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maltatoday | SUNDAY • 12 JUNE 2022 15 EYEWITNESS This is where Thwaites Glacier meets the sea. This pattern, with deeper ice piled high near the center of an ice sheet, and shallower but still low bedrock near the coast, is a recipe for disaster – albeit a very slow-moving disaster. Ice flows under its own weight – something we learned in high school earth science, but give it a thought now. With very tall and very deep ice near Antarctica's centre, a tremendous potential for faster flow exists. By being shallower near the edges, the flow is held back – grinding on the bedrock as it tries to leave, and having a shorter column of ice at the coast squeezing it out- ward. If the ice were to step back far enough, the retreating front would go from "thin" ice – still nearly 3,000 feet thick – to thicker ice toward the center of the continent. At the retreating edge, the ice would flow faster, because the ice is thicker now. By flowing faster, the glacier pulls down the ice behind it, al- lowing it to float, causing more retreat. This is what's known as a positive feedback loop – retreat leading to thicker ice at the front of the glacier, making for faster flow, leading to more retreat. Warming water: The assault from below But how would this retreat begin? Until recently, Thwaites had not changed a lot since it was first mapped in the 1940s. Early on, scientists thought a re- treat would be a result of warmer air and surface melting. But the cause of the changes at Thwait- es seen in satellite data is not so easy to spot from the surface. Beneath the ice, however, at the point where the ice sheet first lifts off the continent and begins to jut out over the ocean as a floating ice shelf, the cause of the retreat becomes evident. Here, ocean water well above the melting point is eroding the base of the ice, erasing it as an ice cube would disappear bobbing in a glass of water. Water that is capable of melt- ing as much as 50 to 100 feet of ice every year meets the edge of the ice sheet here. This erosion lets the ice flow faster, pushing against the floating ice shelf. The ice shelf is one of the re- straining forces holding the ice sheet back. But pressure from the land ice is slowly breaking this ice plate. Like a board splin- tering under too much weight, it is developing huge cracks. When it gives way – and mapping of the fractures and speed of flow suggests this is just a few years away – it will be another step that allows the ice to flow faster, feeding the feedback loop. Up to 10 feet of sea level rise Looking back at the ice-cov- ered continent from our camp this year, it is a sobering view. A huge glacier, flowing toward the coast, and stretching from hori- zon to horizon, rises up to the middle of the West Antarctic Ice Sheet. There is a palpable feeling that the ice is bearing down on the coast. Ice is still ice – it doesn't move that fast no matter what is driv- ing it; but this giant area called West Antarctica could soon be- gin a multicentury decline that would add up to 10 feet to sea level. In the process, the rate of sea level rise would increase sev- eralfold, posing large challenges for people with a stake in coastal cities. Which is pretty much all of us. TheConversation.eu Left: Warming water is reaching under the ice shelf and eroding it from below. Scambos et al 2017 Below: A map of Antarctica seen from above, most of it the ice sheet, shows the velocity of the ice flow ice. Thwaites Glacier is on the left. NASA's Goddard Space Flight Center Scientific Visualization Studio