“This is the gold standard in terms of demonstrating that ice sheets are changing,” said Robin Bell, a professor of geophysics at Columbia University who was not involved in the paper. “You have three measurements, three approaches, from three different instruments, and they all show pretty much the same thing.”
When something is the size of a continent, how can you tell if it’s shrinking? Scientists have a few different tools at their disposal. First, Antarctica is so massive that it exerts its own gravity field, which can be sensed from orbit by satellites like NASA’s GRACE. Second, researchers can shoot radar or lasers at the surface of Antarctica to detect its surface altitude, which they can then combine with knowledge of ice physics and topography to compute its balance.
Finally, they use the “input-output method.” You may have heard of the 16th-century medical pioneer Santorio Santorio, who discovered human metabolism by weighing his food, his urine, his feces, and himself every day for 30 years. The input-output method applies the same idea to a continent: By measuring the velocity of moving glaciers (often with GPS), researchers can calculate how much snow is being added to a glacier and how much is disappearing into the sea.
The new study combines two dozen previous estimates of Antarctica’s changing mass, arrived at through all three techniques. “These are completely independent data sets,” said Csatho, the University at Buffalo professor. Her team worked on the altimetry data. She told me that she was pleased by how the different methods arrived at roughly the same conclusions. “We didn’t know until a few weeks ago where our results would sit relative to each other. It was a very nice surprise to see our results sitting right where they should be,” she said
The study contains two particular pieces of ominous news—especially for Americans. First, it finds that two glaciers in western Antarctica, named Thwaites and Pine Island, are losing mass at a particularly fast clip. Recent research has suggested that these glaciers may be subject to a feedback loop called “marine ice-cliff instability,” in which huge walls of ocean-facing ice buckle under their own weight and tumble into the sea. It’s not yet clear whether marine ice-cliff instability will happen at these two glaciers, but if it does kick in, then Thwaites and Pine Island would begin rapidly disintegrating, catastrophically raising global sea levels. Under that scenario, the two glaciers could increase global sea level by more than four and a half feet by 2100, inundating the homes of more than 150 million Americans.
Rob DeConto, a professor of climatology at the University of Massachusetts, Amherst, published research that first warned of that scenario a few years ago. He said it was too early to know whether marine ice-cliff instability had taken hold. “It’s pretty amazing that there’s been such a big uptick in the pace of mass loss down there, but we still can’t say that it’s because this cliff mechanism is kicking in now,” he told me.