If you want to truly grasp the scale of Earth's polar ice sheets, you need some help from Isaac Newton. Newton taught us the universal law of gravitation, which states that all objects are attracted to one another in relation to their masses (and the distance between them).
The ice sheets covering Antarctica and Greenland are incredibly massive—Antarctica's ice is more than two miles thick in places and 5.4 million square miles in extent. These ice sheets are so large, in fact, that gravitational attraction pulls the surrounding ocean toward them. The sea level therefore rises upward at an angle as you approach an ice sheet, and slopes downward and away as you leave its presence.
This is not good news for humanity. As the ice sheets melt due to global warming, not only do they raise the sea level directly; they also exert a weaker gravitational pull on the surrounding ocean. So water sloshes back toward the continents, where we all live.
"If Antarctica shrinks and puts that water in the ocean, the ocean raises around the world, but then Antarctica is pulling the ocean towards it less strongly," explains the celebrated Penn State University glaciologist Richard Alley on the latest installment of the Inquiring Minds podcast. "And as that extra water around Antarctica spreads around the world, we will get a little more sea level rise in the US than the global average."
Alley, a self-described "registered Republican" and host of the PBS program Earth: The Operators' Manual, spoke on the occasion of truly dire news, of the sort that ice sheet experts like him have been dreading for some time. Last week, we learned from two separate research teams that the ice sheet of West Antarctica, which comprises just one relatively small part of Antarctic ice overall but contains enough frozen water to raise global sea levels by some 10 or 11 feet, has been irrevocably destabilized.
Scientists have long feared that of all the planet's great ice sheets, West Antarctica would be the first to go, because much of it is marine-based—the front edge of the ice sheet is bathing in increasingly warm water, which is melting it from beneath. Here's a helpful visualization of how this process works: