Finally, good news from Antarctica
Earth mantle helps stabilize glaciers
At least since 2014, the news has been dire: The West Antarctic ice sheet is losing ice, and its retreat may be unstoppable. It may be only a matter of time (granted, maybe a very long time) before it adds as much as 10 feet to global sea-level rise. Already, ice loss in the region is accelerating, nearly tripling in the past 10 years alone. But on Thursday, scientists reported they may have discovered a possible mitigating factor, one that could slow or even prevent the ice sheet’s full collapse into the ocean. Namely, Earth’s mantle deep beneath several key West Antarctic glaciers appears to be rebounding, rising upward as the weight of ice on top of it lifts - and doing so at a rapid rate of 41 millimeters per year, or just over 1½ inches. “In this particular region, there is massive ice that is going away, that is melting, and we also have a massive reaction of the Earth,” said Valentina Barletta, a researcher with the Technical University of Denmark who led the work published in Science. “However, this reaction is way more than we expected.” It has long been known that Earth’s crust rebounds as glacial ice lifts from it. But it’s the speed that matters, and the coastal West Antarctic region seems to be showing rates that far exceed other areas of what is known as glacial isostatic adjustment. Barletta and her colleagues, from several institutions in the United States, Denmark and the Netherlands, collected GPS measurements from rock outcroppings in West Antarctica to measure the fast rise and contend that it will help counteract the ice sheet’s collapse in three ways. First, raising the remaining ice will lower the contact between the ocean and the ice sheet, which will decrease its tendency to float away. Second, elevating underwater surfaces can catch the ice and provide friction. Finally, easing the down- ward slope of the seafloor beneath the ice could slow its retreat. The last factor is one reason that West Antarctica was thought to be in unstoppable retreat. The marine-based ice sheet sits on top of a submarine slope that runs downhill toward the center of the Antarctic continent. In such a situation, the process of ice melting from warm ocean water and retreating backward has a tendency to just keep going. “The main result is amazing, very different from the average behavior of the Earth there in this very special region where a lot of ice is melting,” Barletta said. Precisely how much this uplift will protect ice remains to be seen. West Antarctica’s massive glaciers are being melted by a thick, deep layer of warm ocean water that touches them at their bases. The glaciers, which are largely submerged, stand in waters hundreds of meters, or even a kilometer, deep. Thus a small rise of an inch or more every year would not be enough, on its own, to lift them above the warm water layer that is causing the damage. That melting will continue regardless. Still, the point is that there seem to be competing forces. Christopher Harig, a geoscientist at the University of Arizona who commented on the new work by email, said: “On the one hand, we’re investigating new changes and positive feedbacks to the ice itself, such as how surface melt will increase over time and where it can increase ice loss,” he wrote. “On the other hand, here we see the negative feedback from the solid Earth might be even bigger than we thought. So what will happen in West Antarctica in the next century is a very complex question.” Harig and Barletta also pointed out that if the earth is rising more than previously thought below West Antarctica, then West Antarctica is losing more ice than we thought. When scientists measure the mass loss of West Antarctica using gravity readings from satellites, they include an adjustment for any mass increase that is coming from the rising earth. But if there is more solid earth rise than they thought, then the ice mass loss must also have been larger to achieve the same total change in mass.