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 the British Antarctic Survey, focuses on Pine Island Glacier, one of the largest and most threatening in West Antarctica. It is dumping nearly 50 billion tons of ice into the sea each year now – more than any other glacier on the globe except for its next door neighbor, Thwaites – and could ultimately raise sea levels by close to 2 feet all on its own.

This is happening because the glacier has been retreating backward and downhill – the marine-based glacier rests in very deep waters, and the terrain behind where it now 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 ungrounding the glacier from a roughly 800-meter deep ridge, upon which it was resting in a more stable alignment.

The surprise from the new study, though, is the suggestion that the ungrounding 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 of 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 toward 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 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 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 backward, the sediment layering became different, finer and more characteristic of the influence of the ocean and warm circumpolar deep waters.

Thus, the researchers could 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 question is what this sequence of events says about humankind’s responsibility for destabilizing West Antarctic glaciers. The world was less warm in the 1940s, 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.

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