these glaciers could have retreated and sped up like they did in the last 20 years, and did that for decades, even a century.”

Even as the new paper seems likely to spark considerable debate, another just released study of Pine Island raises the stakes even further. Seongsu Jeong of the Ohio State University and colleagues from Ohio State and the University of Michigan suggests that Pine Island has begun a new, different, and troubling form of ice loss in recent years. Rifts are now opening in the center of its floating ice shelf (the part that was once grounded on the ridge), rather than at its front end, and the authors suggest it may be because warm water is carving deeply into that shelf from below.

If this process continues, that “would provide a potential mechanism for rapid ice shelf disintegration,” they write.

So what we know now is this: West Antarctica’s glaciers, and Pine Island glacier in particular, are in retreat, and this is happening even as we are changing the planet in myriad ways with our greenhouse gas emissions. We don’t know all that we could about how long this has been going on, and the further back in time you go the murkier it gets — but it’s still quite the coincidence.

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