One day in 2016, a British glaciologist named John Moore attended a meeting in Cambridge, England, that included a presentation about a glacier on Greenland’s west coast. Typically referred to by its Danish name, Jakobshavn, but also known by its Greenlandic moniker, Sermeq Kujalleq, the glacier functions as a kind of drain, situated on the edge of Greenland’s massive ice sheet, that moves 30 billion to 50 billion tons of icebergs off the island every year. These icebergs, some of them skyscraper-size, calve regularly from the glacier front, crash into a deep fiord and float west into Disko Bay. Then they drift out into the North Atlantic, break apart and melt. The intense activity here, as well its breathtaking location, have earned the area a designation as a UNESCO World Heritage site and made it a powerful attraction for Greenland’s small but vibrant tourist trade.

For scientists, though, Jakobshavn elicits urgency. Glaciologists have identified it as one of the fastest-deteriorating glaciers in the world. And as waves lap higher on the shores of cities like Miami Beach and New York, this far-off ice is partly the reason. Jakobshavn alone was responsible for 4 percent of the rise in global sea levels during the 20th century. It probably contains enough ice to ultimately push sea levels up at least another foot.

What struck Moore that day in Cambridge was not only Jakobshavn’s potential collapse but also the way its ice interacts with the surrounding water. A slide at the meeting showed how warm water nearly a thousand feet below the surface flows from the Atlantic into Disko Bay and eventually makes its way into the fiord and to the glacier front, where it carves away at and weakens the ice. Moore noted something interesting at the bottom of the bay’s entrance: the warm water flows over a sill, a ridge rising several hundred feet above the ocean floor and just over three miles long, akin to a threshold that crosses the floor of a doorway between two rooms. “It’s kind of a pity that sill isn’t just a bit higher,” Moore thought. “Because then it would stop the warm water from coming in and hitting the glacier.” Not long after, he wondered: What if someone made the sill higher?

For the next year, Moore mulled over that question. How hard would that be? How expensive? And how effectively could a raised sill halt the influx of warm water and slow Jakobshavn’s shrinkage?

In 2018, Moore and his colleague Michael Wolovick published an article that proposed looking into building a sea wall 100 meters high, or about 328 feet, on the floor of Disko Bay. Raising the sill, using gravel and sand, could reduce the warm water in the fiord and allow Jakobshavn to thicken naturally and stabilize. Moore believed that such a sea wall might not only decrease Greenland’s contribution to sea-level rise; it might also serve as a trial run for far grander ambitions. If the idea proved workable in the Arctic, it could be translated to Antarctica, where much larger glaciers in the Amundsen Sea, especially one known as Thwaites, threaten to raise sea levels substantially. “Should we spend vast sums to wall off all the world’s coasts,” Moore and Wolovick asked in Nature, “or can we address the problem at its source?”