The Plastic Earth: Connecting climate change to the planet’s shifting crust

Jonathan Shaw in Harvard Magazine:

SO16_image_Page_060_Image_0001Jerry Mitrovica is a solid-earth geophysicist, but the description is inapt. He spends much of his time demonstrating that the earth is not firm at all—it moves. His lab in Cambridge, for example, oscillates up and down by nearly eight inches twice a day. Mitrovica is a pioneer of dynamic topography, the study of such vertical motions. For most people, these ebbs and flows are new ground. But for Mitrovica, who investigates changes large and small to the planet’s shape, on timescales ranging from hours to eons, using evidence that ranges from the history embedded in coral to eclipse records, it’s terra firma. His research, of fundamental importance to earth scientists, also has a public resonance, because his discoveries about the planet’s plasticity, and his explorations of its shape-changing past, bear directly on the problem of melting ice sheets and rising sea levels in an era of rapid climate change.

Fame of the academic variety came early to Mitrovica and mushroomed about a decade ago, when he reminded people what happens to local sea levels in the vicinity of a melting ice sheet, like those covering Greenland and Antarctica. The effect was first described a hundred years ago, but “people had forgotten how big it was,” he says. “It’s big.” If Greenland’s ice sheet melted entirely, sea level would fall 20 to 50 meters at the adjacent coast. That’s counterintuitive, but the ice sheets are so massive (Greenland’s ice, one-tenth the size of the Antarctic ice sheets, weighs on the order of 3,000 trillion tons) that two immediate effects come into play. First, all that ice exerts gravitational pull on the surrounding ocean. When an ice sheet melts, that gravitational influence diminishes, and water moves away from the ice sheet, causing sea levels to drop as far as 2,000 kilometers away. (The drop is most pronounced close to the glacier, because gravity’s effects dissipate with distance.) But because the sea level has fallen where the ice sheet melted, it rises everywhere else beyond that 2,000-kilometer boundary, and on distant shores this rise is far greater than the global average. The effect amplifies the rise in average global sea level attributable to the addition of the meltwater itself to the oceans. (Greenland alone contributed a trillion tons of melted ice from 2011 to 2014.) Second, the land beneath the now-vanished ice sheet slowly rebounds, rising as the weight of the mass above diminishes, a process that continues for thousands of years after the ice sheet is gone. Locally, this doubles the relative drop in sea level. But globally, the uplifting crust pushes water outward, further raising sea levels around the world.

More here.