Other researchers find this possible future somewhat fantastic. “We, as European modelers, are slightly more skeptical of the marine-cliff idea,” Frank Pattyn, a glaciologist at the Free University of Brussels, told me. “It has not been observed, not at such a scale.”
Yet even MICI’s skeptics agree: Our understanding of sea-level rise is rapidly growing more ominous. In its last major report, in 2014, the Intergovernmental Panel on Climate Change projected that oceans could rise two feet by 2100 if greenhouse-gas emissions continue on a worst-case trajectory. That number will almost certainly worsen in the IPCC’s next report, which is due in 2021, Pattyn said. “We are facing sea-level rise that is obviously going to be higher in the mean than what the IPCC’s ‘Fifth Assessment Report’ showed,” he said.
“Nobody’s debating that sea-level rise is happening. It’s back to how much, how fast,” Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography, told me. Even the most optimistic scientists have recently increased their low-end estimates, she said. “It’s healthy to have this debate.”
Read: After decades of losing ice, Antarctica is now hemorrhaging it
There is only one place in the world where MICI is definitely happening: Jakobshavn Glacier, on the west coast of Greenland. (Locals call it Sermeq Kujalleq.) In the 19th century, Jakobshavn was a long river of ice that snaked out of its fjord to meet the surrounding, frozen bay. Now, the bay rarely freezes, and Jakobshavn has retreated miles back into its canyon, forming a tall, brittle cliff face that regularly births icebergs as tall as a house. (Some of those icebergs are so enormous that they get stuck leaving the fjord.) All that ice has to come from somewhere: These days, Jakobshavn empties ice from the center of Greenland twice as quickly as it did during the last century.
Last month, in a large hall at the same AGU conference, several hundred researchers gathered to see a set of presentations billed as a series of updates on new glacier and ice models. It was far closer to a proxy debate on the ice-cliff question. Several of the talks had “marine ice-cliff instability” in the title, and I had heard more than one group of glaciologists gossiping about it days in advance.
Alley, the Penn State glaciologist, addressed the sapphire-colored elephant in the room immediately after taking the dais. As he sees it, it’s just common sense that Antarctic glaciers will develop problematic ice cliffs. The Jakobshavn Glacier, only a few miles wide, has not significantly changed the rate of global sea-level rise. Pine Island Glacier in West Antarctica, on the other hand, is more than 30 miles wide. It holds enough ice to raise sea levels worldwide by about five feet. “What we’ve always relied on is that unzipping one fjord does not affect the global ocean,” Alley said. “What’s different is that here and here and here”—he pointed to glaciers in West Antarctica—“unzipping one fjord will matter a lot.”