It’s not common, across Earth’s history, for ice to sit on the surface as a permanent presence. It’s happened only three times in the past 550 million years, the same time period that complex, multicellular life has ruled the surface. It denotes that we are living in an Ice Age, scientifically, even if glaciers are not choking off Manhattan. The entire span of the human species has happened in an Ice Age. It’s an open question of whether climate change will end the one we are currently in.
But we can already detect one key change in how those two stores of water interrelate. For decades, the biggest driver of sea-level rise was heat itself, because as the ocean gets hotter, it literally takes up more space. (Scientists call this principle “thermal expansion,” and it applies to matter more generally: You demonstrate it at home whenever you run a jar under hot water to loosen the lid.) But in the past few years, meltwater from Greenland and Antarctica has overwhelmed this effect. Oceans are rising today primarily because they have more water in them.
“We’ve been saying all along that ice sheets would become dominant, and that signal is starting to appear,” Dutton said. (Dutton is having a busy week: She won a MacArthur genius grant this morning.)
And while this is a dramatic change, there’s a question in the middle of the report that portends an even more cataclysmic event. Hanging over the report, like an icy Damocletian saber, dangles the question: Will the Antarctic ice sheet collapse?
Read: After decades of losing ice, Antarctica is now hemorrhaging it
In 1978, the glaciologist John Mercer issued a warning in the scientific journal Nature. If people kept burning fossil fuels at the present rate, he wrote, then within 50 years they could set off the “rapid deglaciation” of West Antarctica. The process he identified—called “marine ice-sheet instability”—has haunted climate scientists for the past four decades.
Mercer’s problem begins with a simple fact: Ice floats in water. Many glaciers in West Antarctica have “wet feet,” as Dutton put it, meaning their front face sits in the water. Just like ice in a water glass, these glaciers want to float. But they don’t. The weight of the ice above the waterline keeps the entire glacier stuck to the seafloor.
But as it gets farther from the ocean, the bedrock of West Antarctica slopes downhill. If the glacier were to start retreating, then more and more of its mass would fall below the waterline. Eventually, the mass above the waterline would no longer keep the glacier stuck to the seafloor. The glacier would float off its foundation, the ice floe behind it would quickly spill out into the sea, and the glacier would quickly become so many melting ice cubes.
Once this process starts, it’s irreversible. It has never been observed—because we’ve never observed wrenching global climate change before. But since about 2006, more and more evidence has suggested that Mercer’s process is real and has happened in the past, Oppenheimer said.